Capabilities of Thermodynamic and Kinematic Severe Weather Parameters

I. Introduction and Purpose of Training

	This web module is a prerequisite to completing Topic 7 of the Distance Learning Operations Course (DLOC). Evaluating upper air sounding parameters is an integral part of the convective forecasting and warning process. Historically since the early 1940s, researchers have utilized radiosonde data to determine the thermodynamic and kinematic structure of the atmosphere in which subsequent tornadoes formed. Fawbush and Miller (1952, 1954) investigated mean sounding characteristics from the immediate vicinity of tornadic thunderstorms and established some forecast rules based on the types of air masses in which these storms formed. One of the main results from Fawbush and Miller's work was that a "typical sounding" of the environmental air in the immediate vicinity of a tornadic thunderstorm was characterized by abundant low-level moisture, dry air aloft, and a strong capping inversion separating the two different layers. Subsequent research utilizing radiosonde data (Beebe 1958), (Wills 1969), (Darkow 1969) , Maddox (1976) noted the absence of a capping inversion near the time and place of tornado occurrence. The reason for the disparity in results was due to differences in criteria used for the selection of tornado proximity soundings in the various studies. We now know that the idealized capping inversion erodes prior to initiation of deep, moist convection. The issues of time and space limitations in sampling the pre-storm environment as well as problems resulting from compositing severe parameter predictors from a collection of various proximity soundings have been widely observed and documented (See Maddox, 1976 and Brooks et al., 1994). Each upper air sounding must be examined in detail individually to get the best prediction data available for a given situation. However, many researchers and operational forecasters have agreed upon certain thermodynamic and kinematic conditions favorable for tornadic thunderstorms. These conditions are in the form of various combinations of environmental shear and buoyancy, which act to produce sufficient low-level vorticity and sufficient low-level convergence and updraft to stretch the vorticity into a tornado. Recent research on the tornadogenesis process has focused on three ingredients, which facilitate these processes in various manners (augmented storm-relative helicity, rear-flank downdraft buoyancy, and persistent updrafts) ( See Rasmussen, 2002 and Markowski et al., 2002). Even though direct measurements of many of these parameters are insufficient given today's operational observation networks, the knowledge of the spectrum of parameters values that have been associated with severe thunderstorms, especially tornadoes, is important in establishing a climatological baseline of sounding parameters and a place to start when analyzing real-time data (Rasmussen and Blanchard 1998). Recognition of a particular range of sounding values in the synoptic and mesoscale analysis process of severe weather helps forecasters determine convective mode and in some cases, tornado potential. It is also important to know differences in computational platforms and terminology used when describing parameters such as CAPE and shear. Depending on which numerical model is used and the sounding program utilized in the calculation of the variables, there can be a variety of results. This can be frustrating to the forecaster who needs to predict the meteorological results from the parameter data and not a particular model derivation or a AWIPS computational effect in the parameter data. The goal of this instructional component is to provide forecasters a measure of knowledge on the computations, strengths, and weaknesses associated with some of the most commonly used severe weather thermodynamic and kinematic parameters in operational forecasting and warning today. The emphasis is on supercell parameters associated with predicting tornadoes. Additional treatment of environmental parameters associated with other severe weather hazards is provided in portions of Topic 7 in DLOC. In addition, a section is provided on guidance in applying some of these parameters. The application of many of these parameters over many years of data is still untested. Thus, the local application process is very important to establishing the ultimate capabilities of these parameters for use in severe weather forecasting and warning.
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