Microphysical and Turbulent Structure of Nocturnal Stratocumulus as Observed during ASTEX

Abstract

Measurements of the microphysical and turbulence structure of stratocumulus made during the Atlantic Stratocumulus Transition Experiment are presented. The measurements made from a C-130 aircraft, belonging to the Meteorological Research Flight, on the night of 12?13 June 1992 show that the convection in the boundary layer is driven both by longwave radiative cooling at cloud top and by the surface buoyancy flux. The turbulence kinetic energy budget, velocity and temperature variance, and vertical fluxes are calculated to discover how the turbulence structure varies with height. The vertical velocity variance profile is found to resemble that of a clear convective boundary layer. The entrainment velocity and entrainment fluxes are estimated. The results show that the entrainment is very efficient in the case studied. As a result, the buoyancy production of turbulent kinetic energy in the cloud layer is considerably reduced. Horizontally averaged droplet spectra are calculated to study the relative contribution of small and large droplets to the droplet concentration, liquid water content, and drizzle rate. The observations show that the water vapor flux, liquid water flux, and drizzle rate are all of the same magnitude and, therefore, are important in the moisture budget.