Dryline Dynamics

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9729631 Bannon The dryline is the surface boundary between two distinct air masses that frequently inhabit the south central United States in Spring and early Summer. To the east of the line lies a relatively shallow cool moist maritime air mass and to the west a deeper warm dry continental air mass. This mesoscale phenomenon is significant because it is often linked to the occurrence of severe weather (damaging winds, hail and tornadoes). Thunderstorms tend to develop along and to the east of the dryline. The dryline often migrates several hundred kilometers to the east during the day and then retrogrades westward during the night. This diurnal evolution of the dryline helps make the forecasting of severe weather a major challenge. The purpose of this research is to model the diurnal evolution in structure and position of the dryline. The fundamental approach is to treat the dryline as the western edge of a shallow layer maritime air mass. The dynamics of this air mass will first be studied using a simplified numerical model know as the shallow water model. The sensitivity of the model dryline to mass, momentum, and thermal forcings will be ascertained and compared with existing theories. A slab mixed- layer model of the air mass will then be developed to more accurately represent the major thermodynamic processes. Refinements of the modeling effort include two dimensional studies and an extension to multiple air masses. This theoretical approach of using models of intermediate complexity between analytic theory and sophisticated mesoscale numerical models should be able to capture the essential physics and provide insight into the dynamics of the dryline. The results of the research should provide a better understanding of the dryline and help the forecasting of severe weather. ***