Dynamics of Unusual Extreme Rainfall Events Over Different Altitudes Using Richardson Number

Abstract

To delineate the dynamic stability of a moist atmosphere and to measure stability within a statically stable atmosphere, local Richardson number (Ri) and Brunt-Väisälä (BV) frequency are used in this study. The BV frequency is an important parameter that is widely used in dynamic meteorology and is calculated by replacing potential temperature with equivalent potential temperature in a moist atmosphere. For this purpose, extreme rainfall events were simulated by the WRF model. These events occurred on 9 September 2012 and 28 July 2010 during the summer monsoon in areas of different altitudes (ranging approximately from 100 to 3,500 m) which resulted in flash floods, loss of lives, and property damages in southern and northern parts of Pakistan. In the present case Ri (local Richardson number based on potential temperature) and Rie (local Richardson number based on equivalent potential temperature) are applied to investigate instability (Rie < 1) in a moist atmosphere. The results show that the values of Rie < 1 are mainly located in the lower-to-upper troposphere over the rainfall areas. The analysis shows that Rie is more suitable index for describing the convective instability in a moist saturated atmosphere. The results of Ri and Rie were justified with the European Centre for Medium-Range Weather Forecasts fifth generation reanalysis (ERA5 hourly) available from Climate Data Store. Moreover, negative values of vertical velocity (hPa/s) clearly show large-scale dynamical uplifting associated with instability resulting in a severe thunderstorm. Further, strong synoptic forcings also play a key role to develop deep convection over the region. In addition, maximum convective available potential energy (3,000 J/kg) and K-index (>40) were observed over the areas of high instability that dominate its role in enhancing convection during the events.