sign in sign up
<<
Home , Mesocyclone, Supercell, Waterspout

APRIL 2015 V A N D E N B R O E K E A N D V A N D E N B R O E K E 329

Polarimetric Radar Observations from a -Producing

MATTHEW S. VAN DEN BROEKE Department of Earth and Atmospheric Sciences, University of Nebraska—Lincoln, Lincoln, Nebraska

CYNTHIA A. VAN DEN BROEKE Lincoln, Nebraska

(Manuscript received 20 September 2014, in final form 20 January 2015)

ABSTRACT

A family of four was produced by a convective cell over western on 12 September 2013. This initiated along a boundary north of a mesolow in a low-level cold-air advection regime, and developed characteristics once the second waterspout was in progress. Polarimetric characteristics of the storm, and of the development of supercell character, are presented. These observations represent the first documented polarimetric radar observations of waterspout-producing in the region. Unusually high differential reflectivity values accompanied this storm and its initiating boundary. The high values along the boundary are partially explained by a high density of dragonflies. High differential reflectivity values were present through much of the storm of interest despite very low aerosol concentration at low levels in the lake-influenced air mass. Finally, this case illustrates the importance of environmental awareness on waterspout-favorable days, especially when boundaries are nearby to serve as a potential source of enhanced environmental vertical .

1. Introduction and motivation Conditions favorable for waterspout development in- clude low-level instability, low-level shear, and possibly A waterspout is defined as ‘‘any over a body of slow-moving or intersecting gust fronts (Simpson et al. water’’ (Glickman 2000), and waterspouts display all the 1986). In addition, waterspout-producing lines diversity in behavior, appearance, and origin of their kin typically develop under weak synoptic disturbances in over land. In , waterspouts most com- the presence of differential heating or sea surface tem- monly occur in the Keys (50–500 waterspouts per perature gradients (Golden 1974a; Simpson et al. 1986). year) and along the southeast coast of Florida (Golden There were 46 waterspouts per year on average from 1977) but have been observed on the Great Lakes (e.g., 1994 to 2010 over the Great Lakes (Sioutas et al. 2013). Gay 1921; Hurd 1928), the Great Salt Lake (Simpson Waterspouts were observed on every lake, though Lake et al. 1991), and even Lake Tahoe (Grotjahn 2000). Erie had the highest annual occurrence (Sioutas et al. Golden (1974b) first proposed a five-stage waterspout 2013). On Lake Michigan, 173 waterspouts were ob- life cycle based on observations of water- served from 1993 to 2013 (W. Szilagyi 2014, personal spouts. Most of these waterspouts occurred in a tropical communication). Most waterspouts on the Great Lakes, environment, developed in cloud lines, and were non- including Lake Michigan, occur in the months of August supercellular in origin. In fact, much of the waterspout and September when the water surface temperature is literature based on larger field projects has examined relatively warm (Szilagyi 2004; Sioutas et al. 2013). in the or , and waterspouts Sioutas et al. (2013) identified other conditions favor- forming through primarily nonsupercell processes (e.g., able for waterspout outbreaks on the Great Lakes, such Golden 1974a; Leverson et al. 1977; Simpson et al. 1986). as a 500-hPa long-wave trough or a closed low over the region, increased instability from cold advection, and in Corresponding author address: Matthew S. Van Den Broeke, 306 some cases a land breeze. Bessey Hall, Lincoln, NE 68588-0340. On the afternoon of 12 September 2013, a series of E-mail: [email protected] four waterspouts developed over western Lake Michigan

DOI: 10.1175/WAF-D-14-00114.1

Ó 2015 American Meteorological Society Unauthenticated | Downloaded 10/07/21 09:27 AM UTC 330 AND FORECASTING VOLUME 30

(NWS 2013). The waterspout-producing storm, which of rhv and ZDR were consistent with these expectations was well observed from the polarimetric Weather Sur- when averaged over several points at each of five times veillance Radar-1988 Doppler (WSR-88D) at Milwaukee, examined between 1759 and 1840 UTC (not shown),

Wisconsin (KMKX), appeared to become more super- indicating no consistent, substantial ZDR bias. cellular in nature while the waterspouts were in progress. Radar data were supplemented by additional observa- Using these radar data in conjunction with environmental tions, including routine upper-air and surface observa- and aerosol data, this study provides the following: tions. Rapid Refresh (RAP) model data at 1800 UTC 12 September 2013 were obtained from the National Cli- 1) the first published polarimetric radar observations of maticDataCenter(NCDC).Thesedatawereusedto a waterspout-producing storm in the Great Lakes estimate the sounding and storm-relative helicity (SRH) region, near the waterspout-producing storm. Maps of Lake 2) a chronology of polarimetric features associated with Michigan water surface temperature, estimated using an the transition to supercell convection, Advanced Very High Resolution Radiometer (AVHRR) 3) in situ observations of biological scatterers contrib- satelliteborne instrument, were obtained from the Great uting to high differential reflectivity Z values DR Lakes Environmental Research Laboratory (GLERL). along a boundary, and These data were limited by patchy over 4) the potential occurrence of a drop size distribution southern Lake Michigan, but portions of the lake offshore (DSD) biased toward unusually large liquid drops from Wisconsin and northeastern were cloud free. despite very low observed aerosol concentrations. Aerosol data, including particulate matter with a diameter This case is of particular interest given the small number less than 10 mm (PM10), were obtained from the Envi- of prior observational studies of waterspout-producing ronmental Protection Agency (EPA) for a station near the storms in the , and given the poten- Lake Michigan shoreline (indicated as red star in Fig. 1). tial for substantial human impacts had the storm been displaced only a small distance toward the land. 3. Overview of the synoptic- and local-scale environment 2. Data and methods A long-wave trough over the Great Lakes character- A radar dataset was analyzed from KMKX, which was ized the environment at 1200 UTC 12 September 2013. upgraded to polarimetric capability in April 2012. This The trough axis was located from Hudson Bay through dataset extended from the time a linear reflectivity central and Wisconsin, just west of Lake maximum first appeared east of KMKX (1455 UTC) Michigan (Fig. 2). Two jet streaks were evident at until the storm of interest moved well southeast of 300 hPa: the first on the west side of the trough axis over KMKX (2029 UTC). The storm of interest was within Minnesota and the second to the east over lower Mich- 120 km of KMKX throughout this period, minimizing igan (Fig. 2). As the trough moved eastward through data quality issues inherent at long range. All heights the region, model output indicated that by 1800 UTC noted in this paper are above radar level (ARL). Po- the trailing jet streak was in a favorable location for larimetric radar variables utilized included ZDR, which southeastern Wisconsin to experience synoptic-scale lift, affords an estimate of the reflectivity-weighted mean with strong northwest flow at 300 hPa (Fig. 3a). The axis ratio of scatterers in a sample volume, and copolar eastward-moving trough brought strong 850-hPa cold- cross-correlation coefficient rhv, which provides an in- air advection to the western Great Lakes (Fig. 2). At the dication of scatterer diversity, orientation, and phase surface, a cold front had passed through the region (e.g., Bringi and Chandrasekar 2001). overnight and by 1800 UTC was located from central

