The influence of cloud optical depth on emitted infrared radiation of a spruce forest for different cloud types

From April to July 1996, ground-based measurements of radiation budget comp onents with half-hourly resolution at the Anchorstation Tharandter Wald (ne ar Dresden/Germany) over a homogeneous spruce forest were used to study the effect of clouds on the emitted infrared radiation. The main task of this study was the determination of empirical relations between the cloud optica l depth and the reduction of the emitted infrared radiation. Thus, the clou d optical depth was calculated through an inverse technique based on a tno- stream approximation for radiative transfer. The reductions in the emitted infrared radiation were estimated by comparing cloudy and clear-sky conditi ons (reference days). Three different cloud/weather classes with adequate conditions could be inv estigated: Stratus, Altostratus and Cumulus. The average decrease in emitte d infrared radiation if clouds occur, measured over a spruce forest, for St ratus was -59 Wm(-2), for Altostratus-39 Wm(-2), and for Cumulus -22 Wm(-2) . The maximum influences, which could be observed, were for Stratus -69 Wm( -2), for Altostratus -64 Wm(-2), and for Cumulus -33 Wm(-2). These empirica l relations for cloud optical depth were applied to Meteosat and NOAA-AVHRR data, where a significant improvement in the satellite analysis could be a chieved.