Simulation of mesoscale features associated with intense western disturbances over western Himalayas

Abstract

North India is comprised, in parts, of complex Himalayan mountain ranges having different altitudes and orientations all along this region. Due to the highly variable altitude and orientation of orographic barriers the prevailing weather conditions over the region are complex. The winter season over this region is frequented by eastward-moving low pressure synoptic weather systems called Western Disturbances (WDs). Advance information of these WDs are important for organizations where men and machines are employed to operate in the open, for example, for defence purpose, agriculture, tourism and transport. Future projection of meteorological variables is important during the winter in assessment of cold wave conditions, avalanche release and a critical human comfort index. Therefore, a non-hydrostatic version of the Penn State University/National Center for Atmospheric Research, US, (PSU/NCAR) mesoscale model is used to simulate the characteristic features of the Western Disturbances (WDs) occurring over the Indian region during a winter season. For this study, four cases of active WDs are selected. The model is integrated with 60 km horizontal resolution to simulate the WD features. The model simulations with 60 km horizontal resolution are compared with the National Center for Environmental Prediction/National Center for Atmospheric Research, US, (NCEP/NCAR) reanalyses. It is seen that in all the cases the rate of movement of the system is, in general, a little slower in the simulations. Examining the differences between the predicted and analysed zonal component of the wind reveal that the model simulated zonal winds are generally weaker/under-estimated in the location of the upper trough at 500 hPa or aloft and even in the position of the WDs at lower levels. These results suggest that the model has a systematic easterly bias, though the magnitude is small. In other words the advection simulated in the model is not strong enough to advect the system with the observed speed.