Entrainment rate, cloud fraction, and liquid water path of PBL stratocumulus clouds

Abstract

The large eddy simulation technique is used to search for key factors in determining the entrainment rate, cloud fraction, and liquid water path in the stratocumulus-topped boundary layer (STBL), with the goal of developing simple schemes of calculating these important quantities in climate models. In this study an entrainment rate formula is proposed where the entrainment rate is determined by four variables - total jump of the liquid water potential temperature across the entrainment zone, surface heat flux, net radiative flux away from the top of the STBL, and liquid water path. This study also shows that buoyancy reversal, measured here as the ratio between the equivalent potential temperature jump and the total moisture jump across the cloud top, plays a major role in reducing the simulated cloud amount, both cloud fraction and liquid water path. For cases where no buoyancy reversal occurs, the simulated cloud fraction remains 100% and the liquid water path depends solely on the cloud height. This study raises an interesting feature about what controls the entrainment rate of the STBL. The two cases with a larger surface heat flux studied here show that the net impact of surface heating on the entrainment rate could be negligible if surface heating also leads to enhanced cloud-top evaporation; enhanced evaporation then results in smaller cloud amount and hence smaller radiative forcing for entrainment. Since larger surface heat flux always significantly increases the layer-averaged buoyancy flux and the turbulence intensity, the entrainment rate of the STBL for a given inversion strength is therefore not always directly proportional to the layer-averaged buoyancy flux or to the turbulence intensity.