Rain Measurement History
Earth's climate is affected in part by complex interactions between the Sun and the planet's vast expanse of ocean waters. Understanding their dynamics, and the subsequent formation of water vapor, clouds, rainfall and the release of "latent heat" high into the atmosphere, which then affects global atmospheric circulation, are central to understanding the climate system. For this reason, Earth scientists study ocean storms and measure rainfall.
NASA's Tropical Rainfall Measuring Mission (TRMM) is a link in that understanding. Meteorologists have studied rainfall patterns over the land for decades using ground based radar, special aircraft instruments and weather ground stations. With TRMM, they are able to make extremely precise measurements of rainfall over the ocean, where conventional ground based instruments cannot see.
This is especially important because two thirds of the world's global rain falls in tropical areas, and three fourths of weather producing energy comes from the heat exchanges involved in the rainfall process. Until now, information on the intensity and amount of rainfall over much of the tropics was incomplete.
For scientists, the research isn't a matter of mere academic interest. Understanding rainfall and its variability is crucial to understanding and predicting global climate change. Is our planet getting warmer due to increased levels of carbon dioxide and other greenhouse gases in the atmosphere? Is the threat real? Computer models that predict the future climate still differ in some very substantial ways, with some models predicting little or no warming, while others predict temperature increases that would substantially alter our way of life. With more data and a better understanding of the current climate system driven by the complex interplay of water vapor, clouds, rainfall and the release of energy after it rains scientists are hopeful that they will be able to definitively say. In fact, the U.S. Global Change Research Program classifies understanding the role of clouds and the Earth's energy budget as its highest priority.
The Climate Machine
It all begins with the Sun. One fourth of Earth's energy comes directly from the Sun. The other three fourths are transferred to the atmosphere when warm tropical ocean waters, which the Sun originally heated, evaporate to form huge equatorial cloud clusters that form a belt along the equator as evidenced by satellite photos.
Upon cooling, the vapor condenses into clouds and forms raindrops. When rain falls, the heat originally used to evaporate the water from the Earth's surface is released high into the atmosphere and flows to both poles. Although the latent heat can't be seen or measured directly, rainfall, which is the product of the release, can be measured.
The latent heat then affects Earth's atmospheric circulation and contributes to the weather patterns observed around the globe. Averaged over the entire Earth, the heating released by precipitation is about five times greater than that produced by variations in surface heating. In other words, tropical rainfall working in conjunction with the Sun drives Earth's climate machine.
Before TRMM's launch measurements of the global distribution of rainfall at the Earth's surface had uncertainties of about 50 percent. Unless scientists can better define the amount of rainfall and the energy released when rain occurs, they stand little chance of putting the climate models through the rigorous tests needed to determine whether in fact carbon dioxide and other greenhouse gases in the atmosphere are warming up the planet.
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