Extreme daily rainfall events and their impact on ensemble forecasts of the Indian monsoon

The Indian summer monsoon rainfall is the net result of an ensemble of syno ptic disturbances, many of which are extremely intense. Sporadic systems of ten bring extreme amounts of min over only a few days, which can have sizab le impacts on the estimated seasonal mean rainfall. The statistics of these outlier events are presented both for observed and model-simulated daily r ainfall for the summers of 1986 to 1989. The extreme events cause the wet-d ay probability distribution of daily rainfall to be far from Gaussian, espe cially along the coastal regions of eastern and northwestern India. The gam ma and Weibull distributions provide good fits to the wet-day rainfall dist ribution, whereas the lognormal distribution is too skewed. The impact of e xtreme events on estimates of space and time averages can be reduced by non linearly transforming the daily rainfall amounts. The square root transform ation is shown to improve the predictability of ensemble forecasts of the m ean Indian rainfall for June 1986-89.