Radiative influences on ice crystal and droplet growth within mixed-phase stratus clouds

[1] This study uses a box model and a Lagrangian microphysical parcel model to investigate the influences of radiative heating and cooling on the vapor diffusional growth of liquid drops and ice crystals within mixed-phase clouds. Without radiative effects, the combined influences of drop and ice vapor diffusion lead to slight supersaturations with respect to liquid despite the rapid growth of ice. This allows drops grown on aerosol particles with a critical radius of at least 2.5 mu m to continue growing while drops grown on smaller aerosols evaporate (if activation occurs) leading to a broader drop size distribution. Longwave radiation allows drops with radii > 10 mu m to grow simultaneously with the ice with some drops exceeding 25 mu m in radius. Both the box model and the parcel model suggest that longwave cooling reduces, whereas solar heating increases, glaciation time-scales. Although the maximum possible reduction in the glaciation time-scale due to longwave cooling is as much as 45%, the parcel model shows that reductions on the order of 5 to 15% are more likely. Moreover, infrared cooling increases, and solar heating decreases, the temperature at which the Bergeron process is a maximum. Model simulations suggest the conclusion that radiative influences are most substantial for ice concentrations between 5 and 50 l(-1), precisely when the glaciation time-scale is on the order of the in-cloud time-scale.