The spottiness of rainfall in a desert area

Abstract Between one half and two thirds of the total rainfall in the southernmost, arid part of Israel is of a highly localized type, coming mostly from small convective cells. In a number of storms, the dimensions of such cells could be estimated; the typical diameter was about 5 km. Larger localized systems exist as well. In the instances observed, cells mostly appeared well-separated from each other, both in time and space. Consequently, in a region (or a watershed) of a few hundreds of sq. km., the proportion of the area receiving intensive rainfall on a given day may be as low as 20%. The area covered by individual cells seems to be randomly distributed in space, as long-term average rainfall is uniform throughout the region considered. The number of days on which rain occurs somewhere within that region, is about 50% higher than the number recorded at any single point. Correlation analysis has been used to compare the degree of spottiness in this area and in other climatic regimes. Seasonal differences have also been found. Spottiness is most pronounced in the fall and late spring, whereas spatially uniform rainfall is most likely to occur in mid-winter.