Trends of Extreme Rainfall Events Based on Gridded Data
Goswami et al. (2006) using a rainfall data set for the period 1951-2000 showed that there are significant rising trends of extreme rainfall trends over the northern parts of India. Using a longer time series of daily gridded data set of 1901 -2004 period, Rajeevan et al. (2008) documented an increasing trend of heavy precipitation exceeding 15 cm per day over Central India. But the magnitude of the increasing trend was much smaller than the trend documented by Goswami et al. (2006) using 50 years (1951-2000) of rainfall data. Rajeevan et al. (2008) further documented that there are significant multi-decadal variation in heavy precipitation events over Central India, which may be associated with the variations in sea surface temperature anomalies over the tropical Indian Ocean.
Figure 7 shows the spatial variation in very heavy rainfall (VHR) events (exceeding 15 cm in 24 h) during the monsoon season for the period 1901-2009. The time series of average frequency of VHR events over the Central India for the period 1901-2009 is shown in Fig. 8. From Fig. 8, it can be seen that VHR events
Fig. 7 Spatial variation in frequency of extreme rainfall events (rainfall exceeding 15 cm per 24 h) during the monsoon season (June-September), period: 1901-2009
Fig. 8 Time series of extreme rainfall events (Exceeding 15 cm in 24 h) averaged over Central India (monsoon core zone) for the period 1901-2009. The smoothed line with 9 point filter is also shown in red color (color figure online)
show a multi-decadal variation. They were more frequent in 1920s and 1930s and then in 1980s and 1990s. However, VHR events were below average in 1940s and 1950s. Over the period 1901-2009, there is an increasing trend of about 0.8 events per decade or 6 % per decade. The trend obtained by Goswami et al. (2006) for the period 1951-2000 is 10 % per decade. The trend for the recent period 1951-2004 is 2.2 events per decade or about 14.5 %. These trends are significant at the 99 % confidence level. These variations could be related to global SST variations (Rajeevan et al. 2008).
Pai et al. (2014) using the 0.25° x 0.25° gridded data examined long-term trends of the extreme rainfall over three regions over the country. The study found that during the recent decades, there has been significant decrease in moderate rainfall events, while heavy and very heavy rains have increased in frequency. These results are consistent with the results of Goswami et al. (2006) and Rajeevan et al. (2008). Therefore, the results on extreme rainfall are independent of the resolution of gridded data used for the analysis.