A Large-Eddy Smulation Study of Observed Stratocumulus Clods Under Weak Inversion during POST

Physics of Stratocumulus Top (POST) is a field program designed to understand the physi-cal processes operating within the temperature inversion at the top of stratocumulus clouds. It was conduced in July-August 2008 off the west coast of California. The inversion that caps the stratocumulus has been frequently observed to be strong with more than 10K across the 100 m thickness. Observations made during POST showed, however, that the temperature inversion may be very weak. For example, the soundings on 8 August 2008 demonstrate that the tempera-ture jump across the inversion is only 3K and the water vapor is only 1 g kg-1. The maximum cloud water mixing ratio reaches to 0.7 g kg-1 with the cloud top at 690m and the cloud base at 200m. It is well known that the strength of the inversion has important impact on the cloud and tur-bulence structure due to its effects on the evaporation and turbulence mixing. This work is fo-cused on the processes in the entrainment zone. We apply COAMPS-LES to a nighttime cloud case observed on 8 August. Sounding taken during the flight is used for initial and large-scale conditions. The LES model is configured to cover a 5„e5„e3km domain with horizontal grid spac-ing being 30m and the vertical stretching grid between 15m to 5m. We use a simple longwave (LW) radiation scheme with the LW downward flux above the inversion as the boundary condi-tion. The model results are compared with the available observations. The analysis is focused on the turbulence mixing generated by the wind shear, the radiative cooling and the evaporation