Role of Westerly Jet in Torrential Rainfall During Monsoon Over Northern Pakistan

Pakistan experiences several torrential rainfall events for many years due to interactive role of midlatitude atmospheric and warm moist monsoon circulations. Deadly events such as flash flooding, runoff, and mudslides may trigger by torrential rainfall events which pose significant threat to human assets. This study was undertaken based on three severe weather events (1988, 2010, and 2013) by using percentage departure to identify wet and dry years during the period 1985-2016, which occurred over northern Pakistan. Rainfall data were obtained from 14 synoptic stations of Pakistan Meteorological Department, which were analyzed at daily, monthly, and annual bases. The data sets of Climate Research Unit, Global Precipitation Climatology Centre, and Pakistan Meteorological Department are used to justify the Weather Research and Forecasting model simulation and for analyzing the various atmospheric and diagnostic variables. Results showed teleconnection between large-scale circulation pattern and South Asian summer monsoon during the selected events. Detailed analysis revealed that during all three events, a westerly trough was moving in the north of Pakistan and well-marked monsoon circulations were present at the lower levels. It was observed that the interaction of two weather systems triggered heavy rainfall over area of interest. In addition, high vorticity at all levels and extension of westerly trough to the surface resulted in heavy rainfall and the penetration of upper-level circulations anomalies consists of blocking signature over Eurasia along with Tibetan anticyclone. Hence, the monsoon low-pressure system provides moisture convergence and upper-level westerly trough results in upper-level divergence to enhance the extreme rainfall events.