Entrainment into Mixed Layers. Part II: A New Closure

Abstract

A new entrainment closure for convectively mixed boundary layers, particularly those topped by stratocumulus clouds, is developed and calibrated against results of large eddy simulations carried out by C.-H. Moeng. The closure differs principally in two ways from those developed by J. W. Deardorff, S. Nicholls, and others. The prediction of interface buoyancy flux from its vertical integral is modified by a height-weighting term. This accounts for the greater entrainment produced by buoyancy flux near the cloud top as compared to that generated by surface heating. The relationship between buoyancy flux at the top of a cloud layer and entrainment is reconsidered. Entrainment increases with the “wetness” of the cloud top, which is evidently a function of cloud-top liquid water content and the cloud-top jumps in potential temperature and total water content. This result is related to findings by S. Nicholls and J. D. Turton, though using a somewhat different approach and with different interpretations and an improved closure. The new closure also leads to a new and more stringent definition of cloud-top entrainment instability.