Cyclone Dana that will be forming in the east-central Bay of Bengal by October 23, 2024 is showing uncertainty in its forecast for landfall. The reason for this could be insufficient input information for the forecast models.
The depression that will eventually strengthen to Cyclone Dana formed in the early morning hours of October 22.
On October 22, the India Meteorological Department (IMD) predicted that the cyclone may turn severe in the early morning hours of October 24 and make landfall between Puri in Odisha and Sagar Islands in West Bengal. The direct distance between the two areas is more than 300 km.
The crossing of the Severe Cyclone Dana with wind speeds between 100-110 kilometres / hour (km / hr) and gusts of up to 120 km / hr may happen between the night of October 24 and morning of October 25, according to IMD.
On the Windy weather analysis and visualisation platform, data from the Global Forecasting System (GFS) of the National Centers for Environmental Prediction (NCEP), United States and the European Centre for Medium-Range Weather Forecasts (ECMWF) differ widely in their area and time of landfall for Cyclone Dana.
On October 22, the GFS data projected a landfall close to Contai in West Bengal around 7 am on October 25. The ECMWF data showed a landfall close to Chandbali in Odisha around 9-10 pm on October 24.
“IMD-GFS, NCEP-GFS both indicating extreme north Odisha / south West Bengal landfall, however ECMWF is keeping it Odisha landfall,” the author of the weather blog Vagaries of Weather told Down To Earth (DTE).
Cyclones in the North Indian Ocean region have been quite uncertain in recent years because of various factors such as rapid intensification (RI), even though IMD has kept a tight rein on the tracks, intensities and places of landfall of most of them.
RI is said to occur when a tropical cyclone gains wind speeds of more than 55 km / hr in a 24-hour period.
One reason for the divergent models could be the lack of adequate input observational data, especially from the sea surface and subsurface in the Bay of Bengal.
“Across the world’s ocean basins the measurement of sub surface temperatures is not done very well right now and this is one of the gaps in data and understanding that we have,” Roxy Mathew Koll, climate scientist with the Indian Institute of Tropical Meteorology, Pune, told DTE.
While the monitoring of sea surface temperatures can be done using satellites, the subsurface temperature data needs to come from surface monitoring using ocean monitoring instruments such as buoys and moorings. “We need to also understand how these subsurface temperatures are evolving over time to be able to make better predictions,” said Koll.
India lost a lot of its own buoys and moorings during the COVID-19 pandemic as they could not be maintained and repaired, according to Koll. The data from these instruments placed at certain locations is monitored by the Indian National Centre for Ocean Information Services (INCOIS) based in Hyderabad.
This data of sea surface and subsurface temperatures is essential for accurately predicting characteristics such as RI, track and place of landfall of a cyclone.
“The United States has replaced most of their buoys in the Arabian Sea and the west Indian Ocean regions but not in the Bay of Bengal. But the Indian buoys are active in the Bay of Bengal,” said Koll.
“Around 50 per cent of the buoys that are maintained by INCOIS and National Institute of Ocean Technology are still functional in the Bay of Bengal,” Ajay Kumar B, scientist with INCOIS, told DTE.
“There are two active buoys in the Bay of Bengal in the path of cyclone Dana, one of which is in the middle of the Bay, which are transmitting information crucial for cyclone forecasting,” said Kumar.
Mainly these are wave rider and mooring buoys that give information such as wave heights and sea currents during a cyclone. Kumar did not mention the sea surface temperature data being received from these buoys. There are also tide gauges at various locations along the coast which ferry information about tidal variations of an approaching cyclone.
Another reason for uncertainty in track and place of landfall could be the formation of the cyclone close to the southwest monsoon season as is the case with Cyclone Dana, which has formed within a week of the complete withdrawal of the monsoon winds from India. This occurs because of complex atmospheric activity close to the monsoon season, according to Koll.
Also, difficulties in prediction could arise when the structure of the cyclone system itself is not well-organised. If a cyclone forms from the lower troposphere (0-8 km from sea level) to the upper troposphere then it is well-organised and easier to forecast.
This is because winds in the upper troposphere are stronger and they have control over the cyclone’s structure, making it stable, according to Koll. When the cyclone forms from the lower to middle troposphere then it is not well-organised and therefore quite difficult to predict. This is because winds in the middle troposphere are not as strong.
The data from both GFS and ECMWF on Windy showed that the cyclone is weakly formed in the upper troposphere (8-13 km above sea level).