Sensitivity of global seasonal climate simulations to static or seasonally varying vegetation forcing

An extension of the T42 resolution Florida State University Global Spectral Model (FSUGSM) has been developed to include atmosphere-biosphere interactions, for use in seasonal forecasting and climate studies. The biosphere component is based on the Biosphere-Atmosphere Transfer Scheme (BATS) of Dickinson et al. (1993). Sensitivity to surface parameterization is investigated by running the standard FSUGSM (without BATS) and two formulations with biosphere parameterizations, for boreal summer of 1979. Results are analyzed within three major contexts: large-scale atmospheric response, local surface response, and regional hydrological budgets.; Refinements to the atmospheric model have been made for radiative transfer and suppression of Gibbs waves. The first biosphere formulation used (BATS1) involves various modifications to BATS: 43 vegetation types; separate treatment of temperature of vegetation-covered upper soil; improved computational techniques. In the second case (BATS2), BATS1 is expanded upon to incorporate effects of seasonally varying-vegetation, based on a biweekly climatology of Normalized Difference Vegetation Index (NDVI). Temporally and spatially varying normalization factors are derived from the NDVI climatology to modify BATS vegetation parameters. Hence a different seasonal cycle of vegetation is prescribed for each grid point.; Although large-scale atmospheric responses to vegetation forcing are not pronounced, use of BATS generally maintains Northern Hemisphere upper level jets better and simulates a stronger Somali low-level jet. All models have some difficulty modeling monsoonal flow patterns over land, probably due to orography. Diurnal composites of forecast surface variables at representative grid points reveal systematic differences between model runs, including different phase lags in diurnal cycles of surface temperature and precipitation. Generally, turbulent fluxes with the Control are excessive over semiarid regions. Inclusion of prognostic snow cover in BATS is shown to be advantageous. The seasonally varying vegetation in BATS2 often significantly modifies diurnal cycles.; Regional surface moisture budgets illustrate the importance of a variety of moisture flux processes. Major variations in the hydrologic cycle exist in the 12 regions examined. The Control grossly overestimates rainfall over deserts and underestimates it over the Amazon. BATS runs improve spatial variation of latent heat fluxes over land.