One of the most significant consequences of global warming due to increase in greenhouse gases would be an increase in magnitude and frequency of extreme precipitation events. These increased extreme precipitation events can be attributed to increase in moisture levels, thunderstorm activities and large scale storm activities. Physical considerations and model studies indicate that tropospheric warming leads to an enhancement of moisture content of the atmosphere and are associated with an increase in heavy rainfall events. Extreme rainfall results in landslides, flash floods, and crop damage that have major impacts on society, the economy, and the environment. Although prediction of such extreme weather events is still fraught with uncertainties, a proper assessment of likely future trends would help in setting up infrastructure for disaster preparedness. Recent studies have shown that there is an increasing trend of extreme precipitation events in India.
The exceptionally heavy rainfall of 944 mm over Mumbai (Santacruz) on 26th July, 2005 was extremely unprecedented in nature, which led to many more studies on frequency and variability of heavy rainfall events. The development of a high resolution (1ox1olat./long.) gridded daily rainfall dataset for the Indian region by National Climate Centre (NCC) at India Meteorological Department (IMD) was very helpful in undertaking such studies. The results presented alongside are based on the study about the trend and the frequency of heavy rainfall events over the Indian region and its contribution to total rainfall during the southwest monsoon season for a period of 55 years from 1951 to 2005 using the daily gridded (1×1) rainfall.
The extreme rainfall events are classified into categories based on the amount of rainfall (R) over any grid point in 24 hours. Based on the amount of rainfall in a day IMD has classified it into six categories. However, in our present analysis the six categories identified by IMD has been regrouped into three broad categories with
- Light to rather heavy rainfall (0 < R < 64.4 mm),
- Heavy rainfall (64.4 < R <124.4 mm) and
- Very heavy to exceptionally heavy rainfall (R > 124.4 mm). In this study the last categories with R > 124.4 mm will be referred hereafter as extreme rainfall events.
It is seen that the frequency of extreme rainfall (rainfall > 124.4 mm) shows increasing trend over the Indian monsoon region during the southwest monsoon season from June to September (JJAS) and is significant at 98 per cent level (Fig. 1). It is also found that the increasing trend of contribution from extreme rainfall events during JJAS is balanced by a decreasing trend in category i (rainfall < 64.4 mm/day) rainfall events. Similarly, on monthly scale the frequency of extreme rainfall events show significant (95 per cent level) increasing trend during June and July, whereas during August and September the increasing trend is not significant statistically (Fig. 2). Like the frequency of extreme rainfall event the contribution of extreme rainfall to the total rainfall in a season is also showing highly significant increasing trend during the monsoon season from June to September and during June and July on monthly scale. It is observed that the mean monthly contribution of heavy and extreme rainfall events (rainfall > 64.4 mm in a day) during June-July is 5 to 6 per cent higher than that during August-September and hence contributes significantly to the total rainfall during the first half of the season (June and July).
The majority of earlier studies have attributed the increase of extreme rainfall events to the global warming and climate change. A physical explanation for an increase in heavy precipitation with global warming is provided by Trenberth et al. 2003. (Trenberth K.E., Dai A, Rasmussen R.M., Parsons D.B. 2003; ‘The changing character of precipitation’; Bull. Amer. Meteor. Soc; 84; 1205–1217). In order to understand the physical reason behind this increase of extreme rainfall event the degree of moist convective instability is calculated over the Indian monsoon region during June to September on each day for the 55 monsoon seasons from 1951 to 2005 and the daily mean is calculated over the region by taking the average of 6710 (55 x 122) observations. The mean value is averaged over area bounded by 20-27.5oN, 70-87.5oE. In order to calculate the interannual variability of degree of instability, the number of days out of 122 days in a season with higher than normal degree of convective instability represented by higher than normal magnitude of difference moist static energy (MSE) at 700 hPa and 100 hPa is taken as discussed in Pattanaik (2003) (Pattanaik D.R. ‘Analysis of moist convective instability over Indian monsoon region and neighbourhood’; Mausam; 54:659-670). Thus, the ‘MSE (700 hPa) – MSE (1000 hPa)’ averaged over the central India bounded by 20-27.5oN, 70-87.5oE is calculated during all the 55 years and is shown in Fig. 3. As it may be seen the number of days with greater degree of instability during the monsoon season is showing a significant increasing trend (significant at 99.9 per cent level). The 7 year running mean as superimposed in Fig. 3 also shows a significant increase of frequency of above normal unstable days from about 55 days during the beginning of the period to about 85 days towards the end of the period.
Hence it may be inferred that higher moist convective instability coupled with enough moisture availability during the southwest monsoon season can increase the occurrence of deep convection and hence the frequency of extreme rainfall events. Thus, climate change in the form of the increasing trend in the number of above normal unstable days during the monsoon season may be the cause for the increased frequency of extreme rainfall events over the Indian region.