The influence of clouds on earth radiation budget — a regional study: The North Sea

Abstract The influence of clouds over the North Sea on climate is investigated by analyzing NOAA AVHRR data. The main interest is on high clouds due to their ambivalent behaviour in the radiation field. After a cloud classification, especially for high clouds, and the determination of cloud optical properties, the obtained information was applied to calculate the cloud-climate efficiency. This index is similar to the cloud forcing, but is valid for an individual classified satellite image pixel. The cloud forcing is the sum of the cloud-climate efficiencies over an area. Using NOAA-AVHRR data the annual cycle (October 1989, October 1990, April 1991 – July 1992) of cloud forcing at the top of atmosphere were calculated. Due to the strong dependence on solar insolation, high clouds with the same optical properties lead to an heating or a cooling of the earth/atmosphere system. For thin cirrus clouds the heating effect is well correlated with an increase of the surface temperature. A further approach to compare the increasing/decreasing cloud forcing with an analysis of relative topography 300/1000 hPa shows that an increase is positively correlated with an increase of the temperature in this layer.