On Sunday, May 29, the India Meteorological Department (IMD) announced the onset of monsoon over Kerala, three days ahead of the usual date of June 1. A statement from the IMD, the country’s official weather forecaster, said that all the criteria for the onset of the southwest monsoon have been met and that the monsoon has advanced into the south Arabian Sea, Lakshadweep area and some parts of Tamil Nadu.
But Skymet, a private weather forecasting service provider that had earlier predicted the arrival of the monsoon on May 26, said that the onset criteria have been blatantly set aside, an allegation also raised by weather scientists and amateur observers. The IMD press release mentioned the rainfall criteria for only a single day, while according to its own rules data for two consecutive days should be considered.
“Declaring monsoon based on single day observations amounts to gross violation of standards, never attempted in the past. Any reputed scientific body can ill afford to bend rules and criteria, just to prove the forecast right. Such a step, if taken knowingly, becomes highly objectionable and if otherwise amounts to illusion of knowledge (sic),” Skymet said in a note posted on its website. The IMD had predicted that the monsoon onset would be on May 27.
As per the IMD, a set of three criteria should be met if the monsoon onset has to be declared. One, a rainfall of 2.5 mm or more should be recorded on two consecutive days over 60% of the 14 stations across Kerala. Second, the depth of westerlies (wind blowing from west to east in the middle latitudes) should be strong (in the range of 15 to 20 kts, 25 to 35 kmph) and maintained up to 600 hPa (hPa stands for hectoPascals, a unit of pressure) in the box 0°-10°N and 55° - 85°N. Third, the Outgoing Longwave Radiation (OLR) should be below 200 Watt/sq m in the box confined by 5°-10°N and 70°-75°E. The OLR is a measure of the amount of energy emitted to space by the earth’s surface. An increase in cloud formation would reduce the radiation and OLR values are used as a proxy for convection in tropical and subtropical regions.
The IMD, while announcing the monsoon onset, stated that the depth of the westerly winds extends up to 4.5 km above the mean sea level and their strengths have increased above 15 - 20 kts or 25 to 35 kmph. It also said that the cloudiness over southeast Arabian Sea has increased and average OLR is about 189.7 Watt/sq m. The statement also said there was widespread rainfall activity over Kerala during the past 24 hours and that 10 stations out of 14 received rainfall of 2.5 mm or more.
According to Skymet, while the criteria of westerlies and OLR were aligned with the requirement of onset for the last two days, the most visible manifestation – rainfall – fell short on May 28 and May 30, and onset conditions were fulfilled for only one day – May 29.
TNM reached out to K Santhosh, Director, IMD Thiruvananthapuram, for a response on the allegations that monsoon onset criteria were not met, but he said that the IMD press statement had explained it and he had nothing more to add.
A scientist attached to a weather observatory in Kochi said their radars are not showing winds with normal monsoon speeds.
“I cannot comment on wind speeds in Thiruvananthapuram but in Kochi it is not showing. Also, if you look at the satellite image, the normal monsoon time cloud cover is not there,” he said. Normally during monsoon, we see nimbostratus clouds but it is now mostly cumulus and cumulonimbus clouds, he explained.
“Technically, monsoon has been declared but we may have to wait for more time to receive the kind of rain and winds witnessed during monsoon,” he added.
“Though their own criteria for onset have not been met, IMD had declared onset yesterday which is unfortunate and could affect future reliance on onset data for any study,” tweeted Raj Bhagat P, an earth observer and a specialist in Geo Analytics.
Rather than using fixed thresholds for declaring #monsoon onsets, we should start using (and developing) Monsoon #indices that talk about strength and phase which help us understand the patterns better. pic.twitter.com/ZzrOlmcVKJ
— Raj Bhagat P #Mapper4Life (@rajbhagatt) May 30, 2022
Weather observers said while the cloud cover is not visible over Kerala, it can be seen over places like Lakshadweep and Maldives. “If someone asks whether the OLR criteria have been satisfied, I can’t say they have been,” a weather scientist, who did not want to be identified, told TNM.