THE ESTIMATION OF THERMAL ATMOSPHERIC RADIATION UNDER CLOUDY CONDITIONS

Among the surface radiation budget components that are needed to under stand the climate of the Earth, the thermal atmospheric radiation comp onent is the only one that is not measured routinely, so it must be es timated indirectly. The presence of clouds substantially hampers this task, supplementing the emission of water vapour and carbon dioxide in the lower atmosphere. The methods proposed to estimate thermal atmosp heric radiation often rely on a cloudless sky model and a corrective t erm that takes into account the cloud contribution. Several models for estimating the thermal atmospheric radiation under cloudy conditions have been tested using measurements recorded in Granada (Spain) during a period of 2 years. The cloudless sky model provides estimation with an error of about 5 per cent, whereas the cloudy skies models, after some modifications, estimate thermal atmospheric radiation with an err or of about 6 per cent. Our results show the convenience of a quadrati c dependence on cloud amount for cloudy skies models. In addition to t his, it seems convenient to use available information about the radiat ive properties of clouds to assign different cloud emittances for diff erent types of clouds within each level, especially for middle level c louds.