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Tropical Rainfall Measuring Mission

Phase: Past

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 contributed 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 accomplished 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 was the Precipitation Radar. The Precipitation Radar, built by the Japan Aerospace Exploration Agency (JAXA), (formerly National Space Development Agency (NASDA)), was the first spaceborne instrument designed to provide three-dimensional maps of storm structure. The measurements yielded 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 are being 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 was able to quantify the water vapor, the cloud water, and the rainfall intensity in the atmosphere. The Visible and Infrared Scanner (VIRS) served 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 the TRMM PR was the first radar in space for the purpose of measuring rainfall. Additionally, TRMM carried the Lightning Imaging Sensor (LIS), a small, highly sophisticated instrument that mapped 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.

TRMM was launched on November 27, 1997, on the Japanese H-II vehicle from the Tanegashima Space Center in Tanegashima, Japan. Continuous science data collection began December 8, 1997. Upon completion of the nominal 3-year prime mission, the decision was made to boost the mission from its original altitude of 350km to 402.5km, to reduce fuel consumption and extend the mission life. The TRMM boost was completed August 24, 2001, and the orbit was maintained until 2014, when fuel was depleted and the spacecraft began to descend. Mission operations were terminated in April 2015, the spacecraft re-entered the Earth’s atmosphere and mostly burned up in June 2015.

TRMM exceeded its 3-year design goal by collecting 17 years of rainfall data, creating a benchmark rainfall climatology which is used to test, compare and improve global climate models. The TRMM dataset of rain distribution across the tropics has narrowed considerably the range of uncertainty in previous space-based rainfall estimates. The choice of a precessing, low-inclination orbit (35°) enabled the quantification of the diurnal cycle of precipitation and convective intensity over land and ocean tropics-wide on fine scales (0.25°). TRMM products have provided the first comprehensive estimates of how rainfall is directly related to latent heat release in the atmosphere, a key characteristic in understanding the impact of tropical rainfall on the general circulation of the atmosphere. Based on hydrometeor vertical structure information from the TRMM active and passive sensors, TRMM produced climatologies of latent heating profiles for analysis and comparison with global models. In addition, the lightning sensor delivered a detailed global map of lightning distribution, and combined with the rain data, led to quantifying the lightning/convection relation for land and ocean.

Last Updated: July 15, 2015

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