<title>Alpine cloud climatology: seasonal changes</title>

Abstract

Clouds play an important role in the hydrological cycle and they are a climatic component. It is therefore necessary to known the distribution of clouds. A high-spatial resolution climatology of monthly mean cloud cover has been produced from the daily afternoon passages of the NOAA satellites since September 1991. The APOLLO (AVHRR Processing scheme Over cLouds, Land and Ocean) algorithm has been applied to AVHRR data to obtain a pixel by pixel cloud mask for an area of 560 X 560 km<SUP>2</SUP> over the Alps and their forelands. Cloud cover is calculated for subareas of 14 X 15 km<SUP>2</SUP> for the following classifications: all clouds, thick clouds, thin clouds, thick clouds with low, medium or high tops. This regional cloud climatology reveals topometeorological features like an increased cloud cover in the luff regions at the edges of the Alps. A strong horizontal inhomogeneity on a regional scale is typical in summertime. The seasonal changes of cloud cover are greater in the south than in the north of the Alps due to different climate types. Although the APOLLO algorithm still needs improvement it has proved to be a powerful tool for cloud detection and classification.