Topographic sensitivity of WRF-simulated rainfall patterns over the North West Himalayan region

Abstract

The present study investigates the effect of topography on the rainfall pattern of the North West Himalayan (NWH) region. The Weather Research and Forecasting (WRF) model is employed for conducting four dedicated experiments with varying topography at regional scale, which are assessed with respect to the control experiment of no-change in topography. Categorical validation analysis carried out for the control experiment against the satellite based observational dataset from the Global Precipitation Measurement and Indian National Satellite System, suggests that WRF has reasonable skill in simulating the fine scale (5 km spatial resolution) rainfall pattern over the NWH region. Furthermore, the WRF experiments show that local changes in the topography limited over Himachal Pradesh and Uttarakhand regions result in significant changes to the fine scale rainfall pattern over Himachal Pradesh and Uttarakhand, while the broad scale pattern of rainfall over the rest of the Indian subcontinent remains unaltered. Also the meridional tropospheric temperature gradient did not show any substantial variation as a result of the localised changes in the topography. The WRF model is capable of simulating heavy rainfall along the steep windward slopes as seen in various topographic sensitivity experiments of this study. Analysis of a high rainfall event over Chamoli (Uttarakhand) on 11 July 2017 showed that atmospheric profiles of relative humidity, CAPE, and horizontal and vertical wind speed vary significantly in response to the changes in the topography. It is therefore suggested that the topographic setup of a region plays a major role in shaping up spatial rainfall variability rather than the terrain height alone.