Wednesday, 12 February 2003: 3:45 PM
Low level jets over the mid-Atlantic region during NE-OPS 1999–2002
A number of nocturnal low level jets (LLJ) were documented during the NE-OPS summer intensives of 1999 and 2002. These low-level wind maxima are most pronounced when embedded in southerly, mountain-parallel, synoptic flow, and enhanced by differential heating/cooling under clear sky conditions from the coastal plain to the Piedmont. LLJs are a mesoscale phenomenon with horizontal dimensions on the order of a few hundred kilometers, residing at the top of the nocturnal boundary layer (NBL) typically between 400-800 meters AGL, and exhibiting wind speeds in excess of 15 m/s – nearly twice the geostrophic value. In summers 1999 and 2002, several LLJs were observed by the profiler/RASS at the Philadelphia NE-OPS site, along with simultaneous aloft measurements of water vapor mixing ratio, ozone concentration, temperature, and optical extinction using the Raman lidar, and measurements of meteorological variables, ozone concentration, and PM2.5 using tethered balloons. These observations document the influence that LLJs have on modifying the properties of the residual boundary layer as the inertial oscillation of the ageostrophic wind ensues. As the LLJ rotates from southerly to south-westerly or westerly, incursions of drier air, often with elevated ozone concentrations representative of western source regions, are transported hundreds of kilometers. Periodically, excessive shear below the level of maximum wind speed will induce downward momentum transport resulting in a bursting event at the surface and short-period increases in ozone and PM concentrations. As operational numerical models incorporate more sophisticated boundary layer parameterizations and higher vertical resolution, the general features of LLJs are being simulated. Observations and measurements obtained during NE-OPS LLJ episodes will be compared to Eta-model forecasts. In 2002 especially, Eta forecasts of nocturnal LLJs agreed remarkably with observations and were used with confidence as guidance for these events.