1. Introduction and motivation
A waterspout is defined as “any tornado over a body of water” (Glickman 2000), and waterspouts display all the diversity in behavior, appearance, and origin of their kin over land. In North America, waterspouts most commonly occur in the Florida Keys (50–500 waterspouts per year) and along the southeast coast of Florida (Golden 1977) but have been observed on the Great Lakes (e.g., Gay 1921; Hurd 1928), the Great Salt Lake (Simpson et al. 1991), and even Lake Tahoe (Grotjahn 2000).
Golden (1974b) first proposed a five-stage waterspout life cycle based on observations of Florida Keys waterspouts. Most of these waterspouts occurred in a tropical environment, developed in cloud lines, and were nonsupercellular in origin. In fact, much of the waterspout literature based on larger field projects has examined storms in the tropics or subtropics, and waterspouts forming through primarily nonsupercell processes (e.g., Golden 1974a; Leverson et al. 1977; Simpson et al. 1986). Conditions favorable for waterspout development include low-level instability, low-level shear, and possibly slow-moving or intersecting gust fronts (Simpson et al. 1986). In addition, waterspout-producing cloud lines typically develop under weak synoptic disturbances in the presence of differential heating or sea surface temperature gradients (Golden 1974a; Simpson et al. 1986).
There were 46 waterspouts per year on average from 1994 to 2010 over the Great Lakes (Sioutas et al. 2013). Waterspouts were observed on every lake, though Lake Erie had the highest annual occurrence (Sioutas et al. 2013). On Lake Michigan, 173 waterspouts were observed from 1993 to 2013 (W. Szilagyi 2014, personal communication). Most waterspouts on the Great Lakes, including Lake Michigan, occur in the months of August and September when the water surface temperature is relatively warm (Szilagyi 2004; Sioutas et al. 2013). Sioutas et al. (2013) identified other conditions favorable for waterspout outbreaks on the Great Lakes, such as a 500-hPa long-wave trough or a closed low over the region, increased instability from cold advection, and in some cases a land breeze.
On the afternoon of 12 September 2013, a series of four waterspouts developed over western Lake Michigan (NWS 2013). The waterspout-producing storm, which was well observed from the polarimetric Weather Surveillance Radar-1988 Doppler (WSR-88D) at Milwaukee, Wisconsin (KMKX), appeared to become more supercellular in nature while the waterspouts were in progress. Using these radar data in conjunction with environmental and aerosol data, this study provides the following:
the first published polarimetric radar observations of a waterspout-producing storm in the Great Lakes region,
a chronology of polarimetric features associated with the transition to supercell convection,
in situ observations of biological scatterers contributing to high differential reflectivity ZDR values along a boundary, and
the potential occurrence of a drop size distribution (DSD) biased toward unusually large liquid drops despite very low observed aerosol concentrations.
2. Data and methods
A radar dataset was analyzed from KMKX, which was upgraded to polarimetric capability in April 2012. This dataset extended from the time a linear reflectivity maximum first appeared east of KMKX (1455 UTC) until the storm of interest moved well southeast of KMKX (2029 UTC). The storm of interest was within 120 km of KMKX throughout this period, minimizing data quality issues inherent at long range. All heights noted in this paper are above radar level (ARL). Polarimetric radar variables utilized included ZDR, which affords an estimate of the reflectivity-weighted mean axis ratio of scatterers in a sample volume, and copolar cross-correlation coefficient ρhv, which provides an indication of scatterer diversity, orientation, and phase (e.g., Bringi and Chandrasekar 2001).
Very high ZDR values observed within storms throughout the KMKX domain on this day were initially suspected of being in error, so a scatterer-based calibration procedure was implemented to ensure no large ZDR bias. It was assumed that most hydrometeors should be dry snow aggregates ~1.5 km above the melting level. Typical values of radar variables in such hydrometeors include radar reflectivity factor at a horizontal polarization ZHH between 20 and 35 dBZ, ρhv > 0.97 (often >0.99), and ZDR averaging 0.1–0.2 dB (Ryzhkov et al. 2005a; Picca and Ryzhkov 2012). Values of ρhv and ZDR were consistent with these expectations when averaged over several points at each of five times examined between 1759 and 1840 UTC (not shown), indicating no consistent, substantial ZDR bias.
Radar data were supplemented by additional observations, including routine upper-air and surface observations. Rapid Refresh (RAP) model data at 1800 UTC 12 September 2013 were obtained from the National Climatic Data Center (NCDC). These data were used to estimate the sounding and storm-relative helicity (SRH) near the waterspout-producing storm. Maps of Lake Michigan water surface temperature, estimated using an Advanced Very High Resolution Radiometer (AVHRR) satelliteborne instrument, were obtained from the Great Lakes Environmental Research Laboratory (GLERL). These data were limited by patchy cloud cover over southern Lake Michigan, but portions of the lake offshore from Wisconsin and northeastern Illinois were cloud free. Aerosol data, including particulate matter with a diameter less than 10 μm (PM10), were obtained from the Environmental Protection Agency (EPA) for a station near the Lake Michigan shoreline (indicated as red star in Fig. 1).
