2406 VOLUME126MONTHLY WEATHER REVIEWq 1998 American Meteorological SocietyVariations in Supercell Morphology. Part I: Observations of the Role of Upper-LevelStorm-Relative FlowERIKN. RASMUSSENCooperative Institute for Mesoscale Meteorological Studies, National Severe Storms Laboratory, NOAA, andUniversity of Oklahoma, Norman, OklahomaJERRYM. STRAKASchool of Meteorology, University of Oklahoma, Norman, Oklahoma(Manuscript received 3 September 1996, in final form 21 November 1997)ABSTRACTIt is hypothesized that the precipitation intensity beneath a supercell updraft is strongly influenced by theamount of hydrometeors that are reingested into the updraft after being transported away in the divergent upper-level flow of the anvil. This paper presents the results of a climatological analysis of soundings associated withthree types of isolated supercells having distinctive precipitation distributions, the so-called classic, low-pre-cipitation (LP), and high-precipitation (HP) storms. It is shown that storm-relative flow at 9–10 km above theground is strongest in the environments of LP storms, and relatively weak in the environments of HP storms,with classic storms occurring in environments with intermediate magnitudes of upper storm-relative flow. It isplausible that comparatively strong flow in the anvil-bearing levels of LP storms transports hydrometeors farenough from the updraft that they are relatively unlikely to be reingested into the updraft, leading to greatlydiminished precipitation formation in the updraft itself. Conversely, the weak upper flow near HP storms ap-parently allows a relatively large number of hydrometeors to return to the updraft, leading to the generation ofrelatively large amounts of precipitation in the updraft. It also is apparent that thermodynamic factors such asconvective available potential energy, low-level mixing ratio, and mean relative humidity are of lesser importancein determining storm type from a climatological perspective, although important variations in humidity may notbe well sampled in this study. This climatological analysis does not directly evaluate the stated hypothesis;however, the findings do indicate that further modeling and microphysical observations are warranted.1. Introductiona. Supercell definitionMost recent definitions of supercells incorporate therequirement for a deep, persistent mesocyclone, and alarge degree of correlation between the mesocycloneand an updraft (e.g., Klemp 1987; Doswell and Burgess1993; Brooks et al. 1994b), and hence some common-ality in the dynamics of these storms is likely (e.g.,Rotunno and Klemp 1982, 1985; Weisman and Klemp1982, 1984). From an operational viewpoint, it is notpossible to directly determine the degree of correlationbetween vertical velocity and vorticity because real-timeinformation on vertical velocity in storms is not avail-able using present technology, so those storms contain-Corresponding author address: Erik Rasmussen, NSSL/NOAA3450 Mitchell Lane, Bldg. 3, Room 2034, Boulder, CO 80301.E-mail: [email protected] deep, persistent mesocyclones1must be assumed tobe supercells (Moller et al. 1994).Lost in this definition is the notion that precipitationdistribution is not just a manifestation of the uniqueairflow in supercells, but that it may play a key role ingeneration of the mesocyclones and tornadoes. Indeed,current definitions of what constitutes a supercell do notincorporate the nature of the storm precipitation at all.Recent numerical simulation work has just begun toilluminate the importance of precipitation physics, es-pecially evaporation and the strength and location of thecold pool, in supercell dynamics. For example, Brookset al. (1994b) have found evidence that the mesocycloneand storm-relative flow can alter the location and in-tensity of the evaporatively driven cold pool and thenature of low-level mesocyclone genesis. Although the1It seems the presence of a deep, persistent mesoanticycloneshouldalso be included in the operational criteria for supercell identificationbecause simulated left-moving members of storm split pairs in straightshear have been shown to be mirror-image supercells (Wilhelmsonand Klemp 1978).SEPTEMBER1998 2407RASMUSSEN AND STRAKAFIG. 1. (a) Photograph of an LP supercell and (b) schematic diagramof LP supercells that occurred in the Texas panhandle on 28 May 1994(photo courtesy of W. Faidley, Weatherstock). View to the west.findings presented herein do not agree entirely withthose of Brooks et al. (1994b) as to the cause of vari-ations in the precipitation distribution between the so-called classic (CL) and high-precipitation (HP) super-cells (defined later), the cold pool strength and locationdoes seem to be of fundamental importance. Furtherevidence for the dependence of supercell dynamics andtornadogenesis on cold pool characteristics can be foundin Davies-Jones and Brooks (1993). It is plausible thatstorm propagation, low-level mesocyclone formationand demise, and even midlevel mesocyclone rotationare all dependent to some degree on the location andstrength of low-level horizontal gradients of buoyancy,and these in turn are strongly dependent on the precip-itation distribution in the storm.The frequently observed patterns of the spatial dis-tribution of precipitation, and the inferred motion fields,were fundamental in some of the earliest studies of su-percells (Browning and Donaldson 1963; Browning1964). Eventually, some of the reflectivity patterns doc-umented in these studies became the basis for opera-tional detection of supercell storms (Lemon 1982). Thedirectly observed rotating wind field increased in im-portance as a result of the use of Doppler radar in re-search, as well as numerical simulations. However, theprecipitation processes and structure have been largelyignored as a research topic until quite recently. In lightof the known propensity for CL supercells to producetornadoes, the tendency of HP supercells to producesevere winds and large hail (Moller et al. 1994), andthe relative rarity of tornadoes with low-precipitation(LP) storms, it is appropriate to re-examineprecipitationphysics and the associated effects on storm dynamics.It cannot be assumed that all storms having a deep,persistent mesocyclone have the same propagation char-acteristics, potential for severity, etc.b. The supercell spectrumSince the first supercell thunderstorm descriptions(e.g., Browning 1964), at least three types of supercellsincluding LP