Formation and evolution of the nocturnal LLJ and surface-layer vertical mixing in the SBL during CASES-99

With the cessation of surface heating and convective mixing in the surface layer each evening during CASES-99, a low-level jet (LLJ) developed with speeds peaking from 6 to 20 m/s, generally between 50 and 150 m above ground level (AGL). In the shear zone above and especially below the jet max, flow structure varied from relatively laminar to wavy to fully turbulent. The structure and evolution of the LLJ as well as the flow above and below the max was revealed especially well by NOAA/ETL's High-Resolution Doppler Lidar (HRDL). In this study we trace the structure and strength of the LLJs in time on several nights and relate this evolution to the nature of the flow in the shear zones above and below. The behavior of the flow in these vertical layers is related in a complex manner to the flow and turbulent fluxes in the surface layer as measured by surface mesonet stations and the 55-m tower, and this study will further explore these complex relationships.