Influence of Cirrus Clouds on the Infrared Cooling Rate in the Troposphere and Lower Stratosphere

Abstract

The impact of high-level cirrus clouds upon the infrared cooling rate of the troposphere and lower stratosphere is investigated in this study. Band-by-band infrared cooling rate calculations are carried out for all the major gaseous absorption bands including the water vapor continuum. Influence of a low-level cloud on the IR cooling is investigated in the presence of cirrus clouds of varying thickness. Comparisons with IR cooling rates observed within tropical cirrus are also made using recent data from GATE. Scattering and absorption by ice particles as well as gaseous absorption within the cloud is accounted for by using the discrete-ordinate method to approximate the solution of the radiative transfer equation as it is applied to nonisothermal, inhomogeneous cloudy atmospheres. The numerical problems associated with coordinating the monochromatic discrete-ordinate method with the non-monochromatic statistical random model sub-invervals for gaseous absorption are discussed in detail. Five thicknesses of cirrus clouds are employed in both a standard tropical and a mid-latitude winter atmosphere. It is found that cirrus strongly suppress tropospheric cooling and significantly increase cooling in the stratosphere above 20 km. Cooling within the cirrus is found to be strongly dependent on the presence or absence of low-level clouds. Comparison of the theoretically calculated IR cooling rates to the observed values shows generally good agreement.