Mesoscale features present at 1800 UTC 12 Sep 2013, when convection was already in progress. Station plots include temperature and dewpoint (°C) and wind [knots (kt; 1 kt = 0.51 m s−1); full barb = 10 kt; half barb = 5 kt]. Dewpoint is color shaded, and black contours represent surface pressure (contour interval = 1 hPa). White star represents location of KMKX, and red star represents location of Chicago aerosol monitoring site. Square represents location of RAP sounding in Fig. 3. Triangle shows location of the IBSP Hawk Watch site. Position of radar fine line associated with westward-moving boundary is indicated as a red dashed line, and solid blue line indicates approximate storm track from 1804 to 1900 UTC. Observations plotted using WeatherScope from the Oklahoma Climatological Survey.
Citation: Weather and Forecasting 30, 2; 10.1175/WAF-D-14-00114.1
3. Overview of the synoptic- and local-scale environment
A long-wave trough over the Great Lakes characterized the environment at 1200 UTC 12 September 2013. The trough axis was located from Hudson Bay through central Ontario and Wisconsin, just west of Lake Michigan (Fig. 2). Two jet streaks were evident at 300 hPa: the first on the west side of the trough axis over Minnesota and the second to the east over lower Michigan (Fig. 2). As the trough moved eastward through the region, model output indicated that by 1800 UTC the trailing jet streak was in a favorable location for southeastern Wisconsin to experience synoptic-scale lift, with strong northwest flow at 300 hPa (Fig. 3a). The eastward-moving trough brought strong 850-hPa cold-air advection to the western Great Lakes (Fig. 2). At the surface, a cold front had passed through the region overnight and by 1800 UTC was located from central Illinois through north-central Indiana (Fig. 2). Much of the region was dominated by northwest surface flow and a gradual northward surface temperature decline. These conditions, especially the long-wave trough and surface cold front with attendant northwesterly flow, are typical of a Great Lakes waterspout outbreak environment (Szilagyi 2004; Sioutas et al. 2013). Surface-based instability was relatively weak at 1800 UTC, with typical values of 200–300 J Kg−1 across southeastern Wisconsin and western Lake Michigan (Fig. 3b), according to the 1800 UTC RAP initialization.
Synoptic-scale features on 12 Sep 2013. Shaded purple areas with arrows show locations of 90+-kt 300-hPa jet streaks at 1200 UTC, and dashed black line is the 300-hPa trough axis. Wind barbs are 850-hPa winds (full barb = 10 kt; half barb = 5 kt), and red numbers are 850-hPa temperatures (°C) at each sounding location at 1200 UTC. Blue cold front symbol shows the location of the surface cold front at 1800 UTC.
Citation: Weather and Forecasting 30, 2; 10.1175/WAF-D-14-00114.1
(a) An 1800 UTC 12 Sep 2013 skew temperature–log pressure (skew T–log p) diagram from the RAP model, valid for a point just east of Kenosha, Wisconsin (square in Fig. 1) with wind barbs (kt; flag = 50 kt; full barb = 10 kt; half barb = 5 kt). (b) 1800 UTC surface-based convective available potential energy (SBCAPE) over southern Lake Michigan from the RAP model (contour interval = 100 J Kg−1), with approximate track of the waterspout-producing storm annotated from 1804 to 1900 UTC.
Citation: Weather and Forecasting 30, 2; 10.1175/WAF-D-14-00114.1
Across southeastern Wisconsin and far northeastern Illinois, the morning started with calm to very light winds along the western Lake Michigan shore. By 1455 UTC there was a linear reflectivity maximum oriented from north to south across southeastern Wisconsin, possibly indicating a boundary (Fig. 4a). Surface observations suggested weakly convergent flow as the wind veered from near westerly along the shore to northwest farther inland. A small area of low pressure was present over southeastern Wisconsin and northeastern Illinois at this time. A circulation became evident around this mesolow by 1600 UTC (not shown). At 1800 UTC, an easterly wind component was present near Milwaukee, and westerly flow had developed across far northeastern Illinois and southeastern Wisconsin (Fig. 1).
Base-scan radar reflectivity from KMKX showing evolution of two primary boundaries over southeastern Wisconsin at (a) 1455, (b) 1545, (c) 1634, and (d) 1713 UTC 12 Sep 2013. White arrows are added to annotate boundary locations. Yellow arrows in (d) indicate three newly initiated convective cells.
