A new NASA computer model can now tell exactly where in the world rain
or snow that provides local water originated. Scientists can use this
“water vapor tracer” to improve rainfall and drought forecasts and gain
a deeper understanding of climate change.

The model simulates water movement in the atmosphere around the world,
and traces it from the places where it evaporates to the places where
it falls back to Earth.

“If I see rain or snow in the central U.S., I can now tell you how much
of the moisture came from the Gulf of Mexico, how much came from the
tropical Atlantic Ocean and so on,” said meteorologist Mike Bosilovich
of NASA’s Data Assimilation Office at Goddard Space Flight Center in
Greenbelt, Md. Bosilovich is lead author of the study being published
in the March-April issue of the Journal of Hydrometeorology. “The model
gives us a much clearer picture of how water moves in the atmosphere
than we have ever had before.”

By identifying water vapor movement in the atmosphere, weather
forecasters will better understand how evaporation from a particular
place contributes to local and regional precipitation, leading to more
accurate weather forecasts. The model can actually pinpoint individual
regional sources of atmospheric moisture, rather than combining them.
Bosilovich said if scientists can understand how geographic sources of
atmospheric moisture fluctuate from year to year, they also will have a
clearer picture of how climate changes in the long term.

The atmosphere over North America receives moisture evaporated from
many different water sources. For example, while clouds above the West
Coast generally originate in the Pacific Ocean, those over the Midwest
are more likely to have come from the Gulf of Mexico. Water from
previous storms also evaporates from the land, contributing to the mix.

“You might visualize each region of a continent or ocean as having a
kind of ‘smokestack,'” Bosilovich explained. “Each ‘smokestack’ sends
up a plume of water vapor that mixes with the air.”

But what complicates matters is that these “smokestacks” send up
different-sized plumes of moisture at different times, and changes in
wind and temperature can push them in different directions depending on
the day or season. Until very recently, even the fastest computers had
trouble keeping track of all the variables.

Bosilovich and Siegfried Schubert, who works with Bosilovich, have
demonstrated the model’s capabilities by analyzing the atmospheric
water cycles over India and North America. They chose to analyze the
cycles during the summer months over a period of six years, since both
regions experience monsoons from June through August, and provide a
great deal of moisture to track.

They found that while precipitation in India often comes directly from
the ocean, much of what falls on the United States in the summertime
can be “recycled” moisture — water from previous storms that
evaporates from the ground and then falls again quickly nearby. “The
model could assess how strongly this recycling of water contributed to
floods like the devastating Mississippi River flood of 1993,”
Bosilovich said.

Bosilovich is currently applying the data from the 1993 flood to the
water-vapor tracer model, to gain a better insight into the processes
that generated the flood. Analyzing past weather events will help him
refine his model’s operation, a necessity if it is ever to make
accurate predictions of future weather.

“Currently, the only hard data the model accounts for is sea surface
temperature; everything else is simulated. Our next big job is to work
more observational data into the model, so it can reflect actual global
atmospheric conditions,” said Bosilovich. Such improvements will take
time, but could lead scientists to better understanding of both next
week’s weather and the next century’s climate.

The work is supported by grants from the joint NASA-NOAA Warm Season
Precipitation Initiative and NASA’s Earth Science Enterprise.

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