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Article
Article Type:
Research Article

Structure and Evolution of a Severe Squall Line over Oklahoma

Gerald M. Heymsfield Laboratory for Atmospheres, Goddard Space Flight Center, NASA, Greenbelt, MD 20771

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Steven Schotz General Software Corporation, Landover, MD 20875

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Print Publication:
01 Sep 1985
DOI:
https://doi.org/10.1175/1520-0493(1985)113<1563:SAEOAS>2.0.CO;2
Page(s):
1563–1589
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Abstract

A squall line on 2 May 1979 developed in Oklahoma in proximity to a synoptic-scale cold front. This line is analyzed during its growth and mature periods using radar, satellite, sounding and surface data. Some of the cells produced hail, and many of the cell tops reached 16 km. However, there were no reports of tornadoes. Three main topics are addressed in the paper: 1) examination of squall line and cell propagation mechanisms; 2) the three-dimensional structure of the squall line and individual cells during the mature period and 3) mass and moisture fluxes and precipitation efficiency. Comparison is made between the 2 May case and other tropical and Midwest squall line cases. The 2 May case does not exhibit a “trailing stratiform” anvil during the period mechanism requiring veering environmental wind shear in the lowest levels; and 2) mechanism where the cell motion is eventually governed by the moisture convergence and lifting provided the convergence line.

The motion of the squall line (defined by centroids of cells along the line) follows closely that of a convergence line found to be associated with a synoptic scale cold front. Initially, cells move along the low- to midlevel shear vector, which is directed ∼45° clockwise from the line orientation; then the cells turn to the right (nearly normal to the line). It is postulated that two mechanisms are responsible for this rightward turn of the cells: 1) mechanism requiring veering environmental wind shear in the lowest levels; and 2) mechanism where the cell motion is eventually governed by the moisture convergence and lifting provided the convergence line.

Triple Doppler analysis of a cell along the line indicates maximum updrafts of ∼35 m s−1, and strongest downdrafts at middle to upper levels located between cells along the line. The structure of the squall line is somewhat different from that in the case presented by Newton and other documented squall line studies in that there are not well-organized downdrafts on the rear side at low to midlevels. In addition, low-to midlevel inflow on the rear side of the squall line is apparently absent.

Man and moisture fluxes computed from sounding and radar data indicate magnitudes comparable to previous squall line cases. However, the precipitation efficiency of the squall line is estimated to fall in the range 25–40%, which is somewhat lower than other reported values (>50%). The low precipitation efficiency is suggested to be due in part to large moisture losses at upper levels.

Abstract

A squall line on 2 May 1979 developed in Oklahoma in proximity to a synoptic-scale cold front. This line is analyzed during its growth and mature periods using radar, satellite, sounding and surface data. Some of the cells produced hail, and many of the cell tops reached 16 km. However, there were no reports of tornadoes. Three main topics are addressed in the paper: 1) examination of squall line and cell propagation mechanisms; 2) the three-dimensional structure of the squall line and individual cells during the mature period and 3) mass and moisture fluxes and precipitation efficiency. Comparison is made between the 2 May case and other tropical and Midwest squall line cases. The 2 May case does not exhibit a “trailing stratiform” anvil during the period mechanism requiring veering environmental wind shear in the lowest levels; and 2) mechanism where the cell motion is eventually governed by the moisture convergence and lifting provided the convergence line.

The motion of the squall line (defined by centroids of cells along the line) follows closely that of a convergence line found to be associated with a synoptic scale cold front. Initially, cells move along the low- to midlevel shear vector, which is directed ∼45° clockwise from the line orientation; then the cells turn to the right (nearly normal to the line). It is postulated that two mechanisms are responsible for this rightward turn of the cells: 1) mechanism requiring veering environmental wind shear in the lowest levels; and 2) mechanism where the cell motion is eventually governed by the moisture convergence and lifting provided the convergence line.

Triple Doppler analysis of a cell along the line indicates maximum updrafts of ∼35 m s−1, and strongest downdrafts at middle to upper levels located between cells along the line. The structure of the squall line is somewhat different from that in the case presented by Newton and other documented squall line studies in that there are not well-organized downdrafts on the rear side at low to midlevels. In addition, low-to midlevel inflow on the rear side of the squall line is apparently absent.

Man and moisture fluxes computed from sounding and radar data indicate magnitudes comparable to previous squall line cases. However, the precipitation efficiency of the squall line is estimated to fall in the range 25–40%, which is somewhat lower than other reported values (>50%). The low precipitation efficiency is suggested to be due in part to large moisture losses at upper levels.

SEPTEMBER 1985 GERALD M. HEYMSFIELD AND STEVEN SCHOTZ 1563Structure and Evolution of a Severe Squall Line over Oklahoma GERALD M. HEYMSFIELDLaboratory for Atmospheres, Goddard Space Flight Center, NASA, Greenbelt, MD 20771 STEVEN SCHOTZGeneral Software Corporation, Landover, MD 20875(Manuscript received 24 September 1984, in final form 15 April 1985)ABSTRACT A squall line on 2 May 1979 developed in Oklahoma in proximity to a synoptic-scale cold front. This lineis analyzed during its growth and mature periods using radar, satellite, sounding and surface data. Some of thecells produced hail, and many of the cell tops reached 16 km. However, there were no reports of tornadoes.Three main topics are addressed in the paper: 1) examination of squall line and cell propagation mechanisms;2) the three-dimensional structure of the squall line and individual cells during the mature period and 3) massand moisture fluxes and precipitation efficiency. Comparison is made between the 2 May case and other tropicaland Midwest squall line cases. The 2 May case does not exhibit a "trailing stratiform" anvil during the periodmechanism requiring veering environmental wind shear in the lowest levels; and 2) mechanism where the cellmotion is eventually governed by the moisture convergence and lifting provided the convergence line. The motion of the squall line (defined by centroids of cells along the line) follows closely that of a convergenceline found to be associated with a synoptic scale cold front. Initially, cells move along the low- to midlevel shearvector, which is directed 445- clockwise from the line orientation; then the cells turn to the right (nearlynormal to the line). It is postulated that two mechanisms are responsible for this rightward turn of the cells: 1)mechanism requiring veering environmental wind shear in the lowest levels; and 2) mechanism where the cellmotion is eventually governed by the moisture convergence and lifting provided the convergence line. Triple Doppler analysis of a cell along the line indicates maximum updrafts of ~35 m s-l, and strongestdowndrafts at middle to upper levels located between cells along the line. The structure of the squall line issomewhat different from that in the case presented by Newton and other documented squall line studies in thatthere are not well-organized downdrafts on the rear side at low to midlevels. In addition, low- to midlevel inflowon the rear side of the squall line is apparently absent. Mass and moisture fluxes computed from sounding and radar data indicate magnitudes comparable to previoussquall line cases. However, the precipitation efficiency of the squall line is estimated to fall in the range 2540%, which is somewhat lower than other reported values (&gt;50%). The low precipitation efficiency is suggestedto be due in part to large moisture losses at upper levels.1. Introduction The structure of middle-latitude squall Tines has beenof considerable interest in recent years because thesephenomena often have a long duration and intenserainfall. Lilly (1979) and Houze and Hobbs (1982) review the current knowledge of the structure and dynamics of squall lines. The first detailed study of thestructure of a severe Oklahoma squall line was byNewton (1967), who proposed that squall lines propagate by a combination of translation and discretepropagation of cells. He presented a schematic of thesquall line indicating a tilted updraft and downdraft,and air entraining into the rear of the line from midlevels. It has become apparent from the observationsthat squall lines in midlatitudes form in a wide varietyof situations. Some form in advance of fronts (e.g.,Newton, 1967). Others which form at the dry line frequently have a north-south orientation and propagateeastward (e.g., Ogura and Chen, 1977; Ogura and Liou,1980; Kessinger et al., 1982). Some squall lines formnear cold fronts (Charba, 1974; Sanders and Paine,1975; Koch, 1984; and others) and others form as coldfronts merge with stalled drylines (e.g., Koch andMcCarthy, 1982). Recent theoretical studies indicate the importanceof the vertical shear vector and pressure gradient forcesin the orientation, initiation and propagation of squalllines (Emmanuel, 1983; Miller and Moncrieff, 1983;Raymond, 1984). Miller and Moncrieffsuggest the importance of both the vertical shear and the cross-squallpressure gradient in the dynamics of squall lines. Theyalso report that a major difference between midlatitudeand tropical squall lines is that midlatitude squall linesmove with a "steering level" at mid- to upper levels,whereas tropical squall lines move faster than the tropospheric winds at all levels thereby giving inflow atall levels ahead and outflow behind. Schlesinger (1978),c 1985 American Meteorological Society1564 MONTHLY WEATHER REVIEW VOLUME 113using a three-dimensional model of a supercell, suggested the importance of midlevel horizontal pressuregradient forces in cloud deviate motion, and he foundthat Magnus-type forces due to cloud rotation do notappear to be important. Rotunno and Klemp (1982)show from linear theory that a veering environmentalwind shear produces vertical pressure gradients whichenhance updraft growth on the right flank. The importance of frontal related mesoscale boundary layer convergence regions in the development ofmidlatitude squall lines has been suggested in numerous papers as a precursor condition to squall line development. Ogura and Chen (1977) studied the initiation and growth stage of an Oklahoma squall line.They observed a well-defined convergence line associated with a dry line more than 90 min prior to development of the first echoes. This convergence lineappeared to be quite important in initiating and maintaining the squall line. Lewis et al. (1974) studied anexplosive squall line associated with a well-defined coldfront. An initial mesoscale ascent region was observedin the warm air aloft and behind the surface positionof the cold front. Case studies of tropical squall lines (e.g., Houze,1977; Zipser, 1977) and midlatitude squall lines (e.g.,Ogura and Liou, 1980; Zipser and Matejka, 1981; Kessinger et al., !982) have revealed several features incommon. These characteristics include intense convection at the leading edge of the squall line, an extensive trailing region of stratiform precipitation associatedwith mesoscale ascent, and gust fronts associated withdowndraft outflows. Many of the. case studies of midlatitude