Tropical Rainfall Measuring Mission

The Tropical Rainfall Measuring Mission (TRMM), is the first mission dedicated to measuring tropical and subtropical rainfall through microwave and visible infrared sensors, and includes the first spaceborne rain radar.

Tropical rainfall comprises more than two thirds of global rainfall. It is the primary distributor of heat through the circulation of the atmosphere. Understanding rainfall and its variability is crucial to understanding and predicting global climate change. Our current knowledge of rainfall is poor, especially over the oceans. By use of a low-altitude orbit of 217 miles (350 kilometers), TRMM's complement of state-of-the-art instruments will provide more accurate measurements. These new measurements will increase our knowledge of how rainfall releases heat energy to drive atmospheric circulation.

TRMM's orbit will range between 35 degrees north and 35 degrees south of the equator, allowing TRMM to fly over each position on the Earth's surface at a different local time each day. Scientists can use data from this kind of orbit to calculate rain variations over a 24-hour period; the result will be a data set vastly more informative than any now available.

TRMM is a joint project between the United States and Japan. The National Space Development Agency of Japan (NASDA) will provide the Precipitation Radar (PR) and an H-II rocket to launch the TRMM observatory in Fall 1997 for a three year mission. NASA's Goddard Space Flight Center (GSFC) in Greenbelt, Md., will provide the observatory, four instruments, integration and test of the observatory and will operate the TRMM satellite via the Tracking and Data Relay Satellite System (TDRSS).

Science Objectives

TRMM science objectives are:

To obtain and study multi-year science data sets of tropical and subtropical rainfall measurements;
To understand how interactions between the sea, air and land masses produce changes in global rainfall and climate;
To improve modeling of tropical rainfall processes and their influence on global circulation in order to predict rainfall and variability at various periods of time; and
To test, evaluate and improve satellite rainfall measurement techniques.

TRMM - Importance to Global Change and Human Welfare

Tropical rainfall affects the lives and economics of half the Earth's population residents of developing countries in or near the Earth's tropics. Rainfall variation in the tropics also can affect the weather in locations thousands of miles away, influencing the lives and livelihoods of populations worldwide. For example, tropical rain systems such as typhoons, hurricanes and monsoons produce floods worldwide. The best known effect of tropical short-term climate variations on the world is the El Nino-Southern Oscillation (ENSO) of the Pacific Ocean. In an El Nino, warm water and its coupled cloud systems spread eastward from Indonesia toward South America. Weather conditions associated with El Nino include wetter than normal weather in California, wetter and colder winters than normal in the eastern United States and dryer summer monsoon seasons across the southern hemisphere.

With temperature, rainfall is perhaps the most important factor in defining climate. Excess rainfall can cause flooding and enormous property and crop damage. Still, a deficiency of rainfall can cause drought and crop failure.

Rainfall is also the major source of energy that drives the circulation of the atmosphere. As water changes from liquid to vapor and back to liquid, heat is stored and then released. This latent heat can intensify some weather systems, making tropical rainfall an important determinant of atmospheric circulation and short-term climate changes.

Major advances are occurring in the modeling of rainfall and rain-related cloud processes. The response of cloud systems to their environment is an important link in a chain of climate variations. Scientists are developing cloud models to convert TRMM's signals into rainfall rates.

Role of Rainfall in the Earth System

The atmosphere, oceans and biosphere of our planet are closely coupled components of the global environment, which is often called the Earth system. The unifying element of these large environmental systems is water. It is the active component of the atmosphere, it covers three-quarters of the Earth's surface; and is the basis for all life on our planet. Water is essential to our continued existence.

Because of the unifying role of the global water cycle, studies of rainfall are essential for a deeper understanding of the Earth as a system. TRMM will study these aspects of the Earth system:

Earth Science: Research in the Earth's system components can reveal the interaction of the ocean with the atmosphere, coupled with the many different life forms that make up the Earth's biosphere. Incorporating these interactions in Earth system models will test our understanding of the basic mechanisms, such as rainfall, and will improve simulations of future global trends.
Atmospheric Circulation: Rainfall releases tremendous amounts of latent heat that have enormous impact on weather and climate. Because we do not have adequate rain measurements, existing climate models do not accurately predict the amounts of heat being released. TRMM data will help climate modelers in remedying one of their most serious deficiencies by providing more accurate rain and heat information.
Ocean-Atmosphere Interaction: The ocean influences atmospheric dynamics through evaporation and heat transfer. Tropical clouds can act as a shield against solar radiation impinging on the sea surface. There is a close coupling between the development of large cloud systems and warmth of the sea surface. Interaction between ocean and atmosphere is vital in weather and climate on all scales.
The Biosphere: Tropical forests and their soils are a major source of many of the atmosphere's trace constituents. Together, forests and the atmosphere act as a single water and energy-regulating system. Water received as rainfall returns to the atmosphere through evaporation and transpiration.

