October 1970 749 UDC 551.515.227:651.624.72:551.551.5:551.507.352(769) "1967.09.20" BeulahTEMPERATURES AND TURBULENCE AT TROPOPAUSE LEVELS OVER HURRICANE BEULAH (1 967) DAVID E. WAC0Lockheed-California Company, Burbank, Calif.ABSTRACT Horizontal and vertical temperature variations along with true gust velocity measurements of atmospheric turbu-lence were obtained above hurricane Beulah (1967) by an instrumented U-2. The U-2 flight was part of the US. AirForce High Altitude Clear Air Turbulence program. Pertinent findings include: (1) location of the tropopause justabove the cloud tops at 54,000 ft (100 mb) with a temperature of -86OC, 12'C lower than the mean; (2) a verticaltemperature rise of 11C in a few hundred feet just above the tropopause; (3) horizontal temperature changes up to7C and smooth flight conditions in the stable layer above the cloud tops; and (4) small temperature fluctuations andgenerally turbulent conditions at cloud top level. Aircraft measured winds, although questionable as to their exact directions and speeds, indicated that the flowwas weak and anticyclonic near Beulah's top, becoming increasingly anticyclonic above.1. INTRODUCTIONStratospheric conditions above hurricane Beulah (1967)were measured in detail during 5 half-hour period after thestorm crossed the south Texas coastline. A special instru-mented U-2 aircraft completed two large loops around thehurricane's axis, just above cloud tops, near 54,000 ft.(100 mb). Temperature, mind, and air motions recorded 12times per second are analyzed in this paper together witlhavailable radar, rawinsonde, and other information andcompared with descriptions of previous hurricanes.The flight above Beulah was part of the U.S. Air ForceHigh Altitude Clear Air Turbulence (HICAT) programinitiated in 1962 to provide fine scale measurements of gustvelocity and related atmospheric properties between40,000 and 70,000 ft. In 4 yr from February 1964 toFebruary 1968, a heavily instrumented USAF U-2 com-pleted 285 flights over and near Australia, New Zealand,Panama, the Caribbean, and the British Isles, as well asseveral parts of the United States including Alaska a.ndHawa.ii. Instrumentation included a digital pulse codemodulation (PCM) system, an inertial navigation syst.em,a fixed vane gust probe, and an oscillograph recorder. ARosemount Engineering Model 102 total temperatureprobe was mounted in the aircraft nose. Measurements,analyjis, and meteorological summaries of the flights havebeen given by Crooks et al. (1967), Crooks et al. (1968),Ashburn et a.1. (1968, 1969).While stationed at Patrick Air Force Base, Fla., duringSeptember of 1967 for flights into potential turbulent areasaround Florida, the U-2 was dispatched on Flight 247 t.oBrownsville, Tex., where Beulah was crossing the coastline.Beulah's history has been summarized by Sugg andPelissier (1968). Only the third September storm of thecentury to attain hurricane intensity in the easternCaribbean, Beulah had brushed Hispaniola and Yucatan1 Mention of commercial products does not constitute an endorsement.before landfall on Sept. 20, 1967. The preceding day, aUSAF hurricane reconnaissance aircraft found a centralpressure of 923 mb, and at 1400 GMT (0800 CST) on the20th, 2 hr after landfall, pressure at' Brownsville was 951mb; peak gusts of 95 kt were recorded on an anemometertilted 30" from vertical. Beulah moved about 110 mi north-northwest from Brownsville before turning southwest onthe 21st and broke up in t,he mountains west, of Monterreyon the 22d. Beulah was only t,he. fifth of what, has beenreferred to as t.he "great," hurricanes (Kraft 1966) since1955 to enter the United Stat,es.9. SYNOPTIC SITUATIONA synoptic description of Beulah was obtained fromrawinsonde observations at 0000 GMT on Sept. 20, 1967,t,hs last time at which the observation network wasreasonably complete but 12 hr bofore landfall and 16 hrbefore the U-2 flight, above t,he storm. At, 300 mb, t,hehurricane had a typical cyclonic mind vortex (fig. 1A) anda rather extensive and quite \+arm low-pressure core(fig. 1B). Omr Brownsville, when Beulah was 160 mi tothe southeast, the 