Citation: Weather and Forecasting 30, 2; 10.1175/WAF-D-14-00114.1
Complicating the situation, a westward-moving boundary moved onshore around 1545 UTC to the north of the mesolow and was clearly visible from KMKX (Fig. 4b). East of this boundary, dewpoints were 2°–3°C higher than farther inland. Temperatures on either side of the boundary were initially similar given lake surface temperatures around 22°C (Fig. 5), but after convection initiation (about 1713 UTC; Fig. 4d) the cross-boundary temperature gradient increased substantially, likely because of cold pool development. Moisture in the lake-modified air combined with cold advection aloft may have locally enhanced the conditional instability (e.g., Fig. 3b), making the area east of the boundary more susceptible to convection initiation. The difference in temperature between the water surface and 850 hPa was approximately 11°C, which is 3°C (0.7 standard deviations) below the average value for Great Lakes waterspout outbreaks (Sioutas et al. 2013).
Water surface temperature of southern Lake Michigan (contour interval = 1°F) at 0703 UTC 12 Sep 2013. Gray shading indicates areas that were probably cloudy, decreasing certainty in the temperature estimate. Blue star indicates approximate location of the waterspout-producing storm at 1840 UTC. [Courtesy of the GLERL.]
Citation: Weather and Forecasting 30, 2; 10.1175/WAF-D-14-00114.1
By 1634 UTC, the westward-moving boundary was beginning to interact with the eastward-moving linear reflectivity maximum (Fig. 4c). Surface observations and radial velocity data both indicated convergent flow along the zone where these features interacted, and where convection initiated around 1713 UTC (Fig. 4d). By 1804 UTC, three waves were visible along the boundary in base reflectivity, two of which were associated with zones of enhanced shear in the Doppler velocity field. At this time, the strongest wave along the boundary was located in far southeastern Wisconsin near the shore of Lake Michigan (Fig. 6a).
Radar signatures at the 0.54° elevation angle from KMKX at 1804 UTC 12 Sep 2013. Shown are (a) ZHH (dBZ), (b) ZDR (dB), and (c) ρhv. Circles indicate the recently initiated waterspout-producing cell; distance from KMKX to center of circle is ~80 km. Yellow arrows in (a) and (b) point to locations of two of the rotational signatures along the boundary.
Citation: Weather and Forecasting 30, 2; 10.1175/WAF-D-14-00114.1
4. Observations and results
An approximately chronological discussion of the base radar variables is presented here for the storm of interest, followed by an examination of the polarimetric variables. Microphysical characteristics inferred from the polarimetric radar data are related to environmental characteristics of the post-cold-frontal air mass.
a. Base variable progression
Prior to 1700 UTC, a well-defined westward-moving boundary was oriented northwest–southeast from far southeastern Wisconsin to west of Milwaukee (Fig. 4). Radar reflectivity did not yet indicate precipitating convection associated with the boundary. By 1713 UTC, several convective cells had initiated on the east side of the boundary over southeastern Wisconsin (Fig. 4d), nearly collocated with the zone of maximum shear along the boundary.
At 1804 UTC, 2 min prior to the initial waterspout report (NWS 2013), the base-scan (0.54°) ZHH did not exceed 19.5 dBZ in the cell where the waterspout would occur (~80 km from KMKX; Fig. 6a), though ZHH at 1.49° showed values to 28.5 dBZ associated with this cell (not shown). Meanwhile, a storm was developing 3–4 km to its north. These cells began to merge by 1810 UTC at midlevels (Fig. 7a), each containing well-defined rotation (indicated by radial velocity Vr; Fig. 7b). In addition, spectrum width συ showed an area of substantial turbulence and/or shear near where the waterspout was located (beam centerline elevation ~2.53 km), with a similar area located to the northwest in a nontornadic vortex over land (Fig. 7c; beam centerline elevation ~2.36 km). While this area of midlevel rotation eventually dissipated, a succession of similar vortices over land with boundary-associated convective updrafts through 1830 UTC suggests a tendency for vorticity concentration on this day, possibly enhanced by stretching under strong updrafts and low-level directional shear in the vicinity of the boundary (e.g., Wakimoto and Wilson 1989).
Radar signatures at the 1.49° elevation angle from KMKX at 1810 UTC 12 Sep 2013. Shown are (a) ZHH (dBZ), (b) Vr (m s−1), and (c) συ (m s−1). Circles show updraft regions of developing convective cells, inferred via overlying ZDR columns. Distance from KMKX to center of northern circle is ~75 km and to center of southern circle is ~84 km.
Citation: Weather and Forecasting 30, 2; 10.1175/WAF-D-14-00114.1
A series of pictures was available from the Illinois Beach State Park (IBSP) Hawk Watch site, indicated by a triangle in Fig. 1. Visually, the first waterspout appeared pendant from an extensive cloud line (Fig. 8a; 1817 UTC). By 1820 UTC, the westward-moving boundary apparent at the lowest radar elevation had become more closely collocated with the waterspout-producing storm (Fig. 9e), which was developing strong low-level rotation (Fig. 10a) while maintaining a focused συ maximum (Fig. 10b). The first and second waterspouts briefly overlapped from 1820 to 1824 UTC (Fig. 8b; 1822 UTC). The storm continued to mature, with a broadening mesocyclone evident through time, and the second waterspout became much larger before dissipating (Fig. 8c). By the 1830 UTC scan, two areas of rotation were noted in the radial velocity field, coincident with separate συ maxima (not shown). These may have indicated two broader-scale areas of rotation within the storm. Two separate waterspouts were observed from IBSP from 1831 to 1837 UTC (Fig. 8d; 1835 UTC). Waterspouts disappeared from view as a result of rain obscuration around 1840 UTC (NWS 2013).
