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More Rain Likely in a Warmer World

More Rain Likely in a Warmer World

In a new paper published in the journal Geophysical Research Letters, scientists at NASA’s Goddard Space Flight Center provided compelling evidence that tropical rainfall will increase as sea surface temperature rises. Scientists have long theorized that rising surface temperatures could accelerate the water cycle—the processes of evaporation and precipitation—and data from the joint NASA/NASDA Tropical Rainfall Measuring Mission (TRMM) prove the theory is correct, at least over the tropical Pacific Ocean. The authors of this new paper found that for each degree of rise in sea surface temperature, there is an 8 to 10 percent increase in rainfall efficiency in warm clouds. “Rainfall efficiency” refers to the rate at which a cloud rains out its water content. (Click to read the full NASA press release.)

The false-color maps above, derived from TRMM data, show the liquid water content in found warm clouds (top) and cold clouds (bottom) across the tropical Pacific from January 1-3, 1998. Comparison of the two maps shows there was a much greater distribution of warm clouds than cold clouds. The colors represent the clouds’ liquid water content, ranging from less than 0.06 millimeters (low values, shown in blue) to around 0.24 mm (intermediate values, shown in yellow and mauve) to more than 0.96 mm (high values, shown in red).

Warm clouds tend to be thinner, lower in altitude, and produce a light, warm rain. Cold clouds are typically thicker, higher in elevation, and produce larger raindrops. The researchers concluded that if the tropical oceans continue to warm, as they have over last 20 years, warm tropical rain storms will also continue to become more frequent.

Images courtesy William Lau and Huey-Tzu Jenny Wu, NASA Goddard Space Flight Center, produced using TRMM data