A Theoretical and Observational Study of the Radiative Properties of Cirrus: Results from FIRE 1986

Abstract

A two-stream radiative transfer model is used to examine the radiative properties of cirrus clouds and compare simulations with the observations made during the cirrus FIRE IFO. The sensitivity of cirrus cloud radiative properties to altitude and size distribution changes are examined. The net radiative effect of cirrus in the infrared is largely determined by the amount of ice in the cloud and the surface?cloud base temperature difference (and thus altitude). Increases (decreases) of this temperature difference produce a net radiative heating (cooling). Cloud-top solar heating increases (decreases) with increasing (decreasing) altitude as the optical path of the atmosphere above the cloud layer decreases (increases). The impact of varying concentrations of ice particles less than 100 ?m in diameter is also examined. The addition of these particles greatly enhances the longwave absorption and shortwave albedo of cirrus clouds in a manner that is spectrally dependent. Model simulations using observed microphysical and environmental conditions are compared to measured cirrus cloud radiative properties. Although cloud inhomogeneties are shown to be quite large, broad agreement in the cloud emittance is found between the highly uncertain observations of FIRE, other aircraft observations, and model simulations. Similar comparisons of the solar albedo reveal cirrus clouds to be significantly brighter than predicted by the model. Possible explanations of this brightening anomaly suggest that it may not be possible to use Mie scattering to model the cloud albedo.