Modeling atmospheric longwave radiation at the surface during overcast skies: The role of cloud base height

Abstract

The behavior of the atmospheric downward longwave radiation at the surface under overcast conditions is studied. For optically thick clouds, longwave radiation depends greatly on the cloud base height (CBH), besides temperature and water vapor profiles. The CBH determines the cloud emission temperature and the air layers contributing to the longwave radiation that reaches the surface. Overcast situations observed at Girona (NE Iberian Peninsula) were studied by using a radiative transfer model. The data set includes different seasons, and a large range of CBH (0–5000 m). The atmosphere profiles were taken from the European Center for Medium-Range Weather Forecast analysis. The CBH was determined from ceilometer measurements and also estimated by using a suitable method applied to the vertical profile of relative humidity. The agreement between calculations and pyrgeometer measurements is remarkably good (1.6 ± 6.2 W m−2) if the observed CBH is used; poorer results are obtained with the estimated CBH (4.3 ± 7.0 W m−2). These results are better than those obtained from a simple parameterization based upon ground-level data (1.1 ± 11.6 W m−2), which can be corrected by adding a term that takes into account the CBH (−0.1 ± 7.3 W m−2). At this site, the cloud radiative effect (CRE) at the surface lies in the range 50–80 W m−2, has a clear seasonal behavior (higher CRE in winter), and depends upon the CBH. For the cold and the warm seasons, CRE decreases with CBH at a rate of −5 and −4 W m−2/km, respectively. Results obtained for other climates (subarctic and tropical) are also presented