Prediction Skill of GEFSv12 for Southwest Summer Monsoon Rainfall and Associated Extreme Rainfall Events on Extended Range scale over India

Indian summer monsoon rainfall (ISMR) from June through September (JJAS) contributes 80% of the total annual rainfall in India and controls the agricultural productivity and economy of the country. Extreme rainfall (ER) events are responsible for floods that cause widespread destruction of infrastructure, economic damage, and loss of life. A forecast of the ISMR and associated ER on an extended range (beyond the conventional one-week lead time) is vital for the agronomic economy of the country. In September 2020, NOAA NCEP implemented Global Ensemble Forecast System version 12 (GEFSv12) for various risk management applications. It has generated consistent reanalysis and reforecast data for the period 2000-2019. In the present study, the Raw-GEFSv12 with Day-1 to 16 lead time rainfall forecasts are calibrated using the quantile (QQ) mapping technique against Indian Monsoon Data Assimilation and Analysis (IMDAA) for further improvement. The present study evaluated the prediction skill of Raw and QQ-GEFSv12 for ISMR and ER events over India by using standard skill metrics. The results suggest that the ISMR patterns from Raw and QQ-GEFSv12 with (lead) Day 1 to 16 are similar to IMDAA. However, Raw-GEFSv12 has a dry bias in most parts of prominent rainfall regions. The low to medium-intensity rainfall events from Raw-GEFSv12 is remarkably higher than the IMDAA, while high to very-high-intensity rainfall events from Raw-GEFSv12 are lower than IMDAA. The prediction skill of Raw-GEFSv12 in depicting ISMR and associated ER events decreased with lead time, while the prediction skill is almost equal for all lead times with marginal improvement after calibration.