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Evaluation of operational forecasts from weather research and forecasting model during southwest monsoon 2011 using MET 3.0

This study discusses the skill of rainfall forecasts during monsoon 2011 from the state-of-art Weather Research and Forecasting (WRF) model run real-time at the Meteorological Centre Bangalore. The WRF is run for the limited domain of 9-22° N / 74-87° E covering the southern peninsular India. Rainfall verification is performed using continuous and categorical approaches. It’s verified using India Meteorological Department’s gridded and point rainfall by Model Evaluation Tools package. The results show that model is capable of reproducing seasonal picture of rainfall; maxima over west coast and eastern India and minima over inland peninsular India. Time series of area averaged daily accumulated observed and forecasted rainfall shows that forecast trend matches observed trend on most of the days. However, quantitative forecast amount is less than the observed. The mean error, mean absolute error and root mean square error of rainfall are found to be 3.1 & 1.4, 11.6 & 10.9 and 26.8 and 24.9 mm/day for 24 and 48 hours forecasts. The categorical scores like probability of detection, false alarm ratio, frequency bias, critical success index and Heidke skill scores are calculated for various thresholds. The Heidke skill score is found to be positive and is 0.38 and 0.37 in the lower thresholds up to 10 mm for 24 and 48 hour forecasts. These scores indicate that model’s performance is good for lower rainfall threshold but degrades considerably for higher rainfall thresholds. Hence, an operational forecaster can accept model forecast of a rainy day with confidence but not the intensity.