MIXED
CROPLANDS MAY MAKE SOME AREAS COOLER, WETTER IN SUMMER
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Mesocyclone
"Flying Saucer" | The
variety of the vegetation and crops in the Great Plains and Rocky Mountain states
has helped maintain a cooler, wetter climate, according to a NASA-funded study
using a computer climate model. Hydrometeorologists
Jim Shuttleworth at the University of Arizona, Tucson, Ariz., and Lixin Lu at
Colorado State University, Fort Collins, Colo., found that when they introduced
satellite measurements of the real patterns of vegetation in Great Plains and
Rocky Mountain states into a computer model, the results generated extra convection
in the atmosphere to give a cooler, wetter climate. The
study appears in the June issue of the Journal of Hydrometeorology.
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2 | Lightning
cloud-to-ground | Mixed
vegetation impacts the atmosphere, weather and climate through the proportion
of sunlight that gets reflected from the land and leaves back out to space, the
varying heights of trees and other plants exposed to the wind, and the effectiveness
of different plant types when it comes to evaporating water. For
example, irrigated, lush crop lands with plenty of water in the soil warm the
air less because they use more of the Sun's energy for evaporation, as compared
to hot, dry bare soil. Along with differing temperatures, the varied heights of
plants and trees in a region change the aerodynamics of the atmosphere, creating
more circulation and rising air. When the rising air reaches the dewpoint in the
cooler, upper atmosphere, it condenses into water droplets and forms clouds.
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3 | | "The
mixed vegetation creates areas of different temperatures next to each other, some
warmer and some colder, and this leads to mixing in the atmosphere that gives
rise to clouds and, ultimately, rain," Shuttleworth said. Over
the last two decades, detailed maps of the amount and type of vegetation that
covers the ground have become available through remote sensing. Based on that
information, scientists can enter data to describe vegetation into computer models
that simulate regional climate. In
this study, a climate version of the Regional Atmospheric Modeling System (ClimRAMS)
was used to explore whether more of a mix of vegetation can alter climate in the
United States. The ClimRAMS model simulated the whole of the country with a grid
scale of 200 km (124 miles), but it focused particularly on predicting climate
with a grid scale of 50 km (or 31 miles) in a rectangular area of the Great Plains
and Rocky Mountains.
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4 | | A
U.S. Department of Agriculture soil database was used to define different soil
types in the climate model, and vegetation was classified using an international
land-cover system. Vegetation and land cover types included: mixed crops, short
grass, evergreen needleleaf trees, deciduous broadleaf trees, tall grass, desert,
tundra, irrigated crop, semi-desert, shrub evergreen, mixed woodland and inland
water. The
ClimRAMS normally assumes there is little difference between grasslands in northern
Wyoming and southern Kansas, for instance, but the researchers found that when
they introduced satellite measurements of "leaf area index" (a way of
quantifying how much vegetation is actually present), the more realistic pattern
of vegetation generated extra convection in the atmosphere to give a cooler, wetter
climate. Shuttleworth
and Lu used satellite plant cover data from Kansas, Nebraska, South Dakota, Wyoming
and Colorado for their computer model runs. Computer
model simulations of the growing season that included satellite data of mixed
vegetation showed lower maximum and minimum temperatures in the region, compared
to a model run that contained less detail of plant cover. Over the entire year,
simulated precipitation levels were on average two-thirds of a millimeter per
day more for the model using mixed vegetation, which may account for the cooler
temperatures found during the growing season. In
general, most current computer models that predict Earth's future climate do not
account for the complex mix of vegetation and its atmospheric impacts, and may
be producing forecasts that are too dry and too warm during growing seasons.
"In the future, it will be important to use remote sensing data to enter
the fine details of plant cover into computer models to get more accurate weather
and climate forecasts," Shuttleworth said. Further, more research is needed
to determine if increasing variety of croplands would help lessen drought conditions.
This
research was funded by the NASA Land Surface Hydrology Program. Back
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