Monthly Weather Review, 127, 1954-1966
Extreme daily rainfall events and their impact on ensemble forecasts
of the Indian monsoon
D.B. Stephenson, K. Rupa Kumar, F.J. Doblas-Reyes, J.-F. Royer, F. Chauvin,
and S. Pezzulli
Météo-France/CNRM, 42 Avenue Coriolis
31057 Toulouse Cedex, France
The Indian summer monsoon rainfall is the net result of an ensemble of
synoptic disturbances, many of which are extremely intense. Sporadic systems
often bring extreme amounts of rain over only a few days, that can have
sizable impacts on the estimated seasonal mean rainfall. The statistics
of these outlier events are presented both for observed and model simulated
daily rainfall for the summers of 1986 to 1989. The extreme events cause
the wet-day probability distribution of daily rainfall to be far from Gaussian,
especially along the coastal regions of eastern and northwestern India.
The gamma and Weibull distributions provide good fits to the wet day rainfall
distribution, whereas the lognormal distribution is too skewed. The impact
of extreme events on estimates of space and time averages can be reduced
by non-linearly transforming the daily rainfall amounts. The square-root
transformation is shown to improve the predictability of ensemble forecasts
of the mean Indian rainfall for June 1986-89.