Visual observations of the waterspout-producing storm, taken from location marked by the triangle in Fig. 1. (a) Wide view of the associated cloud line at 1817 UTC, looking east-northeast. (b) First (right) and second (left) waterspouts at 1822 UTC, with dragonflies circled, looking east-northeast. (c) Second waterspout at 1828 UTC after it had become larger and more closely collocated with the updraft, looking east. (d) Second (left) and third (right) waterspouts at 1835 UTC, looking east-southeast. [Images courtesy of J. Sweet.]
Citation: Weather and Forecasting 30, 2; 10.1175/WAF-D-14-00114.1
KMKX (left) ZHH (dBZ) and (right) ZDR (dB) at base-scan level (0.54°) at times indicated to the right. Waterspout-producing storm is circled.
Citation: Weather and Forecasting 30, 2; 10.1175/WAF-D-14-00114.1
Radar signatures at the 0.52° elevation angle from KMKX at 1820 UTC 12 Sep 2013. Shown are (a) Vr (m s−1), (b) συ (m s−1), (c) ρhv, and (d) ZDR (dB). Solid circles mark a strong low-level vortex associated with an ongoing waterspout; distance from KMKX to the center is ~84 km. In (c) and (d), a dashed oval marks the updraft region of the waterspout-producing storm, collocated with a ZDR column extending well above the ambient 0°C level; distance from KMKX to the center is ~81 km.
Citation: Weather and Forecasting 30, 2; 10.1175/WAF-D-14-00114.1
b. Polarimetric variable progression and comparison with base variables
The westward-moving boundary along which most storms initiated on this day was marked by ZDR values exceeding 7 dB (Fig. 6b), as is typical in convergent, along-boundary flow when insects are present (e.g., Achtemeier 1991). Observations from the IBSP Hawk Watch site confirm the presence of insects along the boundary at low levels. An official observing site of the Hawk Migration Association of North America (HMANA), this location was participating in a pilot study with the Migratory Dragonfly Partnership (MDP) to count dragonflies during fall of 2013. The westward-moving boundary passed the observing station around 1830 UTC, sampled by KMKX at a base-scan elevation of ~1.2 km. Photographic and video records showed large numbers of dragonflies around the observing station from 1820 UTC onward (Fig. 8b). During the period 1840–1850 UTC, 1505 dragonflies were counted moving south, more than half of the total for the fall of 2013 (HMANA 2013). Migrating dragonfly swarms have been inferred to elevations exceeding 1 km over the Indian Ocean (e.g., Anderson 2009), though limited observations of migrating dragonflies along the Lake Michigan shoreline have indicated that individuals usually remain below 100 m (Russell at al. 1998). We speculate that a combination of boundary layer lift in the vicinity of the westward-moving boundary and a reflectivity contribution from dragonflies (and possibly other bioscatter species) below beam centerline may have contributed to elevated ZDR values observed by KMKX along the boundary.
By 1804 UTC, the core of the waterspout-producing storm (~80 km from KMKX) was characterized at 0.5° elevation by ZDR values from 2.5 to 4.6 dB, ZHH values less than 20 dBZ, and ρhv values generally exceeding 0.98 (Fig. 6), indicating the dominance of large drops in small concentrations. Once the first waterspout was well under way at 1810 UTC (storm core ~83 km from KMKX), base-scan values of ZDR throughout its parent storm became more uniformly >4 dB, with a few bins >5 dB, though ZHH values were still generally <30 dBZ (Figs. 9a,b). Aloft, a ZDR column (Illingworth et al. 1987; Kumjian and Ryzhkov 2008; and many others) with values to 4.5 dB extended up to 5 km ARL just northwest of the waterspout location, possibly associated with the upstream cell still over land.