squall lineshave dealt with the structure and dynamics on scalescovering the mesoscale and larger, during the matureto decaying stages of their development. Typically,squall lines are a few hundred kilometers in length andadvance rapidly, thereby making it difficult to capturethe cloud scale structure over the lifetime of the squallline. The purpose of this paper is to present the structureof a severe squall line in Oklahoma on 2 May 1979,during its growth and part of its mature period. The squall line developed explosively at about 2315(all times hereafter are in GMT) with extremely rapidgrowth near a synoptic-scale cold front oriented in anortheast to southwest orientation. It formed to thesouthwest of two tornadic thunderstorms first observedahead of the cold front at about 2100 (Heymsfield etal., 1983). The squall line lasted some 6-8 h as it movedinto southeastern Oklahoma. The length of the squallline was about 200 km. Hail caused extensive damage(hail sizes unknown; approximate times 0000-0100)in northwest and central Custer County during thesquall line passage. Distinct thunderstorms developedalong the line, often with lifetimes of at least an hour.Maximum cloud tops determined by stereographicsatellite data (Hasler, 1981) were 16-17 km. A penetration by the T-28 aircraft of the line at midlevelsreported an updraft exceeding 40 m s-t (Heymsfieldand Hjelmfelt, 1981). All of these factors suggest theseverity of the squall line cells. Several different types of data are combined in thisstudy to provide a comprehensive picture of an Oklahoma squall line. The data used here consist of rapidscan (~ 3 min interval) satellite imagery collected fromthe GOES East and West satellites, radar data coveragefrom several Doppler radars, conventional and PortableAutomated Mesonet (PAM) surface data, and specialrawinsondes, all of which are described by Barnes(1981). The National Severe Storms Laboratory(NSSL) was the focal point of the experiment. Bothsatellite and radar data were discontinuous during thesquall line period. The period of rapid scan satellitedata extended until about 0130; however the data weremost useful in studying individual cells during thegrowth period of the squall line. Squall line positionthroughout this paper is definedJkom radar data as theline connecting the cell centroids. The first multipleDoppler radar coverage was at 0020, nearly an hourafter the squall line had been initiated. In this paper, emphasis will be placed on: 1) examination of squall line and cell propagation and thecauses for significant deviation of individual cells tothe right of the environmental winds and vertical windshear vector; 2) the three-dimensional structure of thesquall line and individual cells comprising the line; and3) comparison with other squall line cases. In Section2, we present a description of the development of thesquall line by combining satellite and radar data. Theenvironmental conditions associated with the squallline are described in Section 3. To address some of thequestions regarding the squall line and cell propagation,Section 4 presents analysis of the relative positions ofthe squall line and the synoptic cold front and a convergence line associated with the squall line. In particular, the rightward deviation of the cells is examined.Section 5 presents the three-dimensional structure ofthe squall line during its mature stage computed fromdual and triple Doppler radar over about a 40 minperiod. Section 6 presents estimates of mass and moistu?e fluxes, and precipitation efficiency. These estimatesare compared to previous cases.2. Development of squall line The very early development of the squall line wasdifficult to capture with radar observations because theline was far from all the SESAME Doppler radars(NSSL-Norman (NRO), NSSL-Cimarron (CIM),NCAR-CP3 (CP3), NCAR-CP4 (CP4) and theCHILL~). Thus a combination of radar and satellite I The CHILL (University of Chicago and Illinois State Water Survey) radar was not used because of unretrievable velocity measurements.SEPTEMBER 1985 GERALD M. HEYMSFIELD AND STEVEN SCHOTZ 1565data were used to get a more accurate chronology ofthe squall line initiation and subsequent development.a. Visible satellite-Radar composites Composites of visible images from the GOES-Eastsatellite, and radar echo contours, are shown in Fig. 1,from 2304-0045 during the explosive cloud growthperiod of the squall line. The digital satellite data wereanalyzed on the Atmospheric and Oceanic InformationProcessing System (AOIPS) at Goddard Space FlightCenter. Remapping of the radar data from the CIMradar to satellite coordinates was performed using interactive software described by Heymsfield et al. (1983).A parallax correction for satellite viewing angle wasemployed to correct horizontal positioning between thesatellite and radar data due to cloud height. There isan approximately one-to-one relation between error inassumed cloud height and horizontal position. Radarimages were remapped assuming cloud heights from8-15 km as individual clouds grew to higher heights.These cloud heights were estimated from the satelliteinfrared (IR) temperatures (described later) and thetemperature soundings. Stereographic cloud height information (from Mack et al., 1983, and our own estimates) which are accurate to within ~0.5 km werealso used for this purpose. Thus the horizontal positioning error should not be more than a few kilometers. Analysis of low-elevation reflectivity PPIs indicateda relatively narrow refiectivity band (i.e., the lateralwidth of 30 dBZ contour was a maximum of 15 kmduring the period studied), with a lack of cellular structure. At midlevels, distinct cells were evident at ~2315,with first echoes appearing at ~ 6-7 km altitude. Thus,reflectivity CAPPIs at 6 km (Fig. 1) are used to identifycells, with the exception at 2334 where a 1- PPI ispresented. (Images at 2300 and 2315 do not have reflectivity CAPPIs superimposed because the cells hadnot grown sufficiently to be detectable by radarat 6 km.) The sequence observed in the satellite data associatedwith the squall line initiation and development is asfollows. Several hours before the squall line's rapid intensification period, a line of small congestus (notshown in Fig. 1) is evident along what is later seen tobe a quasi-stationary cold front. The clouds resembleradar-observed "line convection" (Browning and Pardoe, 1973; Carbone, 1982; Koch, 1984), where a narrow band of shallow noncellular precipitation formsnear the leading edge of a surface cold front. Cloudtops from stereographic satellite cloud-height measurements are estimated to be about 4 km. The cloudline at 2300, prior to the intense convection, has a similar orientation to a surface cold front described laterin Section 3. The cells then exhibit explosive growthnearly simultaneously along the line starting about2315. At 2334, the squall line has a number of discretecloud turrets, and the radar contours indicate a majorcell close to the southwest end of the line. Several gapsare evident along the line in the radar contours. By2342, the visible anvil edge has expanded along mostof the length of the squall line, and maximum reflectivities have increased, with several major cells alongthe line. The cloud tops at this time are estimated fromstereographic satellite measurements to be about 14km, which is just above the tropopause level (13.5 kmas indicated in the next section). Cloud material isspreading out as a result of strong upper-level divergence as the cloud tops hit and penetrate the tropopause. At 0004, the width of the visible anvil edge hasincreased to about 60 km. The maximum radar reflectivities along the line have increased to about 55dBZ. Several gaps between radar cells are still noted.By 0034, the visible anvil edge has enlarged further,with a distinct gap in the radar echoes about two-thirdsof the way down the line from the northeastern end.b. Growth of cells in satellite IR data Growth curves from rapid-scan infrared (IR) data(not shown) are presented to provide a precise chronology of the early growth period of the squall line. Inaddition, the IR data are used to supplement the radardata for a better estimation of the intensity of squallline cells during their growth period. Individual cellsin an IR image usually appear as cold areas (i.e., relativeminima of equivalent blackbody temperature TBa) asdescribed by Adler and Fenn (1979). Sometimes however, cold areas associated with cells are not evidentbecause of the large instantaneous field of view (i.e.,the spatial resolution associated with a one pixel measurement) of the satellite data (~ 8 km) in comparisonwith the typical updraft size. Also, as the cells mature,anvil cirrus obscures the cold cloud tops. Growth ratecurves were constructed in the following manner. Distinct cold areas from the GOES-East IR images from2318-0034 were identified along the squall line, andtheir evolution was followed. The time history of TaBfor several cold areas along the squall line is shown inFig. 2. These cold areas are numbered from northeastto southwest along the length of the line, but do notnecessarily correspond to the numbers of radar cellspresented later in Fig. 8. Figure 2 indicates that rapid cooling occurs amongthe cells between about 2320 and 0000, implying considerable growth of the updrafts nearly simultaneouslyalong the entire line. During this period, cooling ratesamong the cells range from ~2-5.7 K min-~, withthe exception of curve 1, which undergoes a brief leveling off. Curves for other cold areas (not shown) havesimilar cooling rates. These values are relatively largeand comparable to growth rates for tornadic storms(Adler and Fenn, 1979). Radar reflectivity data indicatethat about 15 min after the rapid cloud top growthdetected by the IR data, cells experience rapid development of precipitation. This time interval is not un1568 MONTHLY WEATHER REVIEW VOLUME 113TROPOPAUS-2102202301110 ~E 9'r2~08 I I I I I260 / ,(~ &amp;TI~B = 4Kmin-1 ~ &amp;t I I I270 2320 2330 2340 2350 0000 TIME (GMT0010 0020 0030 FIG. 2. IR satellite growth curves. Times of IR images are indicated by triangles at bottom ofthe figure. Height scale determined from CHK sounding represents pseudoadiabatic ascent ofcloud parcel; scale is not continued above 11 km because of ambiguity involved in the temperaturelapse-rate reversal above the tropopause. See text for details.expected given that the growth of precipitation particlesto radar detectable sizes lags the initial updraft development. Based on stereo height measurements, mostof the cells penetrated the tropopause (~ 13.5 km). According to Mack et al. (1983), heights estimated fromTsB minima during the growth of cells underestimatethe stereo-derived heights by ~ 15% due to the coarseresolution of the IR data. We have made additionalstereo-height estimates of the squall line cells. Heightmaxima of cells were found to be greater than 15 kmafter 0000 and as high as 16.5 km. Adler and Fenn (1979) have suggested a method ofusing the growth rate of Tsa minima to estimate meanupdraft intensity. The mean cloud-top vertical velocitycomputed in this manner represents an average overan area equivalent to the satellite instantaneous fieldof view which is about 100 km2. This approach hasbeen used to estimate the cells' intensity in a generalsense. The maximum values of these mean verticalvelocities were computed to be in the range of 3.6-7.4m s-t. Adler and Fenn reported that storms with ascentrates larger than about 4 m S-I were severe. Mack etal. (1983) found that satellite observed ascent rates(from stereographic height measurements) were comparable to echo ascent rates, IR growth rates, and visual(photogrammetric) rates. Thus, the growth determinedhere from the IR data suggests that intense updraftswere associated with the cells along the entire squallline.3. Environmental conditions associated with squall linea. Surface features Figure 3 shows the surface map on 3 May at 0100composited from conventional surface data and thestorm-scale upper air soundings. The wind shift associated with the cold front is just south of Gage, Oklahoma (GAG), with northerly winds behind and moistsoutherly winds in advance of the front. A dry line isSEPTEMBER 1985 GERALD M. HEYMSFIELD AND STEVEN SCHOTZ 15695/03/79 -0100 2/" ,sOt'3-- 3~' 11L."