Radiative Effects of Cirrus Clouds

Abstract

The divergence of net radiation in a tropical atmosphere with cirrus clouds has been examined in terms of two bulk radiative properties of the cloud: effective shortwave optical thickness τ* and broad?band infrared emissivity ?&ast. The effective shortwave optical thickness of the cirrus cloud is the primary factor controlling the radiative energy budget at the earth's surface while the cloud's broad?band infrared emissivity is the primary factor influencing the heat budget of the atmosphere. The net radiative energy budget of the tropical atmosphere over an ocean surface in the presence and absence of a cirrus cloud has been examined. The total net radiative energy at the top of the atmosphere is relatively unchanged from the clear sky value by the presence of a cirrus cloud layer while the surface energy budget shows a significant decrease in the shortwave component. The total net radiative energy loss of the atmosphere when a cirrus layer is present is shown to be 22 to 78% of the loss from a cloudless sky. This reduction of divergence when cirrus are present generates in 24 hr an amount of energy which is 32 to 100% of the estimated net latent heat released to the atmosphere by precipitation processes in a cumulonimbus cloud. The divergence of net radiation through three layers encompassing the atmosphere is shown to be linearly dependent on the broad?band infrared emissivity and effective shortwave optical thickness of cirrus clouds.