Nocturnal low-level jet characteristics over Kansas during CASES-99

Characteristics and evolution of the low-level jet (LLJ) over southeastern Kansas were investigated during the 1999 Cooperative Surface-Atmosphere Exchange Study (CASES-99) field campaign with an instrument complement consisting of a high-resolution Doppler lidar (HRDL), a 60 m instrumented tower, and a triangle of Doppler mini-sodar/profiler combinations. Using this collection of instrumentation we determined the speed U-X, height Z(X) and direction D-X of the LLJ. We investigate here the frequency of occurrence, the spatial distribution, and the evolution through the night, of these LLJ characteristics. The jet of interest in this study was that which generates the shear and turbulence below the jet and near the surface. This was represented by the lowest wind maximum. We found that this wind maximum, which was most often between 7 and 10 m s(-)1, was often at or just below 100 m above ground level as measured by HRDL at the CASES central site. Over the 60 km profiler-sodar array, the topography varied by similar to100 m. The wind speed and direction were relatively constant over this distance (with some tendency for stronger winds at the highest site), but Z(X) was more variable. Z(X) was occasionally about equal at all three sites, indicating that the jet was following the terrain, but more often it seemed to be relatively level, i.e., at about the same height above sea level. Z(X) was also more variable than U-X in the behaviour of the LLJ with time through the night, and on some nights $U-X was remarkably steady. Examples of two nights with strong turbulence below jet level were further investigated using the 60 m tower at the main CASES-99 site. Evidence of TKE increasing with height and downward turbulent transport of TKE indicates that turbulence was primarily generated aloft and mixed downward, supporting the upside-down boundary layer notion in the stable boundary layer.