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 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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