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A WSR-88D Assessment of Tropical Cyclone Outer Rainband Tornadoes

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A WSR-88D Assessment of Tropical Cyclone Outer Rainband Tornadoes SEPTEMBER 1997 SPRATT ET AL. 479 A WSR-88D Assessment of Tropical Cyclone Outer Rainband Tornadoes SCOTT M. SPRATT AND DAVID W. S HARP NEXRAD Weather Service Of®ce, National Weather Service, Melbourne, Florida PAT WELSH AND AL SANDRIK NEXRAD Weather Service Of®ce, National Weather Service, Jacksonville, Florida FRANK ALSHEIMER AND CHARLIE PAXTON NEXRAD Weather Service Of®ce, National Weather Service, Tampa Bay, Florida (Manuscript received 5 January 1996, in ®nal form 15 April 1997) ABSTRACT As part of the National Weather Service (NWS) Modernization and Restructuring Program, WSR-88D (NE- XRAD) Doppler radar installation has been completed at each Weather Service Of®ce in Florida. Recently, this powerful new tool provided unique opportunities for Jacksonville, Tampa Bay, and Melbourne NEXRAD Weather Service Of®ce personnel to investigate tropical cyclone (TC) rainbands for evidence of tornadogenesis. This study provides a radar-based analysis of known tornadic mesocyclones associated with two mature tropical cyclones that were not landfalling in the vicinity of the tornado occurrence, namely, Tropical Storm Gordon (1994) and Hurricane Allison (1995). Based on successful NEXRAD sampling strategies, detailed analyses of storm-scale re¯ectivity and velocity signatures are conducted in the context of establishing preliminary critical criteria for use in the tornado detection and warning process. Important characteristics were found to include detection of discrete, small diameter .50 dBZ echos collocated with storm-relative rotational velocities of 6.5± 15ms21. Rotational features, although often subtle, were identi®able for an average of 30 min prior to tornado production, with total durations of 1±2 h. Near the time of tornado touchdown, the core diameter of the low- level circulation couplets contracted to approximately 1.85 km (1 n mi), leading to an associated increase of shear across the circulation to 0.010 s21 or greater. A comparison between the well-studied Great Plains tornadic supercell and the observed TC-tornado cells revealed a common trait of persistence. While the average depth of rotation associated with the TC-tornado cells (3.5 km) was much more shallow than their midwest counterparts, the ratio of depth of rotation to storm top were comparable. However, the shallow depth and weaker detectable rotation of the TC (tornadic) meso- cyclones greatly reduced the detection capability of the current WSR-88D mesocyclone algorithm when compared to identi®cation of traditional supercells. Based upon the analyzed data, the authors offer several recommendations to assist operational radar mete- orologists with the challenging task of detecting outer rainband tornadoes. Additionally, the authors propose a new WSR-88D scan strategy (volume coverage pattern, VCP) that would provide additional low-level slices in lieu of several current upper-elevation angles. This new VCP would facilitate improved vertical sampling at lower heights where TC mesoscale circulations are most likely to be detected. 1. Introduction centage, tropical cyclones landfalling from the Gulf of Mexico produce more tornadoes than Atlantic storms. In the United States, tornadoes account for as much This is also true for Florida events, where Hagemeyer as 10% of the total deaths associated with tropical cy- and Hodanish (1995) found that 87% of tornado-pro- clones (Novlan and Gray 1974). Most tropical cyclone ducing tropical cyclones have historically approached (TC) tornadoes occur within 24 h of landfall and are from the Gulf of Mexico, placing the state in the pre- favored within a sector 08±1208 in azimuth, relative to ferred sector. true north from the cyclone center (Hill et al. 1966; Gentry (1983) and Weiss (1987) found a bimodal Novlan and Gray 1974; Gentry 1983). By a large per- distribution of tropical cyclone tornadoes with respect to range from the cyclone center. ``Core'' tornadoes occur within inner rainbands or in the eyewall, while the optimum range for ``outer rainband'' tornadoes is Corresponding author address: Scott M. Spratt, National Weather Service, 421 Croton Road, Melbourne, FL 32935. 