Severe weather-related phenomena like cyclones, tornados, cloud bursts, etc., are increasing in number all over the world in the last few years. This may partly be attributable to global warming. However, an exact correlation is yet to be established because of the complex interaction between land, ocean, atmosphere and radiation from the Sun.
A tropical cyclone is a storm system characterised by a low-pressure centre and numerous thunderstorms that produce strong winds and flooding rains. Generally, they form over warm ocean waters within 20 degrees either side of the equator. They feed on heat released when moist air rises, resulting in condensation of water vapour contained in the air; so they lose strength once they move over land. Although their effects on humans can be devastating, they relieve drought conditions.
They also carry heat and energy away from the tropics and towards temperate latitudes, making them an important part of the global atmospheric circulation mechanism. Many tropical cyclones develop from loosely organised cloud systems around a weak disturbance when atmospheric conditions are favourable. Tropical systems move by steering winds in the upper troposphere. Under favourable conditions, the disturbance intensifies by developing into deep clouds around the central low pressure area. India has a long coastline stretching over 7,000 km; so tropical cyclones are frequent.
Most originate as a deep depression in the Bay of Bengal and move in the north-westerly direction. On the west coast, the numbers are limited and the intensity is low. The deadliest cyclone was in 1964, the Rameswaram cyclone that wiped out the ancient township of Dhanushkodi and Talaimannar town in Sri Lanka. The cyclone in 1990 that hit Andhra resulted in over 950 deaths. In the recent past, the most severe cyclone was the one in 1999 that hit the Odisha coast.
Kerala had its worst experience during Cyclone Ockhi near the end of 2017. This caused around 100 deaths and many fishermen went missing. One can say that we were caught totally unawares by the onset of Ockhi. Its origin was over the southwest Bay of Bengal as a deep depression near Cape Comorin in the last week of November 2017. A well-marked low-pressure system was observed over the Comorin Sea at 0530 IST on November 29. This became a depression, crossed the Lankan coast and moved north-westward. Due to favourable conditions, the deep depression turned into a cyclonic storm by 0830 IST on November 30. The IMD issued a warning as early as November 28.
But it was about a deep depression, which in normal circumstances causes heavy rainfall. Nobody in the state took it seriously; though the cyclone warning was issued later, the official mechanism failed to act in a timely manner. This was a lesson to strengthen the disaster management system and the result was that the state was fully prepared to face the recent Cyclone Burevi. However, this cyclone weakened considerably before touching the Tamil Nadu coast. Hence Kerala was totally spared. But it has established that the disaster management authority could indeed take into account IMD’s real-time predictions.
ISRO has contributed significantly to India’s cyclone management system. Space-based systems have become the primary tools for collecting data on cyclones and cloud bursts. Recognising this need, the very first geostationary satellite developed by ISRO had a full-fledged meteorological instrument to take real-time pictures of cloud formation, note temperature and assess the movement. From 1982, this system has been providing visible and IR pictures to IMD, helping the latter make near-time forecasts before 24 hours. The IMD jointly with ISRO developed software for tracking a cyclone and making a forecast in its further movement, identifying the possible location of land impact.
In 2003, the INSAT-3D satellite was operationalised, which, in addition to providing cloud pictures, had a special IR scanner to obtain a vertical profile of temperature, moisture and wind in the atmosphere. This improved resolution in the visible band for the monitoring of mesoscale phenomena and severe local storms and enabled better land-cloud discrimination. The overall objective is to measure the temperature and humidity profiles to obtain a three-dimensional representation of the atmosphere. Today, two satellites, INSAT-3D and INSAT-3DR, provide continuous data to IMD and other researchers in India IMD has the responsibility to establish and operate a ground-based data collection system. From 1950, it has established rain gauges and instruments to collect wind speed and moisture.
In the early days, balloon experiments were the main source of data collection in the atmosphere up to about 20 km. The radiosonde was the most popular system then. With the availability of GPS and satellite-based data collection, this has undergone a sea change. Similarly, the data from the ocean surface is collected using buoys deployed at various locations that capture the temperature and wind. This is relayed through a geostationary platform in real-time.
One of the major spin-offs from ISRO is the Doppler weather radar. Sriharikota, the space port of India, is on the east coast and is frequently haunted by cyclones. So monitoring the clouds and winds surrounding the launch complex is a basic requirement. Initially, the C-band radar was used to monitor the intensity of clouds for a distance of around 50 km. Due to embargoes, ISRO was forced to develop tracking radars on its own. Based on its experience, the design team came up with the idea of using Doppler extraction technique by which the wind profile could also be derived.
The S-band is most suited for this; the first of its kind was developed by ISRO and established in Sriharikota. Later, this technology was transferred to BEL. IMD has several deployed now at key locations. There is a need for establishing a network of at least 100 such radars covering the entire coastal region. ISRO has developed what is called an automatic weather station. The local rainfall, temperature, pressure and wind data are collected and relayed through satellite to the central station for analysis. Nearly 1,000 such stations have been established in different parts of India on a pilot basis. But this is only a small fraction compared to the need of at least 10,000 such stations covering the entire country.
The Indian remote sensing satellites provide valuable data on disaster management, especially on flood-affected zones, and also several vital parameters of the atmosphere like cloud coverage, temperature at different altitudes, aerosols, etc. The parallel development of hardware and software, and modelling the local atmospheric phenomena and global circulation has enabled us to make the forecast not only in real-time but also for the long term, synthesising the data from the satellites of the US, Europe and India and taking inputs from global simulations being carried out by a number of agencies to run our local prediction more accurately.
We are capable of predicting events a week ahead reasonably well and 48 hours ahead very accurately. Communication to remote areas in a vast country like India was a challenge. From 1982, ISRO has been providing the emergency communication transponder, which could relay the disaster warning signal to every nook and corner of India. ISRO has always been sensitive to the needs of society in this vital area of atmospheric studies and weather systems and works hand in hand with IMD to provide accurate weather forecasts and disaster warnings. Though we have powerful systems for observation and prediction, using this data effectively for disaster management has been a big challenge. Proper management systems at state and district levels have to take care of receiving the warnings and implement mitigation measures.
G Madhavan Nair
Former ISRO chairman