A CASE-STUDY OF THE GREAT-PLAINS LOW-LEVEL JET USING WIND PROFILER NETWORK DATA AND A HIGH-RESOLUTION MESOSCALE MODEL

A detailed case study of one complete episode of a typical summertime Great Plains low-level jet (LLJ) using data collected by the NOAA wind profiler demonstration network is presented. The high temporal and sp atial resolution of the data from the profiler network permits a much more detailed picture of the Great Plains LLJ than is possible from pr evious studies of this phenomenon. A three-dimensional mesoscale numer ical model is also used to simulate the episode and to provide informa tion on the physical mechanisms responsible for the initiation, evolut ion, maintenance, and decay of the LLJ. The position and width of the jet core, as well as the diurnal variation of wind speed and direction inside the jet core are well predicted by the model. The analysis and modeling suggest that the diurnal oscillation of horizontal pressure gradient over sloping terrain is secondary to the inertial oscillation mechanism resulting from the release of frictional constraint in the evening and throughout the night in driving this example of the summer time Great Plains LLJ. The meridional variation of the Coriolis parame ter as air moves northward appears to enhance the strength of the jet. A larger amplitude of the diurnal oscillation of the jet speed is fou nd to be associated with drier soil, while rising motion downstream of the jet core is Stronger for wetter soil. This enhanced vertical moti on appears to be associated with latent heat release due to precipitat ion. A horizontal variation of soil moisture content also appears to b e important in reproducing the observed convergence and precipitation patterns in this case.