." :':11..-:' "".': ::?' 18..12 -".'."~ '17' ''.' ..: ..''. 1910/ ~n/ ,'~..~.".~" '. ~.~ ~'~:~':~ :'~'~l: ':&gt; ': t'-:' /- .' , -~',':::~HNT 3 .... - .... ~.......~:''"' ,' 3 .23 2325 -z/COLD FRONT AND DRY LInE................. ~ ':-'.~ANVIL EDGE ' ' CROSS SEGTION23~o1~ ~~~ RADAR (E15dBZ) ~SlTIONS03~ FIG. 3. Surface map on 2-3 May 1979 at 0100. Data are conventional surface data supplementedwith data from special soundings. Shown are the positions of the cold front (northeast-southwestoriented) and dry line (north-south oriented) on 2 May at various times, radar echo, anvil andsounding positions. Temperatures are in Celsius; winds are given as full barb (10 m s-~) and halfbarb 15 m s-~).located in the Texas panhandle. Approximate positionsof radar echoes and the edge of the anvil from the visiblesatellite imagery are also shown in Fig. 3. In addition,the cold front and dry line position deduced from surface data at several times are indicated. The cold frontwas quasi-stationary up to about 2300 and subsequently accelerated southeastward during the formation of the squall line. Although the density of surfacedata is too sparse to obtain a precise frontal position,the squall line is seen to form near the cold front. Thedry line had typical diurnal variation, with eastwardmotion during the early afternoon, and westward motion during the evening.b. Environment conditions ahead and behind the coM front The environment ahead and behind the squall lineis now described based on storm scale and conventionalsoundings. The storm scale soundings were released at90 min intervals beginning at 2300 and ending at 0500.In general, the groups of soundings ahead of and behindthe squall line were all qualitatively similar. Thus, forbrevity, a typical sounding representative of each groupis presented. Figure 4 shows the special networksoundings at CHK and GAG. The CHK and GAGsoundings are representative of the air ahead of andbehind the surface cold front (and hence the squallline, respectively). Figure 5 gives hodographs from thesesoundings. Figure 6 presents profiles of the squall line relativewind components parallel and normal to the squallline (Utr, I)~), and equivalent potential temperature (0e)for the GAG and CHK soundings. Our x'-y' coordinatesystem with absolute wind components u', v' is suchthat x', u', and u'~ are parallel to the squall line orientation (237 o to 57 -) and positive towards the northeast,and y', v', and v~ are perpendicular to the squall lineorientation and positive toward the northwest. Squallline motion is estimated in Section 4 to be ~ 10 m s-~from 327- during the analysis period (2320-0230). The environment ahead of the squall line is convectively unstable, with a warm moist mixed layer andsoutherly wind below the inversion, and drier air withwesterly winds above. The lifted index calculated fromthe actual 500 mb temperature minus the 500 mb parcel temperature is estimated to be -6. Note that thereis a rather deep (0.0-6.0 km) layer of winds v[ into thesquall line at CHK. This "inflow" layer is 2-3 kmdeeper than in Newton's (1967) squall line case. A "linesteering level" (i.e., v~ = 0 according to Miller andMoncrieff, 1983) is also present at about 6 km. Thewind parallel to the line is quite strong (30-40 m s-~)from middle to upper levels. Behind the line, the GAGsounding (Fig. 4) and the 0e profile (Fig. 6) illustratethat the cold front is at about the 800 mb level (--~ 1.61570 MONTHLY WEATHER REVIEW VOLUME 113 100 16.0 I i I I STATION 7929 GAG MAY 2, 1979 2364GMT 160 ~3 ~-~ 13~6 200 ' q./ 11.8 ~ ~ 'x.~ ,.~. ~~ ~ ~ ~x.- ~. ~ ~ ~x. ~~ ~~ ~.-~,~, ~ ~" ~ ~ ~ ~ X'- ~ ,~,_ ~ ~ - ~ _ ..~...;;. ~,-.,**.~. ~ . ~ ,,.~ -.,.~ x ' ~ ~ ~7 ~- -.,__.~,,~,,~_~ ~~*- ~ ~ ~ '~ r,'~, ~ ~ ,., ,% 1~ X ~ 1~ ~ "~ -~ -~ -ffi-~ -~ -~ -10 0 10 ffi ~ ~ TEMPERATURE (C) 16.01.1 I I t I STATION 7924 CHK MAY 2,1979 230EGMT 2O0 ~,_ 11.8 ~ ~0 ~,~ ~-~ !300 ~..~. ~' 9.1 ~ ~ x .~.,; . .~.... ,--- ~ ~ *' x 5-.~'-L', C'&lt;.. - *-' '~ ..( .,~ ~ ~ ,-,, : '\ ,, ~ ~,.-;~. ,~ %, ~ '.'~ - \~ ~ ~.' m ~"x~' ~ ~ ~... ~ [~ ~ ~;','~ , ,~ ' ~,N ~. 1~ ~ ~' -~ -~ -~ -~ -~ -~ -10 0 10 ~ ~ ~ T[M~;~TUR[ ;C) FIO. 4. Soundin~ from CHK (ahead of line) and GAG (~hind~ne). ~le~ed i~n~o~ (~n ~h~) and p~udoa~a~B (thin mlid)~e also shown~ Winds are ~ven ~ full b~b (10 m s-~) and half barb(5 m s-~).km height), and the air is quite dry above the front.The tropopause in the GAG sounding is located at~ 12 km and again there is a "steering level" at 6-7km. It is important to note that the relative flow behindthe line differs significantly from the schematic proposed by Newton (1967) in that it is away from theline at midlevels. Figure 7 presents a vertical cross section at about2330 from Dodge City, Kansas (DDC) to Elmore City,Oklahoma (EMC), which is transverse to the coldfront-squall line orientation. The DDC sounding wasobtained from conventional upper air data, while theOther soundings were from the special SESAME network. The squall line is located between HNT andGAG in a dry environment (relative humidity &lt;30%)from mid- to upper levels, and moist inflow (relativehumidity &gt;60%) ahead of the line. The cold air is againseen to be quite shallow at GAG, with its upper surfaceat about 850 mb (~ 1.5 km). The vertical wind shear is considerably differentahead and behind the line. Ahead of the line, the cloudlayer wind shear (pressure weighted) calculated fromthe lifting condensation level (LCL) (~ 1.5 km) to thetropopause (--13.5 km) is about 2 x 10-3 S-I fromabout 290-; low- to midlevel shear (i.e., the LCL to 6km) is 3.8 x 10-3 $-1 from 277-. Behind the line atGAG, the cloud layer wind shear is 4.8 x 10-3 S-I fromabout 227-. The magnitudes of cloud layer shear aheadof and behind the line are moderate compared to thosedocumented for supercell cases,(2.5-4.5 x 10-3 S-I, inMarwitz, 1972a). The different wind shear ahead andbehind the cold front makes comparison with othercase studies and models more difficult because the environmental wind shear is typically assumed constantacross the line. Given that the squall line orientationis about 237-, the cloud layer shear vector ahead ofthe line is about midway between parallel and perpendicular to the squall line. The shear vector behind theline is nearly parallel to it. This orientation differs fromtropical squall lines (e.g., Houze, 1977) and some midlatitude squall lines (e.g., Ogura and Liou, 1980), wherethe shear vector is typically perpendicular to the line.4. Motion of squall line and individual cells The motion of the squall line and cells comprisingthe squall line is analyzed here for the purposes ofcomparing their motion to that for other squall linesdocumented in the literature, and for determiningwhich mechanisms might govern their motion.a. Line and cell motion j~om radar data1) REFLECTIVITY CELL TRACKING Radar reflectivity data were analyzed in order totrack the movement of the cells in a continuous fashionover an extended period of time (~3 h). The isochronesof the squall line position and the trajectories of individual cells are shown in Fig. 8. Cells along the squallline are defined by a distinct reflectivity maximum withheight and time continuity. Thus some of the smallercells lacking time and height continuity were omitted.The cells labeled Ci-Ci5 were tracked from 2328-0230,using reflectivity CAPPIs constructed from the NROradar data, approximately every 10 min. Average cellspacing is about 15-20 km. The reflectivity data wereinterpolated to grids with horizontal and vertical gridintervals of 1.5 and 1.0 km, respectively. The NROreflectivity data were used for the analysis because ofits good temporal and spatial coverage of the entiresquall line; however, pulse volume averaging tendedto smooth some of the features of cells at distinct ranges.To supplement the cell identification analysis, the CP3and CIM radars were also used when data were availSEPTEMBER 1985 GERALD M. HEYMSFIELD AND STEVEN SCHOTZ 1571G.~.G .~/2/79 ~300 ~ ~ ~'9~--'--.CHK 5/2/79 2300 / ~ ,,~:::~-'~/ )~' 4 58FIG. 5. Hodographs from CHK and GAG. Squall line motion VsL is plotted.able. Because of the large spatial extent of the line, itssouthwest end was not observed well. The movement of the squal.l line was nonuniformalong its length. The approximate eastern half of thesquall line had a continuous band with embedded cellsof high reflectivity, and its motion was fairly uniformbefore 0130. Thus a mean motion of 10.0 m s-~ from327- (shown as Vs~: in Figs. 5 and 8) was estimated byaveraging the motion of this part of the line from~ 2315-0130 after 0130, this portion of the line slowed16141210 8 642-% 310 320 330 340 350 I ! I I I '5/2/79 GAG 2354(BEHIND LINE)ee (K) 310 320 330 340 350 5/2/79 CHK 2305 (AHEAD OF LINE) -10 0 10 20 30 40 , , 1Ur, Vr (ms-) FiG. 6. Squall line relative wind ~omponents from the GAG and CHK soundings. Wind components u~ and v~ are parallel (positive towards the northwest) and perpendicular (positive towardsthe northeast) to the squall line orientation (237- to 57-), respectively. Squall line position wasobtained from radar data. Also plotted are 0~ profiles for each sounding. See text for details.1572 MONTHLY WEATHER REVIEW VOLUME 1132330 GMT 2 MAY 1979 SQUALL LINE. 300 . ~iiz ~' ~~ ~ ~ ~ ~ ~ ....';?~ ...ooo. ,ooo DDC GAG HNT CHK EMC FIG. 7. Vertical cross section normal to the squall line at about2330. Shown are winds, relative humidity (dotted curves) at 15%intervals, and potential temperature (solid curves) at 2 K intervals.Superimposed is the position of the squall line determined from radarreflectivity data. Winds are given as full barb (10 m s-l) and half barb(5 mdown, and the line and cell motion became more difficult to follow. West of C9, the line's motion was quitenonuniform and the reflectivity band had some breaksin it. Thus, no mean motion estimate was attemptedfor this part of the line. Note that a perturbation witha southward bulge developed about two-thirds the waydown the line from the eastern edge of the line. Thisbulge appeared as line convection (shaded region inFig. 8), rather than discrete convective cells at midlevels. It is also interesting to note that Clo and C~ dissipated as they moved into the region of line convectionand C9 and C~3 later develop on the eastern portion ofthis region. Figure 8 illustrates that most of the cells (e.g. C2,C3, C7, and C~2) are longed-lived with lifetimes of atleast 2 h, while others (C~.0 and C~l ) lasted less than anhour. It is important to note that most of the radarcells have an eastward motion initially and then turnto the right of all winds in the troposphere above ~ 1.0km (see Fig. 5). Typical motion of the cells before theirright turn is approximately 13 m s-~ from 275- whichis approximately down the low- to midlevel shear vectorcomputed from the CHK sounding. After the turn,their motion ranges from ~8-12 m s-~ from 320-.Thus, most of the storm cells are right movers relativeto both the wind shear and the cloud layer mean windwhich is 20 m s-~ from 257- between the LCL and thetropopause computed from the CHK sounding. Theirmotion after the turn is in contrast to Raymond's(1984) hypothesis that squall lines propagate down tothe low- to midlevel shear. The earliest cell to turn rightward (~ 2346) is C~, followed successively by cells further down the line. However, C4, C6, Cio, Cii, and C~5 do not make this right turn; instead, with the ex ception of C6, they move eastward and dissipate. These storms do not appear to be the left mover of a storm split (e.g., Rotunno and Klemp, 1982). Note that C6 was in the wake of C7, a relatively intense cell, and weakened considerably after 0030. The above analysis indicates that the squall linepropagation and evolution on 2 May differed significantly in several respects from observations of otherdocumented squall lines. Newton and Fankhauser(1964) f6und from studies of midlatitude squall linesthat new cells