Characteristics of medium range rainfall forecasts of the Asian summer monsoon

Mean characteristics of the rainfall forecasts produced by the global data analysis-forecast system of India are examined for the summer monsoons of 1 993-1996. Further, daily rainfall forecasts (accumulated for 24 h) extendin g from day 1 to day 5 are utilised to compute monthly/seasonal mean forecas t fields to study their consistency and reliability. Global patterns of rai nfall forecasts are also compared with the large scale rainfall climatologi cal fields. In addition, observed rainfall distributions over India are use d for regional verification of the medium range rainfall forecasts. The forecasts appear to reproduce most of the large scale features of rainf all, except the sharp gradients over the Deccan plateau (leeside of the Wes tern Ghats-west coast mountains over the south of Peninsular India) and the Gangetic plains over the north of India. Further, it is found that the for ecast model has a characteristic tendency to reduce the quantum of rainfall over northwest India and the north Indian plains to the south of the Himal ayas. In addition, in this study, certain aspects of the year-to-year varia bility of the predicted rainfall fields and their associated characteristic s are examined along with the other systematic errors of the model. It is f ound that in this case, the sources of systematic errors could at least par tially be eliminated, the rainfall forecasts up to day 3 or so can become a very useful product of interest. Copyright (C) 1999 Royal Meteorological S ociety.