Bandyopadhyay B. K. - Tropical Cyclone Activity over the North Indian Ocean

India Meteorological Department, Mausam Bhawan, Lodi Road, 110003 New Delhi, India

L. S. Rathore

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Tropical Cyclones (TCs) are intense synoptic scale weather systems which originate over warm oceans of the world, develop into massive vortices composed of swirling winds, intense clouds and torrential rains by drawing energy from the ocean. When they move over land, they cause large scale destruction to life and property over the coastal areas of the world. India, with an extensive coastline of about 7500 km is vulnerable to the destructive features associated with landfalling TCs of the North Indian Ocean (NIO) basin comprising of the Bay of Be ngal (BOB) and the Arabian Sea (AS).

Generally, under favourable environmental conditions, a pre-existing low pressure area develops into a cyclonic disturbance (CD) (maximum sustained wind speed (MSW) of 17 knots or more) which intensifies into a TC (MSW: 34 knots or more) or a severe TC (MSW of 48 knots or more) or even a very severe TC (MSW of 64 knots or more). On an average, about 11 CDs develop over the NIO during a year including 9 and 2 over the BOB and AS (Mohapatra et al. ). Subsequently, India Meteorological Department (IMD), the National Weather Service of India was established in 1875 and the important mandate of monitoring and forecasting of TCs over the NIO was vested with it. Since then, IMD is involved in detection, tracking and forecasting the movement and intensity of TCs over the NIO, which has been a highly challenging task during the early years with data sparse sea areas of BOB and AS. Further, the following few peculiarities of the NIO basin added to the challenging task: Despite these constraints, IMD has been striving hard towards reliable early warning services.

In the year 1914, an arrangement was made for receiving voluntary weather observations by wireless directly from ships at sea. Also, weather bulletins were transmitted to the ships by wireless communications since the same year. Thus, an exchange of information between the Indian Meteorological Service and Ships at sea began in 1914, and it continues even to this day. This service, which initially took the form of wireless transmission to individual ships, was soon enlarged into a regular twice-a-day broadcast service. These bulletins were broadcast from coastal radio stations to all ships. Later on NAVTEX stations of shipping authorities of Government of India were used for transmission of coastal weather bulletins (NAVTEX is an international automated direct-printing service for delivery of navigational and meteorological warnings and forecasts, as well as urgent marine safety information to ships). At present, coastal weather bulletins are being transmitted up to 465 km in the sea through NAVTEX stations along east and west coast of India as well as Andaman and Nicobar islands and Lakshadweep islands.

Around 19241925, IMD introduced an arrangement of issuing on loan, meteorological instruments to individual Masters of ship, so as to assist them in taking meteorological observations. Later a system of recruitment of ships for voluntary meteorological work known as Voluntary Observing Fleet (VOF) was commenced in 1946 under the auspices of International Meteorological Organisation. About 200 VOFs have been registered with IMD for taking meteorological observations over the BOB and AS region. With the advent of weather satellites in 1960s, lack of observations over the sea region has been addressed and subsequent technological advancements led to improve satellite observational systems with high resolution state-of-the-art instrumentation for microwave observations over the oceanic region.

Recently, there has been a paradigm shift in early warning services of TCs over the NIO due to various initiatives of IMD and the Ministry of Earth Sciences (MoES), Government of India including the modernisation programme of IMD (Mohapatra et al. ). Under this programme, there has been upgradation of observational systems, monitoring, analysis and prediction techniques, generation and dissemination of warning products, capacity building, outreach programmes, liaison with disaster management agencies, etc. Collaboration among various institutes in national and international levels in all these aspects has also helped tremendously in improving the early warning service of TCs over the NIO.

In this paper, specific international and national collaborative efforts undertaken by IMD so far and the improved results achieved in TC monitoring and prediction over NIO are presented. Section discusses conclusions and future scope.