By 1820 UTC (storm core ~85 km from KMKX), a well-defined convective cell had become apparent at 0.54° elevation (~1.2 km ARL; Fig. 9a). The ZDR values >2.5 dB dominated a larger areal extent than is typically seen within the ZDR arc (e.g., Kumjian and Ryzhkov 2008; Romine et al. 2008; Crowe et al. 2012; Dawson et al. 2014). In particular, a region of large ZDR values at low levels (5–5.5 dB) was located in northwestern portions of the updraft region (Fig. 10d), which was inferred by the presence of a ZDR column overhead. Differential sedimentation under the broad updraft within a region of heavy precipitation may have contributed to the region of largest base-scan ZDR values, as suggested by Kumjian and Ryzhkov (2012). The storm continued to exhibit a column of high ZDR values through the time when waterspouts were last observed around 1840 UTC. The ZDR values of 5.8 dB were present to an altitude of nearly 5.2 km at 1820 UTC, and values of 5.0 dB were present up to nearly 5.5 km at 1825 UTC (not shown), well above the ambient 0°C level (approximately 3.0 km ARL; Fig. 3a). When two intense low-level vortices were observed at 1835 UTC (storm core ~87 km from KMKX), base-scan ZHH in the waterspout-producing cell (altitude ~1.4 km) had increased to at least 63 dBZ in multiple areas north and east of the updraft (Fig. 11a). The southern edge of one such area was collocated with locally lower ZDR values of 0.5–2.5 dB (Fig. 11b) and ρhv values depressed to 0.80–0.91 (Fig. 11c), suggesting hail or melting hail reaching low levels (Straka et al. 2000). The northern side of this area had ρhv values lowered to 0.90 and collocated with ZHH of 60–63 dBZ and ZDR of 2.0–4.25 dB, suggesting water-coated hail (Figs. 11a–c).
As in Fig. 6, but for 1835 UTC 12 Sep. Solid oval shows likely location of hail. Dashed area indicates likely water-coated hail, inferred by collocated high ZHH values, lowered ρhv, and elevated ZDR. Distance from KMKX to center of solid oval is ~93 km and to center of dashed area is ~91 km.
Citation: Weather and Forecasting 30, 2; 10.1175/WAF-D-14-00114.1
Well after the last observed waterspout was obscured by precipitation, the storm maintained widespread ZDR values exceeding 3.5–4 dB at base-scan level, a well-defined ZDR column characterized by high values well above the 0°C level, and a broad mesocyclone. A burst of melting hail or mixed rain and hail reaching the base-scan level was observed from 1840 to 1845 UTC (~92 km from KMKX), during and after the time when the last waterspout became rain obscured (Fig. 12a).
(a) Timeline of waterspouts and radar features observed with the storm of interest. Dashed final waterspout indicates disappearance into precipitation. Dashed mesocyclone and ZDR arc indicate times when these features were present but not well defined. Dashed lowest-scan hail indicates water-coated hail likely present, but no classic polarimetric hail signature. (b) Velocity difference (m s−1) and shear (m s−1 km−1) from KMKX for the mesocyclone of the storm of interest from 1810 to 1855 UTC at the base scan and 1.49° tilts. Dashed lines represent thresholds in the WSR-88D mesocyclone detection algorithm for storms within 100 km of the radar with velocity difference of 30 m s−1 (upper line) and shear of 6 m s−1 km−1 (lower line).
Citation: Weather and Forecasting 30, 2; 10.1175/WAF-D-14-00114.1
c. Radar observations of supercell development
Supercell storms are defined by a “single, quasi-steady rotating updraft, which persists for a period of time much longer than it takes an air parcel to rise from the base of the updraft to its summit” (Glickman 2000). Detection of this characteristic updraft, the mesocyclone (Brown et al. 1975), has been automated utilizing radar observations. One such automated detection scheme is the mesocyclone detection algorithm (MDA) used by WSR-88D (Stumpf et al. 1998). Among several parameters and other qualifications used to test for the presence of a mesocyclone (e.g., vertical and temporal continuity) are velocity difference (VD; a sum of the magnitudes of the maximum inbound and outbound velocities) and a measure of shear (velocity difference divided by the distance between the maximum in- and outbound velocities). For storms within 100 km of a WSR-88D, threshold values of these variables are ≥30 m s−1 for VD and ≥6 m s−1 km−1 for shear (Stumpf et al. 1998). In the waterspout-producing storm, VD met this threshold at the 1.49° elevation angle from 1815 to 1850 UTC (Fig. 12b), at a corresponding altitude of 2.6–3.1 km. Base-scan VD climbed steeply from cell initiation, meeting or nearly meeting the 30 m s−1 criterion from 1825 to 1850 UTC (Fig. 12b). Highest VD values at both levels considered together occurred from 1835 to 1840 UTC. Shear tended to remain more constant throughout the examined time period (Fig. 12b), because the mesocyclone diameter tended to increase with VD through 1850 UTC. Shear at both elevation angles typically exceeded the MDA threshold of Stumpf et al. (1998).
Another approach to determining when a storm may be said to have developed supercell characteristics is via examination of the storm’s polarimetric radar features. Prior studies have examined radar features of classic supercell storms using ZHH, ZDR, and ρhv (e.g., Kumjian and Ryzhkov 2008; Romine et al. 2008; Van Den Broeke et al. 2008; Kumjian et al. 2010), and here we will examine a subset of these features for the waterspout-producing storm. Radar features discussed are plotted on a timeline (Fig. 12a).
