A Synoptic Climatology of the Elevated Mixed-Layer Inversion over the Southern Great Plains in Spring. Part I: Structure, Dynamics, and Seasonal Evolution

Abstract

A synoptic climatology of the atmospheric conditions associated with the creation of the elevated mixed-layer inversion, or lid, over the southern Great Plains of the U.S. (defined as Kansas, Oklahoma, Texas, and portions of the surrounding states) during four spring (April, May, June) seasons from 1983 through 1986 is presented. The lid sounding, also known as a type 1 tornado sounding, is created through the superposition of a potentially warm, nearly dry-adiabatic elevated mixed layer (EML) over a moist, potentially unstable layer. This study examines the situations which are favorable and unfavorable for creation of the lid stratification, using EML and lid occurrence statistics and analyses of various parameters associated with the EML and lid. In addition, we define a set of synoptic types that prevail in this region during the three-month period. The synoptic types are categorized by simultaneously examining the surface isobaric patterns and the predominant 500-mb flow direction over the study region, and designating the flow as either ?favorable? or ?unfavorable? for lid formation based on the implied thermal advection in the layer and the number of lid soundings observed over the region. Our analysis reveals that a typical lid covers only about 20% to 25% of the southern Great Plains, and that a lid coverage greater than 50% occurred on fewer than 2% of the study days. High lid-frequency values expand northward during the season, and the maximum-frequency axis shifts westward. We show that this seasonal change is primarily caused by the northward expansion of the EML-source region from Mexico into the central Rockies and Great Basin, and a westward shift in the mean low-level moist axis. The westward shift in the low-level moist axis is related to the westward expansion of the Bermuda anticyclone. We find that the relative airstream configuration associated with the classic models of lid formation and severe weather over this region occurs most frequently in April and May, and corresponds to a flow type associated with southerly low-level flow and southwest flow aloft. As the season progresses, the expansion of both the EML-source region and low-level moist areas allows the lid to be created with a variety of additional flow configurations. In addition to the classic southwesterly midtropospheric flow type, these configurations include northwest and anticyclonic 500-mb flows by May and June. The dominance in late spring of flow types associated with large scale subsidence leads to an airstream configuration in which the inversion base sinks and the lid strengthens downstream from the source region. This is in stark contrast to the classic lid model where the inversion base rises and the lid weakens in an environment of large-scale vertical ascent.