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(Phone: 202/358-1696)

Krishna Ramanujan
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Caption for Image 1: Distribution of Warm Clouds and Cold Clouds in El Niño Year, 1998

These two images show typical average distributions of warm clouds (top image, mostly in green) and cold clouds (bottom image, mostly in orange) for January 1 to 3, 1998. 1998 was an El Niño year with warmer than average tropical sea surface temperatures. This El Niño year offers an example of warm and cold cloud coverage when tropical oceans warm.

The top image shows a much greater distribution of warm clouds globally, as compared with cold clouds in the bottom image. The units are in millimeters of liquid water in clouds. Warm clouds tend to be thinner and lower and precipitate a light warm rain. Cold clouds are thicker and higher in the atmosphere, and produce larger raindrops. Analyses of images like these with respect to sea surface temperature provide useful information regarding the redistribution of warm clouds vs. cold clouds as a result of global climate change.

These images were derived using data from the TRMM satellite. Credit: Lau and Wu, NASA GSFC

Caption for Item 2: Clouds from Space

This image over Southern Brazil, taken from the space shuttle by an astronaut in February 1984, shows a mixture of cold and warm clouds. Warm clouds, consisting of individual cumuli clouds are found near the bottom of the image. The image does not show whether they produced rain. The tops of these warm clouds are generally found below 3.1 miles (5 kilometers).

Cold clouds, with large anvils rising typically above 6.2 miles (10 km) dominate the top and left portions of the image. The feathery edge of the anvil indicates a cold rain process, called glaciation. These clouds are capable of producing very heavy rain and thunderstorms.

Near the center, about a third and half way from the bottom are cumulus congestus clouds, which can produce rain, and can rise to just above the freezing level near 3.1 miles (5 km), and are still considered warm clouds.

Notice the way warm clouds are also found underneath cold clouds, as indicated near the center of the picture. Credit: NASA-JSC


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February 10, 2004- (date of web publication)


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.


Item 1

High resolution image

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.


This image over Southern Brazil, taken from the space shuttle by an astronaut in February 1984, shows a mixture of cold and warm clouds.

Item 2

High resolution image

The study cites satellite observations 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 water mass in a cloud rains out is the precipitation efficiency. According to the study, when it comes to light warm rains, as sea surface temperature increases, the precipitation efficiency substantially increases.

Computer climate models that predict rainfall have underestimated the efficiency of warm rain. Compared to actual observations from NASA's Tropical Rainfall Measuring Mission (TRMM) satellite, computer models substantially underestimate the precipitation efficiency of light rain. The findings from this study will provide a range of possibilities for warm rain efficiency that will greatly increase a model's accuracy.

"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.

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