Very high ZDR values observed within storms Illinois through north-central (Fig. 2). Much of throughout the KMKX domain on this day were initially the region was dominated by northwest surface flow and suspected of being in error, so a scatterer-based cali- a gradual northward surface temperature decline. These bration procedure was implemented to ensure no large conditions, especially the long-wave trough and surface

ZDR bias. It was assumed that most hydrometeors cold front with attendant northwesterly flow, are typical should be dry aggregates ;1.5 km above the of a Great Lakes waterspout outbreak environment melting level. Typical values of radar variables in such (Szilagyi 2004; Sioutas et al. 2013). Surface-based in- hydrometeors include radar reflectivity factor at a hori- stability was relatively weak at 1800 UTC, with typical 21 zontal polarization ZHH between 20 and 35 dBZ, rhv . values of 200–300 J Kg across southeastern Wisconsin 0.97 (often .0.99), and ZDR averaging 0.1–0.2 dB and western Lake Michigan (Fig. 3b), according to the (Ryzhkov et al. 2005a; Picca and Ryzhkov 2012). Values 1800 UTC RAP initialization.

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC APRIL 2015 V A N D E N B R O E K E A N D V A N D E N B R O E K E 331

FIG. 1. Mesoscale features present at 1800 UTC 12 Sep 2013, when convection was already in progress. Station plots include temperature and 2 dewpoint (8C) and [knots (kt; 1 kt 5 0.51 m s 1); full barb 5 10 kt; half barb 5 5 kt]. Dewpoint is color shaded, and black contours represent surface pressure (contour interval 5 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 from 1804 to 1900 UTC. Observations plotted using WeatherScope from the Climatological Survey.

Across southeastern Wisconsin and far northeastern lake surface temperatures around 228C(Fig. 5), but after Illinois, the morning started with calm to very light convection initiation (about 1713 UTC; Fig. 4d) the along the western Lake Michigan shore. By cross-boundary temperature gradient increased sub- 1455 UTC there was a linear reflectivity maximum ori- stantially, likely because of cold pool development. ented from north to south across southeastern Wisconsin, Moisture in the lake-modified air combined with cold possibly indicating a boundary (Fig. 4a). Surface obser- advection aloft may have locally enhanced the condi- vations suggested weakly convergent flow as the wind tional instability (e.g., Fig. 3b), making the area east of veered from near westerly along the shore to northwest the boundary more susceptible to convection initiation. farther inland. A small area of low pressure was present The difference in temperature between the water sur- over southeastern Wisconsin and northeastern Illinois at face and 850 hPa was approximately 118C, which is 38C this time. A circulation became evident around this (0.7 standard deviations) below the average value for mesolow by 1600 UTC (not shown). At 1800 UTC, an Great Lakes waterspout outbreaks (Sioutas et al. easterly wind component was present near Milwaukee, 2013). and westerly flow had developed across far northeastern By 1634 UTC, the westward-moving boundary was Illinois and southeastern Wisconsin (Fig. 1). beginning to interact with the eastward-moving linear Complicating the situation, a westward-moving reflectivity maximum (Fig. 4c). Surface observations and boundary moved onshore around 1545 UTC to the radial velocity data both indicated convergent flow north of the mesolow and was clearly visible from along the zone where these features interacted, and KMKX (Fig. 4b). East of this boundary, dewpoints were where convection initiated around 1713 UTC (Fig. 4d). 28–38C higher than farther inland. Temperatures on ei- By 1804 UTC, three waves were visible along the ther side of the boundary were initially similar given boundary in base reflectivity, two of which were

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC 332 WEATHER AND FORECASTING VOLUME 30

FIG. 2. Synoptic-scale features on 12 Sep 2013. Shaded purple areas with arrows show lo- cations of 901-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 5 10 kt; half barb 5 5 kt), and red numbers are 850-hPa temperatures (8C) at each sounding location at 1200 UTC. Blue cold front symbol shows the location of the surface cold front at 1800 UTC. associated with zones of enhanced shear in the Doppler exceed 19.5 dBZ in the cell where the waterspout would velocity field. At this time, the strongest wave along the occur (;80 km from KMKX; Fig. 6a), though ZHH at boundary was located in far southeastern Wisconsin 1.498 showed values to 28.5 dBZ associated with this cell near the shore of Lake Michigan (Fig. 6a). (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 rota- 4. Observations and results tion (indicated by radial velocity Vr; Fig. 7b). In addi- An approximately chronological discussion of the tion, spectrum width sy showed an area of substantial base radar variables is presented here for the storm of turbulence and/or shear near where the waterspout was interest, followed by an examination of the polarimetric located (beam centerline elevation ;2.53 km), with variables. Microphysical characteristics inferred from a similar area located to the northwest in a nontornadic the polarimetric radar data are related to environmental over land (Fig. 7c; beam centerline elevation characteristics of the post-cold-frontal air mass. ;2.36 km). While this area of midlevel rotation even- tually dissipated, a succession of similar vortices over a. Base variable progression land with boundary-associated convective updrafts Prior to 1700 UTC, a well-defined westward-moving through 1830 UTC suggests a tendency for vorticity boundary was oriented northwest–southeast from far concentration on this day, possibly enhanced by southeastern Wisconsin to west of Milwaukee (Fig. 4). stretching under strong updrafts and low-level di- Radar reflectivity did not yet indicate precipitating rectional shear in the vicinity of the boundary (e.g., convection associated with the boundary. By 1713 UTC, Wakimoto and Wilson 1989). several convective cells had initiated on the east side of A series of pictures was available from the Illinois the boundary over southeastern Wisconsin (Fig. 4d), Beach State Park (IBSP) Hawk Watch site, indicated by nearly collocated with the zone of maximum shear along atriangleinFig. 1. Visually, the first waterspout appeared the boundary. pendant from an extensive cloud line (Fig. 8a; 1817 UTC). At 1804 UTC, 2 min prior to the initial waterspout By 1820 UTC, the westward-moving boundary apparent report (NWS 2013), the base-scan (0.548) ZHH did not at the lowest radar elevation had become more closely

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC APRIL 2015 V A N D E N B R O E K E A N D V A N D E N B R O E K E 333

FIG. 3. (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 5 50 kt; full barb 5 10 kt; half barb 5 5 kt). (b) 1800 UTC surface-based convective available potential energy (SBCAPE) over southern Lake Michigan from the RAP 2 model (contour interval 5 100 J Kg 1), with approximate track of the waterspout-producing storm annotated from 1804 to 1900 UTC.

