Entrainment Rate, Cloud Fraction, and Liquid Water Path of PBL Stratocumulus Clouds

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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. ... 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. ...

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