Mixed-Layer Budget Analysis of the Diurnal Cycle of Entrainment in Southeast Pacific Stratocumulus

Mixed-layer budgets of boundary layer mass, moisture, and liquid water static energy are estimated from 6 days of data collected at 20°S, 85°W (a region of persistent stratocumulus) during the East Pacific Investigation of Climate (EPIC) stratocumulus cruise in 2001. These budgets are used to estimate a mean diurnal cycle of entrainment and, by diagnosing the fluxes of humidity and liquid water static energy necessary to maintain a mixed-layer structure, of buoyancy flux. Although the entrainment rates suggested by each of the budgets have significant uncertainty, the various methods are consistent in predicting a 6-day mean entrainment rate of 4 ± 1 mm s−1, with higher values at night and very little entrainment around local noon. The diurnal cycle of buoyancy flux suggests that drizzle, while only a small term in the boundary layer moisture budget, significantly reduces subcloud buoyancy flux and may induce weak decoupling of surface and cloud-layer turbulence during the early morning hours, a structure that is maintained throughout the day by shortwave warming. Finally, the diurnal cycle of entrainment diagnosed from three recently proposed entrainment closures is found to be consistent with the observationally derived values.