The Millersville University Weather Researching and Forecasting Modeling System (WRF) is being used to simulate the summertime Mid-Atlantic low-level jet (MALLJ). The MALLJ forms as a result of differential heating and cooling, which is strengthened by the sloping terrain between the Appalachian highlands and the coastal region, followed by the decoupling of the friction layer around sunset. The rapid cessation of turbulent mixing and the reduced frictional influence cause the winds to accelerate in an attempt to adjust to the new force balance. The Coriolis force acting on the wind results in an inertial oscillation of the wind field, which leads to the formation of the LLJ about ¼ pendulum day (1/4 inertial period) after sunset, around midnight in the mid-latitudes. LLJs are known to be significant conveyors of momentum, heat, and moisture, as well as pollutants.

While the Great Plains LLJ (GPLLJ) has been studied extensively, the behavior and evolution of the Mid-Atlantic (MA) LLJ has not received as much attention. Even though the observed speeds of the MALLJ are about half that of the GPLLJ, its ability to transport pollutants along the northeast (I-95) corridor has the potential to have a deleterious influence on the health of this highly populated region. MALLJs are a recurring feature of nearly every major pollution episode in the northeast U.S. This research focuses on characterizing the behavior and evolution of the MALLJ, with an emphasis on the timing, spatial extent, and meteorological conditions attending its formation. The WRF is initialized using NAM-meso-eta fields for a two day period from 1-2 July 2002. The model results will be compared to observations obtained during a field study near Philadelphia, PA (Processes Controlling Urban Air Pollution – PCUAP 2002) for which 915 MHz wind profiler, Raman Lidar, measurements using a tethered balloon, and a suite of surface observations are available.