OBJECTIVE ANALYSIS OF ENVIRONMENTAL CONDITIONS ASSOCIATED WITH SEVERE THUNDERSTORMS AND TORNADOES

Abstract

This study describes objective analysis of the atmospheric conditions that precede or accompany severe thunderstorms and tornadoes. The data used are standard rawinsonde observations and hourly surface reports as they are transmitted over teletypewriter. In analyzing upper air data, spherical coordinates are used with grid points 2½° of lat. and long. apart. Hourly observations are analyzed on a 1¼° grid. The vertical structure of the atmosphere is represented by seven layers between the surface and 100 mb. Observational data are averaged for these layers using all points in the soundings. By use of a nondimensional pressure term as the vertical coordinate, the three layers below 500 mb. slope with the terrain, and the lowest layer contains most boundary processes. The objective analysis procedure fits a first degree polynomial to at least five observations that are nearest to a grid point. A distance weighting factor and upstream-downstream enhancement are used. The analysis method smooths the observations lightly, but has a resolution and accuracy that appear approximately equivalent to those of hand methods. Analyzed quantities include wind components, height, temperature, and moisture. From these a number of kinematic quantities not normally available to forecasters are computed and compared with storm developments. In general, certain quantities that depend on the field of motion appear to be more directly related to storm formation than do synoptic or thermodynamic factors. Objective severe storm indicators that combine different synoptic or kinematic factors are formulated at grid points, and their patterns match areas of storm development reasonably well. The results support the belief that the forecaster's accuracy and efficiency can be increased through greater reliance on computer methods of data processing and analysis.