Atmospheric Radiation and the Gross Character of Stratiform Cloud

Abstract

An elementary model is devised which suggests that atmospheric radiation can be a dominant factor determining the thickness, water content, and general character of some forms of stratified cloud-stratocumulus in particular. For stratocumulus at the top of the mixing layer the theory predicts an equilibrium situation where the upward flux of water vapor E into the cloud is balanced by turbulent entrainment of drier air from above the inversion. The amount of entrainment is determined by radiative cooling which, in turn, is governed by, and generally increases with, the water content and thickness of the cloud. The equilibrium is generally stable and is such that the deck maintains a constant water content but rises overall at a rate determined by E and the relative dryness of the air above cloud top. Any phenomenon which increases the radiative cooling of the cloud should tend to make it thinner, and an actual observation of decreased shortwave absorption confirms this. Conversely, one might expect Sc over the sea to be thicker during the daytime when shortwave heating is significant or when extensive high cloud reduced the longwave cooling. The model suggests that cloud turrets will penetrate the inversion surmounting stratocumulus when the local mixing ratio difference across the inversion exceeds a value of the order of 8 gm kg?1.