One radar feature common to any convective cell with a strong updraft, including supercell thunderstorms, is the weak-echo region (WER) in ZHH, which often takes the form of a bounded weak-echo region (BWER; e.g., Barnes 1978). The storm of interest first exhibited a WER at 1825 UTC in the 2.45° scan (~4.1 km ARL), and this feature was well defined at 1835 UTC (when two waterspouts were occurring) at the 1.49° elevation angle (~2.7 km ARL; Fig. 13a). A WER, sometimes bounded, remained visible through 1845 UTC, but disappeared thereafter. This signature was less pronounced than usual in a storm of this intensity, and only lasted ~20 min. This may be due to the relatively low CAPE environment, which would promote relatively weak vertical motion in the updraft.
Polarimetric signatures from KMKX in the waterspout-producing storm on 12 Sep, indicated by ovals: (a) BWER in ZHH, 1835 UTC, 1.49° elevation angle; (b) strong ZHH gradient along the storm’s forward flank, 1840 UTC, 0.54° elevation angle; (c) ZDR column associated with the updraft region, 1810 UTC, 2.46° elevation angle; (d) ZDR arc along the storm’s forward flank, 1820 UTC, 0.54° elevation angle; and (e) partial ρhv ring around the updraft region, 1835 UTC, 1.49° elevation angle. Ovals correspond to the feature noted in the caption. Blue rectangle in (e) denotes region where hail is present. Distances from KMKX to the center of oval are approximately (a) 88, (b) 92, (c) 79, (d) 85, and (e) 87 km. Distance to center of square in (e) is ~94 km.
Citation: Weather and Forecasting 30, 2; 10.1175/WAF-D-14-00114.1
Supercells are often characterized by a strong forward-flank ZHH gradient (e.g., Kumjian and Ryzhkov 2008; Frame et al. 2009), which became evident in the waterspout-producing storm around 1830 UTC, was exceptionally well defined around 1840 UTC (Fig. 13b), and persisted well after the waterspout was obscured by precipitation.
Differential reflectivity has proven useful in diagnosing features of supercell storms, providing information on drop size distributions (e.g., Feingold and Levin 1987; Gorgucci et al. 2000; Cao et al. 2010), and the presence of hail (e.g., Aydin et al. 1990; Herzegh and Jameson 1992; Dawson et al. 2014). The ZDR column represents liquid drops lofted in an area of strong upward motion (e.g., Herzegh and Jameson 1992), often well above the ambient 0°C level. Such columns may be observed in any deep convection with strong updrafts, and are ubiquitous in supercell storms. In the waterspout-producing storm, a well-defined ZDR column was characteristic from cell initiation, often containing exceptionally high ZDR values of 3–7 dB at an altitude of 3.5–5.5 km (e.g., Fig. 13c). Though the magnitude of ZDR values within the column generally decreased over time, a column remained well defined past the time when waterspouts were observed. Given an estimated ambient 0°C level around 3.0 km ARL (Fig. 3a), liquid drops were likely present well above the ambient 0°C level within the updraft.
Another polarimetric feature common in supercell storms is the ZDR arc (e.g., Kumjian and Ryzhkov 2008, 2009), a band of locally enhanced ZDR values collocated with a strong ZHH gradient along the storm’s forward flank. It may be useful as an indicator that a convective cell is transitioning into a more tornado-favorable phase (e.g., Crowe et al. 2012). A ZDR arc was first present in the 1820 UTC base-level scan (Fig. 13d), but had weakened by 1825 UTC. It was occasionally present through and past the time when waterspouts were observed, but was rarely well defined and did not appear to be a persistent feature in this storm. Radar–storm distance may have reduced the ability to see this signature toward 1900 UTC, as the radar was sampling the forward flank by this time at approximately 1.6 km ARL.
Polarimetrically inferred hail may vary cyclically in observed and numerically simulated supercell storms (e.g., Van Den Broeke et al. 2008; Kumjian and Ryzhkov 2008; Van Den Broeke 2014). In the waterspout-producing storm, the presence of hail was assessed both at the lowest scan and farther aloft. Hail was inferred using the collocation of high-ZHH and suppressed ZDR/ρhv values (e.g., Straka et al. 2000). At the lowest scan, a reduction of ρhv was evident at 1835 UTC in the storm core with the highest ZHH values (up to 60.5 dBZ), but collocated ZDR values were 2.5–3.5 dB (Fig. 11), inconsistent with large or pure hail. Specific differential phase KDP values of 3.5°–4.65° km−1 and output of the WSR-88D hydrometeor classification algorithm (Park et al. 2009) indicated that hail mixed with large drops (possibly melting hail) may have been present (not shown). A similar fallout of hail occurred from 1840 to 1845 UTC, though ZDR values again remained generally high in association with the reflectivity maximum. Beyond 1845 UTC, hail at low levels could not be assessed since the storm became too far from the radar. Aloft, hail was inferred around 4.3 km ARL by 1830 UTC via collocated high ZHH and depressed ρhv values. Hail persisted through approximately 1845 UTC, after which time ZDR values increased in association with the reflectivity maximum (not shown).