develop on the upwind end (with respectto the low-level relative flow) and migrate toward thedownwind end of the squall lines. They postulated theimportance of the environmental-storm relative windsand wind shear. According to their hypothesis, themoisture supply is directed toward the right stormflank. They suggest that large cells deviate to the rightof the mean cloud layer wind more than smaller cellsbecause they require a greater moisture supply. However, new cells on 2 May did not necessarily form nearthe upwind end (relative to mean cloud layer wind) ofthe line as found by Newton and Fankhauser (1964).Cells C~0 and C~, for example, developed more than20 km from the upwind end of the line. In addition,after the cells made their rightward turn, several of them(Ci-Cg) propagated nearly perpendicular to the lineinstead of towards the downwind end. Houze and- Smull ( 1981) found from a comparison between Oguraand Liou's (1980) Oklahoma squall case and GATEtropical squall lines, that both kinds of squall lines progressed by a combination of translation and new celldevelopment; that is, new cells form at the leading edgeof the squall line. Subsequently, they move towardsthe rear of the line and dissipate. Marwitz (1972a) andothers have defined this type of motion as discretepropagation. Discrete propagation has been generallyobserved for squall lines with trailing stratiform precipitation. However, unlike the stratiform type squalllines, most of the cells on 2 May were long-lived anddid not migrate toward the rear of the line and dissipate;rather, the cells here appear to propagate by continuousregeneration, a mechanism usually associated with supercell propagation (e.g., Marwitz, 1972a). The motionof the cells after the rightward turn will be seen in thenext section to be similar to the motion of the surfaceconvergence line (analogous to 'Weaver, 1979) deducedfrom single Doppler radar and PAM surface data. 2) CONVERGENCE LINE MOTION The purpose of this section is to determine the motion of a well-defined convergence line associated withthe developing and mature squall line using singleDoppler radar measurements and PAM surface data.(Multiple Doppler data for the squall line were notavailable before about 0020.) Before and during theSEPTEMBER 1985 GERALD M. HEYMSFIELD AND STEVEN SCHOTZ 1573100SQUALL LINE AND CELL MOTION 2 MAY1979 ~ I I I I I ~ I ~ I ~ t ~ I ~.~, -~' LEGEND ~ .. ~ A ~ I 0042 C1 A ~'~--4~'C-'"~-D~ SE/~2~ /* F B 2334 J 0054 'k /// ~ '%~=G~ C 2338 K 0100 C= A /SC---o ~/E-~..;~ ~ D 2346 L 0111 // //~ G~ / ~i E 0002 M 0130 C3/B'C"o----..._.~E./ ~J/ ~j F 0010 N 0158 / / --"---F----G--.,/~----'~. '~. ./ / / ~, \ _~ ~ G 0019 0 0208 /" /// /.'AcP3 '~ J_ ....~ '~ u~-~ P 0220 ~/ / / z_~ ~ %.- ' ~ ~"' c~-'-7~--; ~-"------' ~J ~L"~ .... / ~ E/___r G____.H/~"'~ ' . /~-'~--~' I. - _ C$ ~ // /// - 1 /// ~ / // ~ v7 // n--I ----------_j / /~ %% - ? /. ..... .~_.,~,==.~--*"--'----~ ~, __ ~**~ '% ...-' ....... '*....s c,~ o,=o C8. ,,~'~ / '""~_/- ~../' ? ~ '-. '~ /C DE-~---F G /' ,/ '~ . ~,~.-. ~,/ r~ / 10/ ~ ; Lv tur "-. A ~ ,,,A--J // ~ / ./ ~ ~/c~ '*~.. /~ / /. / // ~G ~" ~; ; //M... '*-... / Cl1B.~-~i)E...~ . I~,.~..,,.. "....?.. ,NT O0~.~*%N~ ~ ..~ // // ),'~ ...... /JB 'o p /~' ~./ / ~.~-- ~- ,. / /./P~ / 20 ~ - ~ ~/e~ . X-- CELL TRACK ~~ "--'--' ' . ' VSC (.EL) - 0 J~ NARROW REFLE~IV~ BAND~ e PAM STATION ~ e e VCL A RADAR ~ SOUNDING POSIT. AT Z: 5km , M~ ~VsL` CFK: ~ I I I ~ I I ~ *1 ~ I ~ -~ -1~ -1~ -1~ -1~ -1~ -~ ~ -~ x(km) RO. &amp; ~ual] line and cell motions on 2 ~ay 1979 dctc~Jned from mda~ rcficctivJty. Wind vccto~ in lower ~t co~crdenote ~ual] line motion (Fs~), cloud layer mean wind (Fcc) and su~loud layer ~uall Hnc relative wind (Ysc). Tracks ~twccncell positions at times ~ and N a~c dotted because o~ large time ~p Jn data. Note that the location o~ GAG ~al]s just outside o~the uppc~ left co~cr and therefore is not shown-early squall line development, the CP3 velocity measurements detected a large gate-to-gate shear (and hencea wind shift-convergence line) along radials perpendicular to the line? The convergence line was detectedas early as 2300 by CP3, when there were relativelyfew velocity measurements, and there were no reflectivity echoes associated with the squall line. Recall thatthe squall line began its rapid growth period about 15min later. Thus, the velocity measurements initiallywere due to insects and nonprecipitating low-level 2 A strong gate-to-gate shear implies a large gradient (Ov'/Oy') andthus a strong convergence if Ou'/ax' &lt;~ ov'/Oy'. This assumption iscrudely satisfied owing to the two-dimensional appearance of the linein the PAM data (see Section 4.b and Fig. 11)o The convergence linegenerally corresponds to a wind shift line (in absolute coordinates)where the latter was determined from the change in sign of Dopplervelocities along radials and from a wind shift analyzed in the PAMdata. Hereafter, we will refer to the Wind shift-convergence line asjust the convergence line.clouds associated with the moist low-level flow. As timeprogressed, the area of the velocity measurements became larger as precipitation reached the surface. Duringthe mature period of the squall line, the CP4 radarobserved a similar convergence line from about 0000to 0100. This phenomenon was also observed in thePAM surface data which was analyzed from 00300200. Figure 9 shows the position of this convergence lineas a function of time from 2315 to 0014 (using CP3radar) and from 0034 to 0111 (using CP4 radar). Theconvergence line speed was determined by plotting itsposition from successive low-level PPIs of radial velocity using an azimuth normal to the squall line(~ 327 -). The CP3 radar gives an average convergenceline motion of 9.1 m s-~ at 327- and the CP4 radargives 10.4 m s-~ at 327-. In each case, the convergenceline motion varies little over time. The difference inspeed between the CP3 and CP4 estimates may in partbe due to the fact that these estimates were derived1574 MONTHLY WEATHER REVIEW VOLUME 1134O302O3O20!~1 i i I ~ ~ I ~ ~ I ~CP3 RADAR I ~ \ ~edBZ~.AVG.$PEED~ ~ ~ X~ F~O~T.... CP4 RADAR~ _- -AVG. ~PEECONVE~ LINE ~= m~-- -~ CONVERG. LINE ~FRONT, R~R EDGE OF ~ ~DAR ECHO ~- REFLECTIVI~ M~IMUM ~ '(Low Level) ~~ t ~ ~ I ~ ~ ~ ~ I ~ 10 2330 (X)O0 OO30 0100 TIME~ FiG. 9. Radar-derived position of surface windshifi, forward andtrailing edge of detected reflectivity, and centroid of maximum reflectivity. Constructed from low-elevation angle (&lt;2.4-) and constantazimuth (~327-) radial velocityand reflectivity measurements fromCP3 and CP4 radars. Range is given from each of the radars; slopesof the curves indicate advection speed. See text for details.from different locations along the squall line. Isochrones from the PAM data (station positions,in Fig.8) of the surface windshifi line (and convergence line)were also plotted (not shown) over the period from0030 to 0200. The motion in the surface analysis wassimilar (--~ 10.0 m s-~ from 327-) to that from the radardata (Fig. 9). Thus, the convergence line motion is approximately equal to both the squall line motion estimated from the radar reflectivity data (Fig. 8) at thesame positions along the line, and to the motion ofmost of the cells after they make their rightward turn.3) RELATION OF CONVERGENCE LINE TO THE COLD FRONT AND GUST FRONT Because the convergence line motion is similar tothe squall line motion, it might be suggested that theconvergence line influences the cell motion. However,a counter argument could be' raised that the convergence line is just a result of the squall line gust front. Therefore, the question is addressed here whether the convergence line is 1) a squall line-produced gust front, or 2) solely a result of the synoptic-scale cold front shown in Fig. 3, or 3) some combination of 1) and 2). The intent here is to shed some light on this question. During the growth period of the line. (before ~0030), the position of the convergence line relative to the squall line supports hypothesis 2. As mentioned earlier, the convergence line was detected as early as 2300. The satellite IR data (Fig. 2) and stereo cloud top heights (not shown) indicated low cloud tops (&lt;4-5 km) and presumably weak updrafts and downdrafts up until ~2315. Also, radar reflectivity echoes were not evident until 2315. Thus, the convergence line exists pri. or to development of moderate to intense convection and hence does not appear to be initially associated with an organized squall line gust front. During the mature period of the squall line, there issome evidence to support hypothesis 3. It is unclearhowever, given the limited surface observations priorto squall line development, to what extent the coldfront is modified. A detailed analysis of low-level (0.4,1.4 and 2.4- elevation) PPIs of velocity and reflectivityfrom 0030-0115 using the CP4 radar was conducted.These PPIs covered cells C6-C8. For brevity, they aresummarized in the following. The PPIs of radial velocity were examined for features indicative of downdrafts, such as divergent regions comprised of maximaand minima of radial velocity superimposed on thegeneral northerly flow behind the front. In general,downdrafts inferred in this manner were found to pulsate over the period, lacking time and space continuity.These observations suggest that there was not a wellorganized squall line-produced gust front, but ratherthat cell outflows were more localized. One of the morepronounced cell outflows occurred between cells C6and C7, and it is evident in the multiple Doppler analysis presented later in Section 5. At 0030, flow wasgenerally uniform behind the convergence line withabsolute speeds about 10 m s-~. At 0045, a wind maximum ( ~ 18 m s- ~) developed ~.10-15 km behind theconvergence line, just ahead of what appeared to be a. !ine of divergence. As time progressed, the wind max~mum area enlarged and it strengthened, suggestingoutflows from C6 and C7 spreading out toward the convergence line. By 0100, this wind maximum mergedwith the preexisting convergence line, presumably increasing the convergence across the front between thecells. Cells C7 and C8 began their turn to the right nearthis time (Fig. 8). Then the wind maximum diminishedin size. Analysis of the PAM surface data further supports hypothesis 3. PAM station time traces were analyzed to examine the evolution of various parameters as the convergence line and squall line migrated through the PAM network. Figure 10 shows a set of time traces of surface wind speed, wind direction, temperature, dew point temperature, pressure, rain rate and 0e for stationSEPTEMBER 1985 GERALD M. HEYMSFIELD AND STEVEN SCHOTZ 1575 STATION 1 - ~1'~ ?,.. ,, l', i '~ ',/ I , Oo iT',,~~ 25 ~ I I ~~ ' = ~ ~h I .... ~. I ~ ~.~ ~. 2 ~ ~2 15 Ii ~~ i i~ - i ~ ~ i i - 10- i ~ ~ - i ']&amp;, i~ ,~ , ~ - I I - - I I ' '~ 270EEo 180~ eo ~ i ~ ~~ I ~ il / .~ 955 I ~-- '1 I ~~ ~ i , , 1 i I ooo I 1~'15 ' I i i i ~ .......... I ........... ~ ........... [ ........... I ..........iI33O32O2200 2300 0000 0100 0200 0300TIMEFIG. 10. Time traces of various surface parametersfrom PAM station number 1.1 using I min data. A single frontal zone is evident inFig. 