Was significantly influenced by westerly wind in upper atmosphere, which may have led to initial IMD prediction about landfall timing going haywire
The very severe Bay of Bengal cyclone Hamoon, which made landfall as a cyclonic storm near midnight on October 24-25, 2023 south of Chattogram, Bangladesh, accelerated significantly due to westerly winds. This resulted in it making landfall about 16 hours earlier than the IMD predicted timeline, even 12 hours earlier, at noon on October 24, according to experts.
Everything about the cyclone, including landfall timing, location and even speed at landfall, appeared to change within four hours between morning and noon on October 24.
“It (Hamoon) is… likely to weaken gradually while moving northeastwards and cross Bangladesh coast between Khepupara and Chittagong around evening of October 25 as a cyclonic storm with wind speed of 65 to 75 kilometres per hour gusting to 85 kmph,” read the bulletin no 30 of India Meteorological Department (IMD) released at 12 pm on October 24.
However, the next bulletin released four hours later stated, “It… (would) cross the Bangladesh coast close to the south of Chittagong by the early hours of October 25 as a cyclonic storm with a wind speed of 80 to 90 kmph, gusting to 100 kmph”.
Moreover, the landfall category also changed from deep depression, as predicted through 5.30 am on October 24, to a cyclonic storm. The landfall point also shifted south from the initial predicted location in between Khepupara and Chittagong.
According to experts, multiple factors worked in tandem as the cyclone moved over the sea and closer to land, shifting gears literally and finally speeding up significantly in a way that could not be captured by previous models, including those of IMD.
IMD first predicted correctly about landfall timing in its national bulletin number 31, released at 4.15 pm on October 24, barely eight hours before the landfall initiated. “The cyclone quickened considerably under the influence of westerly winds and hence had landfall earlier than initially predicted; it can happen for cyclones,” Mrutyunjay Mohapatra, director general of IMD, told this reporter.
Initially, the high pressure zone over the China region influenced the cyclone Hamoon movement, another IMD expert said. “But since October 24 morning, under influence of westerly trough, it recurved and also quickened significantly, reaching a speed of 22-23 kmph in major part of October 24, instead of 16-18 kmph earlier.
Though IMD anticipated that the westerly trough would play a role, it did not anticipate such a strong impact, according to the expert.
Earlier, IMD deputy director general Sanjib Banerjee told this reporter that the weakening of cyclone Tej in the Arabian Sea may have contributed to the strengthening of its twin in the Bay of Bengal, Hamoon in the latter stages of its movement.
“In a twin cyclonic system — here Tej and Hamoon — often one gets stronger. In this case, Tej was the stronger one. But as it made landfall, shifted and gradually weakened, Hamoon seems to be picking up,” said Banerjee, when IMD first predicted that Hamoon would get stronger than what was earlier predicted.
“I have not seen such an unusual cyclone, within the recurved categories, in the last 30 years,” KJ Ramesh, former director general of IMD, told this reporter on October 27.
“It was unusual on several fronts. Firstly, traditionally, the cyclone track should have been towards Tamil Nadu, Andhra Pradesh and the Odisha coast, but instead it recurved towards Bangladesh, which was captured and predicted by IMD much earlier. But what could not be captured and predicted was the dominating role of westerly wind that pushed the cyclonic system quickly towards the north-east,” explained Ramesh.
Hamoon changed categories at least four times in the 24 hours before the landfall, he pointed out. It first intensified rapidly from a cyclonic storm to a severe cyclonic storm and then to a very severe cyclonic storm. It then weakened quickly to the first severe cyclonic storm and finally to a cyclonic storm before landfall.
An analysis of IMD bulletins showed that Hamoon was just a cyclonic storm as per bulletin number 27 released at 2.30 am on October 24, but it graduated into a severe cyclonic storm as per bulletin number 28 released three hours later.
Subsequently, it became a very severe cyclonic storm, as per bulletin number 30 released at 12 pm on October 24 and then the weakening process began. IMD bulletin no 33, at 8.30 pm on October 24, reported that Hamoon had already weakened into a severe cyclonic storm and close to midnight, when the landfall process started, it had further weakened to a cyclonic storm
The upper-level westerly wind flow, which remains atop the cyclonic system, often has shearing effects on the clouds. This results in the cyclonic system losing strength and the rainfall getting reduced — exactly what happened in the case of Hamoon, Ramesh pointed out.
“The models are run based on inputs and previous experiences. I am sure Hamoon experience will help IMD predict such cyclones better next time,” said a weather expert on October 29.
The cooler than normal seas played a role in Hamoon losing steam quickly before the landfall, said Raghu Murtugudde, a climate scientist at the Indian Institute of Technology, Bombay
“The northern Bay of Bengal has been cooler than normal, which might have played a role in Hamoon losing steam once it recurved away from land under westerly influence. A warm ocean would allow the cyclone to keep going, but as the energy supply was not there, it lost steam and crashed into land,” explained the scientist. “IMD seemed to have missed the ocean conditions and the forecast was off in terms of the landfall timing and location”.
However, it is anyway always difficult to predict the exact movement of a cyclone once it touches land, Murtugudde pointed out.
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