Changing characteristics of precipitation extremes have been reported mainly using parameters like the frequency of occurrence and the magnitude. As climate changes, the spatio-temporal characteristics of rainfall extremes are also likely to get modified. Though there exist studies noting the changing the temporal distribution, the spatial extent of extreme rainfall events has received less attention. We show that 31% of the fractional increase in the number of rainfall extremes of the Indian summer monsoon from 1951 to 2015 is size-change related. We find that the average size of rainfall extreme is significantly increasing after 1980. Depending on the number of connected grids experiencing simultaneous extreme rainfall, we classify them as small, medium and large events. 90 % of large events have occurred after 1980 and these have the strongest rainfall intensity among all types. They are more likely to cause floods, hence important. These events are invariably associated with a monsoon low-pressure system (LPS) and occur in the southwest sector of an LPS within 400 km from its centre. Strong surface pressure anomalies that persist for more than a week are present in different parts of the globe when these events occur, suggesting preferred planetary-scale conditions that favour their formation. Thus giving hopes of their advanced prediction and facilitating flood hazard mitigation.

In recent years India has been increasingly experiencing widespread floods induced by large-scale Extreme Rainfall Events (LEREs). LEREs are mainly associated with monsoon Low-Pressure Systems (LPS). The forecast of these high-flood-potential events, however, has remained challenging. Here, we compare LPSs of the summer monsoon that led to LEREs (LPS-Lg) and strong LPSs that did not result in LEREs (LPS-noLg) over central India for the period 1979-2012. We show that having a strong LPS is not a sufficient condition to produce LEREs, and the LPS-Lg are accompanied by Secondary Cyclonic Vortices (SCVs). The simultaneous existence of LPS and SCV creates a giant mid-tropospheric vortex. SCVs enhance dynamic lifting, static instability, and moisture transport from the Arabian Sea that precondition the atmosphere for deep convection. SCVs also slow down the propagation of LPSs. We show that the interaction of synoptic-scale systems can lead to LEREs even if individual systems aren’t strong enough.

In recent years, India has been increasingly experiencing widespread flooding induced by large-scale extreme rainfall events (LEREs). These events are mainly associated with the monsoon low-pressure systems (LPSs) and occur mostly in their southwestern sector. The forecast of these high-flood-potential events, however, has remained challenging. In this study, we compare the LPSs of the Indian summer monsoon that lead to LEREs (LPS-Lg) and the strong LPSs that did not result in LEREs (LPS-noLg) over central India for the period 1979-2012. We show that having a strong LPS is not a sufficient condition to produce LEREs. Rather, an observed consistent feature is that the LPS-Lgs are accompanied by Secondary Cyclonic Vortices (SCVs). SCVs enhance dynamic lifting, static instability, and moisture transport from the Arabian Sea that precondition the atmosphere for deep convection. The simultaneous existence of an LPS and an SCV creates a giant mid-tropospheric vortex that helps in organizing the deep convection. SCVs also slow down the propagation of LPSs. We show that the interaction of multiple synoptic-scale systems can lead to LERE even if individual systems are not strong enough.