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Extreme Daily Rainfall Events and Their Impact on Ensemble Forecasts of the Indian Monsoon

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  • 1 Météo-France, Toulouse, France
  • | 2 Dipartimento di Studi Geoeconomici, Statistici, Storici per l’Analisi Regionale, Universitá di Roma, “La Sapienza,”Rome, Italy
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Abstract

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, which 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 nonlinearly 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.

Corresponding author address: Dr. D. B. Stephenson, Laboratoire de Statistique et Probabilitiés, Université Paul Sabatier, 118, Route de Narbonne, F-31062 Toulouse Cedex, France.

Email: stephen@cict.fr

Abstract

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, which 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 nonlinearly 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.

Corresponding author address: Dr. D. B. Stephenson, Laboratoire de Statistique et Probabilitiés, Université Paul Sabatier, 118, Route de Narbonne, F-31062 Toulouse Cedex, France.

Email: stephen@cict.fr

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