Pre-squall Mesolow, Mesohigh and Wake Low in Association with Premonsoon Thunderstorms - An Observational Study

Abstract

Observations during premonsoon season of 2008-09 in STORM field experiment are used to examine surface characteristics, in particular surface pressure characteristics, of the convective systems that occurred over the semi-arid part of Gangetic West Bengal. Analyses of automatic weather station data identify transient presquall mesolow, mesohigh (MH) and wake low (WL) during the entire life cycle of the systems. Convective activities are found to be associated with short-lived MH and relatively long-lived WL in 60% events. Average pressure rise of 1.9 - 2.5 mb in MH followed by WL with average pressure drop of 4.2 - 8.9 mb are noticed. Doppler weather radar imageries further reveal that formation of MH & WL takes place in case of organized convective systems. More (less) organized convective systems show more (less) pressure perturbations in MHs & WLs. Most interestingly it has been observed that wind speed shows its maximum value when pressure rises in MH while in WL the environment becomes nearly calm. WLs are also found to be highly localized so that air parcels do not stay for a longer time within the strong pressure gradient fields to achieve higher velocities. As a result these WLs, unlike other locations in USA, do not produce damaging extreme winds in their vicinity.