Mission Description

water cycle in the tropical atmosphere
in the context of climate change.

A. Introduction

This mission was studied in France in the context of GEWEX (Global Energy and Water cycle Experiment). For understanding tropical meteorological and climatic processes, it appeared necessary to obtain reliable statistics on the water and energy budget of the tropical atmosphere and to describe the evolution of its systems (monsoons, cyclones, …) at appropriate time scales. In parallel, tropical atmospheric and oceanic missions were also studied in India, which is directly concerned by these phenomena. The first originality of MEGHA-TROPIQUES is to associate three radiometric instruments allowing to observe simultaneously three interrelated components of the atmospheric engine : water vapour, condensed water (clouds and precipitations), and radiative fluxes. The second is to privilege the sampling of the intertropical zone, accounting for the large time-space variability of the tropical phenomena. Moreover, the MEGHA-TROPIQUES microwave radiometer MADRAS could be one element complementing the constellation of mini-satellites of the Global Precipitation Mission .

Scientific objectives

– to improve the knowledge of the water cycle in the intertropical region, to evaluate its consequences on the energy budget,

– to study the life cycle of tropical convective systems over ocean and continents, the environmental conditions for their appearance and evolution, their water budget, and the associated transports of water vapor.

Applied objectives

– to provide data about the processes leading to dramatic weather events affecting the Tropical countries, as hurricanes, systems producing heavy rainfalls, processes governing monsoons variability or droughts.

Mission Scenario

The key of this mission is the repetitivity of the measurement in the Tropics. The orbit of the platform must be in a low inclination on the equatorial plane. The altitude of the orbit has to be high enough to allow a wide swath of the instruments.

Geophysical parameters to be retrieved:

Atmospheric water cycle elements: Water vapor (integrated and vertical distribution), cloud condensed water content, ice/water, precipitation.

Radiative budget elements: solar reflected and terrestrial emitted fluxes at the top of the atmosphere.

Instrumental payload

The instruments have to be complementary to what exists on geostationary satellites (VIS-IR imagers). Microwave instruments are then fundamental. The main payload instruments are:

– A microwave imager (MADRAS) aimed mainly to study precipitation and cloud properties, including ice at the top of clouds (SSM/I type, with an additional channel at 157 GHz).

– A microwave sounding instrument for the atmospheric water vapor (SAPHIR – 6 channels in the 183 GHz band).

– A radiometer devoted to the measurement of outgoing radiative fluxes at the top of the atmosphere (ScaRaB).