Diagnosis and Numerical Simulation of a Heavy Rainfall Event in Winter over Upper Parts of Pakistan

Using observed, NCEP reanalysis and HRM forecasted data, a diagnostic study has been carried out to find out the reasons for a heavy rainfall that occurred and continued for three consecutive days over north western parts while for two consecutive days over northern and upper Punjab region of Pakistan (Mar 30 to Apr 02, 2007). The development and intensification of the baroclinic wave to the west of Pakistan over the adjoining areas of Afghanistan and Iran; east of Caspian Sea resulted in heavy rain and snow. The study was mostly confined to the domain (50° E – 100° E and 21° N – 50° N) keeping in view the baroclinic wave characteristics and limitations of computational facilitates available. The results of the study showed that the cause of this heavy rainfall was the orographic capture of western upper- level disturbance and moisture (>90% with vertical alignment e.g. from surface to 400hPa levels) in northwestern and northern regions (Hindukush – Himalayan Terrain). A jet stream present at 200hPa caused the development and eastward movement of this baroclinic westerly wave due to strong convergence at lower level and positive divergence at upper level over these regions. Early spring heating of the ground surface provided the ignition to generate the local instability conditions to the existing and advected airmasses. The presence of divergent wind field aloft along with orographic lifting aided in vertical rising of this unstable warm moist air resulting in severe weather conditions.