Turbulence and Cloud-Top Entrainment Rate Estimates in Marine Stratocumulus During VOCALS-REx

Observations made as part of the 2008 VOCALS campaign in the tropical southeastern Pacific during 17 days of nearly homogenous marine stratocumulus clouds are used to explore the temporal evolution of turbulence in the cloud layer and cloud-top entrainment rates. We approach studying turbulence throughout the cloud layer via simplified turbulent kinetic energy (TKE) and heat budget equations. 10-minutes estimates of the terms in the TKE equation are made through a combination of high-resolution ship-based observations and simulated radiative fluxes. Cloud-top entrainment rates (we) are estimated with a boundary layer height budget approach that utilizes observations and ECMWF-Interim reanalysis data. Below cloud top, turbulent buoyancy flux divergence is positive and approximately balanced by heat flux resulting from radiative cooling. At cloud top an imbalance occurs that may be due to entrainment of warm, dry inversion air. The diurnal cycle of turbulent dissipation in the cloud layer reveals an increase in turbulence during the night hours. Turbulence decreases during the day and decoupling occurs around 500m below cloud top. In the afternoon, there is a rapid recoupling accompanied by turbulent mixing. The we diurnal cycle parallels that of cloud-top turbulence with a minimum occurring at midday and an early evening maximum that follows the rapid recoupling of the boundary layer. The mean we from the boundary layer height budget during this 17-day period is 5.5 mm s-1. These results increase our understanding of the daily evolution of marine stratocumulus through the correlations between high-resolution estimates of turbulence and entrainment rates.