59 of 121 rainy days saw deficit, Sept was wettest month: Data

NEW DELHI: According to the India Meteorological Department’s (IMD) gridded rainfall dataset, India received 870 mm of rainfall up to 8:30 am on September 29 in this year’s JuneSeptember monsoon season. This is about 2.32% more than the average of 850.2 mm rainfall for this interval in the 1961-2010 period, which is called the Long Period Average (LPA). These figures differ slightly from those published in IMD’s bulletin because the gridded dataset is based on interpolation of rainfall from multiple stations that have operated consistently over the years while IMD’s bulletin also takes into account newer stations. However, the broad trends do not differ much.

According to the gridded dataset, the cumulative figures hide the intermittent dry and wet spells seen this monsoon season. Fifty nine of 121 days of monsoon so far have seen a deficit compared to the LPA and 62 a surplus. There have also been more extreme intensity rainfall events than usual this year, to the extent that this monsoon season has seen the 30th highest extreme rainfall since 1901.

While monsoon rainfall in June was 15.2% more than the LPA for this month, July and August were much drier. These months had a 5.5% and 19.1% deficit compared to the respective LPA numbers. September has been the wettest month with rainfall as of 8:30 am on September 29 being 35.8% more than the LPA.

To be sure, a simple surplusdeficit breakup does not tell us about the degree of surplus or deficit. This degree can be seen from IMD’s classification of rainfall as normal, excess or deficient. A 19% deviation on either side of the LPA, for instance, is considered normal. A positive deviation of 20% or more is considered excess, and a negative deviation of 20% or more is considered deficient.

There has been more extreme intensity rainfall than usual this monsoon. Intensity is measured by the amount of rainfall that falls in a grid (a box bound by two latitudes and longitudes 0.25 degrees apart in IMD’s dataset) in 24 hours.

The average extreme rainfall (more than 244.5 mm in a grid) over India’s entire area up to September 29 was 6.81 mm in 1961-2010 and 6.94 mm in 20112020. This has been 7.02 mm this year, 3.1% more than the LPA. This is the 30th highest amount since 1901, the earliest year for which IMD has gridded data.

Heavy rainfall (between 35.5 mm and 244.5 mm in a grid) on the other hand has been 4.1% less than the LPA. Moderate intensity (between 7.5 mm and 35.5 mm in a grid) and light intensity rainfall (up to 7.5 mm in a grid) rainfall so far are 6.7% and 8.2% more than the LPA.