Ministry of Earth Sciences Government of National institute of Ocean Technology, India

Rapid mode data transmission by the MoES-NIOT Data buoy during the Cyclone Amphan in

Under the Ministry of Earth Sciences (MoES), Government of India, the Ocean Observations Systems group of National institute of Ocean Technology (NIOT), Chennai India established Moored buoy network in in 1997. A new technique of auto-sensing low Air pressure and high wind speed leading to switch over to rapid mode data transmission is developed for which patent is applied

One of the Indian moored buoy system (WMO TD: 23092, at 17.5oN/ 89.1oE) deployed in Northern Bay of Bengal switched over to high frequency transmission mode during the passage of cyclone Amphan on 19th May 2020. A unique algorithm is implemented in this indigenous buoy system capable of transmitting real time data sets in high frequency mode during cyclone at low pressure and high wind speeds. This buoy system withstood the cyclone pass and also provided valuable Data sets which was disseminated in near real time to nodal agencies for accurate forecast and the land fall. Dr Rajeevan, Secretary MoES acknowledged the accurate forecasts and warnings for Cyclone Amphan by India Meteorology Department, which helped to save precious lives. He noticed buoy data showing strong stirring of water due to strong winds reduce SST a drop by 4 deg. Cyclone Amphan signal was most evident in NIOT-MoES Buoy BD13 (87E, 14 N) located at a distance of 74 km (40 nm) from the cyclone track. Sea level pressure dropped to 967 hPa and wind speed increased to 101 km/hr (28 m/s) on 18.05.2020 12 GMT. Buoy also recorded Significant Wave Height of 6 metres

Buoy measurements during the previous cyclones were analysed in detail to identify a suitable method to identify the approaching cyclones and to trigger into high frequency transmission. The analysis revealed that the quickest response is evident in air pressure and in wind speed in which the measurement and processing are also easy compared to that of waves and currents. It is observed that the air pressure exhibits semi diurnal oscillation and is proposed to utilise the cyclic nature in detecting a cyclone. Air pressure exhibits two peaks and two low values in a day. The air pressure records the highest values at the same time every day at a particular location. The reduction in semi-diurnal oscillation during a cyclone passage is utilised in detecting a cyclone or a low pressure system.

The track of cyclone AMPHAN in Bay of Bengal and the time series observations of air pressure and wind speed with high frequency real time data

The difference between the present and previous day (Day-to-day difference) observation at same time is less than 2-3 hPa in normal conditions. The same value started increasing fast and reached maximum during the peak of the low pressure system. Based on the previous observations 4 hPa is suggested as the threshold to trigger cyclone mode. High frequency transmission is triggered on crossing the threshold and the pressure at the time of trigger is taken as reference. The rapid mode continues until the latest air pressure is more than the reference value and resumes the normal transmission rate.

The moored buoy deployed in the northern BoB with Indian data acquisition system named as Hrudaya triggered high frequency data transmission during the recent super cyclone Amphan in Bay of Bengal. The buoys transmit data at every three hours under normal conditions, while the data is transmitted at every one hour during the cyclone. The high frequency transmission was triggered at 00:00 GMT on 19th May 2020 and continued till 16 GMT on the same day providing 11 additional real time data sets. The rapid mode algorithm enabled the buoy system to provide the critical met-ocean data sets to stake holders with higher frequency in real-time.