Analysis of the surface energy budget at a site of GAME/Tibet using a single-source model

In the GAME/Tibet project, the Anduo site is a typical plateau prairie area that is covered by short and sparse vegetation. From May to September 1998, we measured all the surface energy fluxes at this site to quantitatively describe the surface energy budget, but it was still questionable because of low energy closure ratios in most days of the rainy season. To clarify the surface energy budget at this site, this study proposes a single-source energy partition model, to simplify the processes of heat and water vapor transfer from the surface. The model does not distinguish the contribution of turbulent fluxes from the vegetation, and the one from the ground, and thus has fewer parameters to be calibrated comparing to a dual-source model. The main model parameters are: the soil surface evaporation resistance, the parameter kB(-1) (the logarithm of the ratio of the aerodynamic roughness length to the thermal roughness length), and the surface emissivity. Their values are calibrated by minimizing the discrepancy between measured and model-predicted values of soil temperatures. This tuned single-source model is then validated by the agreement between the measured and simulated net radiation flux, surface soil heat flux, sensible heat flux, and latent heat flux on the days with high energy closure ratios. This model also reproduces these measured fluxes, except latent heat flux on the days with low closure ratios. The simulated latent heat flux is much higher than the measured one. We further show that the measured latent heat flux is untrustable due to an instrumental limitation, while the model provides a reasonable surrogate. Therefore, the single-source concept is applicable to the heat and water vapor transfer on the Tibetan plateau sparse-vegetation surface, and would contribute to a further research on the land-atmosphere interactions over the plateau.