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC 334 WEATHER AND FORECASTING VOLUME 30

FIG. 4. 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. collocated with the waterspout-producing storm exceeding 7 dB (Fig. 6b), as is typical in convergent, (Fig. 9e), which was developing strong low-level rotation along-boundary flow when insects are present (e.g., (Fig. 10a) while maintaining a focused sy maximum Achtemeier 1991). Observations from the IBSP Hawk (Fig. 10b). The first and second waterspouts briefly Watch site confirm the presence of insects along the overlapped from 1820 to 1824 UTC (Fig. 8b; 1822 UTC). boundary at low levels. An official observing site of the The storm continued to mature, with a broadening me- Hawk Migration Association of North America socyclone evident through time, and the second water- (HMANA), this location was participating in a pilot spout became much larger before dissipating (Fig. 8c). study with the Migratory Dragonfly Partnership (MDP) By the 1830 UTC scan, two areas of rotation were noted to count dragonflies during fall of 2013. The westward- in the radial velocity field, coincident with separate sy moving boundary passed the observing station around maxima (not shown). These may have indicated two 1830 UTC, sampled by KMKX at a base-scan elevation broader-scale areas of rotation within the storm. Two of ;1.2 km. Photographic and video records showed separate waterspouts were observed from IBSP from large numbers of dragonflies around the observing sta- 1831 to 1837 UTC (Fig. 8d; 1835 UTC). Waterspouts tion from 1820 UTC onward (Fig. 8b). During the period disappeared from view as a result of obscuration 1840–1850 UTC, 1505 dragonflies were counted moving around 1840 UTC (NWS 2013). south, more than half of the total for the fall of 2013 (HMANA 2013). Migrating dragonfly swarms have b. Polarimetric variable progression and comparison been inferred to elevations exceeding 1 km over the with base variables Indian Ocean (e.g., Anderson 2009), though limited The westward-moving boundary along which most observations of migrating dragonflies along the Lake storms initiated on this day was marked by ZDR values Michigan shoreline have indicated that individuals

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC APRIL 2015 V A N D E N B R O E K E A N D V A N D E N B R O E K E 335

FIG. 5. Water surface temperature of southern Lake Michigan (contour interval 5 18F) 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.] usually remain below 100 m (Russell at al. 1998). We 1987; Kumjian and Ryzhkov 2008; and many others) speculate that a combination of boundary layer lift in the with values to 4.5 dB extended up to 5 km ARL just vicinity of the westward-moving boundary and a re- northwest of the waterspout location, possibly associ- flectivity contribution from dragonflies (and possibly ated with the upstream cell still over land. other bioscatter species) below beam centerline may By 1820 UTC (storm core ;85 km from KMKX), have contributed to elevated ZDR values observed by a well-defined convective cell had become apparent at KMKX along the boundary. 0.548 elevation (;1.2 km ARL; Fig. 9a). The ZDR values By 1804 UTC, the core of the waterspout-producing .2.5 dB dominated a larger areal extent than is typically storm (;80 km from KMKX) was characterized at 0.58 seen within the ZDR arc (e.g., Kumjian and Ryzhkov elevation by ZDR values from 2.5 to 4.6 dB, ZHH values 2008; Romine et al. 2008; Crowe et al. 2012; Dawson less than 20 dBZ, and rhv values generally exceeding et al. 2014). In particular, a region of large ZDR values at 0.98 (Fig. 6), indicating the dominance of large drops in low levels (5–5.5 dB) was located in northwestern por- small concentrations. Once the first waterspout was well tions of the updraft region (Fig. 10d), which was inferred under way at 1810 UTC (storm core ;83 km from by the presence of a ZDR column overhead. Differential KMKX), base-scan values of ZDR throughout its parent sedimentation under the broad updraft within a region storm became more uniformly .4 dB, with a few bins of heavy may have contributed to the re-

.5 dB, though ZHH values were still generally ,30 dBZ gion of largest base-scan ZDR values, as suggested by (Figs. 9a,b). Aloft, a ZDR column (Illingworth et al. Kumjian and Ryzhkov (2012). The storm continued to

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC 336 WEATHER AND FORECASTING VOLUME 30

FIG. 6. Radar signatures at the 0.548 elevation angle from KMKX at 1804 UTC 12 Sep 2013. Shown are (a) ZHH (dBZ), (b) ZDR (dB), and (c) rhv. 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.

exhibit a column of high ZDR values through the time KMKX), base-scan ZHH in the waterspout-producing when waterspouts were last observed around 1840 UTC. cell (altitude ;1.4 km) had increased to at least 63 dBZ

The ZDR values of 5.8 dB were present to an altitude of in multiple areas north and east of the updraft (Fig. 11a). nearly 5.2 km at 1820 UTC, and values of 5.0 dB were The southern edge of one such area was collocated with present up to nearly 5.5 km at 1825 UTC (not shown), locally lower ZDR values of 0.5–2.5 dB (Fig. 11b) and rhv well above the ambient 08C level (approximately 3.0 km values depressed to 0.80–0.91 (Fig. 11c), suggesting ARL; Fig. 3a). When two intense low-level vortices or melting hail reaching low levels (Straka et al. 2000). were observed at 1835 UTC (storm core ;87 km from The northern side of this area had rhv values lowered to

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC APRIL 2015 V A N D E N B R O E K E A N D V A N D E N B R O E K E 337

FIG. 7. Radar signatures at the 1.498 elevation angle from KMKX at 1810 UTC 12 Sep 2013. Shown are (a) ZHH 21 21 (dBZ), (b) Vr (m s ), and (c) sy (m s ). 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.

0.90 and collocated with ZHH of 60–63 dBZ and ZDR of much longer than it takes an air parcel to rise from the 2.0–4.25 dB, suggesting water-coated hail (Figs. 11a–c). base of the updraft to its summit’’ (Glickman 2000). Well after the last observed waterspout was obscured Detection of this characteristic updraft, the mesocy- by precipitation, the storm maintained widespread ZDR clone (Brown et al. 1975), has been automated utilizing values exceeding 3.5–4 dB at base-scan level, a well- radar observations. One such automated detection defined ZDR column characterized by high values well scheme is the detection algorithm (MDA) above the 08C level, and a broad mesocyclone. A burst of used by WSR-88D (Stumpf et al. 1998). Among several melting hail or mixed rain and hail reaching the base- parameters and other qualifications used to test for the scan level was observed from 1840 to 1845 UTC (;92 km presence of a mesocyclone (e.g., vertical and temporal from KMKX), during and after the time when the last continuity) are velocity difference (VD; a sum of the waterspout became rain obscured (Fig. 12a). magnitudes of the maximum inbound and outbound velocities) and a measure of shear (velocity difference c. Radar observations of supercell development divided by the distance between the maximum in- and Supercell storms are defined by a ‘‘single, quasi-steady outbound velocities). For storms within 100 km of rotating updraft, which persists for a period of time a WSR-88D, threshold values of these variables are

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC 338 WEATHER AND FORECASTING VOLUME 30

FIG. 8. 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.]