10 by abrupt changes at 0020-0030 in the parameters.3 There is no evidence of a secondary temperature 3 The smaller temperature drop in Fig. 10 of about 2- at 2342should be noted. This cooling was due to a blockage of the solarradiation (i.e., a shadow) produced by the developing squall line andanvil, which is apparent when the station positions are superimposedon satellite images (see Fig. 1 at 0004).drop which would be associated with a trailing coldfront. Note that 0e falls off abruptly near the frontalzone followed by only a briefleveling-off(0e ~ 330 K)near the time of rainfall. It then gradually decreasesthereafter as the cold air advects into the network. Several of the other PAM traces (not presented) show sustained gusts up to 0-~ 18 m s-1 after the frontal passagenear the time of rainfall. Only a few of these otherstations showed perturbations in 0e associated withthese gusts, which were ~ 3-4- warmer (0e ~ 326-330K) than the general trend associated with the cold advection. Foote and Fankhauser (1973) have used surface 0e to locate source levels of downdraft air by comparing it with the midlevel 0e. Referring back to theprofile Of 0e behind the line (GAG, Fig. 6), the warmingassociated with the gusts suggests that the downdraftair originates at relatively low levels (&lt;2 km). Thestriking feature of the ahead sounding (CHK) is that0e at midlevels is as much as 10- higher than at GAG;thus it is quite plausible that some of the downdraftair comes from midlevels ahead of the line. Downdraftsfeeding from the front of squall lines have been foundin other case studies, e.g., Ogura and Liou (1980) andZipser (1977). The observations of smaller scale downdrafts againsuggest modification of the thermal structure of thecold front. Considering the front to be a gravity current(Charba, 1974; Goff, 1976; Carbone, 1982; Koch,1984), the speed of the front would be expected to bemodified if the downdrafts substantially modified thethermal structure of the front. The frontal motion wasestimated from PAM data using the different gravitycurrent formulae with a reasonable range of coefficients(e.g., Koch, 1984). These gave advection speeds in therange 8-14 m s-1 before 0130, which are consistentwith those observed (~ 10 m s-l). After 0130, however,the observed frontal motion decreased by a few metersper second. This might be a result of the downdraft 0ebeing warmer than that of the low 0e postfrontal airthus decreasing the temperature gradient across thefront. In summary, the above analyses best supportshypothesis 3; that the convergence line is the result ofboth the squall line gust front and the cold front.b. Discussion of cell motion mechanisms Based upon the analysis of the observations presented in the previous sections, we suggest two mechanisms which may act together to explain the cell motion on 2 May. Neither mechanism by itself providesa completely satisfactory explanation for the cell motion. The first mechanism is that proposed by Rotunnoand Klemp (1982), as mentioned in the Introduction.For right-moving cells, their theory requires a veeringenvironmental wind shear with height especially in thelower part of the atmosphere. Figure 5 indicates thatthe wind shear at CHK veers up to about 2 kin. This1576 MONTHLY WEATHER REVIEW VOLUME 113wind shear variation with height was also seen in theother soundings ahead of the line. However, some ofthese soundings show some backing of the wind shearwith height above ~2 km up to midlevels. Since Rotunno and Klemp assume strong veering up to at least4 km in their simulations, it is unclear in what fashionthe backing or lack of significant veering above 2 kmwould affect cell motion according to their explanation.In addition, it may be inappropriate to apply Rotunnoand Klemp's results to this case because they considerisolated supercells in an environment with uniformshear. Furthermore, the presence of a cold front-convergence line with its associated gradient of moistureis not accounted for in their model. The second mechanism is analogous to that inWeaver's (1979) paper. It is suggested here that cells'motion is governed by the moisture convergence provided by the cold front-convergence line. Cells propagate along the advancing surface convergence line,which provides continual lifting of the convectivelyunstable inflow air (shown relative to the line by Vscin Fig. 8). Cell updrafts are' continually regeneratedcausing a net rightward motion of the cells. In addition,downdraft outflows although localized, may have enhanced the convergence and lifting along the frontwhen they merged with it. This enhanced convergenceseemed to occur with C7 and C8. The convergence associated with the front couldprovide a substantial vertical motion. Figure 11 showsa PAM network analysis at 0100 indicating strong convergence and a 0e gradient across the front. The windsindicate that to a rough approximation, Ou'/Ox' ~ Off/Off. Using this assumption, the CP4 velocity data (seeFig. 9 and footnote 2) give a convergence normal tothe front of ~5 x 10-3 s-l over a length scale of 1.5km. Again with the above assumption and using thesquall line advection speed, PAM station wind traces(Fig. 10) give typical convergence values 4 x 10-3 s-~over 2-3 km. Assuming a frontal depth of 1 km, thisimplies a vertical motion ~5 m s-', which is not unreasonable given that Goff (1976) reported values forgust fronts of as large as 8 m s-l from tower data. As mentioned earlier, most of the cells that did notturn to the fight dissipated. Neither of the mechanismswe suggest gives a clear explanation of why some ofthe cells did not turn to the right. We can speculate,however, that these cells might have dissipated becausethey were eventually cut off from the lowqevel moistureconvergence produced by lifting along the convergenceline. One of the more widely accepted mechanisms ofsevere storm deviant motion is that it is the result ofthe development of a mesocyclone. We examined theradial velocity PPIs for evidence of mesocyclones alongthe line. Cell C2 was the only cell with an intense mesocyclone (tangential shear: 1.5 x 10-2 s-~) of comparable magnitude to supercells (Burgess et al., 1975).This cell began to turn near the time when downdrafts120100 23~5 80 ~'17 23. 60 /21, 22.6 _ /23 22.9 40- -100 I I i I3 MAY 19790100 GMT WINDS ANDTEMPERATUREI 5% /14 23.1 23.2~ 23.4....... s.~/ 22.7 CP4 15,/ ~,~ 27 /10 23.1 22.9 ,~ls 22.6 23.1 ~26 22.6.22,,~ /2o t16 22.2 ~.'/'~90 22.5 22.9 22.6 ~ &gt;30 dBZ /25 /24 CP4 CONVERG. LINE_ 23.4 22.4 RADAR . 10 ms-1 -80 -60 -40 -20 x'(km) FiG. ! 1. Surface winds and temperature from P^M network at0 ] 00. Station number (small upper number) and temperature in degrees Celsius (larger lower number) are indicated. Contours of 0e areshown at 2.5 K intervals. Approximate position of frontal boundary(dot-dashed line) is shown. Position of convergence line at 0100 fromCP4 radar (Fig. 9) is indicated.and a mesocyclone developed near the surface. Othercells had weak mesocyclones at low to midlevels. Thusthe explanation that trong rotation in the squall cellsproduces rightward deviation does not seem likely here.5. Three-dimensional wind and reflectivity structure of cells C6-C9 Multiple Doppler rcficctivity and wind analyses arcnow presented to better describe the three-dimensionalstructure of the squall line and for comparison to othersquall lines documented in the literature.a. Methodology and limitations of analysis Doppler radar data were processed at 0030, 0040,0050, and 0115 during the mature period of the squallline. The techniques for processing the multiple Doppler data are described in the Appendix. A triple Doppleranalysis was performed at 0030 using the CP3, CP4,and CIM radars; the other analyses used two Dopplerradars (CP4 and CIM). It should be noted that cellsC6-C9 are relatively far from the radars for multipleDoppler analysis (Fig. 7). Thus the emphasis here ismore on the qualitative results rather than the quantitative results of the analysis. This restriction appliesespecially to the dual Doppler analyses because the dataSEPTEMBER 1985 GERALD M. HEYMSFIELD AND STEVEN SCHOTZ 1577were located such that the angle formed by the radialcomponents from the radars was small. The dualDoppler vertical velocity estimates are therefore usedonly qualitatively, i.e., to depict general structure features and the location of major updrafts and downdrafts. The squall-line scale motion fields were foundto remain qualitatively similar throughout the analysisperiod. Individual cells, on the other hand, underwentconsiderable evolution. Thus the following will emphasize the squall line dynamics more than individualcell dynamics.b. Summary of structure Figure 12 presents CAPPIs (0.5 km, 2.5 km, 5.5 km,8.5 km, 11.5 km) of velocity and reflectivity which arerepresentative of the structure at low levels (0.5-2.5km), midlevels (3.5-7.5 km) and upper levels (8.513.5 kin). Cell positions are indicated at the 8.5 kmlevel. Vectors are shown relative to the squall line motion (10 m s-j, 327-) because our emphasis is primarilyon the squall line dynamics. Cell C3 moves out of thedomain at 0040, as C9 moves into the region and intensifies (see Figs. 8 and 12). It should be noted thatat 0050, and 0114, vectors on the left (southwest) sideof the grid at x' &lt; -70 km for 0040 and 0050 and x'&lt; -50 km for 0114, are less reliable because they werein an unfavorable position for dual Doppler velocityestimation. The following summarizes the generalcharacteristics of the squall line wind structure withemphasis on the CAPPI levels presented in Fig. 12.1) LOw LEVELS The low level reflectivity pattern is fairly noncellularand narrow. There is little evidence of significant trailing stratiform precipitation that is found quite oftenfor squall lines, as mentioned earlier. An analysis ofthe PAM raingage data also indicates that the squallline lacked trailing stratiform precipitation for timeperiods up to about 0200. It should be mentioned,however, that much later (after about 0400) the WSR57 radar data indicate a much wider band of precipitation trailing the squall line; therefore the 2 May squallline may have become more like the stratiform typelater in its lifetime. It is interesting to note that theaverage cross sections presented later in Fig. 13 showthat the radar detected anvil (defined as overall area ofdetected precipitation) expanded with time more tothe rear of the squall line than the front. This expansionmay be related to the squall line appearing to becomemore stratiform in character, but that topic is beyondthe scope of this paper. A wind shift is evident in the Doppler winds at 0.5km altitude. This wind shift is near the position of thesurface wind shift analyzed in the PAM data. In addition to the widespread convergence associated withthe wind shift, there are also localized convergent regions (calculated from finite differencing on wind grids)~3-5 km across with magnitudes 5-7.5 x 10-3 s-~associated with each of the major cells in the region.Note that the development of the downdraft regiondiscussed previously is evident at 0040, and is indicatedby a D in the figure. Simultaneously, cyclonic vorticityassociated with C7 intensified (at x' -- -55 km y'= 105 km). This divergent region and cyclonic vorticitycontinued to intensify at 0.5 km to at least 0050. (Vectors were not analyzed near C, because the CIM radarscanned above this level.) The maximum magnitudeof cyclonic vorticity estimated from the Doppler windsis only about 7 x 10-3 s-~ over a relatively small areathat is about half that ofsupercells (Burgess et al., 1975). 2) MIDLEVELS The cellular structure of the squall line at middle toupper levels is noted by the individual refiectivity cores.This observation is characteristic of midlatitude squalllines, and it suggests distinct updrafts. The overall relative flow ahead of the line has a strong componentinto the line, and it extends up to about 6.5 km in goodagreement with the "inflow" depth deduced from theenvironmental sounding (CHK) shown earlier in Fig.4. The flow into the line suggests greater mixing ofenvironmental air along the front side of the line thanthe rear, where the flow is nearly parallel to the squallline. Also note that the cells appear to act as obstaclesat midlevels, especially around C7 after 0030. 3) UPPER LEVELS At 8.5 km (near the base of the anvil and divergentoutflow), the relative winds behind the line are directedaway from the line (toward the northeast), primarilythe result of updraft air exiting the rear of the squallline. On the other hand, at the 11.5 km level largedivergent outflow regions are evident with each of thecells with flow toward both the front and rear of theline. Thus the radar detected anvil at upper levels isapproximately centered over the squall line, unlikemany tropical squall lines which only have trailing anvils (Houze and Betts, 1981). Note the individual outflow regions strongly interact with each other, and extend out about 30 km transverse to the squall line.They extend from 8.5 km to the top of the observations(13.5 km), with maximum divergences of I to 3(x 10-2 s-t), above 10.5 km. Thus the upper levelsare dominated by strong divergence. Strong convergence occurs, however, between cells as a result of theopposing flow of the divergent regions associated withthe cells. The maximum value of convergence betweencells is about 1.3 x 10-2 s-t. 4) VERTICAL STRUCTURE AND DRAFTS Locations of centroids of major updrafts and downdrafts are depicted on the 8.5 km CAPPIs in Fig. 12.1578 MONTHLY WEATHER REVIEW VOLUME 113136.2124.2~112.288.276.2136.2 124.2~ 112.288.276.2 ' I5/03/79OO300.5 KM[ I ~ I ~ I ] I 35.O M/S~ J ~-85.2 -73.2'l ' I '5/03/7900500.5 KM f f '~ ~ J~' ,.