Thursday, 6 June 2002
Drop Equilibrium in the Presence of Solar Absorption
Discussions of the radiative influences on the growth of drops within stratus clouds have primarily focused on the effects of longwave radiation. The impacts of solar absorption, though potentially important, have been largely ignored. In particular, solar absorption modifies the growth rate of cloud drops by effectively raising the supersaturation needed for drops to grow from the vapor. The magnitude and sign of this modification is dependent upon the strength of the infrared emission and solar absorption and how these quantities vary with drop size. Hence, in this work we examine the strength of the competing influences of infrared and solar radiative fluxes in the vicinity of cloud top. Simple, 1-D stratus clouds are created and used in conjunction with a radiative transfer model that accounts for cloud optical properties. As one might expect, solar absorption reduces the radiative cooling that small drops experience. However, we show that drops with radii as small as 40 microns can experience a net heating effect. This raises the supersaturation needed for a drop to experience a zero net vapor growth rate. In some cases, the supersaturation necessary becomes as larger as a few percent. The potential consequences of this result are also explored.