ANALYSIS OF THE SPATIAL COVERAGE OF BRITISH RAINFALL FIELDS

Based on UK radar data, the distribution of coverage of an area by rai nfall fields is analysed including the influence of spatial and tempor al resolution of the data, size of the area, and topographic and clima tic variations. The aim is to derive a method allowing the choice of a value of the coverage, for example within an atmospheric general circ ulation model. Two cases are distinguished: that of a coverage which i s stationary with respect to the size of the area and that of a varyin g coverage for areas smaller than a critical size which is generally b etween 60 km x 60 km and 70 km x 70 km. For the stationary coverage, a proposed method consists of the selection, based on topography, of an average coverage <(epsilon)over bar>, for the given period, and the c hoice of a value of the coverage epsilon from a distribution of mean < (epsilon)over bar>, presenting some dependence on the season and the t opography. The self-similarity of the coverage process for a wide rang e of area sizes leads to the determination of estimates of the fractal dimension and the examination of the temporal variation of this dimen sion. For the non-stationary coverage (within the mesoscale-gamma and lower mesoscale-beta ranges), parabolic curves allow a good approximat ion to the decrease of coverage with area size.