A method is presented that has been developed for classifying cloud and precipitation for a specific synoptic situation affecting south-east Europe during winter and spring: a depression system that gives rise to a heavy rainfall weather situation. The classification algorithm used for classifying clouds and precipitation is based on a combination of spectral channels and the multispectral box classification technique using visible and infrared Meteosat data. In order to increase the accuracy of the classification algorithm, the appropriate cloud types are selected according to the specific synoptic weather situation. The classification algorithm produces nine classes, four of which involve precipitation. These are: (1) clear or insignificant clouds, (2) low thin cloud with no rain, (3) low or middle thin cloud with no rain, (4) low or middle thick cloud with no rain, (5) middle or high cloud with no rain, (6) middle or high cloud with the possibility of rain, (7) middle or high cloud with light-moderate precipitation, (8) middle-high cloud with moderate-heavy precipitation, (9) heavy thunderstorm. The classifier has been tested for its accuracy (ground truth) using comparisons between actual meteorological weather reports and classification results derived from the algorithm applied. For the estimation of classification accuracy, the omission/commission method is applied between the observed and the classification produced values. The case study of 26 March 1998 is presented. The classifier used has proved to be very reliable for classifying major cloud types and precipitation during the synoptic situation of depression systems approaching the south Balkan peninsula from the west. It has been tested for different intensities of rainfall as well as for heavy thunderstorms and the results are very satisfactory. The results produced can be used to support the forecaster's daily work. The need for the extraction of such classification products lies in the fact that detailed weather information is demanded for regions in Greece devoid of surface observations, such as the Aegean Sea, the Ionian Sea, the South Cretan Sea and remote mountainous areas. That information has been found to be useful, especially in cases where significant weather systems are approaching Greece from the west or south-west, where surface observations over the sea are not available and an early knowledge of the heaviness of precipitation is needed, even before that weather system is in range of any land-based weather radar. Moreover, the single use of isolated channels (i.e. infrared or visible) could involve the danger of errors in the interpretation of the satellite image.
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