Characteristics of the quiescent dryline from a case study perspective



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The dryline is a low-level atmospheric boundary that is characterized by a strong horizontal moisture gradient across a narrow zone. Numerous previous studies have already established the dryline’s role of producing lift and convergence necessary for the development of thunderstorms. One of the many forecasting challenges facing the operational meteorologist is determining the evolution of the high plains dryline.

This dissertation examines a type of dryline called the "quiescent" dryline. The quiescent dryline exists in a synoptic environment characterized by relatively weak environmental wind. This dryline is observed to advance eastward during the day and retreat westward at night, often repeating this cycle for several days. The exact nature of the motion and evolution of the quiescent dryline is dependent on many factors that occur at different scales in the atmosphere-land/soil system.

In order to perform the study, a field experiment was designed to investigate the mesoscale (~10 to 100 km) structure and evolution of the quiescent drylines. The West Texas Mesonet, a network of instrumented towers centered on Lubbock, Texas was utilized along with several additional towers deployed for the experiment. Data was collected and analyzed for several drylines that occurred across the South Plains of West Texas during the spring of 2002. A framework was created to identify and characterize advancing and retreating drylines through the objective analysis of several meteorological quantities. The results showed that the dryline could be identified through strong gradients of moisture, along with moisture advection and convergence, in agreement with previous studies. Relatively high values of moisture advection and convergence were observed in conjunction with retreating drylines. Mobile instrumented platforms were also utilized to examine a quasi-stationary dryline, obtaining detailed measurements of moisture gradients in the vicinity of the dryline at scales less than one kilometer. Additionally, the drylines were simulated by two common meteorological mesoscale models and their forecast skill was evaluated. The results show that simulations with higher horizontal resolutions are not necessarily more accurate in forecasting the dryline evolution.