300-mb temperat.ure was only -28"C,higher by 5" than the mean September sounding for theCaribbean (Jordan 1958). The -30C isotherm encircledthe hurricane at distances of about 400 mi, within whichwinds were generally light, only 10 to 25 kt; Brownsvillereported 35 kt. The general situation mas similar to thepattern around hurricane Cleo (1958) for which LaSeurand Hawkins (1963) found positive temperature anomaliesincreasing with height, and also quite widespread.At 100 mb (fig. IC), flow was anticyclonic for severalhundred miles around the hurricane, but weak (4 kt overBrownsville) ; whether the flow immediately above the corewas cyclonic cannot be established for lack of observations.The contours on the 100-mb map are doubly smoothed:first, height's at grid points 200 mi apart were interpolatedfrom charts prepared using the Northern Hemisphere data750 MONTHLY WEATHER REVIEW Vol. 98, No. 10, 002 20 SEP 1967FIGURE 1.-(A) 300-mb contours and winds, (B) 300-mb isotherms,(C) 100-mb contours and winds, (D) 100-mb isotherms, and (E)100-mb temperature departure from normal, all for 0000 GMTon Sept. 20, 1967. Contours in hundreds of feet; winds in knots;and isotherms and temperatures in degrees Celsius.October 1970 David E. Wac0 751tabulations and teletypewriter data, and then values forfour grid points were averaged and plotted. Such smooth-ing is needed t,o reduce inaccuracies arising from themeasurement unreliabilities at this height.Temperatures at 100 mb were lower than the meanvalues for Sept,ember (Muench and Borden 1962) forhundreds of miles in all directions (figs. 1D and IE), andthe core itself was quite cold, as has generally been foundin the upper levels of int'ense hurricanes (Penn 1966;Koteswaram 1967; Miller 1967; and Gentry 1967, 1968).Although at 0000 GMT the cold core appeared to be some-what south of the surface conter, inflight' measurements of-86C at 1600 GMT, when the hurricane had passed itsmaximum intensity, suggest that the coldest area \\-asdirectly above the surface cent8er.3. CLOUDSThe extent of the cirrus deck associated nh hurricaneBeulah gives some idea as to the massiveness of tho storm.The U-2 pilot observed cirrus-type clouds with 46,000-ft(147-mb) tops extending 275 mi northeast of t,he storm'scenter and estending south as far' as tho aye could see.This is of the same order as the cirrus shiold over hurricaneDaisy, 1958 (Malkus et al. 1961), which had a radius of230 mi. Cloud tops 50 mi from Beulah's center on thenorth side were 50,000 ft (121 mb), and an occasionalcumulonimbus tower protruded 500 ft above the cirrus.The height' of the cirrus continued to increase radially in-ward to 53,000 ft (105 mb) 10 to 15 mi from the hurricane'scenter, then rose abruptly in wall-like fashion to 54,000ft (100 mb). The cloud structure inward from this "rw~ll"deserves special attention and is discussed in detail in thenext few paragraphs.While flying above the hurricane at 65,000 ft (57 mb),the pilot. observed a dark circular band in t,he cirrus deckwith a diameter of about 25 mi. After the descent, it, wasdiscovered that the band more or less coincided with themall described above. The band or wall gave a visualpicture of the eye, which was completely obscured by thecirrus shield. The cloud surface inward from the wall wasrelat'ively flat instead of dome-shaped as has sometimesbeen suggested (e.g., Kotesn-aram 1967).One cumulonimbus tower protruded above the cirrus inthe northwest quadrant' of the hurricane. On t.he westernedge of t,he wall, the 54,000-ft (100-mb) cap ext,ended out,over the surrounding area in an unorganized fashion.It should be noted that,, in locating the hurricane'scent.er, a perpendicular eye wall was assumed, an observa-tion noted by Malkus and Riehl (1960) and Miller (1967).4. AIRCRAFT OBSERVATIONSThe flight track of the U-2 aircraft is shown in figure 2along with t.he track of Beulah. The plane approachedBeulah's northwestern side at 65,000 ft (57 mb) and flewthree counterclockwise loops around the center. For halfof the first loop, the aircraft, continued at 65,000 ft to apoint almost due east of the center. From