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EarthQuest Winter 1988 The Tropical Rainfall Measuring Mission The Tropical Rainfall Measuring Mission (TRMM) is proposed space system for measuring tropical rainfall and its variations. Such measurements are critically needed for progress in our knowledge of the overall earth system. Evaporation of water vapor into the atmosphere and its condensation to produce rainfall are at the heart of the earth's hydrological cyclee, which is, in turn, essential for life on the planet. The placement of human settlements has been dictated more by the availability of an ample water supply than by any other factor. The nature of the earth's vegetative cover is controlled, to a very large extent, by moisture conditions and particularly by rainfall. A principal driver for global circulation in the troposphere is heat released by condensation of water. The nature of this heat release, coupled with the resulting motions in the atmosphere, concentrates rainfall in the tropics in a thin latitude band called the intertropical convergence zone. We now know that year-to-year variations in the latent heat released in this region are a leading cause of year-to-year global climate variations, including the El Niño/Souther Oscillation phenomenon. During the past year, NASA and the Science and Technology Agency of Japan have been studying the feasibility of implementing TRMM as a joint space mission with the goal of obtaining improved measurements of rainfall in the tropics. A basic product would be the data set of 30-day average rainfall and its vertical distribution over 5 x 5 km areas. The quantification of global or regional rainfall is a major challenge, due to its high temporal and spatial variability. Moreover, most of the rain in the tropics falls in regions that are relatively inaccessible to in situ measurements. The quantitative distribution of rainfall over the tropical oceans is uncertain to a multiplicative factor of about two to three. Remote sensing from space has proven to be very useful in studying rainfall. Spaceborne measurements of the outgoing long-wave radiation now provide our best present estimates of rainfall over the tropical oceans, aided by passive microwave measurements from the electronically scanning microwave radiometer on Nimbus 5, from the scanning multichannel microwave radiometer on Nimbus 7, and from the special sensor for microwave imaging on the Defense Meteorological Satellite. These microwave measurements are obtained from polar sun-synchronous orbits. As a result they do not give representative estimates of the daily rainfall in any one place. The same will be true for the passive microwave measurements to be made on the Eos polar platforms. Present microwave measurements are also more reliable over the oceans that over land, where differences in surface emissivity complicate data interpretation. TRMM will carry three principal instruments. One is a cross-track scanning multichannel passive microwave radiometer, which will give good measurements of rainfall over the oceans. TRMM will be the first space mission to include as well a cross-track scanning two-frequency precipitation radar, which will yield good measurements of the vertical distribution of rainfall over both land and sea. The third payload package is an advanced very high-resolution radiometer (AVHRR). Data from this device compared with rainfall measurements from the passive microwave and radar instruments can be used to better interpret visible/infrared measurements form past and future operational satellites. The TRMM orbit will be circular at an altitude of 320 km and at an inclination of 35 degrees. This orbit will give intensive coverage in the tropics, allow extraction of the diurnal cycle in climatological rainfall, and all permit recovery by the Space Shuttle for refurbishment and later deployment on the Space Station. Ad advantage of the relatively low orbital altitude is a smaller instrumental "footprint" and hence higher spatial precision, allowing measurement of rainfall variations over small spatial scales. An extensive ground-truth program is also planned for testing the remote-sensing algorithms. The possibility of a joint U.S./Japanese mission to measure tropical rainfall was set tinto motion in September 1986 by a letter of invitation from the NASA Associate Administrator for Space Science and Applications to the Commissioner of Japan. Resulting studies have concluded that a shared mission appears to be possible. Japan would build the rainfall radar and launch TRMM on its new H-2 rocket. The U.S. would build the spacecraft, the passive microwave instrumentation, and the AVHRR, and supply the TRMM data system. With this division of tasks, a TRMM mission seems possible in 1994, so that these data would available during the later stages of the Tropical Oceans and Global Atmosphere program. Funds have yet to be committed to this program by either country. Japan is expected soon to officially outline its proposed participation in the TRMM program. The U.S. portion of TRMM is expected to be a candidate for an FY 1990 NASA new start as a component of the new Earth Probe program: a series of small specialized art science satellites. For additional information on TRMM, contact Marvin Gellar, Code 610, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA.
