Radiative influences on drop and cloud condensation nuclei equilibrium in stratocumulus

Abstract

Radiative heating and cooling occurs throughout most of a stratocumulus cloud layer. Shortwave (SW) heating of individual drops can be strong enough that drop temperatures at equilibrium deviate by as much as 6°C from the surrounding environment. These large temperature differences lead to substantial errors when the classical equation for vapor growth is used. A new form of the vapor growth equation is derived for cases of strong radiative heating. The new equation is used to assess the equilibrium supersaturation (seq) state of drops and cloud condensation nuclei (CCN) within a stratocumulus cloud. Longwave (LW) cloud top cooling has two primary effects on seq. It tends to reduce seq for large drops to values as low as −6%, providing for drop growth at subsaturations. It also tends to reduce the critical supersaturation and size at which larger CCN activate to become growing cloud drops. In contrast, LW heating of cloud base suppresses the growth of cloud drops and CCN. Larger CCN (sizes between 0.5 and 5 μm) lose their Köhler curve maximum, indicating restricted CCN growth. Solar heating produces seq up to 20% for drops with sizes between 500 and 1000 μm, indicating strong evaporation of drizzle drops. Since solar absorption increases more slowly with drop size than LW emission, a minimum appears in the Köhler curves at sizes between 20 and 100 μm, suggesting a preferred size range for vapor growth in SW heated clouds. Like LW heating, SW heating causes suppression of growth for larger CCN.