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The satellite called Megha Tropiques

Almost 10 years after India and France first joined hands to build a satellite for studying cloud formation and rainfall in the tropics, the Megha Tropiques spacecraft is expected to take to the skies aboard a Polar Satellite Launch Vehicle some time later this year.

It was in November 1999 that the Indian Space Research Organisation (ISRO) and France’s space agency, Centre National d’Etudes Spatiales (CNES), signed a Statement of Intent to build a spacecraft “aimed at enhancing the understanding of tropical weather and climate.” The name chosen for the satellite reflects both its mission and its roots in the two countries, juxtaposing a Sanskrit word for clouds with a French one for the tropics.

However, there soon followed a prolonged period of considerable uncertainty and there were times when it seemed that the project could fall through. Among the changes that saved Megha Tropiques was the decision to build the spacecraft around the basic structure that ISRO had developed for its remote sensing satellites instead of using France’s Proteus platform as originally planned.

The two space agencies finally signed a Memorandum of Understanding to proceed with development of the satellite five years ago.

As its name indicates, Megha Tropiques is a satellite that is intended to watch over the tropics. “Earth’s climate is driven by energy from the Sun,” pointed out J. Srinivasan of the Centre for Atmospheric and Oceanic Sciences at the Indian Institute of Science, who is the principal investigator looking after the satellite’s scientific programme. “Energy absorbed from the Sun is highest at the equator and lowest at poles.”

As water vapour rises up into the atmosphere, forms clouds and then falls as rain or snow, it transfers energy from Earth’s surface up to a height of several kilometres. The gases in the atmosphere absorb little of the energy that comes directly from the Sun, Prof. Srinivasan told this correspondent. If there were no water vapour, conduction from Earth’s surface would heat the air to perhaps a height of only a hundred metres or so.

The tropics therefore provide the massive “heat engine” that drives circulation of both the atmosphere and oceans. The spacecraft’s four instruments will help scientists understand components of this heat engine and how each component affects weather and climate. These instruments will continuously measure the energy balance of the earth and atmosphere, check how much water vapour is present in the atmosphere, survey what sort of clouds are growing and which ones are dissipating and provide estimates of how much rain occurs.

The satellite’s orbit, a low-inclined one of just 20 degrees to the equator, has been chosen to keep it largely over the tropics. (By contrast, many earth-viewing satellites are usually in orbits that take them from pole to pole.) As a result, the satellite’s instruments can monitor rapidly changing processes in the tropics, said Prof. Srinivasan.

The satellite will accurately measure the vertical profile of water vapour in the atmosphere in two different ways, he said. Such data is crucial for figuring out how clouds and weather systems in the tropics evolve. The distribution of water vapour in the atmosphere is one of the factors that govern whether or not powerful cyclones form, he pointed out.

A three-day international conference on the Megha Tropiques begins in Bangalore on Monday (March 23).

For six months after Megha Tropiques is launched, data sent back by the satellite will be calibrated and checked for quality. Thereafter, all data from the satellite will be made freely available to scientists across the world over the Internet, said Prof. Srinivasan. By doing so, data from Megha Tropiques can be combined with information from other scientific satellites currently circling the globe such as NASA’s CloudSat and the Franco-American Calipso and Ocean Surface Topography Mission (OSTM)/Jason-2missions as well as India’s Oceansat-2 that will be launched later this year.

Such analysis will increase our understanding of processes occurring in the atmosphere and ocean, he noted. That, in turn, will make it possible to greatly improve our existing weather and climate prediction models, with enormously practical benefits to India and all the countries in the tropics, added Prof. Srinivasan.

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