Intense showers, long dry spells: An exceptional monsoon for Delhi
According to the Regional Meteorological Centre, New Delhi, the Capital’s Safdarjung station recorded 94.7 mm rainfall in the 24 hours up to 8:30 AM on 11 September. To put this in perspective, this is 8.6% of the total rainfall of 1100 mm received by the city-state till Saturday morning.
The gridded dataset of the India Meteorological Department shows the rain in Delhi at 672 mm till Friday morning; the corresponding figure for Safdarjung is 1005.3 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 672 mm 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.
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 8:30 AM on September 10, on 18 days, rain in excess of 11.87 mm fell in the Capital. On nine of these days, rain in excess of 32.60 mm 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 2011-2020, 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 long-term 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. IMD 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 82 mm in the 2011-2020 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%. This year, the numbers (as mentioned above) were 391.1 mm and 58.2%
To be sure, does not necessarily mean that rainfall in Delhi became less intense in the 2011-2020 decade. If the increase in intensity of rainfall in Delhi was such that it fell in fewer hours than before, it would not be measured by the gridded dataset. This is because this data is collated only for 24-hour rainfall.
But 2021 is an outlier even in terms of the 24-hour metric. By the end of the monsoon season on September 30, it will likely be even more so.