A 'Wetter' Water Cycle
The Earth's pattern of evaporation and precipitation, known as the water cycle, will intensify due to warming temperatures, according to William Lau and Huey-Tzu Jenny Wu of NASA's Goddard Space Flight Center.

The rate that moisture in a cloud is converted into rainfall is known as precipitation efficiency. This study found that when sea surface temperatures warm, the precipitation efficiency for light rains dramatically increases. Compared to actual observations from the TRMM satellite, computer models used in weather prediction repeatedly underestimated this efficiency.

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

Warm vs. Cold Rain
Warm rain is created when water droplets form around airborne particles, such as dust, and produce clouds. As the droplets collide, they form raindrops, which grow large and heavy enough to fall out as warm rain. The researchers discovered that for every degree rise in sea surface temperature, a cloud produces as much as 10 percent more light-to-moderate rainfall.

Cold rains are associated with brief, heavy downpours. Here, strong vertical winds carry larger raindrops high into the atmosphere, where they freeze and continue to grow. This process is driven mostly by the speed of the winds and less on sea surface temperature.

When water vapor condenses it releases heat, warming the lower atmosphere, as observed during warm rain. But, this warming also makes the air lighter and rise faster, creating strong vertical winds that produce more cold rain.

The study found warm rains account for about 30 percent of the total global rainfall and 72 percent of the total rain over tropical oceans, pointing to the major role of warm rains in the water cycle.

Additional research is needed to better understand the relationship between warming sea surface temperatures and increased warm-rain precipitation, and to determine how cold rain processes might be impacted.

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. TRMM was launched in Nov. 1997 and is a joint mission with the National Space Development Agency of Japan (NASDA).

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