Some precipitation characteristics of supercell storms may also be manifest in ρhv, including a partial or full ring of low values around the updraft region in mixed-phase hydrometeors (e.g., Kumjian et al. 2010). A partial ρhv ring was evident at 1835 UTC (Fig. 13e), surrounding the WER, and remained evident at 1840 UTC, then disappeared until 1855 UTC when it was again well defined. Taken together (Fig. 12), the radar observations indicate a storm with most pronounced supercell character from approximately 1830 to 1845 UTC.
d. Microphysics and the storm environment
As described above, relative to prior studies ZDR values were unusual throughout the examined portion of the waterspout-producing storm’s life in two primary ways: 1) the large areal extent of ZDR > 4 dB at low levels (e.g., not just confined to the typical ZDR arc region) and 2) the very high ZDR values (5–7 dB) within the ZDR column. Given good data quality and no apparent large calibration offset, at low levels these high ZDR values may be attributed to very large liquid drops in low concentration. Such large-drop dominant distributions at low levels are partially a result of differential sedimentation and updraft-induced sorting (e.g., Kumjian and Ryzhkov 2012), though we do not believe these mechanisms contribute to the unusual nature of the ZDR distribution in the storm of interest relative to similar storms presented in the literature. Within ZDR columns, high ZDR values were typically collocated with ρhv values depressed to 0.90–0.95, suggesting mixed-phase particles. Additional environmental factors potentially contributing to the ZDR distribution include the vertical wind and moisture profiles, and the regional aerosol distribution.
In modeling studies, the vertical moisture profile has been shown to affect supercell evolution and microphysics (e.g., Gilmore and Wicker 1998; James and Markowski 2010; Van Den Broeke 2014). The environment of many supercell storms, including the waterspout-producing storm, is characterized by relatively dry air at low levels, a moist layer centered near 840 hPa, and a relatively deep dry layer above (Fig. 3). On days with such a deep dry layer aloft, small ice crystals may sublimate before falling very far, leaving hydrometeor distributions dominated by melting graupel and frozen raindrops and increasing the average drop size below the melting level (Van Den Broeke 2014). This may represent one possible mechanism contributing to high ZDR values at low levels in many supercell storms, but does not account for the possible influence of aggregation, coalescence, or droplet breakup.
High values of ZDR within the ZDR arc may be related to the storm-relative vertical wind profile. The ZDR arc has been hypothesized to occur because of raindrop and hail size sorting along the forward flank with an appropriate storm-relative vertical wind profile (Ryzhkov et al. 2005b; Kumjian and Ryzhkov 2009; Dawson et al. 2014, 2015). Resulting collections of large, isolated drops have been found under the ZDR arc (Schuur et al. 2001). Efficiency of the size-sorting mechanism, and thus magnitude of ZDR values, is hypothesized to depend upon the magnitude of the mean storm-relative wind, which may be manifest in the SRH value (Kumjian and Ryzhkov 2009; Dawson et al. 2015). A 0–3-km SRH value of 146 m2 s−2 was estimated near the storm location from the 1800 UTC RAP model vertical wind profile (Fig. 3a), using low-level wind on the lakeward side of the boundary and radar-estimated motion of the mature storm. This estimate may not fully account for the presence of the boundary. In addition, rapid development of a rotating updraft in this storm suggests an environment with high values of preexisting vertical vorticity. Thus, high observed ZDR values within the ZDR arc of this storm may have been an indication of the local wind profile, including enhanced storm-relative low-level winds nearby, but this does not explain widespread high ZDR values elsewhere in the storm.
Finally, aerosol distributions may be important to thunderstorm DSDs. Recent modeling studies have noted larger liquid drops in nonsupercell environments with increased aerosol loading, due to increased riming leading to larger ice particles (e.g., Storer et al. 2010; Lebo and Morrison 2014). Polarimetric radar observations have yielded similar conclusions from tropical convection (May et al. 2011). Also, recent simulations of polarimetric radar variables in a midlatitude hailstorm indicated no ZDR columns above the freezing level in clean clouds (Khain et al. 2014). In the waterspout-producing storm, however, enhanced ZDR values to high elevation were associated with a relatively clean air mass at low levels. Available aerosol data for this case included several PM10 time series from the region. While neglecting vertical aerosol variability, which is critical to convective microphysics (e.g., Lebo 2014), such data provide one means of examining the ambient aerosol distribution, and have not been widely utilized in prior polarimetric studies of convection. The PM10 time series from Chicago, Illinois (red star in Fig. 1), was examined throughout September 2013 to ascertain whether aerosol concentration was anomalous in the air mass in which the storm of interest initiated (Fig. 14). One peak in PM10 was collocated with the air mass immediately inland of the westward-moving boundary on 12 September. Behind this boundary, as air from over Lake Michigan moved inland, PM10 values dropped substantially to near 10 μg m−3 (Fig. 14), one of the most pristine air masses of 2013 at this site. Given ZDR columns well above the ambient 0°C level and anomalously high ZDR values throughout the waterspout-producing storm despite an environment characterized by low aerosol loading, more detailed observational study is needed to determine typical impacts of aerosol variability on microphysics and polarimetric signatures of midlatitude supercell storms.
