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

The large eddy simulation technique is used to search for key factors in de termining the entrainment rate, cloud fraction, and liquid water path in th e stratocumulus-topped boundary layer (STBL), with the goal of developing s imple 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 potent ial temperature across the entrainment zone, surface heat flux, net radiati ve flux away from the top of the STBL, and liquid water path. This study al so shows that buoyancy reversal, measured here as the ratio between the equ ivalent potential temperature jump and the total moisture jump across the c loud top, plays a major role in reducing the simulated cloud amount, both c loud fraction and liquid water path. For cases where no buoyancy reversal o ccurs, 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 entrainmen t rate of the STBL. The two cases with a larger surface heat Aux studied he re show that the net impact of surface heating on the entrainment rate coul d be negligible if surface heating also leads to enhanced cloud-top evapora tion; enhanced evaporation then results in smaller cloud amount and hence s maller radiative forcing for entrainment. Since larger surface heat flux al ways significantly increases the layer-averaged buoyancy flux and the turbu lence intensity, the entrainment rare of the STBL for a given inversion str ength is therefore not always directly proportional to the layer-averaged b uoyancy flux or to the turbulence intensity.