PROGRAM TITLE: Earth Probes/Tropical Rainfall Measuring Mission (TRMM) ACTIVITY STREAMS: Observations & Data Management SCIENCE ELEMENT:Climate and Hydrologic Systems Biogeochemical Dynamics Ecological Systems and Dynamics NATIONAL AERONAUTICS AND SPACE ADMINISTRATION SCIENTIFIC MERIT: TRMM is a joint program between the U.S. and Japan to obtain a minimum of 3 years of climatologically significant observations of rainfall in the tropics beginning in 1997. This mission will play a significant role in global change studies, especially in developing an interdisciplinary understanding of atmospheric circulation, ocean-atmospheric coupling, and tropical biology. TRMM will help overcome one of the greatest weaknesses in learning about climate change (such as global warming) by improving large-scale computer models of atmospheric circulation. General Circulation Models (GCM's), used for regular shorter term forecasts, currently use a crude representation of precipitation causing discrepancies among their predictors of rain and its associated release of latent heat. The process of rainfall releases tremendous amounts of latent heat and the linkages between tropical and mid-latitude weather patterns depend critically on the altitude at which this latent heat reaches a maximum. The El Nino-Southern Oscillation (ENSO) marks the disruption of normal patterns and its effects are felt worldwide through global circulation anomalies. The TRMM mission will provide climate modelers the means to remedy one of their most serious deficiencies. TRMM will also improve the general understanding of ocean- atmosphere interaction. Wind stress on the sea surface is a major driver of ocean circulation, and the addition of fresh water by precipitation affects sea-surface temperature, salinity, and marine biochemistry. Conversely, the ocean influences atmospheric dynamics through evaporation and heat transfer. Also, tropical clouds can act as a shield against solar radiation impinging on the sea surface. There is apparently close coupling between the development of large cloud systems and warmth of the sea surface temperature, a coupling exhibited in early ENSO development. The intensive observation of this coupling in the TOGA-COARE field campaign of 1992-93 will enhance the usefulness of the TRMM data in understanding and predicting ENSO events, as well as for more general studies of the ocean-atmosphere interaction, so vital in weather and climate studies. The tropical rainfall measured by TRMM is also closely coupled to tropical forest processes. Tropical forests typically receive enough water to meet transpiration needs each year, and widespread cloudiness usually shields them from direct sunlight. These factors encourage high growth rates and maintenance of a large standing biomass, so that most of the water received as rainfall returns to the atmosphere through evaporation and transpiration. Since a large portion of this returned water is recycled, the tropical forest and atmosphere behave in some respects like a single water and energy regulating system. Moreover, tropical forests and their soils are a major source of the atmosphere's trace constituents, including carbon monoxide, methane and non-methane hydrocarbons, which may play a part in oxidizing capacity and atmospheric acidity. STAKEHOLDERS: The TRMM is a joint mission with Japan who will provide the launch vehicle and the Precipitation Radar instrument. There will also be broad international participation in the ground- based validation of the TRMM measurements, which is directed toward an understanding of the coupled tropical ocean-atmosphere system, with particular emphasis on ENSO events. The Global Energy and Water cycle Experiment (GEWEX), an initiative of the WCRP, will also employ and validate TRMM data. Other international programs, such as the International Geosphere-Biosphere Program (IGBP), will use TRMM data for investigation of hydrologic processes important to global change. POLICY RELEVANCE: Primary - Climate Change and Global Warming Secondary - Ecological & Biodiversity - Seasonal and Interannual Prediction PROGRAM CONTACT:Lenwood G. Clark Dr. John Theon Office of Mission to Planet Earth, Code YS NASA Headquarters, Code YF 358-0274 tel: 202-358-0786 fax: 202-358-2769 E-mail: L.CLARK.NASA/omnet