Precipitation forecast verification of the Indian summer monsoon with intercomparison of three diverse regions

The Indian summer monsoon precipitation forecast, as well as its verification, are always of great interest because of their socioeconomic impact on the Indian subcontinent. The present work highlights the verification of quantitative precipitation forecasts of the Global Spectral Model, running at the National Center for Medium Range Weather Forecasting (NCMRWF), Noida, India. Studies like pattern correlation and anomaly correlation over all of India confirm that the model is applicable over the subcontinent. Some comparative studies are done for three diverse regions like West Bengal, Andhra Pradesh, and Rajasthan. Verification studies include both measure-oriented methods like root-mean-square error (RMSE) and Pearson’s correlation coefficient and distribution-oriented methods like bias score, false alarm ratio, probability of detection, and true skill score. The distribution-oriented verification (yes–no) is done for the daily threshold precipitation of 0.254, 2.54, 6.4, 12.8, 19.2, 32.0, 44.8, and 64.0 mm. Two years of data from 1997 and 1999 for Andhra Pradesh and Rajasthan and 5 yr of data from 1997 to 2001 are used for West Bengal. The distribution of model output during a severe rainfall situation over West Bengal is also examined to understand the usefulness of the model forecasts during those events. It can be concluded that the model is most efficient in predicting precipitation in the 2.54–12.8-mm range but the efficiency decreases rapidly for higher thresholds. Performance of the model during active and break phases of the monsoon is examined and it is found to be reasonably good. On the whole, it can be concluded that the performance of the NCMRWF model is reasonably good for day-1 forecasts and the weekly rainfall forecast is quite good for all forecast lead times.