USE OF A LAND-SURFACE-TRANSFER SCHEME (LSX) IN A GLOBAL CLIMATE MODEL- THE RESPONSE TO DOUBLING STOMATAL-RESISTANCE

One response of vegetation to future increases in atmospheric CO2 may be a widespread increase in stomatal resistance. Such a response would increase plant water usage efficiency while still allowing CO2 assimi lation at current rates. The associated reduction in transpiration rat es has the potential of causing significant modifications in climate o n regional and global scales. This paper describes the effects of a un iform doubling of the stomatal resistance parameterization in a global climate model (GENESIS). The model includes a land-surface transfer s cheme (LSX) that accounts for the physical effects of vegetation, incl uding stomatal resistance and transpiration, which is described in det ail in an appendix. The atmospheric general circulation model is a hea vily modified version of the NCAR Community Climate Model version 1 wi th new treatments of clouds, penetrative convection, planetary boundar y layer mixing, solar radiation, the diurnal cycle, and semi-Lagrangia n transport of water vapor. The other surface models include multi-lay er models of soil, snow and sea ice, and a 50-m slab ocean mixed layer . The effects of doubling the stomatal resistance parameterization are largest in heavily forested regions: tropical South America, and part s of the Northern Hemispheric boreal forests in Canada, Russia and Sib eria in summer. The primary surface changes are a decrease in evapotra nspiration, an increase in upward sensible heat flux, and a surface-ai r warming. Secondary effects include shifts in the ITCZ which cause la rge increases in precipitation, soil moisture and runoff in western tr opical South America, and decreases in these quantities in northern su btropical Africa. Noticeable changes in relative humidity, cloudiness and meridional circulation occur throughout the troposphere. The globa l effects on atmospheric temperature and specific humidity are small f ractions of those found in other doubled CO2 experiments. However, unl ike doubled CO2 the signs of those changes combine to give relatively large reductions in relative humidity and cloudiness. IL is suggested that the stomatal-resistance effect and other plant responses to large -scale environmental perturbations should be included in models of fut ure climate.