The initiation and maintenance of convection along synoptically-quiescent drylines

Deep moist convection occurs frequently along drylines in the central and southern Plains of the United States, yet the forecasting of where and when that convection will form, and if it will sustain itself, is still not well understood. The dryline features highly variable vertical velocity along its length, presenting a particular challenge for forecasters attempting to pinpoint locally the threat of hazards related to deep convection. Schaefer (1986) found that 70% of the dryline days in April, May, and June were convectively active. Convection associated with the dryline accounts for a large amount of the total rainfall over the area in the spring and summer. Therefore, QPF skill is paramount to regional interests, such as agriculture. Dryline convection is also typically associated with severe weather. Consequently, the forecasting of convection initiation (CI) is of particular importance to public safety. This study seeks to identify mesoscale forcing factors pertinent to the initiation and maintenance of deep convection near drylines.

Because of the narrow updrafts associated with the dryline, the buoyancy of these updrafts is susceptible to dilution by entrainment. Previous work (Griesinger and Weiss (2006)) suggests that there is a positive correlation between CI and the 850 – 500 hPa lapse rate as well as a negative correlation between CI and the 850 – 700 hPa lapse rate. Suggested reasons for the correlation focus on entrainment in these layers.

The large variation in vegetation and soil moisture across the Texas Panhandle can create gradients in sensible and latent heat flux. These gradients have been shown to produce mesoscale circulations that possibly contribute to CI. For example in a study by Hane et al. (1997), the gradient in sensible heat flux across the Texas Panhandle contributed to a mesoscale circulation that later produced a cloud line due to increased boundary-layer convergence and moisture. We seek to expand the current understanding of how thermally-direct land-use circulations affect dryline structure, propagation, and CI.

This study uses idealized simulations from the Advanced Research Weather Research and Forecasting (ARW) Model to identify mesoscale influences on the dryline. A control simulation was constructed on a 900 x 450 km domain using a hyperbolic terrain profile similar to that over the region surrounding the Caprock Escarpment in West Texas (Peckham and Wicker (2000)). Sensitivity studies were performed to assess the impact of vertical variations in temperature, moisture, and wind on the structure and propagation of dryline circulations. The results of these simulations and others relating the effects of horizontal heterogeneities in soil moisture and vegetation will be presented.