Transverse Ageostrophic Circulations Associated with Elevated Mixed Layers

Abstract

An elevated mixed layer is a principal component of the conceptual model recently proposed by Carlson and others to explain the evolution of a severe storm environment over the Southern Plains of the United States during springtime. Elevated mixed layers are most likely to be found downwind of strongly heated arid land areas (often plateaus), which favor the growth of a deep mixing layer with high potential temperature. The lower and lateral boundaries of elevated mixed layers are distinctly defined respectively by a statically stable layer, referred to as a lid inversion, and a midlevel front. These boundaries mark the zonesof transition between the airstream defining the elevated mixed layer and other airstreams of differing geographical origin and, consequently, thermodynamic properties. In this study, the Sawyer-Eliassen secondary circulation equation is used to diagnose the transverse ageostrophic circulations that are associated with the dynamical forcing implied by the above conceptual model of the elevated mixed layer structure. The diagnoses are based upon subjective analyses of the elevated mixed layer identified in the SESAME IV dataset at 2100 GMT 9 May 1979 and upon analytically specified patterns reproducing many of the main features of the subjective analyses. The outcome of the diagnoses indicates that a combination of confluence and anticyclonic shear forces a thermally direct circulation centered in the midlevel front and an indirect cell centered in the upper region of the elevated mixed layer,which results in a zone of rising motion between the cells at the edge of the elevated mixed layer. Additional tests, which compare the ageostrophic circulations derived from analytically specified fields in which the elevated mixed layer structure is defined in detail and in which it is highly smoothed, indicate that the above circulation pattern is enhanced by increased baroclinicity in the midlevel frontal zone and the diminished static stability in the elevated mixed layer.