This study is conducted to understand the underlying physical processes responsible for the extreme precipitation event of September, This Years that caused severe flooding in Jhelum River, the worst one since 1959. Reanalysis data ERA15 of European Centre for Medium Range Weather Forecasts (ECMWF), which is available at 06-h intervals with a resolution of 2.5° × 2.5°, and 17 pressure levels, is used to drive the model as both the initial and lateral boundary conditions. The performance of the model is demonstrated by integrating it from December 01, 1991 to October 31, 1992 with a
resolution of 0.44° × 0.44° and 0.22° × 0.22° covering the whole South Asia domain.
CRU data together with real time observational precipitation and temperature data from meteorological stations representating the Jhelum River basin is used to compare and verify the model outputs. The model simulated realistically the temporal pattern of the area averaged precipitation, monthly mean precipitation spatial pattern and the daily precipitation intensity distribution. The model not only captured the fluctuations in the daily Maximum and Minimum temperature but also reproduced well the large scale monsoon circulations responsible for the moisture convergence in the catchment areas resulting in heavy precipitation.
The extremely heavy precipitation occurred due to the interaction of intense monsoon depression with an active westerly wave passing over the area. At higher resolution i.e. 0.22° × 0.22°, the model suitably resolved the complex topography and land-sea contrast, and hence the simulated results are found very close to the observations.
Time series of area averaged daily precipitation over the Jhelum River basin region. Station observed precipitation data from meteorological stations representating the Jhelum River basin is used for comparison.The model reproduced reasonably the trends and fluctuations of precipitation over the above region, except for an overestimation of daily precipitation in the later June and early July and an underestimation in the mid July, early August and September. These underestimations and overestimations are responsible for the biases in the monthly mean daily precipitation. It can be seen that the extreme daily precipitation is captured well by the model at higher resolution because the
topography is best resolved at higher resolution.
In order to realistically simulate the monthly mean precipitation, the model should be able to simulate reasonably the individual precipitation events. Thus the performance of the regional climate model PRECIS in the simulation of precipitation was further evaluated by calculating the BIAS, spatial correlation coefficient between the observed and model simulated precipitation in the 4 months over the Jhelum River basin region.
At 25km horizontal resolution the model overestimated (positive bias) the area averaged monthly mean precipitation in the 4 months over the Jhelum River basin while at 50km horizontal resolution the model underestimated (negative bias) the monthly mean precipitation in June, July and August. The maximum bias is in June and these reduced in the later months, probably because the land surface processes and the associated forcings are better resolved in the later months. The spatial correlation coefficient between the observed and model simulated precipitation is found to be relatively lower in June and July compared to August and September. In September 94% was achieved over the Jhelum River basin.
The observed and model simulated surface air temperature variation at 50km horizontal resolution in the 4 months over the Jhelum River. The trends and fluctuations in the surface air temperature simulated by the model at 25km resolution resemble to that of the 50km resolution and hence the results are not shown here. We calculated the area averaged daily maximum and minimum surface air temperature in the 4 months over the Jhelum River basin region. In general PRECIS captured the trends and fluctuations in the daily maximum and minimum surface air temperature over the Jhelum River basin except for a warm bias (less then 2°C) in the daily maximum temperature in June and minimum temperature in June and August and a cold bias (0.8°C-3.5°C) in both maximum and minimum temperature in the later months.