BHUBANESWAR: As Cyclone Dana was fast dissipating hours after making landfall near Bhitarkanika National Park of Odisha on Thursday midnight, it left in its wake a raging debate over the prediction of its intensity. Did it manifest as strongly as projected by the India Meteorological Department (IMD) was the poser.
According to the national weather forecaster, it crossed close to Habalikhati near Bhitarkanika and Dhamra, packing wind of 110 km per hour. Damage to kutcha houses as well as scores of uprooted trees that impacted low tension power lines vouched for the gale force.
However, for a cyclone that was billed as a severe category storm, Dana did not induce commensurate rain. The IMD had predicted extremely heavy rainfall of over 200 mm across several districts on Thursday and Friday.
In the last 24 hours, only Chandbali received very heavy rainfall of 158.6 mm. Similarly, heavy rains occurred in only four places - Paradip 76.4 mm, Bhadrak 70.2 mm, Balasore 68.8 mm and Jajpur 68.2 mm. A number of coastal and adjoining districts received light rains despite prediction of very heavy showers.
Weather experts pointed out that the tropical weather system did not gather sufficient strength due to factors such as presence of multiple high-pressure zones, unfavourable sea surface temperature during its passage across the Bay of Bengal apart from vertical wind shear. This was not articulated by the IMD though.
Associate professor of IIT’s School of Earth, Ocean and Climate Sciences Sandeep Pattnaik said environmental parameters such as diminished vertical wind shear, elevated oceanic heat potential, and increased sea surface temperatures enhance moisture availability, thereby facilitating moist convection and the formation of intense cloud structures.
“Additionally, the Madden-Julian Oscillation (MJO) was situated in a conducive phase over the east-central Bay of Bengal, which enabled the system to escalate from a low-pressure region to cyclonic storm Dana,” he explained.
However, a high pressure zone over the Arabian Sea led to influx of hot and arid north-westerly winds towards the cyclone which was moving into the north-western Bay of Bengal. The incursion of dry air into the cyclone’s core impeded moist convection, acting as a countervailing force against further intensification. Another high-pressure system over Myanmar persistently pushed the cyclone towards the Odisha coast, he pointed out.
“Due to an insufficient expanse of oceanic distance and inhospitable environment, the cyclone did not maturate into an organised system while approaching the Odisha coastline and was unable to surmount the resistance posed by north-westerly winds to penetrate into the state,” Pattnaik noted.
SOA University’s Centre for Environment and Climate (CEC) director Sarat Chandra Sahu said environmental conditions were conducive for the system till it was over the east-central Bay of Bengal.
“After it reached the northwest Bay of Bengal, a high pressure area over the Arabian Sea triggered dry northwesterly winds towards the system. The dry northwesterly winds pulled moisture from the surface and up to the mid-level which did not help the storm to thrive,” he said.
Dana developed an eye only for a brief period and eventually could not sustain strength to trigger heavy rains ahead and during landfall. It rapidly weakened into a deep depression by 2.30 pm on the day.