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NASA predicts more tropical rain in a warmer world


As the tropical oceans continue to heat up, following a 20-year trend, warm rains in the tropics are likely to become more frequent, according to NASA scientists.

In a study by William Lau and Huey-Tzu Jenny Wu, of NASA’s Goddard Space Flight Center, Greenbelt, Md., the authors offer early proof of a long-held theory that patterns of evaporation and precipitation, known as the water cycle, may accelerate in some areas due to warming temperatures. The research appears in the current issue of Geophysical Research Letters.

The study cites observations from NASA’s Tropical Rainfall Measuring Mission (TRMM) satellite showing the rate that warm rain depletes clouds of water is substantially higher than computer models predicted. This research may help increase the accuracy of models that forecast rainfall and climate. The rate that water mass in a cloud rains out is known as the precipitation efficiency. According to the study, when it comes to light warm rains, as sea surface temperatures increase, the precipitation efficiency substantially increases as well.

"We believe there is a scenario where in a warmer climate there will be more warm rain. And more warm rain will be associated with a more vigorous water cycle and extreme weather patterns," Lau said.

The process that creates warm rain begins when water droplets condense around airborne particles and clouds are created. The droplets collide, combine and grow to form raindrops. The raindrops grow large and heavy enough to fall out as warm rain. The study claims, for each degree rise in sea surface temperature, the rate a cloud loses its water to moderate-to- light warm rainfall over the tropical oceans increases by eight to 10 percent.

Cold rains are generally associated with heavy downpour. They are generated when strong updrafts carry bigger drops higher up into the atmosphere, where they freeze and grow. These drops are very large by the time they fall. Once updrafts take these large drops high enough, and freezing takes place, the process of rainfall is more dependent on the velocity of the updraft and less on sea surface temperatures. Since the process of condensation releases heat, warm rains heat the lower atmosphere. More warm rains are likely to make the air lighter and rise faster, creating updrafts producing more cold rain.

The study found warm rains account for approximately 31 percent of the total global rain amount and 72 percent of the total rain area over tropical oceans, implying warm rains play a crucial role in the overall water cycle. Light warm rains appear to occur much more frequently, and cover more area, than cold rains, even though they drop less water per shower. The total precipitation from all types of warm rains accounts for a substantial portion of the total rainfall.

In a warmer climate, it is possible there will be more warm rain and fewer clouds. If the amount of water entering into clouds stays constant and rainfall efficiency increases, then there will be less water in the clouds and more warm rains.

More study is needed to better understand the relationship between increased warm-rain precipitation efficiency and a rise in sea surface temperatures, and to determine how cold rain might be affected by an increase in warm rain and a decrease in cloud water amounts.

NASA’s Earth Science Enterprise is dedicated to understanding the Earth as an integrated system and applying Earth System Science to improve prediction of climate, weather and natural hazards using the unique vantage point of space.

Krishna Ramanujan | GSFC
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