Remote sounding of the cirrus optical depth and temperature from 3.7 and 11 micrometer windows

Abstract

On the basis of forward IR radiation transfer analyses for an atmosphere containing semi-transparent, non-black cirrus, parameterization equations are derived for the retrieval of the cloud optical depth and cloud temperature utilizing AVHRR 3.7 and 10.8 μm channels. The retrieval techniques developed involve the use of either dual-frequency or dual-scanning angle radiance observations. We show that the cloud optical depth and cloud temperature may be inferred successively from the observed brightness temperature differences using these two techniques: Numerical experiments and error analyses demonstrate that the dual-frequency method is specifically appropriate for optically thin cirrus cases (τ<1). In case the optical depth of cirrus is close to 3, combination of dual-frequency and dual-scanning angle methods is shown to give reasonable accuracy for the cloud optical depth and temperature retrieval.