there, the pilotdescended to 55,000 ft (95 mb) on the western side. Thenext two loops mere flown, for the most part, at cloud-toplevel (approximately 54,000 ft or 100 mb).The positions of Beulah at 1221, 1840, and 1945 GMTmere established from radar fixes at Brownsville. The1624 Gm location was estimated from the pilot's observa-tion of the circular pattern within the clouds.HORIZONTAL TEMPERATURE VARIATIONFigure 2 shows time histories of temperature and pres-sure altitude from eight 3-min runs (segments, each approx-ima,tely 22 mi) along the tJ-2'~ flight path in the immedi-ate vicinity of Beulah. The locat,ion of the runs in referenceto the hurricane's position is illust,rated in the diagram oft,he flight track. The runs are discussed helow.Run 1-The start of t.his run was at 55,000 ft (95 mb)along t,he westsrn edge and about 1,000 ft above the eyecloud area. By the end of the run, the pilot had descendedto just abore the cloud t,ops. A sharp drop in temperatureof 4.5'C in 7 sec (approximately 1 mi) occurred when thepilot descended to within 700 ft of the cloud tops. Thetemperature continued t,o fall to -S5OC at cloud-toplevel. There vas a t'ctal t~emperature drop of 10'C in t,he750 ft of descent.Run 2"This run commenced about 10 mi south-southeast of the hurricane's center and was completed3 mi west-southwest of the center. The altitude was keptfairly constant at' 54,300 ft' (98 mb). The temperature of"86C near the beginning of the run equaled the lowest onany part of the flight. A sudden 4C rise took place aft,erthe start of the run, and the rise continued to -77Cabout, 3 mi before the end; the temperature then droppedsharply to -84C. The cold portions appear to havecoincided with the led of t,he cloud tops. The pilotobserved that the tops were lower in the mea of warming.The lowering of the t,ropopause along with the, cloudscould oxplain the higher temperatures. Simpson (1952)noted in the case of t,yphoon Marge in '1951 that thetropopause lou-ered as the hurricane approached, thenrose to a masimum over the eye. Since the U-2's altitude\vas kept. fairly constant,-observnt,ions were likely higherabove the tropopa,use where the cJouds were lower.Runs 3 to 5"The temperature fluctuated between-82O and -85C in the early part of run 3 when theplane was in the vicinity of the hurricane's center heading1mrt.h. Tho temperature rose slowly to -81C on thenortall side (run 4) and gradually lowered as the planeheaded south (run 5). On t,he north side, the pilot de-scended while turning to stay near tho cloud tops (run 4),which could account] for lower temperatures than wererecorded on the south side (run 2) where the plane'saltitude was kept fairly constant while in the turn.Rwn. "his run was flown in a southern directionalong the western side of the eye. During the first half ofthe run, the pilot flew within the wispy portion of thecirrus. Temperatures were low throughout the run, with752 MONTHLY WEATHER REVIEW Vol. 98, No. 10TEMPERATURE,1619-75-c RUN 4-8o-C TEMPERATURE-85T A4K-I...............# TRACK OF BEULAH- - - - - - - -FLIGHT TRACK OF U-2DURING DESCENT TO 55,000 FTe 3 MINUTE "RUNS" FOR WHICHTEMPERATURE IS ANALYZED 54K-ALTITUDE / 53K/ 52 K16125 1626 1627 1628-75-c RUN 6-80-c TEMPERATURE-85C 55K-ALTITUDE -- 54K , 53KI 11628 1659 ,,aE 1630 1631-757 RUN 7-80C-85CW-/ALTITUDE - 54KI53K1631 1652 ,,LE 1653 1634::l-=e+;zx;;l TEMPERATUREALTITUDEI I1634I635 16361637FIGURE 2.-Time histories of temperature (deg Celsius) and pressure altitude (feet) for runs 1 through 8 and the flight, track of the u-2 aircraft over hurricane Beulah.only 2" fluctuations. A slight cooling took place in themiddle of the run as the pilot climbed above the cirrus.Run 6 was flown in proximity to run 1, with the exceptionof being closer to the cloud tops. Hence, the 10C dropin temperature during run.1 was probably due to tlie air-craft's descending from the stratosphere into the tropo-sphere. Run 6, on the other hand, was entirely belowthe tropopause.Run '?