21 21 21 $30 m s for VD and $6ms km for shear (Stumpf the weak-echo region (WER) in ZHH, which often takes et al. 1998). In the waterspout-producing storm, VD met the form of a bounded weak-echo region (BWER; e.g., this threshold at the 1.498 elevation angle from 1815 to Barnes 1978). The storm of interest first exhibited 1850 UTC (Fig. 12b), at a corresponding altitude of 2.6– a WER at 1825 UTC in the 2.458 scan (;4.1 km ARL), 3.1 km. Base-scan VD climbed steeply from cell initia- and this feature was well defined at 1835 UTC (when 2 tion, meeting or nearly meeting the 30 m s 1 criterion two waterspouts were occurring) at the 1.498 elevation from 1825 to 1850 UTC (Fig. 12b). Highest VD values at angle (;2.7 km ARL; Fig. 13a). A WER, sometimes both levels considered together occurred from 1835 to bounded, remained visible through 1845 UTC, but dis- 1840 UTC. Shear tended to remain more constant appeared thereafter. This signature was less pronounced throughout the examined time period (Fig. 12b), because than usual in a storm of this intensity, and only lasted the mesocyclone diameter tended to increase with VD ;20 min. This may be due to the relatively low CAPE through 1850 UTC. Shear at both elevation angles typi- environment, which would promote relatively weak cally exceeded the MDA threshold of Stumpf et al. (1998). vertical motion in the updraft. Another approach to determining when a storm may are often characterized by a strong be said to have developed supercell characteristics is via forward-flank ZHH gradient (e.g., Kumjian and Ryzhkov examination of the storm’s polarimetric radar features. 2008; Frame et al. 2009), which became evident in the Prior studies have examined radar features of classic waterspout-producing storm around 1830 UTC, was supercell storms using ZHH, ZDR, and rhv (e.g., Kumjian exceptionally well defined around 1840 UTC (Fig. 13b), and Ryzhkov 2008; Romine et al. 2008; Van Den Broeke and persisted well after the waterspout was obscured by et al. 2008; Kumjian et al. 2010), and here we will ex- precipitation. amine a subset of these features for the waterspout- Differential reflectivity has proven useful in di- producing storm. Radar features discussed are plotted agnosing features of supercell storms, providing in- on a timeline (Fig. 12a). formation on drop size distributions (e.g., Feingold and One radar feature common to any convective cell with Levin 1987; Gorgucci et al. 2000; Cao et al. 2010), and a strong updraft, including supercell , is the presence of hail (e.g., Aydin et al. 1990; Herzegh

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC APRIL 2015 V A N D E N B R O E K E A N D V A N D E N B R O E K E 339

FIG. 9. KMKX (left) ZHH (dBZ) and (right) ZDR (dB) at base-scan level (0.548) at times indicated to the right. Waterspout-producing storm is circled.

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC 340 WEATHER AND FORECASTING VOLUME 30

FIG. 10. Radar signatures at the 0.528 elevation angle from KMKX at 1820 UTC 12 Sep 2013. Shown are (a) Vr 21 21 (m s ), (b) sy (m s ), (c) rhv, 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 08C level; distance from KMKX to the center is ;81 km.

and Jameson 1992; Dawson et al. 2014). The ZDR col- Another polarimetric feature common in supercell umn represents liquid drops lofted in an area of strong storms is the ZDR arc (e.g., Kumjian and Ryzhkov 2008, upward motion (e.g., Herzegh and Jameson 1992), often 2009), a band of locally enhanced ZDR values collo- well above the ambient 08C level. Such columns may be catedwithastrongZHH gradient along the storm’s observed in any deep convection with strong updrafts, forward flank. It may be useful as an indicator that and are ubiquitous in supercell storms. In the a convective cell is transitioning into a more tornado- waterspout-producing storm, a well-defined ZDR col- favorable phase (e.g., Crowe et al. 2012). A ZDR arc umn was characteristic from cell initiation, often con- was first present in the 1820 UTC base-level scan taining exceptionally high ZDR values of 3–7 dB at an (Fig. 13d), but had weakened by 1825 UTC. It was altitude of 3.5–5.5 km (e.g., Fig. 13c). Though the mag- occasionally present through and past the time when nitude of ZDR values within the column generally de- waterspouts were observed, but was rarely well defined creased over time, a column remained well defined past and did not appear to be a persistent feature in this the time when waterspouts were observed. Given an storm. Radar–storm distance may have reduced the estimated ambient 08C level around 3.0 km ARL ability to see this signature toward 1900 UTC, as the (Fig. 3a), liquid drops were likely present well above the radar was sampling the forward flank by this time at ambient 08C level within the updraft. approximately 1.6 km ARL.

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC APRIL 2015 V A N D E N B R O E K E A N D V A N D E N B R O E K E 341

FIG. 11. 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 rhv, and elevated ZDR. Distance from KMKX to center of solid oval is ;93 km and to center of dashed area is ;91 km.

21 Polarimetrically inferred hail may vary cyclically in phase KDP values of 3.58–4.658 km and output of the observed and numerically simulated supercell storms WSR-88D hydrometeor classification algorithm (Park (e.g., Van Den Broeke et al. 2008; Kumjian and Ryzhkov et al. 2009) indicated that hail mixed with large drops 2008; Van Den Broeke 2014). In the waterspout- (possibly melting hail) may have been present (not producing storm, the presence of hail was assessed shown). A similar fallout of hail occurred from 1840 to both at the lowest scan and farther aloft. Hail was in- 1845 UTC, though ZDR values again remained generally ferred using the collocation of high-ZHH and suppressed high in association with the reflectivity maximum. Be- ZDR/rhv values (e.g., Straka et al. 2000). At the lowest yond 1845 UTC, hail at low levels could not be assessed scan, a reduction of rhv was evident at 1835 UTC in the since the storm became too far from the radar. Aloft, storm core with the highest ZHH values (up to 60.5 dBZ), hail was inferred around 4.3 km ARL by 1830 UTC via but collocated ZDR values were 2.5–3.5 dB (Fig. 11), collocated high ZHH and depressed rhv values. Hail inconsistent with large or pure hail. Specific differential persisted through approximately 1845 UTC, after which

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC 342 WEATHER AND FORECASTING VOLUME 30

FIG. 12. (a) Timeline of waterspouts and radar features observed with the storm of interest. Dashed final wa-

terspout 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 2 2 2 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 1.498 tilts. Dashed lines represent thresholds in the WSR-88D mesocyclone detection algorithm for storms within 100 km of the radar with velocity 2 2 2 difference of 30 m s 1 (upper line) and shear of 6 m s 1 km 1 (lower line).

time ZDR values increased in association with the re- d. Microphysics and the storm environment flectivity maximum (not shown).