200±400 km from the center, but they can occur at even E-mail: [email protected] greater distances. Weiss (1987) found that more than q1997 American Meteorological Society Unauthenticated | Downloaded 09/24/21 06:58 AM UTC 480 WEATHER AND FORECASTING VOLUME 12 74% of postlandfall tornado occurrences reported from largest contributing factor to the genesis of tornadic 1964 to 1983 were associated with outer band con- cells within tropical cyclones and that buoyant insta- vection, while in a study of Florida tornadoes asso- bility played only a minor role. In model simulations ciated with tropical cyclones between 1882 and 1994, of TC-spawned supercells, Weisman and McCaul Hagemeyer and Hodanish (1995) stated that 25 of 61 (1995) found that the upward dynamic pressure gra- cases involved tornadoes that occurred on the ``outer dient force contributed about three times as much to fringes of systems.'' It is at these greater distances from the updraft speed as buoyancy. Collectively, these the TC center, where the severe weather awareness may sources have served to raise the level of understanding not be high, that tornadoes could be the primary threat and anticipation of the TC-tornado threat through cli- (Weiss 1987; McCaul 1991). matological patterns, statistics, and buoyancy/shear as- On several occasions airborne Doppler radars aboard sessments. National Oceanic and Atmospheric Administration re- Even with an increased recognition of favorable syn- search aircraft have detected small-scale vortices (also optic-scale TC-tornado environments, many of these referred to as mesovorticies or mesocyclones) within tornadoes still occur without of®cial National Weather convective eyewall bands of intensifying strong hur- Service (NWS) warnings or with little (or negative) ricanes (Marks and Houze 1984; Black and Marks lead time. To increase warning skill, an improved un- 1991; Willoughby and Black 1996). Stewart and Lyons derstanding of storm-scale characteristics and radar in- (1996) more recently observed similar features during terpretation utilizing WSR-88 Doppler technology is a WSR-88D interrogation of Tropical Storm Ed's 1993 essential. This can be achieved by further de®ning the landfall on Guam. While these inner rainband vorticies structure and evolution of TC±tornadoes through mod- can be signi®cant to the local warning meteorologist el simulation efforts (e.g., McCaul 1990; Weisman and (Fujita 1993; Wakimoto and Black 1994), this paper McCaul 1995) and through case study analyses (e.g., will strictly focus upon outer rainband mesocyclones McCaul 1987; Snell and McCaul 1993; Zubrick and and associated tornadoes, as observed during TC's Belville 1993; Cammarata et al. 1996). Speci®c to out- Gordon and Allison. er band tornadogenesis, McCaul (1987), McCaul et al. The occurrence of multiple tornadoes, or tornado (1993), and Snell and McCaul (1993) have made sig- outbreaks, associated with tropical cyclones has been ni®cant contributions via the analysis of data from sev- a problem facing operational meteorologists for some eral 10-cm/WSR-57 radars and a Doppler-enhanced time. Multiple tornadoes are often observed to be as- 5-cm/WSR-74C radar during the postlandfall tornado sociated with the strongest convective cells within the outbreaks associated with Hurricane's Danny (1985) outer rainbands (Hill et al. 1966; Fujita et al. 1972), and Andrew (1992). The present paper will build upon where convergence and vorticity gradients are the the foundation established by the previously sited au- greatest. Several studies (Novlan and Gray 1974; Gen- thors. try 1983; Weiss 1985) have described characteristics Radar signatures of multiple tornadic mesocyclones of the tropical cyclone environment that have occa- associated with banded convection of Tropical Storm sionally become conducive for the occurrence of mul- Gordon (1994) and Hurricane Allison (1995) will be tiple tornado events. McCaul (1991) analyzed 1296 investigated using products from three Florida 10-cm/ rawinsonde observations (raobs) that were within 3 h WSR-88Ds. During each event, outer band convection and 200 km of an observed TC-tornado and computed resulted in an ``outbreak'' of tornadoes as de®ned by the spatial distribution of potential buoyant energy (or Hagemeyer and Hodanish (1995), within a highly convective available potential energy, CAPE) and 0± sheared and weakly buoyant environment. The major- 3-km storm-relative helicity (SRH), which provides an ity of these tornadoes occurred very near the coast and estimate of a cell's potential of developing a rotating were weak (F0±F1) with short tracks; however, a brief updraft. Results showed the SRH was maximized to F2 tornado did produce numerous casualties and con- the right of the cyclone with increasing CAPE at great- siderable destruction. er distances. Restricting raobs to those within 2 h and Section 2 will provide a background of some sam- 40 km of hurricane-spawned tornadoes and made on pling considerations of the WSR-88D related to the the outer side of any rainbands in which the tornadoes observation of tropical, low-topped rotational cells. were embedded, McCaul produced a composite of 10 Sections 3 through 6 will present short synoptic over- soundings with a buoyancy (CAPE) of only 253 J kg21 views and storm-scale discussions of each tornado and an approximate SRH of 300 m2 s22. In a compar- event by examining the temporal
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