Surface energy balance complexity in GCM land surface models. Part II: coupled simulations

Global coupled simulations with the Bureau of Meteorology Research Centre c limate model and the CHAmeleon Surface Model (CHASM) are used to examine ho w four general extensions to the representation of the basic land surface e nergy balance affect simulated land-atmosphere interface variables: evapora tion., precipitation, skin temperature and air temperature. The impacts of including separate surface energy balance calculations for: vegetated and n on-vegetated portions of the land surface; an explicit parametrisation of c anopy resistance; explicit bare ground evaporation; and explicit canopy int erception are isolated and quantified. The hypothesis that these aspects of surface energy balance parametrisation do not contain substantial informat ion at the monthly time scale (and are therefore not important to consider in a land surface model) is shown to be false. Considerable sensitivity to each of the four general surface energy balance extensions is identified in average pointwise monthly changes for important land-atmosphere interface variables. Average pointwise changes in monthly precipitation;ind land evap oration are equal to about 40 and 31-37% of the global-average precipitatio n and land evaporation respectively. Average pointwise changes fur land sur face skin temperature and lowest model layer air temperature are about 2 an d 0.9 K respectively. The average pointwise change and average pointwise bi ases are statistically significant at 95% in all cases. Substantial changes to zonally average variables are also identified. We demonstrate how the g lobally averaged surface resistance parameter can vary from 150 to 25 s/m d epending on which aspects of the surface energy balance are treated implici tly. We also show that if interception is treated implicitly, the effective surface resistance must vary geographically in order to capture the behavi our of a model which treats this process explicitly. The implication of the se results for the design of land surface models is discussed.