Influence of the biosphere on the global circulation and hydrologic cycle - a GCM simulation experiment

Abstract

The Simple Biosphere model (SiB) of Sellers et al. was implemented in the NASA/Goddard Laboratory for Atmospheres (GLA) General Circulation Model (GCM). SiB calculates the land surface fluxes of heat, moisture and momentum, as well as the near-surface fluxes of shortwave and longwave radiation with an explicit parameterization of the interactions among atmosphere, vegetation and soil processes. The influence of including SiB in the GLA GCM is evaluated by comparing the monthly simulations of the GCM with and without SiB. The latter parameterizes the surface drag coefficient as a simple function of topography and also uses two hypothetical slabs of soil: one for diurnal temperature variations and one for annual soil moisture variations, that are necessary for calculating the land surface fluxes of heat and moisture. Ensemble sets of four July and four January simulations were made with the GLA GCM. Each set contains two runs: one with and one without SiB in the GCM. The simulated years were 1979, 1980, 1981 and 1982 and each 47-day model integration was started from the analyzed observations for the antecedent 15 December 00Z and 15 June 00Z for January and July, respectively. After allowing an initial adjustment period of about 16 days, the last 31 days of each integration were taken to represent the desired simulation. The results of SiB vis-a-vis noSiB comparison show that SiB simulates much lower evapotranspiration rates over land and generates significantly different values of surface fluxes for both vegetated and bare soil regions. These differences in the surface fluxes are accompanied by large and statistically significant changes in the simulated rainfall, particularly in the tropics and the summer hemisphere. An analysis of the monthly mean diurnal cycle of surface fluxes shows that both the surface fluxes and rainfall values for the SiB-GCM are realistic in different regions: densely vegetated, sparsely vegetated and bare soil (including dry deserts). Despite some shortcomings of the SiB-GCM simulations, such as insufficient latitudinal displacement of the ITCZ in both the summer and winter simulations, it is evident that the SiB provides a better estimate of the surface fluxes on the whole, leading to improvements of the simulated circulation and rainfall climatology of the Earth. © 1990.