Mixed Croplands May Make Some Areas Cooler, Wetter In Summer

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.

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.

"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.

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.