Satellite-derived hourly precipitation values over India and neighboring areas are examined during the summer monsoon season of 2004 to determine the observed patterns of diurnal variations. These are compared with the patterns found in the forecasts from the global spectral model in operation at the National Centre for Medium Range Weather Forecasting in India. The observed hourly precipitation shows that maximum amounts are recorded over most areas of India during the afternoon hours, coinciding with the maximum in surface temperature. This pattern is modified in areas where local mesoscale events like katabatic winds or land-sea breezes produce strong convergence patterns and associated convection. The model forecasts weaken the mesoscale effects on precipitation and the convection due to ground heating seems to start in the model 2-3 h before the time it is observed by the satellites. The frequency and amount of precipitation increases with the forecast length but the hour of maximum precipitation remains almost the same. Harmonic analysis of the frequency of observed precipitation shows that the diurnal cycle predominates in both magnitude and the amount of variance explained. The semidiurnal cycle is considerably smaller in magnitude and explains significant variance only over a small area. Other cycles of smaller periodicity are unimportant in the diurnal variation of precipitation. A similar result is also obtained for the model forecasts except that the spatial distributions of amplitude and variance explained are different from that obtained from the observed data. The spatial distribution and values remain almost the same with forecast length.
For the summer monsoon seasons of 1995, 1996, and 1997 the day-1 to day-4 forecasts of precipitation from both the National Centre for Medium Range Weather Forecasting (NCMRWF) and the European Centre for Medium-Range Forecasts (ECMWF) models reproduce the main features of the observed precipitation pattern when averaged over the whole season. On average, less than 30% of all rain gauge stations in India report rain on a given day during the monsoon season. The number of observed rainy days increases to 41% after spatial averaging over ECMWF model grid boxes and to 50% after spatial averaging over NCMRWF model grid boxes. The NCMRWF model forecasts have 10%–15% more rainy days, mostly in the light or moderate precipitation categories, when compared with the spatial average of observed values. Seasonal accumulated values of all of India’s average precipitation show a slight increase with the forecast lead time for the NCMRWF model and a small decrease for the ECMWF model. The weekly accumulated values of forecast precipitation from both models, averaged over the whole of India, are in good phase relationship (∼0.9 in most cases) with the observed value for forecasts with a lead time up to day 4. Values of statistical parameters, based on the frequency of occurrence in various classes, indicate that the NCMRWF model has some skill in predicting precipitation over India during the summer monsoon. The NCMRWF model forecasts have higher trend correlation with the observed precipitation over India than do the ECMWF model forecasts. The mean error in precipitation is, however, much less in the ECMWF model forecasts, and the spatial distribution of seasonal average medium-range forecasts of ECMWF is closer to that observed along the west coast mountain ridgeline.
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.
The observed and the model forecast rainfall and other atmospheric fields are compared To determine the skill of the National Centre for Medium Range Weather Forecasting (NCMRWF) model in forecasting precipitation and circulation features over India, we compared observed and forecast rainfall and other atmospheric fields for the boreal summer monsoon season of 2004. This season was an important period within the period 4 of the Coordinated Enhanced Observing Period (CEOP). For comparison of observed rainfall with predicted values, station values are averaged over the area represented by each grid point of the model. For other fields, comparisons were made between the analysis and forecast values at the same horizontal resolution. The model showed considerable skill in predicting the daily and seasonal accumulated rainfall amounts when averaged over the whole of India. The predicted allIndia average precipitation tends to increase in magnitude with increasing forecast period. The model forecasts that were undertaken especially for the CEOP project reproduced the important features of the summer monsoon over India. The onset of monsoon is captured in terms of both the convective precipitation and diabatic heating fields forecast by the model. Variations in monsoon activity between July and August are also well simulated in the monthly averaged forecast fields for these two months.
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