Mesoscale objective analysis of daily rainfall with satellite and conventional data over Indian summer monsoon region

A mesoscale objective analysis scheme for producing daily rainfall analysis on a regular latitude/longitude grid over the Indian monsoon region is described. The Barnes scheme is applied to interpolate irregularly distributed daily rainfall data on to a regular grid. The spatial resolution of the interpolated arrays is 0.25 degrees of latitude by 0.25 degrees of longitude. Daily rainfall derived from INSAT IR radiances and raingauge observations are combined to produce this analysis. Some objectively determined constraints are employed in this study: (i) weights are determined as a function of data spacing, (ii) in order to achieve convergence of the analysed values three passes through the data are considered and there is automatic elimination of wavelengths smaller than twice the average data spacing. The case of a typical westward moving monsoon depression during the 1994 monsoon season is selected to represent the characteristics of the analysed rainfall. Objective analyses of six days (16 to 21 August 1994) have been carried out using Barnes three pass scheme. The weighting function scale length parameter (c, denominator in the exponential Gaussian weight function) is varied from over a range of values and the root mean square (rms) errors are computed to select the appropriate value of c. The value of c depends on the number of correction passes being performed and on the density of the observations. The characteristics of the output field from this analysis system have been examined by comparing the analysed rainfall with the observed values. The heavy rainfall over the Western Ghat of India has been clearly brought out in this analysis.