Although the role of individual land surface characteristics, such as soil moisture and albedo, in the climate system has been widely recognized, the effects of vegetation biophysical processes (VBP) are not yet fully understood. The Simplified Simple Biosphere Model (SSiB) has been coupled to the UCLA atmospheric general circulation model (UCLA AGCM/SSiB-1) to investigate the role and mechanism of land/atmosphere interactions. The coupled system was run for 6-years using climatological sea surface temperature (SST) and the results compared with those obtained with the same AGCM except for the use of an earlier land surface parameterization (UCLA GCM/CNTL). In this earlier version, climatological surface albedo and ground wetness are prescribed and surface temperature is obtained by using a simple single layer energy balance model, The UCLA AGCM/SSiB-1 results indicate a substantial impact from the explicit representation of VBP. The systematic bias in the CNTL precipitation climatology is reduced, especially over land. The annual mean precipitation bias was reduced by 60% globally and 69% over land. The improvement is consistent for all seasons. Improvements are most clear in monsoon climate regions and areas characterized by large landmass, such as the boreal forest areas over the Eurasian and North American continent. These also appeared to be the regions sensitive to climate/VBP interactions with the NCEP GCM in other studies.

To further test the VBP effect, we ran a simulation (UCLA AGCM/SSiB-2) with a more recent version of the land surface parameterization. SSiB-2 includes Collatz et al.'s CO2 photosynthesis model and a new methodology to scale leaf photosynthetic assimilation to canopy level, which includes leaf-shading effects. The simulations with the two versions of SSiB produced similar values of global and annual mean precipitation, but the seasonal cycle of precipitation in UCLA/SSiB-2 is improved over some regions. The improvement is particularly clear, for example, over the North American and Eurasian boreal forest areas during the northern spring season. A third experiment is carried out to isolate the soil moisture effect.

These results indicate that under unstable atmospheric conditions, not only low frequency mean forcings from the land surface, such as monthly mean albedo, but also VBP perturbation processes are important to the continental scale precipitation. A realistic representation of land surface processes in the GCM, therefore, is important for global water and energy studies. It is particular crucial to the monsoon climate regions and areas characterized by large landmass such as the boreal forest zone.