Some precipitation characteristics of supercell storms As described above, relative to prior studies ZDR may also be manifest in rhv, including a partial or full values were unusual throughout the examined portion ring of low values around the updraft region in mixed- of the waterspout-producing storm’s life in two primary phase hydrometeors (e.g., Kumjian et al. 2010). A partial ways: 1) the large areal extent of ZDR . 4 dB at low rhv ring was evident at 1835 UTC (Fig. 13e), surrounding levels (e.g., not just confined to the typical ZDR arc re- the WER, and remained evident at 1840 UTC, then gion) and 2) the very high ZDR values (5–7 dB) within disappeared until 1855 UTC when it was again well the ZDR column. Given good data quality and no ap- defined. Taken together (Fig. 12), the radar observations parent large calibration offset, at low levels these indicate a storm with most pronounced supercell char- high ZDR values may be attributed to very large liquid acter from approximately 1830 to 1845 UTC. drops in low concentration. Such large-drop dominant

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC APRIL 2015 V A N D E N B R O E K E A N D V A N D E N B R O E K E 343

FIG. 13. Polarimetric signatures from KMKX in the waterspout-producing storm on 12 Sep, indicated by ovals:

(a) BWER in ZHH, 1835 UTC, 1.498 elevation angle; (b) strong ZHH gradient along the storm’s forward flank, 1840 UTC, 0.548 elevation angle; (c) ZDR column associated with the updraft region, 1810 UTC, 2.468 elevation angle; (d) ZDR arc along the storm’s forward flank, 1820 UTC, 0.548 elevation angle; and (e) partial rhv ring around the updraft region, 1835 UTC, 1.498 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.

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC 344 WEATHER AND FORECASTING VOLUME 30 distributions at low levels are partially a result of dif- low-level winds nearby, but this does not explain wide- ferential sedimentation and updraft-induced sorting spread high ZDR values elsewhere in the storm. (e.g., Kumjian and Ryzhkov 2012), though we do not Finally, aerosol distributions may be important to believe these mechanisms contribute to the unusual thunderstorm DSDs. Recent modeling studies have nature of the ZDR distribution in the storm of interest noted larger liquid drops in nonsupercell environments relative to similar storms presented in the literature. with increased aerosol loading, due to increased riming

Within ZDR columns, high ZDR values were typically leading to larger ice particles (e.g., Storer et al. 2010; collocated with rhv values depressed to 0.90–0.95, sug- Lebo and Morrison 2014). Polarimetric radar observa- gesting mixed-phase particles. Additional environmental tions have yielded similar conclusions from tropical factors potentially contributing to the ZDR distribution convection (May et al. 2011). Also, recent simulations of include the vertical wind and moisture profiles, and the polarimetric radar variables in a midlatitude hailstorm regional aerosol distribution. indicated no ZDR columns above the freezing level in In modeling studies, the vertical moisture profile has clean (Khain et al. 2014). In the waterspout- been shown to affect supercell evolution and micro- producing storm, however, enhanced ZDR values to high physics (e.g., Gilmore and Wicker 1998; James and elevation were associated with a relatively clean air mass Markowski 2010; Van Den Broeke 2014). The envi- at low levels. Available aerosol data for this case in- ronment of many supercell storms, including the cluded several PM10 time series from the region. While waterspout-producing storm, is characterized by rela- neglecting vertical aerosol variability, which is critical to tively dry air at low levels, a moist layer centered near convective microphysics (e.g., Lebo 2014), such data 840 hPa, and a relatively deep dry layer above (Fig. 3). provide one means of examining the ambient aerosol On days with such a deep dry layer aloft, small ice distribution, and have not been widely utilized in prior crystals may sublimate before falling very far, leaving polarimetric studies of convection. The PM10 time se- hydrometeor distributions dominated by melting grau- ries from Chicago, Illinois (red star in Fig. 1), was ex- pel and frozen raindrops and increasing the average amined throughout September 2013 to ascertain drop size below the melting level (Van Den Broeke 2014). whether aerosol concentration was anomalous in the air This may represent one possible mechanism contributing mass in which the storm of interest initiated (Fig. 14). to high ZDR values at low levels in many supercell storms, One peak in PM10 was collocated with the air mass but does not account for the possible influence of aggre- immediately inland of the westward-moving boundary gation, coalescence, or droplet breakup. on 12 September. Behind this boundary, as air from over

High values of ZDR within the ZDR arc may be related Lake Michigan moved inland, PM10 values dropped 23 to the storm-relative vertical wind profile. The ZDR arc substantially to near 10 mgm (Fig. 14), one of the most has been hypothesized to occur because of raindrop and pristine air masses of 2013 at this site. Given ZDR col- hail size sorting along the forward flank with an appro- umns well above the ambient 08C level and anomalously priate storm-relative vertical wind profile (Ryzhkov high ZDR values throughout the waterspout-producing et al. 2005b; Kumjian and Ryzhkov 2009; Dawson et al. storm despite an environment characterized by low 2014, 2015). Resulting collections of large, isolated aerosol loading, more detailed observational study is drops have been found under the ZDR arc (Schuur et al. needed to determine typical impacts of aerosol vari- 2001). Efficiency of the size-sorting mechanism, and thus ability on microphysics and polarimetric signatures of magnitude of ZDR values, is hypothesized to depend midlatitude supercell storms. upon the magnitude of the mean storm-relative wind, which may be manifest in the SRH value (Kumjian and 5. Discussion Ryzhkov 2009; Dawson et al. 2015). A 0–3-km SRH 2 value of 146 m2 s 2 was estimated near the storm loca- On 12 September 2013, convection initiated near tion from the 1800 UTC RAP model vertical wind a westward-moving boundary north of a mesolow along profile (Fig. 3a), using low-level wind on the lakeward the western Lake Michigan shoreline. The synoptic- side of the boundary and radar-estimated motion of the scale environment, including a long-wave trough nearly mature storm. This estimate may not fully account for overhead and low-level cold-air advection north of the presence of the boundary. In addition, rapid de- a surface cold front, was typical of waterspout outbreaks velopment of a rotating updraft in this storm suggests an in the Great Lakes region. One convective cell produced environment with high values of preexisting vertical four waterspouts and briefly took on supercell charac- vorticity. Thus, high observed ZDR values within the teristics. Supercell indicators included a mesocyclone, a ZDR arc of this storm may have been an indication of the strong forward-flank ZHH gradient, and an intermittent local wind profile, including enhanced storm-relative ZDR arc.

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC APRIL 2015 V A N D E N B R O E K E A N D V A N D E N B R O E K E 345

FIG. 14. Average hourly Chicago PM10 from 6 to 18 Sep 2013.

