Low level jet development during a numerically simulated return flow event

Abstract

The evolution of the southerly low level jet during a return flow event is studied using output from the Penn State/NCAR Mesoscale Model (Version 4). Three geographically different southerly low level jets (LLJ's) develop in the simulation: one over the southern Plains of the United States, a second near the Mexican Plateau southwest of Brownsville, Texas, and a third over the western Gulf of Mexico. The evolution of the lower troposphere over the southern Plains is much like the conceptual model for a severe storm environment produced by Carlson and Ludlam (1968). The LLJ over the Plains is found to form first as an inertial oscillation, and later as a response to lee troughing and to the shape of an elevated mixed layer that forms over the region. Over Mexico, the temperature structure over the Plateau is responsible for a pressure gradient that supports a LLJ east of the Plateau. The LLJ encompasses a smaller area than the LLJ over the southern Plains, and 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. An eastward protrusion in the model topography north of Veracruz, Mexico forces the low level flow to accelerate downstream of the obstacle. This simulation shows that the structure of the lower tropospheric air flow during a period of return flow is complex. When mid-level westerlies are weak, mesoscale processes govern the development of low level jets. As the westerly winds increase in response to an approaching upper level disturbance, synoptic influences overwhelm the mesoscale processes leading to a single larger scale low level jet.