-This leg was on the eastern side of the hurricane,fairly close to the cloud tops. Temperature fluctuationsup to 7C occurred with only a 250-ft altitude change.Temperatures were lowest in the southeast portion of thehurricane near the beginning of the run (-84C) and inthe area about 7 mi east of the eye center (-86C). Therewere two zones of warming, again appearing to be abovethe tropopause. The first was attributed to the plane'sascent above the clouds, the second tmo tho dropping-offof the cloud tops and the accompanying lowering of thetropopause as the pilot directed the aircraft away from theeye center.Run 8-The temperature was "83C near the begin-ning of the run, 12 mi northeast of the hurricane's center.The pilot descended during the run to stay close to theclouds. A gradual temperature rise took place to -80C.This was most likely due to the increased distance of theaircraft from the hurricane's center and a subsequentlowering of the tropopause.Su,mmary-The lowest temperatures above Beulah ap-peared to coincide with the position of the highest clouds,or, more precisely, a few hundred feet above their tops.October 1970 David E. Wac0753Since the cirrus cap was relatively flat above the hur-ricane, the coldest zone was uniformly distributed over awide area rather than in one or more particular locations.Warming that took place away from the hurricane'scenter corresponded to a lowering of the tropopause.VERTICAL TEMPERATURE STRUCTUREThe change in temperature with height above Beulahrevealed interesting features. Temperatures were measuredby the U-2 between 1605 and 1639 GMT on September 20.They are 20-sec averages, read every minute on the descentfrom 66,000 to 55,000 ft and every 30 sec thereafter. Theresults are presented in figure 3 with the soundings forMonterrey, Mexico, and Victoria, Tex., and the meanSeptember sounding for the West Indics area (Jordan1958). The Monterrey sounding was taken at 0000 GMT onthe 20th when the hurricane was 240 mi to the southeastand Victoria's at 0600 GMT when Beulah was 225 mi south.A certain amount of caution should be taken in com-paring the U-2's measured temperature profile wit'h atrue vertical sounding. First, the readings were taken overan area 25 by 40 mi. Second, t'he plane never act'uallypenetrated the clouds while over the hurricane; hence, allreadings below about 53,500 ft, (102 mb) were outwardfrom t,he eye cloud area. Nerert,heless, the profile revealssome interesting features, namely:1. The thick layer cooler than the moan,2. A t,ropopause at 54,000 ft, (100 mb), about 5,000 ft(27 mb) higher than t,he mean, with a temperature of- 86"C, 12C lower than the mean for that level, and3. An 11C temperature rise in only a few hundred feetabove the tropopause. These observations agree closelywith U-2 measurements over hurricane Isbell in 1964(Gentry 1967) where a tropopause of 102 mb (- 85C) anda st.rong inversion and subsequent cooling above thetropopause were observed.The soundings for Monterrey and Vict,oria, alt'houghtaken over 200 mi from the hurricane's center, disp1a.ysome of the features found on the U-2 t.emIwraturcprofile. Evident' are the high tropopause and large verticalextent of below-normal t,emperatures. Also present is thcsharp rise in temperature above the tropopause, 10C in2,500 ft, at Monterrey and 9C in 1,200 ft, at. Victorin.WIND MEASUREMENTSDespite an osci1logra.ph malfunct2ion that led to someerroneous readings, directions of winds greater than 20kt and the relative windspeeds Ltrc sufficiently trustworthyt,o support, the general conclusions:1. Both the highest level of measurement,s, 65,000 ft(57 mb), and lowest led, 54,000 ft (100 mb), had windsanticyclonic outward from the hurricane center, with acurvature similar t'o that observed in the clouds below.2. The lower level winds tended to decrease radiallyinward, becoming very weak near the center (t4he sametrend anwarinz to a lesser derrree at the umer level).U-2 TEMPERATURESO InA InTurbulenceSmooth AlrMONTERREY, MEX.SOUNDING1 1, '\50K I I I I I I I I I1\\-80" "70 TEMPERATURE (OC)FIGURE 3.