Early waterspouts were produced in association with observation that some waterspouts may derive their a rapidly growing cumulus, consistent with prior obser- vorticity primarily from a low-level source (e.g., Verlinde vations (e.g., Wakimoto and Lew 1993). A ZDR arc was 1997). In the Great Lakes event documented herein, evident by 1820 UTC (Kumjian and Ryzhkov 2009), and weak initial low-level ZHH and updraft overhead (in- a WER became well defined by 1825 UTC, indicating ferred by high ZDR values aloft) provide further evi- a strengthening updraft (Fig. 12a). The greatest number dence that the initial waterspouts formed via stretching of radar-diagnosed supercell indicators was present of preexisting vertical vorticity along the boundary. This from approximately 1830 to 1845 UTC (Fig. 12), during formation mechanism is similar to that observed for which time several waterspouts were produced. Thus, it (e.g., Wakimoto and Wilson 1989), and has seems probable that the storm’s first waterspout was been observed before along boundaries (e.g., Brady and mesocyclone independent and driven primarily by low- Szoke 1989; Snow and Wyatt 1998; Collins et al. 2000). level convergence and stretching of preexisting envi- In environments characterized by large low-level in- ronmental vorticity under a rapidly growing updraft, stability leading to rapid vertical acceleration collocated with a transition toward more supercell-characteristic with a preexisting vorticity maximum along a boundary, processes during the second waterspout. Portions of the resulting vortex may become quite intense (e.g., later waterspout life cycles may have also been influ- Caruso and Davies 2005; Pfost et al. 2005). enced by bursts of hail reaching low levels, which may The presence of bioscatter, supported by observed locally concentrate low-level vertical vorticity (e.g., Van high dragonfly density (e.g., Fig. 8b), appeared to

Den Broeke 2014). These bursts of hail occurred in the contribute to high ZDR values near the initiating storm core rather than in the right-rear quadrant relative boundary. Within the convection, very large liquid to a midlevel WER, as would be the case with a de- drops appeared to be responsible, and high values in scending reflectivity core (DRC; Rasmussen et al. 2006). the ZDR arc region may have partially resulted from Early waterspouts followed a similar pattern to those the local wind profile. Aerosol concentration was ex- documented by Collins et al. (2000) in a Florida non- tremely low in the lake-influenced air mass at low supercell thunderstorm. There, a convective cell rapidly levels, a condition not previously associated with high developed along a low-level boundary and formed concentrations of large drops in convective clouds. a tornado initially over land. The rapid vorticity con- Future work should include study of potential polari- centration was attributed to stretching under a strong metrically observable aerosol effects on DSDs and updraft and low-level convergence under a downdraft ZDR columns in midlatitude convection, and could (Collins et al. 2000). This is consistent with the utilize similar PM10 measurements.

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC 346 WEATHER AND FORECASTING VOLUME 30

Operational nowcasters can take useful findings and Bringi, V. N., and V. Chandrasekar, 2001: Polarimetric Doppler cautions from this event. Though only one storm along : Principles and Applications. Cambridge Uni- the lake-induced boundary took on supercell charac- versity Press, 636 pp. Brown, R. A., D. W. Burgess, J. K. Carter, L. R. Lemon, and teristics for a substantial length of time and produced D. Sirmans, 1975: NSSL dual- measurements in several waterspouts, other convective cells along the tornadic storms: A preview. Bull. Amer. Meteor. Soc., 56, 524–526, same boundary exhibited brief supercell structures and doi:10.1175/1520-0477(1975)056,0524:NDDRMI.2.0.CO;2. low-level rotation. While low-level vortices produced Cao, Q., G. Zhang, E. A. Brandes, and T. J. Schuur, 2010: Polari- under similar scenarios (e.g., Marquis et al. 2007)maybe metric radar rain estimation through retrieval of drop size distribution using a Bayesian approach. J. Appl. Meteor. due to stretching and are relatively weak (e.g., Wakimoto Climatol., 49, 973–990, doi:10.1175/2009JAMC2227.1. and Wilson 1989), it is possible to get intense vortices if Caruso, J. M., and J. M. Davies, 2005: Tornadoes in non- preexisting rotation along a boundary superimposes with mesocyclone environments with pre-existing vertical vorticity a convective updraft (e.g., and Wilhelmson along convergence boundaries. Electron. J. Operational 2007). This may happen even if the broader synoptic Meteor., 6 (4), 1–36. [Available online at http://www.nwas.org/ ej/pdf/2005-EJ4.pdf.] environment does not seem favorable for strong torna- Collins, W. G., C. H. Paxton, and J. H. Golden, 2000: The 12 July does. Even in the absence of tornadoes, these preexisting 1995 Pinellas County, Florida, tornado/waterspout. Wea. Fore- boundary-associated circulation patterns may be favored casting, 15, 122–134, doi:10.1175/1520-0434(2000)015,0122: locations for rapid convective development (e.g., Wilson TJPCFT.2.0.CO;2. et al. 1992; Lee and Wilhelmson 1997). Thus, it is rec- Crowe, C. C., C. J. Schultz, M. Kumjian, L. D. Carey, and W. A. Peterson, 2012: Use of dual-polarization signatures in di- ommended that nowcasters pay special attention to en- agnosing tornadic potential. Electron. J. Oper. Meteor., 13 (5), vironments featuring a similar synoptic situation and 57–78. [Available online at http://www.nwas.org/ej/pdf/ a boundary with rotational signatures (e.g., Fig. 6a) prior 2012-EJ5.pdf.] to convection initiation. Dawson, D. T., E. R. Mansell, Y. Jung, L. J. Wicker, M. R. Kumjian, and M. Xue, 2014: Low-level ZDR signatures in su- percell forward flanks: The role of size sorting and melting of Acknowledgments. MVDB was supported by an aca- hail. J. Atmos. Sci., 71, 276–299, doi:10.1175/JAS-D-13-0118.1. demic appointment at the University of Nebraska— ——, ——, and M. R. Kumjian, 2015: Does cause Lincoln. We thank those who provided visual observa- hydrometeor size sorting? J. Atmos. Sci., 72, 340–348, tions and photographs of the event: Brian and Christine doi:10.1175/JAS-D-14-0084.1. Whittier, and Janice Sweet and R. David Johnson of the Feingold, G., and Z. Levin, 1987: Application of the lognormal raindrop distribution to differential reflectivity radar mea- Illinois Beach State Park Hawk Watch site. Andrew surement (ZDR). J. Atmos. Oceanic Technol., 4, 377–382, Kalin helped with plotting the 1800 UTC RAP sounding doi:10.1175/1520-0426(1987)004,0377:AOTLRD.2.0.CO;2. (Fig. 3a). Three anonymous peer reviewers provided Frame, J., P. Markowski, Y. Richardson, J. Straka, and J. Wurman, excellent suggestions that substantially strengthened the 2009: Polarimetric and dual-Doppler radar observations of manuscript and clarified the presentation. the Lipscomb County, , supercell thunderstorm on 23 May 2002. Mon. Wea. Rev., 137, 544–561, doi:10.1175/ 2008MWR2425.1. REFERENCES Gay, E., 1921: Waterspouts on . Mon. Wea. Rev., 49, 409, doi:10.1175/1520-0493(1921)49,409a:WOLO.2.0.CO;2. Achtemeier, G. L., 1991: The use of insects as tracers for ‘‘clear-air’’ Gilmore, M. S., and L. J. Wicker, 1998: The influence of midtropo- boundary-layer studies by Doppler radar. J. Atmos. Oceanic spheric dryness on supercell morphology and evolution. Mon. Technol., 8, 746–765, doi:10.1175/1520-0426(1991)008,0746: Wea. Rev., 126, 943–958, doi:10.1175/1520-0493(1998)126,0943: TUOIAT.2.0.CO;2. TIOMDO.2.0.CO;2. Anderson, R. C., 2009: Do dragonflies migrate across the western Glickman, T., Ed., 2000: Glossary of . 2nd ed. Amer. Indian Ocean? J. Trop. Ecol., 25, 347–358, doi:10.1017/ Meteor. Soc., 855 pp. [Available online at http://glossary. S0266467409006087. ametsoc.org/.] Aydin, K., Y. Zhao, and T. A. Seliga, 1990: A differential reflectivity Golden, J. H., 1974a: Scale-interaction implications for the radar hail measurement technique: Observations during waterspout cycle. J. Appl. Meteor., 13, 693–709, doi:10.1175/ the Denver hailstorm of 13 June 1984. J. Atmos. Oceanic 1520-0450(1974)013,0693:SIIFTW.2.0.CO;2. Technol., 7, 104–113, doi:10.1175/1520-0426(1990)007,0104: ——, 1974b: The life cycle of Florida Keys’ waterspouts. J. Appl. ADRRHM.2.0.CO;2. Meteor., 13, 676–692, doi:10.1175/1520-0450(1974)013,0676: Barnes, S. L., 1978: Oklahoma thunderstorms on 29–30 April TLCOFK.2.0.CO;2. 1970. Part I: Morphology of a tornadic storm. Mon. Wea. ——, 1977: An assessment of waterspout frequencies along the U.S. Rev., 106, 673–684, doi:10.1175/1520-0493(1978)106,0673: East and Gulf Coasts. J. Appl. Meteor., 16, 231–236, OTOAPI.2.0.CO;2. doi:10.1175/1520-0450(1977)016,0231:AAOWFA.2.0.CO;2. Brady, R. H., and E. J. Szoke, 1989: A case study of nonmesocyclone Gorgucci, E., G. Scarchilli, V. Chandrasekar, and V. N. Bringi, tornado development in northeast : Similarities 2000: Measurement of mean raindrop shape from polarimetric to waterspout formation. Mon. Wea. Rev., 117, 843–856, radar observations. J. Atmos. Sci., 57, 3406–3413, doi:10.1175/ doi:10.1175/1520-0493(1989)117,0843:ACSONT.2.0.CO;2. 1520-0469(2000)057,3406:MOMRSF.2.0.CO;2.

