RADIATIVE EFFECT OF CIRRUS CLOUDS IN METEOSAT 2ND-GENERATION SPINNINGENHANCED VISIBLE AND INFRARED IMAGER CHANNELS

Radiances at the top of the atmosphere are calculated for three channe ls at lambda=0.635, 3.8 and 8.7 mu m of the Spinning Enhanced Visible and Infrared Imager which is to be flown as part of the Meteosat Secon d-Generation Programme. Special emphasis is put on the influence of na tural cirrus clouds. Their optical properties are determined by means of a geometric ray-tracing code assuming hexagonal ice crystals. Radia tive transfer is modeled by a successive order of scattering program i n the visible spectral range and a combination of the radiance samplin g method and the discrete ordinate method in the infrared. It is found that significant errors are introduced if spherical particles are ass umed and if the small particle fraction of the size spectrum is neglec ted. 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 r atio and cloud homogeneity. Further work is recommended to study the p ossibility of using spectrally averaged radiances for deriving cirrus cloud properties before designing any detailed retrieval algorithm.