-Vertical distribution of temperature from aircraftmeasurements between 1605 and 1639 GMT on Sept. 20, 1967;temperature soundings for Monterrey, Mexico (0000 GMT onSept,ember 20), and Victoria, Tex. (0600 GMT on September 20),and the mean September sounding for the Caribbean are plottedfor comparison.The anticyclonic circulation near the top of Beulah doesnot support the hypot,hesis of Simpson (1952) and othersthat, the cyclonic vort,ex increases in the cold core.TURBULENCETurbulence encountered by the U-2 over Beulah variedfrom 1-ery light to light-moderate. Most of the t~urbulenceoccurred near the level of the cloud tops. Smooth condi-t,ions prevailed st all itudes higher than 1,500 ft abovethe cloud tops, and only very light tmbulencc was found int,he lsyer from 500 to 1,500 ft above the tops.Aft8er leaving t,he hurricane area, the pilot experiencedmoderate turbulence at 51,000 ft (1 16 mb), 45 mi northeastof the eye. During tfhis portion of the flight, the aircraft'slocation was 500 ft below the top of the cirrus deck.The horizontal t,emperature varia,tion above Beulah, asmeasured by tho U-2, was related both to the the aircraft'sproximitv tlo t:hc cloud t,ops and to the presence or absence11 CI c3 .I~~ of turbulence. Only small horizontal changes occurred754MONTHLY WEATHER REVIEWVol. 98, No. 10during portions of the flight near the cloud tops wheremost of the turbulence was experienced. The absence oflarge temperature variations was probably because theflight was entirely below the tropopause in a region wherethe vertical temperature gradient was small and littlemixing took place between the stratospheric and tropo-spheric air.The zone in which the U-2 measured the largest hori-zontal temperature changes, located only a few hundredfeet above the cloud tops, had relatively little turbulence.This was likely related to the aircraft's position in respectto the tropopause. Smooth flight conditions might beexpected in a stable area, such as existed in the inversionabove the vertical overshoot of t.he cumulonimbus clouds.At the same time, this layer had a rather strong verticaltemperature gradient (fig. 3), and large horizontal tem-perature changes would be likely, especially consideringthe unevenness of a tropopause surface above cumulonim-bus clouds. In actuality, the aircraft mas probably sam-pling alternating tropospheric and stratospheric air whileflying in a horizontal path due to the uneven tropopause.5. SUMMARYThe following meteorological features associated withhurricane Beulah were found in the upper troposphereand lower stratosphere through analysis of radiosonde dataand measurements by a U-2 aircraft:1. A warm core cyclonic circulation encircled by aperipheral ridge at the 300-mb level,2. Large zones of negative temperature anomalies bothin the vertical and horizontal near the 100-mb level,3. A tropopause just above the cloud tops at 54,000 ft(100 mb) that was 5,000 ft higher than the mean and witha temperature of -86"C, 12OC lower than the mean forthe season at this level,4. An 11C temperature rise in only a vertical rise of afew hundred feet above the layer where convective motionreached its highest limit (the bottom of this inversionappeared to be located about 400 ft above the cloud tops;this 400-ft layer represented the overshoot of rising air inthe cumulonimbus towers, and warming within the inver-sion layer probably resulted from subsiding air above thelevel of divergence), and5. Weak but apparently anticyclonic winds at cloudtop level.Light to moderate turbulence was observed when theaircraft flew near cloud-top level, whereas relativelysmooth air prevailed a few hundred feet above this.Within the turbulent zone, ascending motion in the over-shoot layer above the clouds prevented the downwardpenetration of warmer air. Consequently, temperatureswere uniformly lower in the turbulent zones. On the otherhand, some rather large horizontal temperature changesoccurred slightly above the cloud-top level where theU-2 experienced relatively smooth flight conditions.These changes were probably related to the large tem-perature change in only a few hundred feet above thetropopause and the nonuniformity of the tropopausesurface. Under these conditions, a flight track along ahorizontal path near the tropopause surface could result inthe sampling of air