Average hourly Chicago PM10 from 6 to 18 Sep 2013.
Citation: Weather and Forecasting 30, 2; 10.1175/WAF-D-14-00114.1
5. Discussion
On 12 September 2013, convection initiated near a westward-moving boundary north of a mesolow along the western Lake Michigan shoreline. The synoptic-scale environment, including a long-wave trough nearly overhead and low-level cold-air advection north of a surface cold front, was typical of waterspout outbreaks in the Great Lakes region. One convective cell produced four waterspouts and briefly took on supercell characteristics. Supercell indicators included a mesocyclone, a strong forward-flank ZHH gradient, and an intermittent ZDR arc.
Early waterspouts were produced in association with a rapidly growing cumulus, consistent with prior observations (e.g., Wakimoto and Lew 1993). A ZDR arc was evident by 1820 UTC (Kumjian and Ryzhkov 2009), and a WER became well defined by 1825 UTC, indicating a strengthening updraft (Fig. 12a). The greatest number of radar-diagnosed supercell indicators was present from approximately 1830 to 1845 UTC (Fig. 12), during which time several waterspouts were produced. Thus, it seems probable that the storm’s first waterspout was mesocyclone independent and driven primarily by low-level convergence and stretching of preexisting environmental vorticity under a rapidly growing updraft, with a transition toward more supercell-characteristic processes during the second waterspout. Portions of later waterspout life cycles may have also been influenced by bursts of hail reaching low levels, which may locally concentrate low-level vertical vorticity (e.g., Van Den Broeke 2014). These bursts of hail occurred in the storm core rather than in the right-rear quadrant relative to a midlevel WER, as would be the case with a descending reflectivity core (DRC; Rasmussen et al. 2006).
Early waterspouts followed a similar pattern to those documented by Collins et al. (2000) in a Florida nonsupercell thunderstorm. There, a convective cell rapidly developed along a low-level boundary and formed a tornado initially over land. The rapid vorticity concentration was attributed to stretching under a strong updraft and low-level convergence under a downdraft (Collins et al. 2000). This is consistent with the observation that some waterspouts may derive their vorticity primarily from a low-level source (e.g., Verlinde 1997). In the Great Lakes event documented herein, weak initial low-level ZHH and updraft overhead (inferred by high ZDR values aloft) provide further evidence that the initial waterspouts formed via stretching of preexisting vertical vorticity along the boundary. This formation mechanism is similar to that observed for landspouts (e.g., Wakimoto and Wilson 1989), and has been observed before along boundaries (e.g., Brady and Szoke 1989; Snow and Wyatt 1998; Collins et al. 2000). In environments characterized by large low-level instability leading to rapid vertical acceleration collocated with a preexisting vorticity maximum along a boundary, the resulting vortex may become quite intense (e.g., Caruso and Davies 2005; Pfost et al. 2005).
The presence of bioscatter, supported by observed high dragonfly density (e.g., Fig. 8b), appeared to contribute to high ZDR values near the initiating boundary. Within the convection, very large liquid drops appeared to be responsible, and high values in the ZDR arc region may have partially resulted from the local wind profile. Aerosol concentration was extremely low in the lake-influenced air mass at low levels, a condition not previously associated with high concentrations of large drops in convective clouds. Future work should include study of potential polarimetrically observable aerosol effects on DSDs and ZDR columns in midlatitude convection, and could utilize similar PM10 measurements.
Operational nowcasters can take useful findings and cautions from this event. Though only one storm along the lake-induced boundary took on supercell characteristics for a substantial length of time and produced several waterspouts, other convective cells along the same boundary exhibited brief supercell structures and low-level rotation. While low-level vortices produced under similar scenarios (e.g., Marquis et al. 2007) may be due to stretching and are relatively weak (e.g., Wakimoto and Wilson 1989), it is possible to get intense vortices if preexisting rotation along a boundary superimposes with a convective updraft (e.g., Houston and Wilhelmson 2007). This may happen even if the broader synoptic environment does not seem favorable for strong tornadoes. Even in the absence of tornadoes, these preexisting boundary-associated circulation patterns may be favored locations for rapid convective development (e.g., Wilson et al. 1992; Lee and Wilhelmson 1997). Thus, it is recommended that nowcasters pay special attention to environments featuring a similar synoptic situation and a boundary with rotational signatures (e.g., Fig. 6a) prior to convection initiation.
Acknowledgments
MVDB was supported by an academic appointment at the University of Nebraska—Lincoln. We thank those who provided visual observations and photographs of the event: Brian and Christine Whittier, and Janice Sweet and R. David Johnson of the Illinois Beach State Park Hawk Watch site. Andrew Kalin helped with plotting the 1800 UTC RAP sounding (Fig. 3a). Three anonymous peer reviewers provided excellent suggestions that substantially strengthened the manuscript and clarified the presentation.
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