Model sensitivity analysis study for western disturbances over the Himalayas

Western disturbances (WDs) are extratropical synoptic scale weather systems which cause significant precipitation over the Himalayas and surrounding areas during winter (December, January and February, DJF). Three intense WDs, 13-17 January 2002, 05-08 February 2002, and 11-13 February 2002, are chosen as two of the WDs are extensively studied by Hatwar et al. (Curr Sci 88:913-920, 2005) and one independent WD (Indian Meteorological Department, Delhi, Mausam 54(1):346-347, 2003) is considered. Firstly, it is planned to study model sensitivity with these WD cases, which are simulated with different combinations of cloud microphysics, planetary boundary layer and cumulus parameterization schemes in weather research and forecasting model to assess a better suite for the WD simulations. Sensitivity and error analyses carried out with different observations, show that the combination of Eta Ferrier or Eta Grid-scale cloud and precipitation microphysics scheme, Yonsei University scheme and Kain-Fritsch scheme has shown consistently lower error values. Further, the results suggest, that the model simulations of a WD capture the spatial distribution of precipitation, locations of low pressure region and the circulation patterns very well. It is observed that the WD system comprises of low pressure region in the vertical atmospheric column in form of a stationary surface low and a depression in the subtropical westerly jet moving eastwards. Further, the growth of convective cyclonic systems over the steep topographical region of the Himalayas is depicted by the increased positive vorticity and high values of CAPE, alluding to the propensity of WDs to cause orographically forced precipitation. WDs and associated precipitation show varied but significant impacts on the Indian winter climate such as snow cover variation and cold wave or fog conditions along with impact on winter crop production.