Low-Level Jet Development during a Numerically Simulated Return Flow Event
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The evolution of the southerly low-level jet (LLJ) during a return flow event is studied using output from the Penn State/NCAR Mesoscale Model (Version 4). Three geographically different southerly LLJs develop in the simulation: one over the southern Plains of the United States, a second southwest of Brownsville, Texas, and a third over the western Gulf of Mexico. The LLJ over the Plains is found to form first as an inertial oscillation and later as a response to lee troughing and an elevated mixed layer that develops over the region. Over Mexico, the temperature structure over the Altiplanicie Mexicana (Mexican High Plain) is responsible for a locally intense low-level pressure gradient east of the High Plain which remains nearly stationary over two diurnal cycles. The LLJ over the western Gulf of Mexico results largely from topographic blocking of the low-level southerly flow by the eastern end of the Neovolcanic Cordillera northwest of Veracruz, Mexico. The evolution of the lower troposphere over the southern Plains resembles the Carlson and Ludlam conceptual model for a severe storm environment, but the structure of the return flow is complex. When midlevel westerlies are weak, mesoscale and boundary layer processes govern the development of LLJs. As the west and southwesterly winds increase with an approaching upper-level disturbance, synoptic influences overwhelm the mesoscale processes leading to a single, larger-scale LLJ.