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC APRIL 2015 V A N D E N B R O E K E A N D V A N D E N B R O E K E 347

Grotjahn, R., 2000: Multiple waterspouts on Lake Tahoe. Bull. Amer. May, P. T., V. N. Bringi, and M. Thurai, 2011: Do we observe Meteor. Soc., 81, 695–702, doi:10.1175/1520-0477(2000)081,0695: aerosol impacts on DSDs in strongly forced tropical thun- MWALT.2.3.CO;2. derstorms? J. Atmos. Sci., 68, 1902–1910, doi:10.1175/ Herzegh, P. H., and A. R. Jameson, 1992: Observing precipitation 2011JAS3617.1. through dual-polarization radar measurements. Bull. Amer. Me- NWS, cited 2013: Waterspouts near Kenosha September 12th, teor. Soc., 73, 1365–1374, doi:10.1175/1520-0477(1992)073,1365: 2013 (Updated 9/25). NOAA/. OPTDPR.2.0.CO;2. [Available online at http://www.crh.noaa.gov/news/display_ HMANA, cited 2013: HawkCount: Illinois Beach State Park: Day’s cmsstory.php?wfo5mkx&storyid597117&source52.] summary: Sep 12, 2013. Hawk Migration Association of North Park, H., A. V. Ryzhkov, D. S. Zrnic, and K.-E. Kim, 2009: The America. [Available online at https://www.hawkcount.org/day_ hydrometeor classification algorithm for the polarimetric summary.php?rsite5527&rmonth509&rday512&ryear52013.] WSR-88D: Description and application to an MCS. Wea. Houston, A. L., and R. B. Wilhelmson, 2007: Observational anal- Forecasting, 24, 730–748, doi:10.1175/2008WAF2222205.1. ysis of the 27 May 1997 central Texas tornadic event. Part II: Pfost, R. L., P. Santos Jr., and T. E. Warner, 2005: South Florida Tornadoes. Mon. Wea. Rev., 135, 727–735, doi:10.1175/ seabreeze/outflow boundary tornadoes. Electron. J. Oper. MWR3301.1. Meteor., 6 (7), 1–29. [Available online at http://www.nwas.org/ Hurd, W. E., 1928: Waterspouts. Mon. Wea. Rev., 56, 207–211, ej/2005-EJ6/.] doi:10.1175/1520-0493(1928)56,207:W.2.0.CO;2. Picca, J., and A. Ryzhkov, 2012: A dual-wavelength polarimetric Illingworth, A. J., J. W. F. Goddard, and S. M. Cherry, 1987: Po- analysis of the 16 May 2010 extreme larization radar studies of precipitation development in con- hailstorm. Mon. Wea. Rev., 140, 1385–1403, doi:10.1175/ vective storms. Quart. J. Roy. Meteor. Soc., 113, 469–489, MWR-D-11-00112.1. doi:10.1002/qj.49711347604. Rasmussen, E. N., J. M. Straka, M. S. Gilmore, and R. Davies- James, R. P., and P. M. Markowski, 2010: A numerical in- Jones, 2006: A preliminary survey of rear-flank descending vestigation of the effects of dry air aloft on deep convection. reflectivity cores in supercell storms. Wea. Forecasting, 21, Mon. Wea. Rev., 138, 140–161, doi:10.1175/2009MWR3018.1. 923–938, doi:10.1175/WAF962.1. Khain, A. P., E. Ilotovich, N. Benmoshe, V. Phillips, A. V. Romine, G. S., D. W. Burgess, and R. B. Wilhelmson, 2008: A dual- Ryzhkov, and M. R. Kumjian, 2014: Effects of aerosols on size polarization-radar-based assessment of the 8 May 2003 and structure of hail and on the polarimetric radar signatures. Oklahoma City area tornadic supercell. Mon. Wea. Rev., 136, Proc. 14th Conf. on , Boston, MA, Amer. Me- 2849–2870, doi:10.1175/2008MWR2330.1. teor. Soc., 198. [Available online at https://ams.confex.com/ Russell, R. W., M. L. May, K. L. Soltesz, and J. W. Fitzpatrick, ams/14CLOUD14ATRAD/webprogram/Paper250197.html.] 1998: Massive swarm migrations of dragonflies (Odonata) in Kumjian, M. R., and A. V. Ryzhkov, 2008: Polarimetric signatures eastern North America. Amer. Midl. Nat., 140, 325–342, in supercell thunderstorms. J. Appl. Meteor. Climatol., 47, doi:10.1674/0003-0031(1998)140[0325:MSMODO]2.0.CO;2. 1940–1961, doi:10.1175/2007JAMC1874.1. Ryzhkov, A. V., S. E. Giangrande, V. M. Melnikov, and T. J. ——, and ——, 2009: Storm-relative helicity revealed from polar- Schuur, 2005a: Calibration issues of dual-polarization radar imetric radar measurements. J. Atmos. Sci., 66, 667–685, measurements. J. Atmos. Oceanic Technol., 22, 1138–1155, doi:10.1175/2008JAS2815.1. doi:10.1175/JTECH1772.1. ——, and ——, 2012: The impact of size sorting on the polarimetric ——, T. J. Schuur, D. W. Burgess, and D. S. Zrnic, 2005b: Polari- radar variables. J. Atmos. Sci., 69, 2042–2060, doi:10.1175/ metric tornado detection. J. Appl. Meteor., 44, 557–570, JAS-D-11-0125.1. doi:10.1175/JAM2235.1. ——, ——, V. M. Melnikov, and T. J. Schuur, 2010: Rapid-scan Schuur, T. J., A. V. Ryzhkov, D. S. Zrnic, and M. Schönhuber, 2001: super-resolution observations of a cyclic supercell with a dual- Drop size distributions measured by a 2D video disdrometer: polarization WSR-88D. Mon. Wea. Rev., 138, 3762–3786, Comparison with dual-polarization radar data. J. Appl. Me- doi:10.1175/2010MWR3322.1. teor., 40, 1019–1034, doi:10.1175/1520-0450(2001)040,1019: Lebo, Z. J., 2014: The sensitivity of a numerically simulated ide- DSDMBA.2.0.CO;2. alized line to the vertical distribution of aerosols. Simpson, J., B. R. Morton, M. C. McCumber, and R. S. Penc, 1986: J. Atmos. Sci., 71, 4581–4596, doi:10.1175/JAS-D-14-0068.1. Observations and mechanisms of GATE waterspouts. J. At- ——, and H. Morrison, 2014: Dynamical effects of aerosol per- mos. Sci., 43, 753–782, doi:10.1175/1520-0469(1986)043,0753: turbations on simulated idealized squall lines. Mon. Wea. Rev., OAMOGW.2.0.CO;2. 142, 991–1009, doi:10.1175/MWR-D-13-00156.1. ——, G. Roff, B. R. Morton, K. Labas, G. Dietachmayer, Lee, B. D., and R. B. Wilhelmson, 1997: The numerical simulation M. McCumber, and R. Penc, 1991: A Great Salt Lake of non-supercell . Part I: Initiation and evolu- waterspout. Mon. Wea. Rev., 119, 2741–2770, doi:10.1175/ tion of pretornadic misocyclone circulations along a dry 1520-0493-119-12-2740.1. outflow boundary. J. Atmos. Sci., 54, 32–60, doi:10.1175/ Sioutas, M., W. Szilagyi, and A. Keul, 2013: Waterspout outbreaks 1520-0469(1997)054,0032:TNSONS.2.0.CO;2. over areas of and North America: Environment and Leverson, V. H., P. Sinclair, and J. Golden, 1977: Waterspout wind, predictability. Atmos. Res., 123, 167–179, doi:10.1016/ temperature and pressure structure deduced from aircraft j.atmosres.2012.09.013. measurements. Mon. Wea. Rev., 105, 725–733, doi:10.1175/ Snow, J. T., and A. L. Wyatt, 1998: Back to basics: The tornado, 1520-0493(1977)105,0725:WWTAPS.2.0.CO;2. nature’s most violent wind: Part 2—Formation and Marquis, J. N., Y. P. Richardson, and J. M. Wurman, 2007: Kine- current research. Weather, 53, 66–72, doi:10.1002/ matic observations of misocyclones along boundaries j.1477-8696.1998.tb03962.x. during IHOP. Mon. Wea. Rev., 135, 1749–1768, doi:10.1175/ Storer, R. L., S. C. van den Heever, and G. L. Stephens, 2010: MWR3367.1. Modeling aerosol impacts on convective storms in different

