A Synoptic Climatology of the Elevated Mixed-Layer Inversion over the Southern Great Plains in Spring. Part III: Relationship to Severe-Storms Climatology

Abstract

This study investigates the relationships between the occurrence of the lid (also known as a type 1 tornado sounding) and the occurrence of severe storms over Kansas, Oklahoma, and Texas during the spring season (defined as the months of April, May, and June). The period of this study covers four seasons from 1983 through 1986. The size distribution of severe storm events is examined in relation to the occurrence/size of the antecedent lid over the study region. Days in which no severe weather was observed over the region (defined as ?non-event? days) are included in order to examine the occurrence/size of the lid on these days as well. The relationships between the occurrence/size of severe-storm outbreaks and the antecedent lid are also examined using conceptual models of the life cycle of lid development and dissipation, for both early and late spring. Composite mean analyses of key meteorological parameters and geographic frequency composites of the elevated mixed layer, buoyant instability (as defined by an unstable value of the Lifted Index or buoyancy term in the Lid Strength Index), and severe-weather events are constructed for different stages of lid development. These composites are then utilized to determine geographic relationships among these parameters. The results show that the size distribution of severe-weather events has a peak at the 1600 km2 (a 40- ? 40- km grid square) category; this peak strengthens during the season (especially from May to June). Also found is a relationship between the occurrence and size of the lid at 1200 UTC and the occurrence and size of subsequent severe-storm events, where this relationship is most well-defined in April and deteriorates rapidly from May to June. We hypothesize that the deterioration of this relationship is due to factors such as the increasing horizontal extent of the low-level moist layer (and buoyant instability) during the spring, and changes in the synoptic-scale circulation from baroclinic waves in the westerlies to subtropical anticyclones and weak cyclonic disturbances. In some late spring synoptic patterns, severe weather is not only associated with the presence of a widespread lid, but also is found to exist in an environment containing a variety of sounding types; these include lid soundings, uncapped soundings (with a zero or negative lid strength term), and even a subsidence-type sounding that is buoyantly unstable. In such environments, features such as the low-level jet and surface troughs may provide a sufficient lifting mechanism to allow the development of deep convection to occur.