alternately at the bottom and top ofthe inversion layer. The absence of any moderate turbu-lence above the cloud-top level may be partially explainedby the intense inversion that could have had a dampeningeffect on vertical mixing.ACKNOWLEDGMENTSThe author is indebted to Messrs. F. A. Mitchell, J. M. Rapp,and W. M. Crooks of the Lockheed-California Company for theirassistance in the reduction and interpolation of the HICAT dataand to H. F. Hawkins of the National Hurricane Research Lab-oratory for supplying pertinent synoptic data. Special acknowledg-ment is due Dr. Arnold Court of San Fernando Valley State Collegefor the helpful comments stemming from his review of the paperand to Maj. Kenneth J. Mason who piloted the single engine U-2at near-maximum range and altitude over Beulah when this violenthurricane was near its maximum intensity. Portions of the flightwere conducted without benefit of radio communications. Enginefailure or flameout over the area of 160-kt winds and extremeturbulence would not have permitted bailing out or gliding theaircraft to safety.REFERENCESAshburn, E. V., Prophet, D. T., and Waco, D. E., "High AltitudeClear Air Turbulence Models for Aircraft Design and Operation,"AFFDL Technical Report 68-79, Air Force Flight DynamicsLaboratory, Wright-Patterson Air Force Base, Ohio, July 1968,Ashburn, E. V., Waco, D. E., and Mitchell, F. A., "Development ofHigh Altitude Clear Air Turbulence Models,'' APFDL TechnicalReport 69-70, Air Force Flight Dynamics Laboratory, Wright-Patterson Air Force Base, Ohio, Sept. 1969, 72 pp.Crooks, W. M., Hoblit, F. M., and Mitchell, F. A., "High AltitudeClear Air Turbulence Measurements and Meteorological COR*lations," AFFDL Technical Report 68-127, Air Force FlightDynamics Laboratory, Wright-Patterson Air Force Base, Ohio,Sept. 1968, 762 pp.Crooks, W. M., Hoblit, F. M., and Prophet, D. T., "ProjectHICAT-An Investigation of High Altitude Clear Air Turbu-lence," AFFDL Technical Report 67-123, Air Force Flight Dy-namics Laboratory, Wright-Patterson Air Force Base, Ohio, Nov.1967, 1130 pp.Gentry, R. Cecil, `Y3tructure of the Upper Troposphere and LowerStratosphere in the Vicinity of Hurricane Isbell, 1964," Papersin Meteorology and Geophysics, Vol. 18, No. 4, MeteorologicalResearch Institute, Tokyo, Dec. 1967, pp. 293-310.Gentry, R. Cecil, "Research Activities at the National HurricaneResearch Laboratory,'' Weatherwise, Vol. 21, No. 4, Aug. 1968,Jordan, C. L., "Mean Soundings for the West Indies Area," Journalof Meteorology, Vol. 15, No. 1, Feb. 1958, pp. 91-97.Koteswaram, P., "On the Structure of Hurricanes in the UpperTroposphere and Lower Stratosphere," Monthly Weather Review,Vol. 95, NO. 8, Aug. 1967, pp. 541-564.Kraft, Raymond H., "Great Hurricanes, 1955-1965," MarinersWeather Log, Vol. 10, No. 6, Nov. 1966, pp. 200-202.LaSuer, N. E., and Hawkins, H. F., "An Analysis of Hurricane Cleo(1958) Based on Data From Research Reconnaissance Aircraft,"Monthly Weather Review, Vol. 91, Nos. 10-12, 0ct.-Dec. 1963, pp.Malkus, Joanne S., and Riehl, Herbert, "On the Dynamics andEnergy Transformations in Steady-State Hurricanes," Tellus,120 pp.pp. 144-151.694-709.Vol. 12, NO. 1, Feb. 1960, pp. 1-20.
Abstract
Horizontal and vertical temperature variations along with true gust velocity measurements of atmospheric turbulence were obtained above hurricane Beulah (1967) by an instrumented U–2. The U–2 flight was part of the U.S. Air Force High Altitude Clear Air Turbulence program. Pertinent findings include: (1) location of the tropopause just above the cloud tops at 54,000 ft (100 mb) with a temperature of −86°C, 12°C lower than the mean; (2) a vertical temperature rise of 11°C in a few hundred feet just above the tropopause; (3) horizontal temperature changes up to 7°C and smooth flight conditions in the stable layer above the cloud tops; and (4) small temperature fluctuations and generally turbulent conditions at cloud top level.
Aircraft measured winds, although questionable as to their exact directions and speeds, indicated that the flow was weak and anticyclonic near Beulah's top, becoming increasingly anticyclonic above.