~~' ~~ ~ ~ ~ ~J t t ' ~/ ..... *~x~x------~ f f f ~~ ~ ~ ~ ~, , c~ ~ ~ --~ * STATION ~ ~ ~ ~ . . ~~%~% . . ~ ' , .~ ~ --~--~--~--~//--~--%~.. ~. ~ /~ ..... /~-- ~ t ~ ~~ ~/~/- ~ ~////~f'F 7~ ~ //~//// ~/7~ / / - ~N ~/~'~/ ~ ~ ~ ~-61.2 -49.2 -37.2 -25.2 I [ I ' I ' I 1 J '""'~'~ t ~'"'~-- STATION ~CP3 -.~, ~ '~ ~-'-"l .?- ' :---~:: :-I ~ / - ~ '.. ~, O~ .,' - -~..,.--.-~'- '. L--'~; ~ .; '~. ,'~.-~.: :z ~-~___--- '-./...&lt;: .~'~---.~--~_-.6cZ t ' ~ ' - .~..... ' -. - ~. ~.. _-..-_--;--_~ '~ ~,,..-..~,l. - /x~: ,,./t t x----', -'~--/-f-~,t~ ~,'- _J "_Z'"'! "," ~- 35.0 M/$ I ~ I I ~ I ~ ~ I -85.2 -73.2 -61.2 -49.2 -37.2 -25.2 X' (KM)136'f124.2~112.:88.;76.2126.2114.2 5/03Y79 OO40 0.5 KM xT,/ ,X,, l/l./' _7 _7 ./ 35.0 M/S ~ J J . I , I I ~ I i i I-85.2 -73.2 -61.2 -49.2 -37.2 -25.2 [ ' I ' I ' I ' I I I '5/03/7901150.5 KM - // - cP3 * ~ / STATION -~.'I ~-~'' I ~' / ~ ~ I ~, ~ X k/ ~(~-.... ,.. ~._-~2.~x...~ ~~ ,,, ~.... ~ '_. ~ ,/ /~~/~. w ~ f .~" .... ~--~z~~~: ~ ~~~~~-,~~:; :, ~~~ ~ ~ ,o2 X~~' . ''~' ' I-- ~ , "' ~'~~'h t ~'I ~ ~ ,~ ~-- ~~' -~_~ , ~;, ; , , ? , t , [ -75.2 -63.2 -51.2 -39.2 -27.2 -15.2 X' (KM) FIG. 12. Squall line relative horizontal vectors from dual- (0040, 0050, 0115) and triple- (0030) Doppler radar analyses. Reflectivitycontours presented are 15 dBZ (dashed), 30 dBZ (thin solid), and 45 dBZ (thick solid). Wind vectors are plotted at 3.6 km intervals, referencevector magnitude (35 m s-~ or 50 m s-I) is shown in lower left comer. On the 8.5 km level at each time, arrows labeled C6, etc., point tocentroids of cells, and updrafts and downdrafts are indicated by plus and minus signs, respectively. On the 0.5 km level, PAM station windbarbs centered at dots are plotted; a full barb is 10 m s-~ and a half barb is 5 m s-I. PAM station I is indicated. The radar locations are asfollows: CIM (x' = -12.1 km, y' = 39.5 km), CP3 (x' = -30.3 kin, y' = 112.3 km), and CP4 (x' = -56.7 km, y' = 67.7 km). See text forfurther details.To further elucidate the vertical structure of the squallline, 'we constructed a two dimensional cross sectionof the squall line. Figure 13 shows 0030 and 0115 meancross sections of wind vectors, streamlines, and reflectivity normal to the squall line. These cross sectionsare averaged along the line where triple Doppler estimates of vertical velocity are available at 0030 and at0115 where velocity estimates are reliable. The averagedregion covers about a 30 km wide Section at 0030 anda 40 km section at 0115. Cross sections through individual cell updrafts do not show significantly differentupdraft/downdraft structure, except magnitudes ofupdrafts are larger. Major updrafts are coincident with the reflectivitycores at midlevels, as has been found often in supercellcases (e.g., Heymsfield, 1978). These updrafts generallySEPTEMBER 1985 GERALD M. HEYMSFIELD AND STEVEN SCHOTZ ' 1579136.2t124.288.2 '5/03/792.5 KM 35.0 M/S 76.2 I , t -85.2 -73.2 5/03/79 0050136.2 2.5 KM, I , I I I I I -61.2 -49.2 -37.2 -25.2i I ' I I I ' I124: lOO.2- ( 88. 2 I 35.0 M/$ -8~.2 -73.2 -5~ .2 49.2 -37.2 -25.2 X' (KM L I I .-~.--'~'--'--' '--~~ .... I"-. ,-"'~ _f'~C"~~ ",.Z,C "": _;E~"~--~,,2.? -85.2 -73.2 -61.2 -49.2 -37.2 -25.2 I ' - I ' I ' I ' I ' I 5/03/79 0'115126.2 2.5 KM114.2 . 76.2 - 35.0 M/S 66.2 -75.2 -63.2 -51.2 -39.2 -27.2 -15.2 X' (KM)FIG. 12. (Continued)extend from the bottom (1.5 km) to the top (13.5 km)of the data. Comparison of our cross sections to thetwo-dimensional model results reported by Miller andMoncrieff (1983) reveal updraft streamlines similar tothose in their Fig. 10a, with two branches at upperlevels. At 0030, C? has a calculated updraft maximumof about 35 m s-: at a height of about 7.5 km (at x'= -60 km, y' = 110 km). This magnitude confirmsthe conclusion drawn earlier from the satellite measurements that the squall cells had intense updrafts, Itis also of comparable magnitude' to maximum valuesestimated by Kessinger et al.. (1982) for their squallline case study. The updraft of C7 at 0030 and updrafts at otheranalysis times (position shown by a plus sign in Fig.12, 8.5 km level), tilt up to 45- from the vertical fromlow to midlevels and become quite erect from middleto upper levels. Other studies of squall lines (e.g., Newton, 1967; Ogura and Liou; 1980 and Kessinger et al.,1982) have also found appreciable front-to-back slope(with respect to the squall line); however, these caseshave a stronger shear component normal to the line.The tilt of updrafts in squall lines as well as that observed in midlatitude supercell cases has been explainedin part by the conservation of low-level momentum asit is transported vertically 'by updrafts. This explanation1580 MONTHLY WEATHER REVIEW VOLUME 113136.2124.2~ 112.288.;76.;136.~124.288.276.2 I 5/03/79 0030 5.5 KM 50.0 M/S I , I I I I I i I i I~5.2 -73.2 -61.2 ~9.2 -37.2 -25.25/03/~9oo~o5.5 KM50,0 M/S I I I J I i I I I J I-85.2 -73.2 -61.2 ~9,2 -37.2 -25.2 L I ' 1/ 5/03/791 / 0040 /36.2~ 5.5 KML124.2 11 2.2 SO.O76.2 I I I I J I I I I I I -85.2 -73.2 -61.2 ~9.2 -37.2 -25.2 I S/0~/79 0115 5.5 KM 26.2 14.2 " ~ 78.~- '- ~8.2 50.0 M/$ I [ I ~ I ~ I i I [ I -75.2 -63.2 -51.2 -39.2 -27.2 -15.2 x' IKMIFIG. 12. (Continued)seems plausible here given that the inflow has a component directed into the squall line from low to midlevels. However, Seitter and Kuo (1983) have recentlyproposed that water loading may also contribute to theupdraft slope. Furthermore, Thorpe et al. (1982) foundfrom their two-dimensional model that upshear updraftslope could only be maintained with strong low-levelwind shear and weak shear aloft. This type of shearprofile normal to the line was present on 2 May. Major downdrafts are generally located between thecells and are associated with upper-level convergenceresulting from the interacting outflows of adjacent cells.This observation differs from isolated supercell cases(e.g., Lemon and Doswell, 1979), where the downdraftsupwind of cells were suggested to be due to the obstruction of midlevel flow caused by cells. At 0030, thestrongest estimated downdraft located between C? andC8 is ---10 m s-] and extends from ~-10.5 km downto 3.5 km. It is important to note that intense downdrafts originating from midlevels are not evident, ingeneral, along the rear side of the squall line--in contrast to Newton (1967) and Ogura and Liou's (1980)observations and the model results reported by Moncrieff and Miller (1983). This absence may be due inpart to the lack of low- to midlevel relative flow intothe rear of the line as seen in Fig. 12. The absence ofSEPTEMBER 1985 GERALD M. HEYMSFIELD AND STEVEN SCHOTZ 1581 136.2 124.2~112.2 100.: I ~ I ~ I ~ I ~' I ; ~I ' s/O3/TS ~ _~/-.,~./J. ~.7 C6 C3 0030 t';8 /.~/~,~ ~,,'~,~"-,/"'/'~/'~ .Y"/ / ~/~- 8 5 KM I /~//~/~//~ ~ - , ~ / , c,/)~~ ~~~~-~'~-, ,~,,,,~, /~//~/ZZZ/ t t //P t [.~Z// (/////V / / ~-~-~/7~4~d ~ C f C C ~t///~~-/////~ ~/~:~ ~ ~ ~/~ ///~/~ 7~)~/'~ ~ 7~/~' ~ ~ ~ ~ ~:~~,//~ ~/ S J~ ; / ~ ~ , V if; ~ ,~ ? ~F/~/7t / ~ t ( ~////J//~ %~ / , , ~ ~ /~/y ; / ~. 't~ -y-p,~;/z, , - , ,,.' - : ~--k' ~ ~: ~&gt;/~ ....... ~ 577;::::;;::~///~ ....~// / ~ : ~ ~ ~ [ I i I [ I ] I ~ I [ I t 5/03/79 C9 C8 "'" ~"~,.-.-?',,-'Z ' i __._ ,- C7 C6 C3 ~.~'J-~--')'--"~_ UU4U ' /), / ......... ~ ...~.....,~*.2F..sK. / /,, ~,.,..~.x,.-~ I / / ;, ~-..-//~-,l-/,-///.7 " / -F ' p.-'~'~'/tl'//A,////~ / r'- I/ ~'//J~/~///; ~ ! "/-////~.~ '- -, / _-'~ , .~) , , t[~/'/// /k/.'//N// t!.... - "//' --4 .... '1'-'--',~ ~ ~'/X('/~ ~P- . - 1. - / /~ z~t ~ /, T~ 7_~ ?..~'/'2~ - %~: . / t / ~ / ?,~. ~ :it: ,~: lJ:' ~,~112.2 - ' 'q. ' '"' / '~ _~/~ ~;'~ :.?~~:~~J~ -_d~ ~-~ ~t~.~ z :,:.~.:: .~~oo.2 - '" ' ~'~ .... ~ ........ ~ ~ /*, \~ ,~ ~ ~~ .- ,; ~ ..~'-~.~-~2.~- .i .i 71 ....... - _ '~ \ % ,~ .... ...~_.~,':'~'-. --._~_,~..,~. ........... ~~ ..... ~ ,, ~ ~ ~ ........~~: ~ ............. ~~~.~--. ~. .......... - ~ ; ; ; ; ~%~ . : : : ~ ~ ~ ~ X ~ ~ : ~ ; ~; ; : : : : : ;~ ~ ~ ~~ ~ - ~ --~--~ _ ~ ~~; ~ ; ; : : ;~ - - ~%~~~ 50.0 M/S '~ ~ ~ ~ ~ .... ~ % : -, %~ ~ 76 2 -- 50.0 M/S , - _ 76.2- I ~ I ~ I ''r- I '~1 ; I - ' I ~ I ~ I 'l I I I ~ I -85.2 -73.2 -61.2 ~9.2 -37.2 -25.2 -85.2 -73.2 ~1.2 ~9.2 -37.2 -25.2 ~ ~7~~~~~~ ~~~~~~S~9~ ~ I ~ ~ ' --, ~, I ~ l- ~ I I ' 7 ~q-~////) ~//~ ~ / /5/03/ 9~ / /~ _/ ...... ' / ~ ~/ / J / /~//~ ~ ~ 1-115 ] ,'~/~/// I 136.2 126~/8's KM ~ C9 C8 C~ ~'~/''' ~ ///// / '~V , , I ~ . // ~ I , ~',/~1. - 4- ,~" /. l F"-' ////1' q'_;~'_~ ' ~ / q,'"'T q ' ';:::""" ~' ~/~/ I~ ,.. :1',: "~~~:'~l~//~ .... .....,... ~o~ : - //' ' V_~ t ~ ~ ~~~ 4 "='~ ~-. ."~' ~~ ~ ~ ~~/ I . fi ~ -, ~ ~ / AI X//t~///// fi ~ Y~;_~~ ~/~ I.. ,,.: ":-' ,o.o.. .... &gt; "---, ,,.._. - - , ,- -- -- -- - - : - : : - ' : - ~, ~, ?"',-'-i, d~.~'-,~.= -85.2 -73.2 -61.2 ~9.2 -37.2 -25.2 -75.2 -63.2 -51.2 -39.2 X' (KM) X' (KM)FIG, 12, (Continued)relative flow into the rear of the line was also foundfrom an examination of several RHI sections of squallline relative radial velocity, oriented from ~ 315-360-(a RHI of 327- azimuth is approximately normal tothe squall line) in azimuth, at several times from nearinitiation into the mature period using CP3 and CP4radars. In addition, the relative wind profile behind theline from the sounding (Fig. 6) showed an absence offlow into the rear of the line. The above explanationdoes not however rule out downdrafts behind the lineresulting from precipitation loading and evaporation,as suggested by Kessinger et al. (1982), or downdraftair originating from the front of the line (e.g., Zipser,1977). These mechanisms may account for the morelocalized downdrafis inferred from the single Doppleranalyses discussed earlier. As mentioned earlier, thereis evidence that some of the downdraft air may comefrom ahead of the line.6. Estimation of mass and moisture fluxes Estimation of the mass and moisture fluxes of the 2May 1979 squall line is helpful in understanding itsstructure and how it compares with previous studies.Several studies have estimated the mass and moisturebudgets of squall lines, for example: midwest squall1582 MONTHLY WEATHER REVIEW VOLUME 113136.2124.2 I ' I ' I ' I ' I ' I5/03/7911.5 KM ~.-"' .."l./,.~~ / tt 7 I/... ft I /' /M \ ,'4 / /136.2124.2112.2 - &gt; 50.0 M/S 76.2 [ ~ I I [ I I t [ &amp; I 76.2 -85.2 -73.2 -81,2 -49.2 -37.2 -25.2 I I ' I ~ I ] I ] I ] I i 5/03/79 0060 136.2 11.6 KM 126.2,24.2l- t ', x'ff , - i //, , q~112.2 102.2 / ' ' M ~ ' t. ' ~' 't/~/~-~//~ ,, .~1 76.~ 66.2-85.2 -73.2 -81.2 -49.2 -37.2 -25.2 X' (KM)5/03/79O04O11.5 KMI [ I ' I [ I ' I &gt; ~0.0 M/S-i I : I , I i I , I i I -85.2 -73.2 -81.2 -49.2 -37.2 -25.2 I ' I ' 'l i I ' I ] I 5/03/79 0115 11.5 KM , ~ , /ZX/.f Z ~,, t Z/IZ/.L/./ ~ ~ ,17LZAAAL . _ ,74' .." - ..///+ [/.LLZLI, L4 ... I / .' ,,~///t. .--////tJ t/,,Z! ~/, /". , ,,//.,/,~ tiJ/.ZZZZZA///* ~ Z-; ',, - , ,,--?'/t i,Z/7./Z/./,-?'/t .~ '!_ ,~ ,,/, ~//...-.'.,,' t .T'/'-ffr~////..,c"_/~' ~ '~ 1Z ,. ,-' .,,'-/.,,'~-. , ,',&gt;'t /~ ///5~LY_ / ~(Vr t ~_'~1, ], \"---'"-"~",,-'"--"."- - l;II "-."