The Great Plains of the western United States (U.S.) has been identified as a region of regular occurrences of a nocturnal low-level jet (LLJ), arising from early-evening accelerations of the late-afternoon mixed-layer flow. The LLJ is thus one of the “key elements of the warm season regional climate over North America.” This nocturnal LLJ is important as a source for wind energy in the Central U.S. and for its role in transporting moisture from the Gulf of Mexico to produce summertime precipitation in the region. In addition to the importance of the mean LLJ wind for these applications, the LLJ has also been shown to be a control on turbulent fluxes between the surface and the atmosphere. The acceleration of the LLJ produces a shear zone below the jet in which turbulence and turbulent fluxes are generated. These processes are not well represented in NWP and climate models, and thus are a source for error in surface-atmosphere interaction processes, lasting over approximately half of the diurnal cycle each day. In published climatologies of the LLJ in the Great Plains, the frequency of occurrence, height and speed of the jet have been determined using rawinsonde and radar-wind-profiler data, which often exhibit biases in nocturnal observations and were often unavailable at the lowest few hundred meters. In this study we use NOAA's scanning High Resolution Doppler Lidar (HRDL) to investigate LLJ properties over the Great Plains. The high temporal and spatial resolution of HRDL data from the surface up through several hundred meters AGL, permits a much more detailed study of LLJ characteristics than is possible from previous studies of this phenomenon. Analysis of the HRDL streamwise velocity profiles, obtained from the Cooperative Surface-Atmosphere Exchange study in eastern Kansas in October 1999 (CASES-99) and from a measurement campaign in early September 2003 in the southeastern Colorado, provides an opportunity to test the generality of the Great Plains LLJ climatology against High Plains sites. Intercomparison of HRDL data with sonic anemometers and Doppler sodar observations show good agreement in wind speed between all instruments.

The results will show that most of the jets occur below 500 m with prevalent southerly direction in both experiments. Histogram distributions of LLJ most probable heights were below 200 m AGL in both cases, and thus probably were not included in the previous climatologies. Results of recent studies showing relationships between LLJ properties and the generation of turbulent mixing will also be presented.