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P>P 3.2 OBJECTIVE ENVIRONMENTAL ANALYSES and CONVECTIVE MODES for U.S P 3.2 OBJECTIVE ENVIRONMENTAL ANALYSES AND CONVECTIVE MODES FOR U.S. TROPICAL CYCLONE TORNADOES FROM 2003-2008 Roger Edwards1, Andrew R. Dean, Richard L. Thompson and Bryan T. Smith Storm Prediction Center, Norman, OK 1. INTRODUCTION and BACKGROUND contribution to buoyancy, and render those reduced tornado trends that do exist in that region less Tropical cyclone (TC) tornado prediction remains a dependent on time of day (e.g., Smith 1965). substantial challenge, both on its own and compared to advances made in prognostically relevant b. Meso-β to convective-scale influences understanding of midlatitude supercells in particular. An ingredients-based approach (e.g., Johns and Supercells within TCs tend to be smaller in vertical Doswell 1992), used for supercell tornado and horizontal extent than those of midlatitude environments in midlatitudes, likewise can be applied systems, more similar to the dimensions of to TCs, focusing on moisture, instability, vertical shear midlatitude “mini-supercells” (e.g., Kennedy et al. and lift for that majority of TC tornado situations 1993, Burgess et al. 1995), and that term has resulting from supercells. Being rich in low level occasionally been used in the TC setting (e.g., Suzuki moisture, the main factors influencing the occurrence et al. 2000). This spatial compression appears to be of supercell tornadoes in TCs are the relative related, in part, to the warm-core nature of the TC, its distribution and overlap of shear, instability (as resultant weak thermal lapse rates aloft and the indicated by buoyancy) and various forms of resultant concentration of buoyancy in the lowest few convective bands and boundaries as foci for kilometers AGL. This also corresponds quite well to convection. the layer where vertical shear is maximized in the environment, along with peak perturbation pressure a. Cyclone-scale influences (non-hydrostatic) forcing on the storm scale (McCaul and Weisman 1996). TC tornado occurrence tends to decrease inward toward the TC center from some loosely defined TC tornado environments typically are maximum density located generally outside the radius characterized by small convective inhibition (e.g., of gale winds. Outer-band TC tornadoes often are McCaul 1991), indicating only weak lift is necessary to supercellular (e.g., Spratt et al. 1997)and occur in produce deep convection in favorably buoyant areas. regimes generally rightward or eastward of the This lift is manifested often in the form of spiral center’s track (with some exceptions), characterized convergence bands, and sometimes around in situ by strong lower-tropospheric vertical shear and baroclinic boundaries away from the immediate TC favorable CAPE (McCaul 1991). Though inner-band center that have been associated with marked TC tornadoes have occurred with many TCs, three changes in tornado distribution of TCs (Edwards and important factors contribute to a lower frequency of Pietrycha 1996). Whether embedded in convective occurrence of discrete supercells and TC tornadoes clusters or in bands, the tornado potential in inward toward the eyewall of a mature hurricane, a supercells may increase upon, or soon following, their trend documented in multiple climatologies from Hill et interaction with low level boundaries embedded in the al. (1966) to Edwards (2010), especially near and TC envelope― such as fronts and wind shift lines, before TC landfall. First, although a hurricane’s winds where backed winds and relatively maximized increase with proximity to center, vertical shear tends convective boundary-layer vertical and horizontal to decrease (e.g., McCaul 1991, Molinari and Vollaro vorticity commonly are present. This phenomenon 2008). Second, as illustrated by McCaul (1991), was well-documented with midlatitude supercells in buoyancy tends to diminish markedly from a TC’s VORTEX field observations (e.g., Markowski et al. outer fringes inward toward center. Third, convective 1998, Rasmussen et al. 2000), and has been applied mode nearer to center tends toward the nonsupercell, to various forms of in situ and pre-existing boundaries with more continuous banding structures, greater in the landfalling TC environment (e.g., Rao et al. coverage of relatively amorphous rain shields, and 2005, Edwards and Pietrycha 2006). ultimately (in sufficiently well-developed storms) one or two eyewalls whose annular radii and degrees of Discrete, tornadic supercells also may develop closure may vary with time, and from storm to storm. outside well-defined precipitation bands in the weakly The relatively dense precipitation patterns near capped free environment, whether supported by center, concurrent with the presence of thicker deep- diurnal heating over land or (especially