An assessment of real-time extended range forecast of 2013 Indian summer monsoon

Several aspects of real-time forecast of Indian summer monsoon (ISM) in 3-4 pentad lead time (extended range) are discussed in this study to explore the operational capability of the Climate Forecast System model version 2 (CFSv2) developed by National Centre for Environmental Prediction (NCEP). 2013 summer monsoon was a near excess monsoon year in terms of seasonal mean and was a result of rich diversity of phenomena including strong intraseasonal variations and intense northward propagations over the Indian region. Eleven-member forecasts were made at every 5-day interval during the June-September monsoon season which included monsoon onset and withdrawal phases. The ensemble members were created by perturbing the initial conditions at each start time. In addition to the CFSv2 forecasts, we also carried out forecasts using the atmospheric-only component (GFSv2) forced with CFSv2-derived sea surface temperature (SST) subjected to a bias correction based on historical observations (GFSbc runs). Both the CFSv2 and GFSbc runs were able to predict the progression of ISM over the Indian region and the subsequent intraseasonal oscillations (active and break phases). The analysis for an extreme event (Uttarakhand flood) and monsoon revival (MR) towards the end of the season was also performed. Comparison between the two runs shows that active and break spells were predicted with good fidelity over the Indian region, though GFSbc outperforms CFSv2 on several occasions. Thus, improvement of the operational monsoon forecast over Indian region using NCEP CFSv2 requires better representation of air-sea interaction and mean states of ocean.