Interactions among Turbulence, Radiation and Microphysics in Arctic Stratus Clouds

Abstract

The Arctic Stratus Experiment, conducted during June 1980 over the Beaufort Sea, produced an extensive set of simultaneous measurements of boundary layer structure, radiation fluxes, and cloud microphysical properties. In this paper these data are used to determine the interactions between mixing, radiative transfer, and cloud microphysics for four cloud decks. The thermodynamic structure and fluxes of the thermodynamic quantities in the cloudy boundary layer are examined, including liquid water fluxes. Net radiative heating profiles are also determined. A detailed analysis of the fine-scale structure of the cloud microphysics is presented, including correlations between the cloud microphysical parameters (droplet concentration, liquid water content, mean radius, spectral dispersion, and the 95% volume liquid water drop radius), which are used to infer the nature of the mixing processes and the local effects of radiative heating/cooling. A comparison is then made with other observations and existing model conceptions of the cloudy boundary layer and cloud microphysical processes. Due to the large static stability and frequent occurrence of a humidity inversion, these clouds are not maintained by surface fluxes of moisture. The net radiative cooling at the cloud top is balanced differently for each of the cases examined, although in all four cases at least a portion of the radiative cooling was found to promote mixed-layer convection. The effects of turbulent entrainment do not penetrate beyond 50 m below mean cloud top, therefore not directly affecting the evolution of the drop spectra except for right near cloud top. Significant liquid water production due to radiative cooling is indicated by the profiles of buoyancy flux, entropy flux, water fluxes, and vertical velocity variance, and also by the large drop spectral dispersions and the correlations between the cloud microphysical parameters. Liquid water fluxes are determined to be nearly as large as the vapor fluxes. The liquid water flux divergences introduce significant structure into the profiles of liquid water content and drop spectra, and also enhance coalescence processes in the lower portion of the clouds.