Radiative effect of cirrus clouds in Meteosat Second Generation Spinning Enhanced Visible and Infrared Imager channels

Abstract

Radiances at the top of the atmosphere are calculated for three channels at λ = 0.635, 3.8 and 8.7 μm of the Spinning Enhanced Visible and Infrared Imager which is to be flown as part of the Meteosat Second-Generation Programme. Special emphasis is put on the influence of natural cirrus clouds. Their optical properties are determined by means of a geometric ray-tracing code assuming hexagonal ice crystals. Radiative transfer is modeled by a successive order of scattering program in the visible spectral range and a combination of the radiance sampling method and the discrete ordinate method in the infrared. It is found that significant errors are introduced if spherical particles are assumed and if the small particle fraction of the size spectrum is neglected. A comparison of several realistic particle size distributions of cirrus clouds shows that the radiances at the top of the atmosphere do not differ significantly. Thus the derivation of cirrus microphysics certainly has to meet very demanding requirements on signal-to-noise ratio and cloud homogeneity. Further work is recommended to study the possibility of using spectrally averaged radiances for deriving cirrus cloud properties before designing any detailed retrieval algorithm. Copyright 1998 by the American Geophysical Union.