Freak weather: ‘Extreme’ rainfall events are the big worry, data shows
NEW DELHI: According to the Regional Meteorological Center, New Delhi, the Capital’s Safdarjung station recorded 94.7mm rainfall in the 24 hours up to 8:30am on September 11. To put this in perspective, this is 8.6% of the total rainfall of 1,100mm received by the city-state till Saturday morning.
The gridded dataset of the India Meteorological Department shows the rain in Delhi at 672mm till Friday morning; the corresponding figure for Safdarjung is 1,005.3mm.
The 672mm is, in absolute terms, the 20th highest rainfall in the Capital in the past 120 years in the June 1-September 10 period.
Of this 672mm, 391.1mm or 58.2% was classified “heavy” or “extreme” rainfall. This share is the fifth highest since 1901. The other four years are 1912, 1933, 1975, and 1978. In other words, Delhi has seen the most patchily intense monsoon rains in 43 years in 2021.
NEW DELHI: According to the Regional Meteorological Center, New Delhi, the Capital’s Safdarjung station recorded 94.7mm rainfall in the 24 hours up to 8.30am on September 11. To put this in perspective, this is 8.6% of the total rainfall of 1,100 mm received by the city-state till Saturday morning.
The gridded dataset of the India Meteorological Department shows the rain in Delhi at 672mm till Friday morning; the corresponding figure for Safdarjung is 1,005.3 mm. The difference is because the gridded dataset is based on interpolation of rainfall from multiple stations that have operated consistently over the years to generate the area average of rainfall in a grid. There are two such grids in Delhi, data from which are averaged to generate average rainfall.
And here is where it starts to get interesting.
The 672mm is, in absolute terms, the 20th highest rainfall in the Capital in the past 120 years in the June 1- September 10 period.
Of this 672 mm, 391.1 mm or 58.2% was classified “heavy” or “extreme” rainfall. This share is the fifth highest since 1901. The other four years are 1912, 1933, 1975, and 1978. In other words, Delhi has seen the most patchily intense monsoon rains in 43 years in 2021.
How patchily intense?
The maximum LPA (long period average) or expected rainfall for any day up to September 10 in Delhi is 11.87 mm. Of the 102 days of the monsoon up to 8.30am on September 10, on 18 days, rain in excess of 11.87mm fell in the Capital. On nine of these days, rain in excess of 32.60mm fell. In contrast to this, there was no rainfall on 44 days.
This combination of extremely dry and extremely wet days in 2021 is exceptional compared to historical averages. In the 1961-2010 period, it took 7.54 days on average for 50% of the monsoon rainfall up to September 10 to fall in Delhi.
In 2011-2020 decade, it took 8 days on average, while in 2021, this mark was reached in just 6 days.
Similarly, 99% of the monsoon rainfall up to September 10 fell in 43.14 days on average in 1961-2010, in 43.3 days in 20112020, but in just 35 days in 2021.
The heavy rains of Friday-saturday (which will reflect in the gridded dataset 24 hours later) will skew these proportions even more. The reason HT used the gridded dataset is because it’s the only way to perform a longterm analysis of monsoon trends in Delhi.
To return to the 2021 monsoon’s patchy intensity in Delhi, it’s interesting to note that the long-term trend in rainfall in the Capital — to the extent that it can be measured in the gridded dataset which only gives data for 24-hour intervals — is that the share of high and extreme intensity rainfall (more than 35.5 mm in a day in a grid) and its share in total rainfall has actually been declining in Delhi.
The Met department measures deviations in rainfall for any interval by comparing it with the average rainfall for that interval in the 1961-2010 period or the Long Period Average (LPA).
Average “heavy” and “extreme” intensity monsoon rainfall was 82mm in the 20112020 decade in Delhi compared to the LPA of 234 mm. Its share in total rainfall in the decade was 19% compared to the LPA of 36%.