near shore) layer cloud cover associated with the storm’s central the relatively high surface θe characteristic of the dense overcast (CDO), restrict diurnal heating and its maritime tropical air mass at night. Several excellent 1 Corresponding author address: Roger Edwards, Storm Prediction Center, National Weather Center, 120 Boren Blvd #2300, Norman, OK 73072; E-mail: [email protected] examples of discrete supercells, with or without sufficient diabatic surface heating needed to banded or clustered convection nearby, are found in substantially magnify CAPE in a sounding, given many recent studies, e.g., Suzuki et al. (2000), ambient thermal lapse rates that are only slightly Edwards et al. (2000), McCaul et al. (2004), and above moist adiabatic through most of the Schneider and Sharp (2007). troposphere (e.g., the composite sounding profile of McCaul 1991). Meanwhile, drying aloft also has been The placement of discrete supercells, versus implicated in aforementioned cold pool generation in those embedded in convective bands, may affect their outer bands (e.g., Barnes et al. 1983, Powell 1990b). tornado potential via thunderstorm-scale processes. Resulting differential heating between the band axis This was one of the motivators for determination of and any relatively cloud deprived slot outside the convective modes herein. Numerical simulations inward edge of a rainband may generate thermal have indicated weak cold pools with discrete boundaries suitable for supercell maintenance as supercells in the TC environment (McCaul and described above, and may contribute to tornado Weisman 1996), related largely to the occurrence such as has been documented in the characteristically high moisture content of the lower inward side of inner and outer bands (e.g., McCaul troposphere and resultant lack of evaporationally 1987; Rao et al. 2005). aided θe deficit in the near-surface downdraft. McCaul and Weisman proposed this as a possible This study presents some preliminary analytic reason for the relative weakness of TC tornadoes results for the near-convective environments and compared to those arising in baroclinic perturbations storm modes for TC tornado events during 2003- of midlatitudes. Their simulations, however, did not 2008, and as such, may be taken as a Part II for involve environmental inhomogeneities in thermal or Edwards (2008). For clarity, “storm” will refer to those kinematic fields, such as the boundary situations convective elements, on horizontal scales of 100-101 described above, where tornado potential may be km, specifically responsible for tornadoes. This term enhanced. Such inhomogeneities likely are not is used instead of “cell” since (as shown herein) some always resolvable using routinely available tornadic modes are not discretely cellular. “TC” will operational datasets. Despite their apparent be used to refer to the tropical cyclone at large. weakness or absence in small, discrete TC supercells, convectively-generated thermal 2. DATA and METHODS inhomogeneities may be created and reinforced in a collective sense by cold pools of nearly continuous, TC tornado data comes from the 1995-2009 training convection in spiral bands. Barnes et al. “TCTOR” database (Edwards 2010, this volume). (1983) documented 12 K θe deficits in the subcloud Because TCTOR for 2010 is not finalized as of this layer of spiral bands, indicating the existence of such writing, and no TC tornadoes were recorded in the processes. conterminous U.S. during 2009, our analysis period stops at the end of 2008. The three most prolific TC- The plausibility of cooling processes with bands tornado seasons in the entire TCTOR dataset―2004, was reinforced by the buoy data analyses of Cione et 2005 and 2008―fall within the subset analyzed in this al. (2000), who documented 1) increased sea-air study. thermal deficit outside the relatively thermally homogeneous (horizontally, as well as air-sea) inner The number of tornado events examined (664) core region of hurricanes, and 2) thermal deficits with actually is less than that contained in the 2003-2008 passage of strong convective bands (e.g., their Fig. temporal subset of TCTOR (762), because of the 5a). Among counterbalancing factors are restrictions spatial/temporal gridding and filtering technique imposed on supercell development and longevity by detailed in Smith et al. (2010, this volume). For this cell mergers and absorptions into somewhat more purpose, a tornado event (hereafter “tornado”) stable precipitation areas, each of which may be quite constitutes the tornado report that was assigned the common in such close quarters. A related lack of maximum F Scale rating in each 40 km grid square, discrete cells with inward extent toward the eyewall for the analysis hour containing the report (e.g., a has been
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