CONSTRAINTS ON DROPLET GROWTH IN RADIATIVELY COOLED STRATOCUMULUS CLOUDS

Radiative cooling near the top of a layer cloud plays a dominant role in droplet condensation growth. The impact of this cooling on the evolution of small droplets and the formation of precipitation-sized drops is calculated using a microphysical model that includes radiatively driven condensation and coalescence. The cloud top radiative environment used for these calculations is determined using a mixed-layer model of a marine stratocumulus cloud with a subsiding, radiatively cooled inversion. Calculations of the radiatively driven equilibrium supersaturation show that net long wave emission by cloud droplets produces supersaturations below 0.04% for typical nocturnal conditions. While supersaturations as low as this will force evaporation for droplets smaller than approximate to 5 mu m, radiatively enhanced growth for larger droplets can reduce the time required to produce precipitation-sized particles by a factor of 2-4, compared with droplets in a quiescent cloud without flux divergence. The impact of this radiative enhancement on the acceleration of coalescence is equivalent to that produced in updrafts of 0.1 - 0.5 m s(-1), and varies linearly with the total emitted flux (the ''radiative exchange'').