The data provided by the Megha-Tropiques atmospheric research satellite will help provide insights into the patterns of rainfall in the world's tropical regions.

After a difficult and, at times, uncertain gestation lasting over a decade, the Indo-French atmospheric research satellite, Megha-Tropiques, is at last ready to leave aboard the Polar Satellite Launch Vehicle (PSLV), from Sriharikota.

Prior to the two countries joining hands in this effort, scientists in both countries, along with their respective space agencies, had been independently considering similar sorts of missions. In the late 1980s and the early 1990s, the French were examining the possibilities for a ‘Tropiques' satellite. The Indians, for their part, were thinking of a ‘Climatsat' satellite around the mid-1990s.

The idea of merging these efforts came about as a result of contacts between scientists in the two countries.

In 1998, the space agencies, the Indian Space Research Organisation (ISRO) and the Centre National d'Études Spatiales (CNES), decided to carry out a feasibility study. The following year, they signed a Statement of Intent. They would, in the words of a press release issued on the occasion, pursue cooperation on a mission “aimed at enhancing the understanding of tropical weather and climate.”

The name chosen for the satellite, Megha-Tropiques, reflected the mission's goals. ‘Megha,' the Sanskrit word for clouds, underscoring a key focus of the satellite, and the French word ‘Tropiques' denoting its concentration on the tropical region.

Subsequently, the ISRO and the CNES signed a Memorandum of Understanding (MoU) in 2001 to undertake a detailed design of the satellite. But then, amid heavy cuts in the French space agency's budget, the satellite's fate became decidedly uncertain. Finally, in late 2004, the agencies signed a second MoU that gave the green signal to proceed with development of the satellite.

The use of satellites to watch over the weather and collect related data began with the American TIROS-1 that was launched in April 1960. Over 300 satellites followed, carrying weather cameras and a variety of other sensors to measure various parameters of the oceans and the atmosphere.

With their capacity for global coverage, satellites made it possible to monitor and follow events over the oceans. Satellite imagery showed, for instance, that the heavy rains of the Indian monsoon were brought by a vast band of clouds that arose near the equator and then steadily moved northward over the subcontinent. The information sent back has helped unravel the complex ways in which the ocean and the atmosphere influence each other to produce the world's climate and weather.

The U.S.-Japanese Tropical Rainfall Measuring Mission (TRMM), in particular, which was launched in 1997, has contributed greatly to studying rainfall patterns in the tropics, including the powerful storms that sweep these parts of the world. Although the satellite was expected to function for only three to five years, it still continues to be in service. In many ways, the Megha-Tropiques will be following in the footsteps of this ageing satellite whose days are numbered.

The famous American meteorologist Jule Charney showed many years ago that the tropics have inherently more predictability in them, remarked Roddam Narasimha, a distinguished scientist and a member of the Space Commission that oversees the Indian space programme. He is also chairman from the Indian side of the joint scientific working group for the Megha Tropiques.

“But we are not able to cash in on that predictability.” One reason was that a sufficiently deep analysis of basic scientific problems had not been done. As a result, models from all over the world don't have the same skill over the tropics, particularly on the monsoons, that they have at higher latitudes, he pointed out.

In an orbit inclined more closely to the equator than the TRMM, the Megha-Tropiques will enjoy much greater coverage of the tropics. The satellite can have up to six passes a day over places that are within about 20° of the equator.

Such frequent coverage is essential to study how, say, a cloud system in the Bay of Bengal evolves, observed J. Srinivasan, chairman of the Divecha Centre for Climate Change at the Indian Institute of Science in Bangalore, who is the principal investigator for the mission from the Indian side. Such a system typically goes from birth, through growth, to death in 12 hours or less. The satellite could capture data about the cloud several times during that period.

The equatorial region gets more energy from the sun than places closer to the poles. The surplus energy from the tropics is moved to other parts of the globe by ocean currents and atmospheric circulation.

As water vapour condenses to form clouds and then falls as rain, it releases the heat that went into its evaporation. Such release of latent heat provides a large part of the energy that drives global atmospheric circulation.

The sensors on the Megha-Tropiques have been designed to provide information about the energy and water cycles that drive cloud systems in the tropics.

The satellite lacks the active radar that the TRMM possesses, which gives a more accurate estimate of the amount of rainfall occurring. But, like the latter, a passive sensor on the Megha-Tropiques, the Microwave Analysis and Detection of Rain and Atmospheric Structures (MADRAS), will be able to ‘see' into clouds by picking up faint microwave signals given off by the earth and the atmosphere. This instrument will gather data on winds, water vapour and rain as well as about liquid water and ice in clouds.

However, the Megha-Tropiques scores over the TRMM in its ability to map the vertical profile of water vapour. It does so with two sensors.

One of those instruments, known by the acronym SAPHIR, will measure water vapour at six vertical levels from the ground to a height of about 12 km.

This instrument will have a horizontal resolution of 10 km. The other instrument will use the bending of radio signals broadcast by GPS navigation satellites to measure water vapour at some 100 vertical levels but with poor horizontal resolution.

The vertical distribution of water vapour is crucial in triggering the formation of deep clouds that produce a great deal of rain, pointed out Dr. Srinivasan. “At present, we don't know how much water vapour has to be present at what heights in the atmosphere in order for deep clouds to develop.” Figuring this out could hold the key to accurately predicting heavy rainfall events, such as the deluge that swamped Mumbai in 2005, a problem that is occurring time and again during the monsoon.

The Scanner for Radiation Budget Measurement (SCARAB) on the satellite will measure sunlight reflected back into space by clouds as well as the radiation emitted by the heating up of the earth and the atmosphere. A cloud system needs to retain enough energy to keep itself going. But when clouds become very deep and highly reflective, so much radiation is lost to space that the system slowly dies, observed Dr. Srinivasan.

The data provided by the satellite will help scientists better understand not just the vagaries of the Indian monsoon but also of rainfall in other tropical regions of the world. There are scientists from 11 countries in Asia, Africa, Europe and the Americas on the 21 international science teams that have been formed for the Megha-Tropiques mission, according to Rémy Roca of the Laboratoire de Météorologie Dynamique in Paris, who is the principal investigator for the mission on the French side.

After the satellite was launched, its sensors would be carefully calibrated over the subsequent three months. Then, for the next six months, data from the satellite would be supplied to members of the international science teams. After that, the data would be made freely accessible to all scientists. There were a number of groups across the world who wanted to incorporate the data into their models for weather and storm forecasting, he added.

The benefits from the Megha-Tropiques will be enhanced by its data being used in conjunction with those from other satellites. The Indo-French satellite is part of the Global Precipitation Measurement (GPM) mission based on an international network of satellites. The GPM Core Observatory, which will carry an active radar, is currently expected to be launched in mid-2013.

This GPM satellite was in effect a follow-on of the TRMM mission but would orbit at higher latitudes, noted Robert A. Houze, Jr., an atmospheric scientist at the University of Washington, in Seattle, U.S.

Its active radar would be able to measure precipitation and show vertical structures in the atmosphere more precisely than the Megha-Tropiques. The GPM satellite could provide a kind of calibration for the latter. “So the two together will make a very nice programme,” said Dr. Houze, who heads one of the Megha-Tropiques science teams.

The Megha-Tropiques will be “an incredible opportunity to advance process and predictive understanding of the monsoons,” remarked Raghu Murtugudde, a climate scientist at the University of Maryland in the U.S., in an email. “Doing the water content, water vapour profiles and radiation is like having the complete recipe for a megha-dish! It will be delicious.”

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