THE IMPACT OF IMPLEMENTING THE BARE ESSENTIALS OF SURFACE TRANSFER LAND-SURFACE SCHEME INTO THE BMRC GCM

This study describes the first order impacts of incorporating a comple x land-surface scheme, the bare essentials of surface transfer (BEST), into the Australian Bureau of Meteorology Research Centre (BMRC) glob al atmospheric general circulation model (GCM). Land seasonal climatol ogies averaged over the last six years of integrations after equilibri um from the GCM with BEST and without BEST (the control) are compared, The modeled results are evaluated with comprehensive sources of data, including the layer-cloud climatologies from the international satell ite cloud climatology project (ISCCP) data from 1983 to 1991 and the s urface-observed global data of Warren et al., a five-year climatology of surface albedo estimated from earth radiation budget experiment (ER BE) top-of-the-atmosphere (TOA) radiative fluxes, global grid point da tasets of precipitation, and the climatological analyses of surface ev aporation and albedo, Emphasis is placed on the surface evaluation of simulations of land-surface conditions such as surface roughness, surf ace albedo and the surface wetness factor, and on their effects on sur face evaporation, precipitation, layer-cloud and surface temperature. The improvements due to the inclusion of BEST are: a realistic geograp hical distribution of surface roughness, a decrease in surface albedo over areas with seasonal snow cover, and an increase in surface albedo over snow-free land. The simulated reduction in surface evaporation d ue, in part, to the biophysical control of vegetation, is also consist ent with the previous studies. Since the control climate has a dry bia s, the overall simulations from the GCM with BEST are degraded, except for significant improvements for the northern winter hemisphere becau se of the realistic vegetation-masking effects. The implications of ou r results for synergistic developments of other aspects of model param eterization schemes such as boundary layer dynamics, clouds, convectio n and rainfall are discussed.