Implementing surface parameter aggregation rules in the CCM3 global climate model: regional responses at the land surface

The land-surface parameters required as input to a GCM grid box (typically a few degrees) are often set to be those of the dominant vegetation type wi thin the grid box. This paper discusses the use and effect of aggregation r ules for specifying effective values of these land cover parameters by taki ng into account the relative proportion of each land-cover type within each individual grid box. Global land-cover classification data at 1 km resolut ion were used to define Biosphere Atmosphere Transfer Scheme (BATS) specifi c aggregate (using aggregation rules) land-cover parameters. Comparison of the values of the aggregate parameters and those defined using the single d ominant vegetation type (default parameters) shows significant differences in some regions, particularly in the semi-desert and in forested regions, e .g, the Sahara Desert and the tropical forest of South America. These two d ifferent sets of parameters were used as input data for two 10-year simulat ions of the NCAR CCM3 model coupled to the BATS land-surface scheme. Statis tical analyses comparing the results of the two model runs showed that the resulting effects on the land-surface diagnostics are significant only in s pecific regions. For example, the sensible heat flux in the Sahara Desert c alculated for the aggregate parameter run increased due to the marked incre ase in the minimum stomatal resistance and the decrease in fractional veget ation cover in the aggregate parameters over the default parameters. The mo delled global precipitation and surface air temperature fields were compare d to observations: there is a general improvement in the performance of the aggregate parameter run over the default parameter run in areas where the differences between the aggregate and default parameter run are significant . However, most of the difference between the modelled and observed fields is attributable to other model deficiencies. It can be concluded that the u se of aggregation rules to derive land-surface parameters results in signif icant changes in modelled climate and in some improvements in the land-surf ace diagnostics in selected regions. There is also some evidence that there is a response in the global circulation pattern, which is a focus of furth er work.