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B. K. Basu

Abstract

The ensemble of seasonal (120 days) simulations of the Northern Hemispheric summer for the reanalysis period of the European Centre for Medium-Range Weather Forecasts has been examined to assess the extent to which the characteristic features of the Indian summer monsoon can be reproduced in these simulations. The present simulations could reproduce a better distribution of the seasonal average precipitation over India in comparison with the earlier Atmospheric Model Intercomparison Project simulations. The interannual variation in the seasonal total of the spatially averaged precipitation over India has predictability. The 10-day-average precipitation values did not show any impact of the El Niño or La Niña events or any periodicity in the amount of precipitation. The intraseasonal variability did not produce any distinct pattern for 10-day-average rainfall during the excess or deficient years. The simulated patterns over India correspond to the weak phase of summer monsoon with excess precipitation over the northeastern part of the country and adjoining China. The cloud cover is less over the central parts of India and near-ground maximum temperature is higher. A simulated motion field reproduces the typical features of the Indian summer monsoon including the low-frequency seasonal migration of the Tibetan anticyclone at 200 hPa.

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B. K. Basu

Abstract

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.

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B. K. Basu

Abstract

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.

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