Features

A peek into the clouds

Ranganath Ramarao Navalgund

doi:10.1038/nindia.2009.108 Published online 22 April 2009

(L to R) R. R. Navalgund, ISRO chairman G. Madhavan Nair, Member of the Space Commission R. Narasimha and Renaut Didier of CNES at the Bangalore conference.

How can we scientifically retrieve and understand various parameters in the tropical environment? How best can we apply them to the study of tropical systems in short term weather and of the climate in the long term? These were questions, answers to which were sought at an international conference on the Indo-French project on Megha-Tropiques satellite (Megha is Sanskrit for clouds, Tropiques French for tropics) in Bangalore (March 23-25, 2009).

As understandable, the project was conceived to understand and predict spatial and temporal variations of clouds and rainfall in the tropical atmosphere. The satellite is in its advanced stages of development and scheduled for launch during 2009-10.

The three day conference jointly organised by the Indian Space Research Organisation (ISRO) and French Space Agency CNES was divided into seven technical sessions — its science and applications, retrieval and validation and Megha-Tropiques and NASA's Global Precipitation Measurement (GPM) mission. The calibration and the validation plans including identification of various sites, necessary instrumentation and methodologies were discussed in great detail. The scientific community evinced keen interest in exploring the possibility of inclusion of Megha-Tropiques(MT) in the GPM constellation.

The conference was attended by about 35 scientists from various international agencies from France, Japan, Brazil, UK and USA besides 150 senior scientists from Indian research and development agencies and Universities.

The satellite

Megha-Tropiques will be placed in a low inclination orbit of 200 which will facilitate high temporal re-visit capability over the tropical region. This orbit will enhance observational capability of the rapidly developing tropical systems and will enable monitoring of weather and climate over the entire tropics.

The objectives of mission are to improve the understanding of processes related to large tropical convective systems and the life cycle, to improve the determination of atmospheric energy and water budget in the tropical area at various time & space scales, and impact of tropical climate and their predictability: drought, monsoon variability, floods and tropical cyclones.

It will carry four scientific payloads of which Microwave Analysis and Detection of Rain and Atmospheric Structures (MADR/AS) is a five-frequency-nine-channel passive radiometer. This will measure precipitation, cloud micro-physics, ocean surface winds, total water vapour and liquid water content of the atmosphere.

The second payload, Scanner for Radiative Budget (ScaRab) is a cross track scanning radiometer providing data on the radiative budget of tropical convective systems. The third, Soundeur Atmospherique du Profil d'humidite intertropicale par Radiometrie (SAPHIR), is a multi-channel cross track millimeter wave humidity sounder for atmospheric profiling in the inter-tropical region. The fourth payload is a GPS Radio Occultation Sensor (GPS-ROS), capable of measuring water vapour and temperature profiles in the tropics.

While the first three sensors are being developed by CNES, the GPS-ROS will be provided by the Italian Space Agency. The MT mission will measure the vertical profile of water vapour by two totally different techniques.

Space Applications Centre (SAC)at Ahmedabad is the nodal Centre for Megha Tropiques geophysical parameter retrieval, science and applications. It is also involved in the development of electronics, integration of CNES supplied part of the payload, integration of the scan mechanism with MADR/AS payload and overall testing. The data shall be received at the ISTRAC (ISRO) station at Bangalore and by CNES earth stations at Kourou and Hartebeestoek in South Africa.

Scientific rationale

Significant temporal variations are seen in clouds and rainfall in different time-scales ranging from few hours (diurnal), sub-seasonal (few weeks), inter-annual to several years. On the inter-annual scale, the occurrence of El Nino and variability of monsoon rainfall between good and bad monsoon years are some of the prominent features. Variation of the monsoon on the inter-annual scale is linked to variation on sub-seasonal scales between active spells and breaks in the monsoon. The sub-seasonal variation is in turn embedded in the planetary scale systems.

There has been no satellite mission to simultaneously measure all the major factors controlling the energy and moisture budget.

During the past four decades many countries have launched more than 400 meteorological satellites to understand and predict the variability of weather and climate. About two decades ago, it was realised that tropical clouds play an important role in modulating global weather and climate. They are organised in a variety of scales ranging from a few kilometers to many thousands of kilometers with a lifetime between a few hours to several weeks. These clouds bring rain that sustains life but they can also cause floods that destroy life. The study of these clouds is important not only because they have an impact on our life but also because they are responsible for the transport of energy and moisture (both horizontally and vertically) in the atmosphere.

The earth's radiation budget plays a crucial role in the variability and modulation of the climate. The excess solar radiation absorbed in the tropics is transported to the poles through large-scale circulation of the atmosphere. Changes in the radiation budget triggered by natural or anthropogenic factors can bring about profound changes in the global climate. The increasing emission of greenhouse gases by human beings is believed to have had a discernible impact on the global climate and contributed substantially to global warming in the last several decades. The worldwide concern about global warming has accelerated research on the effect of radiation budget on global climate.

The main energy source for the atmospheric thermal engine is in the tropical regions. The latent heat released during condensation of water vapour in clouds provides the major driving force for the atmospheric circulation in the tropics. In particular, the deep convective clouds in the Tropical Convergence Zones (TCZ) play a major role in the water and energy transport in the tropics. The prominent facets of variation on the inter-annual scale such as El Nino and fluctuations of the Indian monsoon rainfall are associated with large space-time variations of the TCZ.

The sub-seasonal variations are also associated with variations of the TCZ and embedded synoptic scale and meso-scale systems. The radiation budget plays an important role in the dynamics of the TCZ. Hence, knowledge of the variation of different components of the radiation budget and energy exchanges is important to understand the fluctuation of clouds and rainfall in different time-scales.

The energy and moisture budget of the tropical atmosphere controls the amount of precipitation in the tropics. Major factors that control the energy and moisture budget are surface evapouration, net radiation at the top of the atmosphere, integrated water vapour and the vertical profile of temperature and humidity. Some satellites have measured most of these quantities during the past 20 years. There has, however, been no satellite mission that measures all these quantities simultaneously. The simultaneous measurement of these quantities will be essential to understand the energy and moisture budget of tropical convective systems.

Thus the main requirement is to complement the existing information by more intensive and simultaneous radiometric measurements for atmospheric water cycle and corresponding radiation budget for cloud-radiative interaction properties and ensure a high temporal sampling in order to characterise the life-cycle of convective systems. The mission will look at providing all-weather capability to provide a sustainable data base for characterisation of the processes in the atmosphere and ocean.