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC 348 WEATHER AND FORECASTING VOLUME 30

environments. J. Atmos. Sci., 67, 3904–3915, doi:10.1175/ ——, J. M. Straka, and E. N. Rasmussen, 2008: Polarimetric radar 2010JAS3363.1. observations at low levels during tornado life cycles in a small Straka, J. M., D. S. Zrnic, and A. V. Ryzhkov, 2000: Bulk sample of classic southern plains supercells. J. Appl. Meteor. hydrometeor classification and quantification using po- Climatol., 47, 1232–1247, doi:10.1175/2007JAMC1714.1. larimetric radar data: Synthesis of relations. J. Appl. Me- Verlinde, J., 1997: Airborne Doppler radar analysis of a TOGA teor., 39, 1341–1372, doi:10.1175/1520-0450(2000)039,1341: COARE waterspout storm. Mon. Wea. Rev., 125, 3008–3017, BHCAQU.2.0.CO;2. doi:10.1175/1520-0493(1997)125,3008:ADRAOA.2.0.CO;2. Stumpf, G. J., A. Witt, E. D. Mitchell, P. L. Spencer, J. T. Johnson, Wakimoto, R. M., and J. W. Wilson, 1989: Non-supercell M. D. Eilts, K. W. Thomas, and D. W. Burgess, 1998: The tornadoes. Mon. Wea. Rev., 117, 1113–1140, doi:10.1175/ National Severe Storms Laboratory mesocyclone detection 1520-0493(1989)117,1113:NST.2.0.CO;2. algorithm for the WSR-88D. Wea. Forecasting, 13, 304–326, ——, and J. K. Lew, 1993: Observations of a Florida waterspout doi:10.1175/1520-0434(1998)013,0304:TNSSLM.2.0.CO;2. during CaPE. Wea. Forecasting, 8, 412–423, doi:10.1175/ Szilagyi, W., 2004: The great waterspout outbreak of 2003. Mar. 1520-0434(1993)008,0412:OOAFWD.2.0.CO;2. Wea. Log, 48 (3). [Available online at http://www.vos.noaa. Wilson,J.W.,G.B.Foote,N.A.Crook,J.C.Fankhauser,C.G. gov/MWL/dec_04/waterspout.shtml.] Wade, J. D. Tuttle, C. K. Mueller, and S. K. Krueger, 1992: Van Den Broeke, M. S., 2014: Effects of mid- and upper-level The role of boundary-layer convergence zones and hori- drying on microphysics of simulated supercell storms. Elec- zontal rolls in the initiation of thunderstorms: A case tron. J. Severe Storms Meteor., 9 (3). [Available online at http:// study. Mon. Wea. Rev., 120, 1785–1815, doi:10.1175/ www.ejssm.org/ojs/index.php/ejssm/issue/view/53.] 1520-0493(1992)120,1785:TROBLC.2.0.CO;2.

Unauthenticated | Downloaded 10/07/21 09:27 AM UTC