A157 Role of topography on winter precipitation over the western Himalayas

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The Himalayan region is characterized by pronounced topographic variability and landuse change from west to east with large variation in precipitation and temperature patterns. Over western part (WH) almost one third of annual precipitation is received during winter (Dec - Feb: DJF) by cyclonic storms embedded in westerlies called Western Disturbance (WD) in Indian parlance. In the present paper, regional winter climate over the WH region is discussed based on two regional climate models, REMO and HadRMS, outputs at finer resolution of 25km. Simulations from 18 years (1990 - 2007) suit with ERAINTERM as initial and boundary condition is analyzed during winter (DJF) to assess if models could present the regional climate of the WH. Composite of contrasting wet and dry winter precipitation years show strengthening of westerly at 200 and 500 hPa during wet winters. At 850hPa, strengthening of meridional wind is seen. At 200hPa higher zonal wind with significant region from East Africa to western India is seen. Since model could reproduce large scale fields - a case study of an intensive WD (20-22 Dec 2006) is carried out with fine scale model outputs to assess interplay of WH topography with WD. Model could simulate vertical wind structure and 500hPa low pressure circulation of WD. Higher vertical wind shear in lower troposphere (Fig. 1) and stronger meridional wind from surface to 500hPa along the Kashmir valley is discernable during peak of WD - 21 Dec 2006. Also, increase of specific humidity up to mid troposphere along the valley topographic boundaries is seen. Validation with corresponding observation shows that model simulation produce wet bias over/at high elevation sites of the WH. This propensity for overestimating precipitation over/at high mountainous regions could be due to insufficient high elevation observations over mountainous regions. This associated uncertainty in precipitation thresholds - which may still need much finer downscaling over the mountainous region - and simultaneous model internal variability need further investigations.
2011-10-31

A157 Role of topography on winter precipitation over the western Himalayas

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