Elevated Mixed Layers in the Regional Severe Storm Environment: Conceptual Model and Case Studies

A conceptual model of the evolution of the severe local storm environment is discussed in conjunction with three case studies drawn from data obtained during the 1979 SESAME field program. The conceptual model describes how a particular configuration of topography and air flow can produce a low-level restraining inversion or “lid” which focuses the location and even enhances the intensity of severe local storms. While capping inversions have often been attributed to subsidence, in this model it is demonstrated that the lid frequently originates from differential advection of a hot, dry mixed layer from an elevated plateau over a cooler, moister layer advected northward ahead of a trough in the westerlies. For either the elevated mixed layer or subsidence inversion, this type of vertical stratification suppresses release of convective instability while, nevertheless, allowing the latent instability of the boundary layer to increase with time. However, for elevated mixed layer lids, the differing source regions of the elevated mixed layer and a cooler airstream to its north and west result in sharp horizontal gradients of temperature and static stability in a zone of confluence at the lateral boundary of the lid. Intense convection may occur along the lid edge partly due to differential advection in which moist boundary layer air flows out from beneath the lid (a process called underrunning), but also as a result of lifting on the cold side of the lid edge zone. Each of the three cases studied (10–11 April, 9–10 May and 25–26 April, all 1979) generally conformed to the conceptual model. In the first two cases the lid formed by advection of a deep mixed layer from Mexico over moist air from the Gulf of Mexico. A crucial factor in the underrunning was the establishment of a transverse ageostrophic circulation across the lateral boundary of the lid. This circulation was a response to the migration of a mesoscale jet streak which aided the process of the destabilization by inducing the low-level air to flow into a region of low static stability outside the lid and by providing the necessary ascent there to release the instability. In a third case, that of 25–26 April, the convective outbreak was confined to a narrow region along the lateral boundary of the lid. Although there was evidence of a transverse circulation across the lid boundary with ascent occurring outside the lid, the ageostrophic component of the low-level wind was relatively weak in this case compared to the strong geostrophic flow across the lid edge in the first two cases. The 25–26 April case also deviated from the other two in that the source region of the elevated mixed layer was the arid southwestern United States rather than Mexico.