Determining entrainment rate and the role of entrainment in stratocumulus clouds
McDowell, David W.
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An important process in predicting the evolution of the boundary layer is the entrainment rate, which has received little verification using observed data. The entrainment rate is therefore computed using aircraft measurements obtained off the coast of California during FIRE in 1987. The entrainment zone is defined and determined to be typically less than 10 meters deep. The structure above the boundary layer is found very complex and consists of a layered structure located in the first few hundred meters above the cloud top. These layers are 20-130 meters deep and posses properties that relate the layers to boundary layer processes. A conceptual model is presented to explain the formation of these layers. The added presence of the layered structure above the cloud top complicates the determination of jump conditions and thus the calculation of entrainment velocity. In addition, jumps in conserved quantities vary considerably between soundings, which questions the validity of using a simple average to calculate the entrainment rate. Therefore, a new method for calculating entrainment velocity is presented, which decreases the variation in jump conditions. This method is physically based and yields an entrainment rate with significantly less uncertainty.
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