On the spatial extent of extreme rainfall events over India during the summer monsoon

Abstract

The spatial extent of extreme rainfall events directly affects the damages caused and impacts of such events. However, it has been less explored in the literature. This thesis examines extreme rainfall events of different sizes over central India during the Indian summer monsoon for the period 1951-2015. It utilizes a 2D connected component labelling algorithm to identify the sizes of events in the daily 1 1 gridded rainfall dataset provided by the India Meteorological Department. In the first part, this work revisits the observed trends in extreme rainfall events with the improved definition of events that accounts for their sizes. It shows that while 60% of the fractional increase in the area covered by daily extreme rainfall during the study period is due to a rise in the number of events, the rest is contributed by their increasing size. The increase after 1990 is, however, mainly due to the increase in the average size and not the frequency of their occurrences. This reveals the changing spatial characteristics of rainfall extremes over the study region that was not noted before. To get a mechanistic view of extremes of different sizes, we classify them as small (area . 104 km2 ), medium (104 gt area . 7 104 km2) and large events (area gt 7 104 km2 ). A majority of these events (gt80%) are associated with synoptic-scale monsoon low-pressure systems (LPSs) but are accompanied by different background conditions. In the second part, I propose a physical mechanism for large-sized extreme rainfall events. All of the largesized events are produced by LPSs within 400 km of their center, with a clear preference to the south-western sector. Another consistent synoptic feature for large-sized events are the presence of secondary cyclonic vortices (SCVs) to the west of LPSs and the extratropical forcing. The interaction of two cyclonic vortices forms conditions favourable for long-lived, organized, and slow-moving convective systems that produce large-sized extreme rainfall events in the region between them...