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-AtmosphereExchange Study (CASES–99) field campaign with an instrument complement consisting of ahigh-resolution Doppler lidar (HRDL), a 60 m instrumented tower, and a triangle of Dopplermini-sodar/profiler combinations. Using this collection of instrumentation we determined thespeed UX, height ZX and direction DX of the LLJ. We investigate here the frequencyof occurrence, the spatial distribution, and the evolution through the night, of these LLJcharacteristics. The jet of interest in this study was that which generates the shear and turbulencebelow 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 CASEScentral site. Over the 60 km profiler–sodararray, the topography varied by ∼100 m. The wind speed anddirection were relatively constant over this distance (with some tendency for strongerwinds at the highest site), but ZX was more variable. ZX was occasionally about equal at allthree sites, indicating that the jet was following the terrain, but more often it seemed to berelatively level, i.e., at about the same height above sea level. ZX was also more variable thanUX in the behaviour of the LLJ with time through the night, and on some nights $UX wasremarkably steady. Examples of two nights with strong turbulence below jet level were furtherinvestigated using the 60 m tower at the main CASES–99 site. Evidence of TKE increasing withheight and downward turbulent transport of TKE indicates that turbulence was primarilygenerated aloft and mixed downward, supporting the upside–down boundary layer notion in thestable boundary layer.