A Real-Data Numerical Study of the Development of Precipitation along the Edge of an Elevated Mixed Layer

Abstract

An 18-h numerical simulation of the weather associated with the severe-storm outbreak in the region of the Texas-Oklahoma panhandles, during the AVE-SESAME IV study period (9?10 May 1979), was performed using the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model. This simulation and the related sensitivity tests provided the four-dimensional data sets that were used to reach a better understanding of the processes that were involved in this case in the development of severe convection along the edge of the elevated mixed layer (EML). The sensitivity studies were performed to isolate the contributions of differential surface forcing, latent heating and the low-level moisture gradient to the development of the underrunning, its intensification, and the heavy rainfall. These studies showed that the differential surface heating at the edge of the EML is the most important single factor responsible for initiating the underrunning, and therefore the precipitation, during this case. Compared to the precipitation amounts produced by the complete model during the 9-h period of heavy precipitation (2100?0600 GMT), only 3% was produced after the elimination of the surface differential heating associated with the cloud-cover and soil moisture-availability gradients. The elimination of the latent-heating feedback in the model atmosphere caused a decrease in the 18-h precipitation amounts of ?50%. Finally, the strong gradient in the low-level mixing ratio along the edge of the EML had a surprisingly important direct dynamic influence on the underrunning, and consequently on the precipitation.