Extreme rain events, severe floods and hydrometeorological disasters are serious issues that is believed, will intensify the hydrological cycle under a changing global climate regime. Experiences suggest that the extreme rain events are large scale embedded (for the country as a whole), long period intense rainfall activities during the summer monsoon period. Every year some parts of the country experience extreme rain events, surprisingly even during large scale droughts. Location and time of occurrence of extreme rainstorms and associated floods vary drastically from one year to another. An unprecedented rain event on 26-27 July 2005 occurred at Santacruz, Mumbai and caused enormous loss to life and property. It raised apprehensions whether spatio-temporal features of extreme rain events over the country were changing. This motivated several research programmes on point-and-area-averaged extreme rainfalls. Studies such as these assist the understanding of the association between atmospheric circulation and extreme rainfall. On each day of the monsoon period, 10.4 per cent area of the country is computed to experience wet conditions. Wet conditions are calculated on the basis of actual daily rainfall over a 1 degree-latitude-longitude-grid (1×1° grid). If actual rainfall exceeds the daily-mean-monsoon-rainfall, a particular 1×1° grid is declared under wet condition and the day as a wet day. The daily-mean-monsoon-rainfall varies in India from less than 2 mm per day in the extreme north west to more than 24 mm per day along the central West Coast. For the country, 5 January is the driest day with only 2 per cent area under wet condition and 21 July through 10 August are the wettest days during which 32 per cent area experiences wet conditions.
Large scale wet spell and main monsoon wet spell
After obtaining the total area of the country under wet condition for each day of an individual year, there are five computational steps to identify large-scale-wet-spells (LSWSs). A LSWS is defined as ‘a continuous period during which on individual days more than 10.4 per cent of the area in the country is under wet conditions’.
- For each day, percentage area of the country under wet condition is calculated (total wet area divided by total geographical area, 3.3×106 sq km);
- The actual daily percentage area is normalised by dividing with the normal percentage area (10.4 per cent);
- The normalised percentage area is smoothened out by applying the 5 point low-pass binomial filter; and
- Finally, the spell is identified as a LSWS if the normalised, smoothed percentage area is equal to or greater than 1.0; otherwise, it is identified as a large-scale-dry-spell (LSDS).
Normally, six LSWSs and five intervening LSDSs occur over India each year from 3 April through 15 November. The third and the longest LSWS (91 days) occurs during 18 June through 16 September and is referred to as the main-monsoon-wet-spell (MMWS). Spatio-temporal distribution of seasonal rainfall indicates that 70 per cent of the all India monsoon rain occurs over 26.32 percentage area of India (PAI) during the MMWS, and the other 30 per cent is spread over 73.68 PAI during the 122 days of the monsoon period from June through September.
Large scale extreme rainfall events
In India seven types of extreme rainfall events occur during the MMWS and are categorised into 1- to 25 day or 1- to 150 day durations on the basis of rainfall intensity, rainfall aerial extent, rainwater and spatio-temporal characteristics.
On 26-27 July 2005, the India Meteorological Department (IMD) observatory at Santacruz International Airport, Mumbai recorded the highest 1-day rainfall of 944 mm. The rainfall for the 1×1° grid around Santacruz was 473.3 mm, the highest 1-day rainfall amount for the grid during the period from 1951-2007. To understand whether the unprecedented extreme rain event over Mumbai was an isolated episodic extreme rain event or a realisation of a long-term trend in the spatio-temporal extreme rain events over the country, the annual maximum 1×1° grid rainfall series (1951-2007) for 1-, 2-, 3-…150-day duration was developed. This was done by screening year wise highest rainfall amount from among the 365/366 daily rainfall of the 287 1×1° grids. From the series, it emerged that the mean of 1-day 1×1° grid spatio-temporal extreme rain events is 340.7 mm, and it increases in an exponential manner to 4612.3 mm for the 150-day duration. In addition, compared to the period 1951-1977, the 1- to 5-day extreme rainfall has increased by 9 to 15 per cent during 1978-2007 and the longer duration extremes (6- to 150-day) by 1 to 6 per cent. From the results it appears that, the unprecedented rainfall over Mumbai on 26-27 July 2005 is a realisation of a significant rising trend in the 1- to 5-day 1×1° grid spatio-temporal extreme rain events across the country.
Using satellite and radar inputs combined with synoptic and thermodynamic analysis, it was concluded that formation of mesoscale convective systems over Mumbai region comprising super thunderstorm cells and their interaction with the synoptic scale low pressure area from the Bay of Bengal led to the concentrated very high intensity rainfall. Attempts have been made with some success to simulate this extreme rainfall event using numerical weather prediction (NWP) models.
It has earlier been established that active monsoon conditions over north Konkan are usually associated with a trough off the west coast of India, formation of lows/depressions over north Bay of Bengal, presence of mid-tropospheric cyclonic circulation (MTC) off north Maharashtra-south Gujarat coast and a strong pressure gradient along the coast.
The major convergences in the oriental monsoon circulation
Local (point) unprecedented rain events are embedded in long period large scale extreme rainfalls. During active phases of the oriental monsoon circulation, seventeen convection-convergence zones develop which produce rainfall (Fig 1). Following changes in the monsoon circulation can be seen much before the occurrence of intense spatio-temporal extreme rain events over any location across of the country.
- Intensification of the monsoon circulation (low level convergence) into heat low over Middle East and Siberia-Mongolia, upper level divergence from the anticyclone over the Tibet-Turkey sector, outflows from the anticyclone in all directions, subsidence over the eight deep highs and return flows from the deep highs converging into the heat low;
- Accumulation of huge moisture in the atmosphere and thickening of the moist layer;
- Integration of the WCIC, the IGPC and GKC and development of a huge low-pressure spread over Indian subcontinent, Middle East and China-Mongolia sector; and
- Occurrence of depression/cyclone over north west India.
During 2005, intensification of the monsoon circulation started around 12 July and attained a peak round 26-27 July, which continued up to 10 August with small variations.
Warming of global troposphere causes intensification of vertical moisture flux convergence, which causes frequent occurrence of small scale, short period intense extreme rain events, while their horizontal integration causes large scale, long period heavy rain spells. Monitoring global tropospheric conditions on a real time basis using adequate network of surface and upper air observations with quality instruments is essential to forecast short range (few hours to few days) behaviour of the convergence zones. In the era of changing global climatic conditions, analyses of different types of extreme rain events are expected to provide critical information to the researchers and users alike about characteristics of extreme rain events.