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Tropical Rainfall Measuring Mission

Phase: Operating

Launch Date: November 27, 1997

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Program(s):Earth Systematic Missions

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Greenhouse gases and global warming continue to be one of the major environmental concerns in the U.S. and around the world. But the scientists still disagree on such big questions as: How much warming will there be? What other quantities, such as rainfall, might be affected? And, where will the changes occur?

To predict climate changes that might occur due to greenhouse gases, scientists use very sophisticated computer models. They try to use all the data they can possibly find to describe climate as it is today and then they introduce changes into the models such as the introduction of greenhouse gases and see what happens. Before they can do this with confidence, however, they have to be sure that the models are properly describing the climate as it is today. Otherwise, if the models don't even represent the current climate accurately, why should we believe predictions made with them?

The Tropical Rainfall Measuring Mission (or TRMM) is a NASA satellite that provides more information both to test and to improve those models. TRMM is particularly devoted to determining rainfall in the tropics and subtropics of the Earth. These regions make up about two thirds of the total rainfall on Earth and are responsible for driving our weather and climate system. TRMM will contribute to a better understanding of where and how much the winds blow, where the clouds form and rain occurs, where floods and droughts will occur, and how the winds drive the ocean currents. TRMM will do this not just by providing rainfall data but, more importantly, by providing information on heat released into the atmosphere as part of the process that leads to rain.

Most of the heat energy that drives the atmospheric circulation comes as the result of evaporation of water from the ocean surface. (Only about one-fourth of the energy comes directly from the Sun.) Energy from the Sun passes through the atmosphere to the ocean surface where much of it is absorbed and causes the liquid water there to become the gas we call water vapor. The amount of heat required to turn the liquid water into gas is called latent heat of evaporation. It is called latent because it is hidden away in the water vapor molecules but can be released later on as the water vapor rises into the atmosphere and condenses back into liquid water droplets in the clouds or falls back to Earth as rain. In the tropics huge equatorial cloud clusters and hurricanes involving lots of violent convective thunderstorms are the visible evidence of latent heat release.

Among the three primary instruments on TRMM, the most innovative is the Precipitation Radar. The Precipitation Radar, built by the National Space Development Agency (NASDA) of Japan, will be the first spaceborne instrument designed to provide three-dimensional maps of storm structure. The measurements should yield invaluable information on the intensity and distribution of the rain, on the rain type, on the storm depth and on the height at which the snow melts into rain. The estimates of the heat released into the atmosphere at different heights based on these measurements can be used to improve models of the global atmospheric circulation. The TRMM Microwave Imager (TMI) is a passive microwave sensor designed to provide quantitative rainfall information over a wide swath under the TRMM satellite. By carefully measuring the minute amounts of microwave energy emitted by the Earth and its atmosphere, TMI will be able to quantify the water vapor, the cloud water, and the rainfall intensity in the atmosphere. The Visible and Infrared Scanner (VIRS) will serve as an indirect indicator of rainfall, and will also tie in TRMM measurements with other satellite measurements. Other instruments similar to TMI and VIRS have operated in space before, but to date there has not been any radar in space for the purpose of measuring rainfall. Additionally, TRMM will carry the Clouds and the Earth's Radiant Energy System Instrument (CERES) and the Lightning Imaging Sensor (LIS). The data from the CERES instrument will be used to study the energy exchanged between the Sun; the Earth's atmosphere, surface and clouds; and space. Finally the Lightning Imaging Sensor is a small, highly sophisticated instrument that will detect and locate lightning over the tropical region of the globe. Looking down from a vantage point aboard the TRMM observatory, 218 miles (350 kilometers) above the Earth, the sensor expands scientists' capabilities for surveying lightning and thunderstorm activity on a global scale. It will help pave the way for future geostationary lightning mappers. From their stationary position in orbit, these future lightning sensors would provide continuous coverage of the continental United States, nearby oceans and parts of Central America. Researchers hope that future sensors will deliver day and night lightning information to a forecaster's work-station within 30 seconds of occurrence--providing an invaluable tool for storm "nowcasting" as well as for issuing severe storm warnings. These instruments all can function individually or in combination with one another.

By using TRMM measurements, scientists hope to better understand what the climate system is today and how the energy associated with rainfall interacts with other aspects of the global climate.