The environment recycles a large portion of this returned water. During a rainstorm, clouds transport these atmospheric trace constituents into the free atmosphere while cleaner air flows down to the surface. This exchange reveals a direct link between tropical rainfall and the global cycles of carbon, nitrogen and sulfur, which are important in biological processes.

TRMM Observatory and Instruments

GSFC will design, build and test the observatory "in house" at its Greenbelt, Md., facility. At launch, the observatory will weigh 7,920 lbs. (3,600 kg). It is about 17 feet tall (approximately 5 meters) and 12 feet (3.6 meters) in diameter. A gallium arsenide solar array/nickel cadmium battery power subsystem will provide 1,100 watts of load power to the satellite.

A three-axis attitude control subsystem will stabilize the observatory and keep the instruments pointing toward Earth to within 0.2 degrees. A command and data handling subsystem will provide onboard commanding, data collection, processing and storage. This subsystem will use state-of-the-art technology employing a fiber optic data bus and solid state recorders.

A reaction control subsystem will maintain the orbit at approximately 217 miles (350 km). Data for each orbit will be stored on board and transmitted to the ground by the communication subsystem through TDRSS once per orbit.

The observatory instruments for primary rainfall measurements are a precipitation radar, a multi-frequency microwave radiometer and a visible/infrared radiometer. For observations related to precipitation, NASA has added a Lightning Imaging Sensor (LIS) and a Clouds and the Earth's Radiant Energy System (CERES). A brief description of the five instruments follows:

The PR will determine the vertical distribution of precipitation by measuring the "radar reflectivity" of the cloud systems and the weakening of a signal as it passes through the precipitation. A unique feature of the PR is the measurement of rain over land, where passive microwave channels have more difficulty.
The TRMM Microwave Imager (TMI) is a multi-channel radiometer, whose signals in combination can measure rainfall quite accurately over oceans and somewhat less accurately over the land. The TMI and PR data, will yield the primary precipitation data sets.
The VIRS measures radiance in five bandwidths from the visible through the infrared spectral regions. Scientists use Infrared (IR) data to make rough estimates of tropical precipitation. The VIRS, PR and TMI data will help improve the techniques by which scientists use IR data from other satellites to calculate rainfall. This is the third component of TRMM's rain package.
The LIS is an optical telescope and filter imaging system that will investigate the distribution and variability of both atmospheric and cloud-to-ground lightning over the Earth. These instruments will contribute to our understanding of storm dynamics and will be correlated to levels of precipitation and the release of latent heat.
The CERES is a visible/infrared sensor designed especially to measure energy rising from the surface of the Earth and the atmosphere including its constituents (e.g., clouds and aerosols). This energy, when balanced against the energy received by the Earth from the Sun, constitutes the Earth's radiation budget. Understanding the radiation budget, from the top of the atmosphere to the Earth's surface, is important to understanding climate and its variability.

Science Data Retrieval and Transmission Process

TRMM will have its own unique Science Data and Information System (TSDIS) at the GSFC, to process the information from the satellite. TSDIS will analyze the rainfall data and also provide validation from nearly a dozen ground radar sites. The data products will be available to the science community from the Earth Science Data and Information System (ESDIS) about six months after launch.

TRMM data will be provided on five levels, on a scale of zero to four, representing the different processing done at each stage after Goddard receives the data. Level zero is the time ordered and quality checked raw data received from the satellite. The higher levels result from combining hardware and software calibrations with the data to form the final products.
Through TSDIS, Goddard will provide pre-processing of rainfall data (PR, TMI and VIRS) and all ground data and transmit it to appropriate centers and to the ESDIS archives.
The data also will be transmitted to Japan, where higher level processing will be performed at NASDA's Earth Observation Center. It will then be distributed to scientists in the fields of climatology, meteorology, hydrology and other disciplines in Japan and the United States.

TRMM Validation

Space-based measurements of such an elusive parameter as rainfall, even with the advanced technology now available, is a real challenge because rainfall varies so widely over time and space. To ensure credible results, the TRMM project is developing a global ground validation program. The project is establishing cooperative sites around the tropics, covering areas that receive different amounts of rainfall, with precipitation radars and rain gauges to verify, and calibrate the TRMM measurements. Routine collection of radar and rain gauge data is already well underway from Darwin, Australia; the Marshall Island/Atol of kwajalein; Cape Canaveral and the Kennedy Space Center, Fla.; southern Texas; Phuket and Chang Mai, Thailand; Israel; Taiwan; and southern Brazil. The validation program supports TRMM algorithm development and includes an extensive precipitaion research component.

TRMM: A Comprehensive Program

TRMM is planned as a three-year, systematic program designed to increase the extent and accuracy of rainfall measurements. It is a timely collaboration that meets the research objectives of both the United States and Japan. The mission will make major contributions to several international Earth Science Programs and is an essential component of NASA's long range plan to study global change from space. The TRMM project is part of NASA's Mission to Planet Earth, a long term, coordinated research effort to study the Earth as a global environmental system. TRMM is managed by Goddard's Flight Projects Directorate for NASA's Office of Mission to Planet Earth, Washington, D.C.