- ' ' $0.0 M/$ I I I I I I I , I ~ I -75.2 -63.2 -51.2 -39.2 -27.2 -15.2 x' (KM)FiG. 12. (Continued)lines (Newton, 1966, California squall lines (Carbone,1982), and tropical squall lines (Gamache and Houze,1983). These previous calculations assume two-dimensionality, i.e., variations along the line are considered small compared to across the line. On the otherhand, calculations for multicellular or supercell thunderstorms (e.g. Braham, 1952; Fankhauser, 1971) assume three-dimensional moisture and wind fields. Recently, Doppler calculated wind fields were used in estimation of mass flux measurements' (Frank and Foote,1982). We will use various approaches (two- and threedimensional) to grossly estimate the squall line massand moisture fluxes.a. Mass fluxes The upward mass flux is defined as the rate at whichair is transported vertically by updrafts. Essentially, thelow-level influx into the line is assumed to be uniformalong the line and to be completely transported upwardas it enters the updraft region (Carbone, 1982); i.e., thefollowing should hold at cloud base if the estimates arereasonable: Fa (influx) = Fa (upward), (1)where F~ is the mass flux. The inflow and upward massfluxes are estimated respectively from the environSEPTEMBER 1985 GERALD M. HEYMSFIELD AND STEVEN SCHOTZ 158313.511.5 9,5 7.5 5.5 3.5 1.5 IIi1~111111111~1111111111111111111~111111111~11111111111111~- 5~03/79 0030 GMT ' TRANSVERSE TO LINE .... .....,.~-..~... MEAN SECTION / / \ AHEAD OF .... . ,.- .- .- 7'7 .- .- 79.8 91.8 103.8 115.8 127.8 139.8 3Orals Y (kin) 13.5 5103/79 0115 GMT ~ _ -- TRANSVERSE TO LINE - MEAN SECTION 30m/s Y Ikml F[O. ! ~. Vertical cross sections transverse to line ~t 00~0 and 01 ! $. Sections ~rc ~vcm~cd ~lon~~ ~0 km somerit o- the line ~t 00~0shown as in Ei~. 12. Strc~miincs in the pl~nc ~rc shown.mental sounding (CHK in Fig. 4), and from the Doppler vertical velocity fields at 0030 in the previous section. 1) MASS INFLUX For a gross .calculation of mass influx, we assumetwo-dimensionality (Newton, 1966) for estimating thesquall line mass influx. The mass flux into a length Lof the squall line is given by F~ (influx) = L ,o(z)VtrdJ, (2)where o(z) is the density of the inflow air, V'r(Z) is thewind component normal to the line, and Zt is the topof the inflow layer (~6 km) defined at the level wherefir(Z) becomes negative. The 2305 CHK wind sounding(Fig. 6) is used for calculation of (2). In this discussionwe limit our calculations to a 20 km section (i.e., L= 20 km) of the squall line defined by -65 km &lt; x'&lt; -45 km in Fig. 12. [The value of L used by Newton(1966) was defined as a mean spacing between the cells.The L used is somewhat arbitrary, and since the intercell spacing was 15-20 km, we used the same valueas Newton (1966).]1584 MONTHLY WEATHER REVIEW VOLUME 113 Figure 14a shows the air flux calculation accordingto (2), with a maximum value of 9.2 x 10TM g s-~ at~6 km altitude. This value would be an upper limitbecause some environmental flow into the line presumably flows around cells along the line rather thaninto their updrafts. Soundings at later times and otherlocations southeast of the squall line give mass fluxes(and moisture fluxes to be discussed later) within 20%of the CHK sounding. 2) UPWARD MASS FLUX The 'upward mass flux is calculated from the Dopplerfields using a method similar to Frank and Foote11982): Fa (upward) = ~ p(z)wi~(z)3~l, (3) Awhere wo are the grid values of vertical motion, andzL4 = 1.44 km2 is the area of a grid cell in the Dopplerwind fields. The summation in (3) is performed for allgrid cells with w &gt; 3 m s-j, within the region boundedby -65 km &lt; x' &lt; -45 km and y' between the frontand rear of the squall line data region (see Fig. 12).Use of w = 0 gives only a few percent difference at alllevels, but it includes sometimes undesirable noisy estimates of w near the data edges. Downward mass fluxis defined similarly to (3) when the summation is overgrid cells with w &lt; -3 m s-~. Upward and downwardmass fluxes are shown in Fig. 14a. Also, the entrainment rate (e.g., Simpson and Wiggert, 1969), definedas 1/Fa(dFa/dz), is plotted. The upward mass flux indicates significant inflow of environmental air up to~7.5 km, with a maximum magnitude of 9.5 x 10TMg s-~. The influx above cloud base defined by the LCL(z = 1.5 km) is manifested by a significant increase inthe updraft area (not shown). Presumably, near cloudbase, the updraft area is small, in part due to our 3 ms-~ definition and also because the upward flux is provided by the narrow convergence line. The squall lineand individual cells apparently entrain air much abovethe LCL. The agreement of flux estimates in (1) is surprisinglygood, considering the simplistic approach used. It nevertheless indicates that the sounding method gives atleast reasonable estimates of the mass influx for thesquall line. One significant difference between ourevaluation of (1) and the Carbone (1982) study is thathis integration was performed up to cloud base, whileours was taken up to midlevels. Table 1 gives values of mass fluxes (and moisturefluxes to be discussed later) from previous studies ofvarious types of squall lines. In comparing the differentcases, it is necessary to normalize the different valuesso they correspond to our assumed L = 20 km length.For the tropical flux values, Gamache and Houze11982, 1983) use L = 150 km and a time interval of 9h. For mass influx, only the squall line mass transportin Gamache and Houze's 11982) Fig. 14 was considered; moisture fluxes are taken from Gamache andHouze's 11983) Fig. 12 [C, for influx and (R, + Re) fortotal rainout]. Thus, their values are multiplied by 4.1x 10-3 g kg-t s-~. Values from Newton 11966) use hisFig. 12. The mass fluxes calculated for the 2 May squallline are found to be about 30% larger than other squallline cases. The entrainment rate curve (Fig. 14a) is nonuniformwith height, with strongest entrainment ('~0.9 km-l)at low levels and strongest detrainment (,~ -0.6 km- i)occurring at upper levels; the crossover between entrainment and detrainment occurs at about 7.5 km,which can be defined as the base of the anvil. A nonuniform entrainment rate with height is suggested byprevious observations (e.g. Braham, 1952); however,it has been found to be constant with height in Doppler'~' 8 - MASS FLUXES // "~ '~-'--~ - (gs-1 x 10n) ( 6.- ....... CHK SOUNDINGX, . -- , UP (DOPPLER) ":--- .o.v. ,,,o,.,., .., \ 2 110'1 km'1) i _..~-'~'~?:~ - ': , I ~ I ,'1 , I ~ I ~ ~,1/'~....4"'; I , I -8 -4 0 4 8q ( g/Kg ) -~MOISTURE FLUX ( gs-1 x 10 9 ) _ 5 10 15FIG. 14. Mass and moisture fluxes. Given are (A) mass fluxes and entrainment rate and (B) moisture flux and mixi.ng ratio q. See text for details.SEPTEMBER 1985 GERALD M. HEYMSFIELD AND STEVEN SCHOTZ 1585 TABLE 1. Previous values of mass-moisture fluxes. Mass influx Moisture influx Rainout Precipitation L Squall line Investigator (10~- g s-') (108 g s-') efficiency (kin) typeCarbone (1982) 77% CaliforniaGamache and Houze (1982) 66 150 TropicalGamache and Houze (1983) 100 59 ~ 59% 150 TropicalNewton (1966) 70 88 47 53% 20 Oklahoma2 May 1979 100 76 18 25-40% 20 Oklahomaobservations of an isolated storm (Kropfli and Miller;1976) and assumed to be constant in one-dimensionalcloud models (Simpson and Wiggert, 1969) when thecloud radius is assumed constant.b. Moisture fiuxes Estimates of moisture fluxes in Midwest squall lineshave been made by Newton (1966) and Ogura andLiou (1980). Braham (1952) has given physical descriptions and an evaluation of the various moisturesources and sinks acting in an air mass thunderstorm,and this serves in part as a basis for the following. Themoisture conservation for a quasi-steady state squallline can be stated as:F,~ (rainout) = F,, (moisture influx) - F,~ (anvil outflow) - F,~ (vapor loss by entrainment) - Fr~ (vapor carried by downdrafts), (4)where Fm is the moisture flux. The terms that can beestimated best with the 2 May observations are therainout and the moisture inflow. The precipitation efficiency can be calculated from (e.g., Foote and Fankhauser, 1973) F,~ (rainout) Precipitation Efficiency - x 100%. (5) Fm (influx)Since we cannot estimate the remaining terms in (4)with acceptable accuracy, the precipitation efficiencywill be the main emphasis of the following. Foote andFankhauser (1973) found that the efficiency is dependent on the stage of development of thunderstorms,and the use of instantaneous rather than time-averagedresults can given misleading results. With the squallline case here, this should not be as significant a problem because of its long lifetime. 1) MOISTURE INFLUX The moisture inflow is estimated from the environmental sounding: gm (influx) L f' = -- q(z)V'r(z)dz, (6) g dsfcwhere q is the mixing ratio. Figure 14b shows curvesof q and Fm calculated from the 2305 CHK sounding(Fig. 4). The values of q below the LCL are 10-18 gkg-~, and they decrease to near zero at about 5 kmaltitude. The moisture flux has a maximum value of7.6 x 109 g s-l, and 80% of the moisture influx entersthe squall line below the LCL. This is in contrast tothe mass influx which extends over a much deeperlayer. An independent estimate of Fm can also be madefrom the Doppler observations at z = 1.5 km and anappropriate estimate of q. This estimate, which usedthe saturated mixing ratio qs at cloud base from theCHK sounding (Fig. 4), gave Fm ~ qsFa (upward)= (14.6 g kg-~) (5 x 10~ g s-I) = 7.3 x 109 g s-~. Thevalue of Fro obtained from this latter method is almostidentical (perhaps fortuitously) to the estimate fromthe soundings. Both values are of similar magnitude(see Table 1) to the Oklahoma squall line studied byNewton (1966) and the tropical squall line (Gamacheand Houze, 1983). The Oklahoma squall line valuesare in good agreement in part because of the similarmoisture in the environment. The agreement betweenthe moisture flux value for the tropical squall line (Gamache and Houze, 1983) and 2 May is surprising sincethe presquall moisture profiles in both cases are comparable, but convective updraft magnitudes in thetropical squall line are only about 10% of those in ourcase. The flux magnitudes are close in magnitude because of the much larger width (normal to the line) ofthe tropical updrafts (75-100 km) compared to the updraft region (~ 10 km) on 2 May. 2) RAINOUT ESTIMATION The estimation of rainout near the surface can beobtained by various methods (Foote and Fankhauser,1973) using surface rainfall traces and low-level radardata. Using both the low level reflectivity data and thePAM data, Fm (rainout) is evaluated. Estimation of rainout was made from the z = 1.5km CAPPIs of radar reflectivity data according to F,, (rainout) -- p~ ~ RigA,t, (7) Awhere Rii are the grid values of rain rate, A and AA aredefined as for (3), and p,~ is the density of water. The1586 MONTHLY WEATHER REVIEW VOLUME 113rain rate R is defined by z = 200R1'6, where z is theradar reflectivity in mm6 m-3. The summation is overvalues with reflectivity &gt; 10 dBZ. This method givesFm (rainout) values (in units of 109 g s-1) of 2.2, 2.7,2.2, and 2.7 at 0030, 0040, 0050, and 0114, respectively. Because of the narrow band of rain associated withthe squall line, use of surface data was considered lessaccurate than the radar-estimated approach. Some stations had no rainfall associated with the line passage,while others had total rainfall of 25 mm. Thus a roughestimate of the rainout was made using time traces ofrainfall from PAM stations according to F,~ (rainout) = Pw R*(y')dy'dx' = CLpw R(t)dt, (8)where R* is the station rain rate, L is the length of thesquall line segment (=20 km), Y is the width of theline (normal to it), T is the interval of the squall linerainfall, and C is the advection speed of the squall line.The station rain rate R* is determined directly fromthe PAM measurements from the I min data. Themean network Fm (rainout) determined from (8) was1.8 x 109 g s-I, with a range of values from 0-5.6x 109 g s-~ over the 27 stations. These estimates areabout 20% lower than estimates from the radar data.The PAM data may be lower because of evaporationin the dry air below cloud base. The precipitation efficiency has values of 25-40%when calculated using (2) and both the radar and PAMnetwork estimates of Fm (rainout). These values aresomewhat lower; than for Newton's (1966) estimate of53%. Marwitz (1972b) found that for thunderstorms,precipitation efficiency decreased with increased vertical wind shear. The 2 May squall line efficiency ismuch lower than that estimated from Marwitz's curveof efficiency versus shear (~60%) in Foote and Fankhauser's (1973) Fig. 20. Both Newton (1966) and Footeand Fankhauser suggest that some of the storm inefficiency (35-50%) can arise from evaporation of precipitation in downdrafts. We speculate, however, thatthe relatively lo.w precipitation efficiency is due tomoisture losses through the anvil loss term in (4). Thistermcan be expressed as losses due either to divergence oradvection of moisture. During its growth period, thesquall line anvil showed the very rapid expansion normal to it (see Fig. 1). In addition, the u~ component(Fig. 6) which is 20-30 m s-~ at upper levels, impliesthat substantial amounts of precipitation would betransported downwind along the line. Thus both components of the anvil loss term are suggested to be important. No attempt was made to evaluate this termbecause of inadequate estimate of q and the complexinteractions of the cell outflows at upper levels. Thehigher precipitation efficiency in Table 1 associatedwith tropical squall lines (59%) is perhaps related tothe absence of strong vertical shear and the considerablyweaker updrafts.7. Summary and conclusions The squall line on 2 May 1979 developed in proximity to a cold front in northwest Oklahoma whichsubsequently moved southeastward. A synthesis of radar, satellite, sounding, and surface data was presentedfrom the initiation of the squall line through part ofits mature period. The combined use of satellite andradar data was found to bc of considerable value in thedescription of the general structure of the squall lineand the evolution of cells comprising the line. Cellsproduced hail, were nontornadic, and had maximumheights of about 16 km. A number of similarities anddifferences were found between the 2 May case andother midlatitudc and tropical squall line cases. Findings regarding the evolution and movement ofthe squall line and its ceils are: 1) Cells exhibited explosive growth nearly simultaneously along the line. Growth rates and mean cloudtop ascent rates estimated from satellite IR data suggestintense updrafts along the entire line, which compareto rates documented for other severe storms (Adler andFenn, 1979). Most of the cells penetrated the tropopause based on satellite stereographic height estimates. 2) The 2 May squall line did not exhibit trailingstratiform precipitation during its developing and earlymature periods. This trailing region has been commonly observed for tropical squall lines and for somemidlatitude squall lines. Later in the lifetime of thesquall line there may have been a stratiform region,but this period was beyond the times analyzed in thispaper. At low levels, there was a large resemblance to"line convection" (Browning and Pardoe, 1973, etc.)in the radar echoes, while at higher levels, discrete convective cells were observed. 3) The squall line had nonuniform movement alongits length. However about the eastern half of the squallline moved nearly uniformly and had a speed and direction which was in general similar to a low level convergenqe line deduced from single Doppler radar andsurface (PAM) data. This convergence line was lessthan 10 km ahead of the squall line. It was suggestedto be associated with the synoptic scale cold front,which became modified by squall line downdrafts asthe line matured. 4) Although some new cells developed during theperiod analyzed, most of the cells along the line didnot dissipate over the 3 h period they were tracked.SEPTEMBER I985 GERALD M. HEYMSFIELD AND STEVEN SCHOTZ 1587Thus the cells comprising the squall line resembledsupercells in that they propagated by continuous regeneration. Unlike in stratiform type squall lines (e.g.,Houze, 1977), discrete cell propagation was not evident. 5) Many of the cells moved initially with an eastward direction along the low- to midlevel shear vectorand then turned southeastward, to the right of the lowto midlevel shear vector and all tropospheric windsabove ~ 1 km. After the turn, the cells slowed downin a fashion similar to supercells (e.g., Lemon and Doswell, 1979). It was postulated that two mechanismswere responsible for the rightward turn of the cells: (i)the mechanism proposed by Rotunno and Klemp(1982), requiring veering environmental wind shear inthe lowest levels; and (ii) the mechanism in which thecell motion is eventually governed by the moistureconvergence and lifting provides the convergence line,analogous to Weaver's (1979) findings for midlatitudethunderstorms. Conclusions regarding the three-dimensional structure of a few selected cells during the mature period ofthe squall line from the multiple Doppler analysis are: 1) One of the cells developed a weak mesocycloneduring the analysis period. The other cells, however,had weak low-level vorticity in comparison to documented supercells even though they moved considerably to the right of the shear vector. This finding wassurprising given that right moving supercells generallyhave strong associated mesocyclones (e.g., Lemon andDoswell, 1979). 2) The cells exhibited a relatively deep "inflow"layer (---6.5 km) on the front side of the line. UnlikeNewton's (1967) model and tropical cases, the flow onthe rear side at mi~l-levels was nearly parallel to, ratherthan into the line. 3) Updrafts were generally colocated with reflectivity maxima at middle to upper levels. Triple Dopplerestimates gave a maximum updraft magnitude of ~ 35m s-I. This magnitude is consistent with the rapidgrowth rates observed in the satellite IR data and ingood agreement with the values of vertical velocity reported by in situ aircraft measurements (Heymsfieldand Hjelmfelt, 1981) that were taken ~ 30 min earlierat ~8 km altitude. In the two-dimensional mean vertical cross sections of the squall line, updrafts split intotwo branches, as suggested in modeling work reportedby Miller and Moncrieff (1983). 4) Strongest downdrafts occurred from middle toupper levels and resulted from the strong convergenceproduced between the divergent outflow regions of adjacent cells. These downdrafts had magnitudes ~ 10m s-l. The nature of these downdrafts is considerablydifferent than those for isolated supercells proposed byLemon and Doswell (1979). Given the resolution limitations of the data, analyzed downdrafts were weaknear the surface. Unlike other midlatitude squall lines(Newton, 1967; Houze and Smull, 1982) and squallline models (e.g., Miller and Moncrieff, 1983), majormidlevel downdrafts were in general absent along therear of line. This observation may be due in part tothe lack of environmental flow into the rear of the lineat midlevels. It was suggested that some of the downdraft air near the surface may have come from aheadof the line, similar to that for tropical squall lines (Zipser, 1977). Conclusions regarding the mass and moisture fluxcalculations are: 1) Mass flux magnitudes computed from soundingsand the multiple Doppler analysis are in good agreement, with a maximum magnitude of about 9.5 x 10~g s-~. This value is of similar magnitude to other documented cases for hail storms and squall lines. Moistureflux estimates are also of the same magnitude as thosedocumented for other cases. 2) The precipitation efficiency was estimated to fallin the range of 25-40%. This range is somewhat lowerthan efficiencies reported (&gt;50%) for other midlatitudeand tropical squall lines. It was speculated that thislower efficiency was due in part to upper-level lossesdue to moisture transport downwind from the cell updrafts. The differences between the 2 May squall line andother midlatitude squall lines brought to light in thisresearch, may be partially due to the orientation of thecloud layer wind-shear vector with the squall line. On2 May, this shear vector is about midway between parallel and perpendicular to the squall line ahead of theline and para!lel to it behind the line. In many of theother cases (e.g., Ogura and Liou, 1980), it is morenormal to the line. Another important difference between the 2 May squall line and other midlatitude andtropical case studies is the presence of a pre-existingcold front in close proximity to the squall line. It wouldtherefore be valuable for squall line simulations toconsider more realistic initial conditions, such as different shear orientations than the classical profile(Newton, 1967), inhomogeneous moisture, and windshear across the squall line, and the presence of externalforcing such as that provided by the convergence line. Another important question to be addressed is whythe squall cells were nontornadic with weak mesocyclones, while tornadic storms formed earlier northeastof the squall line. Once again, this may be related tothe effect of the convergence line on the squall linecell's downdraft structure. Acknowledgments. The authors gratefully acknowledge discussions and suggestions regarding the paperby Drs. Joanne Simpson, Robert Adler, Steven Koch,and Wei-Kuo Tao. We are appreciative of the variouspeople in the Severe Storms Group at Goddard who1588 MONTHLY WEATHER REVIEW VOLUME 113helped in the data reduction and preliminary analysis.Mr. Roy Blackmer helped in obtaining stereo cloudtop heights and compositing of the radar and satellitedata. Dr. Koch and Mr. Robert Golus provided software to reduce the PAM data, and they were quitehelpful in a number of discussions regarding its analysis. The various groups participating in the 2 MaySESAME data collection effort are greatly appreciated.Radar data were provided by Mr. Bill Bumgarner ofNSSL, and Mr. Jim Wilson of NCAR/FOF. Specialsounding data were provided by Professor J. J. Stephensof Florida State University. APPENDIX Doppler Analysis Techniques The analyses of radar data were performed on a VAX11/780 computer using interactive processing softwaredeveloped at NASA/Goddard Space Flight Center. Thisprogram performed all analysis functions, such as tapeinput, interpolation (Cressman or Barnes), calculationof vertical ,velocity and kinematic quantities, etc., withuser-specified options. The analysis grid used here was61 x 59 horizontally, by 16 levels, with horizontal andvertical grid spacings of 1.2 km and 1.0 km, respectively. The boundaries of this grid are shown superposed on the 0034 satellite image in Fig. 1. Typicaldata volume scan times were 3-4 min. Gate spacingswere 150 m for CIM and 150-170 m for CP3 and CP4;data were stored only every 300 m for CP3 and CP4and 150 m for CIM. CAPPIs were interpolated from0.5 km to 16.5 km in height using a Barnes filter (seeBrandes, 1977) with' one pass. Data were advected usingthe approximate motion of C7, obtained from Fig. 7as 13.5 m s-l, from 276-. Horizontal velocities werecomputed, making an advection correction suggestedby Gal-Chen (1982). Vertical velocity calculations weremade for the dual and triple radar cases, and bothanalyses used the analastic continuity equation (Rayet al., 1975). The dual-Doppler analyses used an iteration scheme (Brandes, 1977) to compute w. The topboundary condition was specified as follows. The uppermost vertical velocity boundary condition assumedzero vertical velocity and divergence at 1 km abovethe radar data. Divergence was assumed to increaselinearly from this level to the first level where therewere data. If no data were encountered below 10.5 km,the integration path was terminated.REFERENCESAdler, R. F., and D. D. Fenn, 1979: Thunderstorm vertical velocitiesestimated from satellite data. J. Atmos. Sci., 36, 1747-1754.Barnes, S. L., 1981: SESAME 1979 data user's guide. NOAA/ERL, Boulder, CO, 236 pp.Braham, R. R., Jr., 1952: The water and energy budgets of the thun derstorm and their relation to thunderstorm development. J. Meteor., 9, 227-242.Brandes, E. A., 1977: Flow in severe thunderstorms observed by dualDoppler radar. Mon. Wea. Rev., 105, 113-120.Browning, K. A., and C. W. Pardoe, 1973: Structure of low-level jet streams ahead of mid-latitude cold fronts. Quart. J. Roy. Meteor. Soc., 99, 619-638.Burgess, D. W., L. R. Lemon and R. A. Brown, 1975: Tornado char acteristics revealed by Doppler radar. Geophys. Res. Lett., 2, 183-184.Carbone, R., 1982: A severe frontal rainband. Part I: Stormwide hydrodynamic structure. J. Atmos. 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