Impact of initial and boundary conditions on simulations of western disturbances and associated precipitation

Western Himalayas (WH) is characterized by variable topography and heterogeneous land use. During winter, it receives enormous amount of precipitation due to eastward moving extratropical cyclones, called western disturbances (WDs), in Indian parlance. This variable altitude and orientation of orographic barriers has a complex interplay with WDs in defining precipitation over the WH. To understand such complexities, three WDs are considered to study interaction with the Himalayan orography using the advanced regional prediction system. Two simulation strategies are performed and presented-first to illustrate the impact of different initial and boundary conditions and second to illustrate the impact of different horizontal model resolution with same model configuration. In the first strategy, three different initial and boundary conditions-the National Center for Environmental Prediction-Global Forecast System, USA (NCEP-GFS) (1) analysis (2) 0000UTC forecast and the National Center for Medium Range Weather Forecast, India-T80 spectral model (NCMRWF-T80) (3) 0000UTC forecast-are provided to the same model configuration. In the second strategy, outputs from model simulated with NCMRWF-T80 spectral model forecast at coarser horizontal model resolution of 30 km (hereafter called Experiment I) are used as input initial and boundary conditions for simulation at finer horizontal model resolution of 10 km (hereafter called Experiment II). Though there are many other dynamical factors, but in the present study, it is shown that model-simulated precipitation is sensitive to the initial and boundary conditions. Simulations at coarse resolution could capture the weather system, but detailed spatial distribution along the orography is better illustrated at finer resolution model simulation. Also, Experiment II could simulate precipitation over different ranges of the western Himalayas depicting orographic forcings.