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Rainfall is a key parameter in relation to the life, health and productivity of the biosphere, and in particular to human life and activity of all forms. However, rainfall is a rare event, instantaneously affecting no more than about 1 to 1 1/2 percent of the global surface and is so variable in space and time that it is difficult to monitor by in situ means. Especially over sea areas, but also over many thinly populated land areas, information on rainfall remains essentially partial and fragmentary.

As part of the AVICENNE Programme of the European Commission, the Centre for Remote Sensing in the University of Bristol, UK is collaborating with institutes in Italy, Jordan, Turkey and Syria on a Project entitled ''Analysed Climatology of Rainfall obtained from Satellite and Surface Data for the Mediterranean Basin (ACROSS).'' This aims to produce the first unified satellite and raingauge climatology of rainfall over the Mediterranean Sea and countries bordering it to the east and north in order to permit a much more satisfactory evaluation and interpretation than has been possible of important over-land and over-water rainfall anomalies hitherto. For this purpose, ground based and satellite data are being collected for coastal and sea areas respectively from the Straits of Gibraltar to (and including) the Black Sea. The surface data will be used to provide rainfall analyses over the land areas themselves, and, in conjunction with French radar data, to calibrate rainfall analyses of imagery from passive microwave satellites, for the period from 1978 through 1995. The satellite data used for the over-water rainfall evaluation are from the Nimbus-7 Scanning Multichannel Microwave Radiometer (from 1978 to mid 1987) and the DMSP Block 5D2 Special Sensor Microwave Imager (SSM/I) from mid 1987 through 1995. The results will be used to prepare a unified climatology of rainfall over the study region, to describe the observed spatial and temporal variations of rainfall through analyses of the resulting climatology, to identify and map the most significant departures from the long-term mean rainfall as well as the characteristic space and time scales of the extremes, and to investigate the possibility of using the passive microwave data to test the major effects of rainfall anomalies on surfaces, e.g. through desertification.

This paper introduces the ACROSS Project, describes and explains both the methodology being followed and the design of the end-products, all of which are expected to make a significant contribution to the knowledge of rainfall and its extremes in the Mediterranean Region as a whole. Some preliminary results are presented.