Sensitivity of ground heat flux to vegetation cover fraction and leaf area index

Two land-surface models that participated in the recent Project for Intercomparison of Land-Surface Parameterization Schemes (PILPS, phase 2c) are used to investigate the sensitivity of the ground heat flux to the vegetation cover fraction and leaf area index (LAI). The two models are the Biosphere-Atmosphere Transfer Scheme (BATS) and the model developed at the Institute of Atmospheric Physics, Chinese Academy of Sciences (IAP94). The impacts of including transmission of solar radiation through the canopy in the BATS model are also assessed. The ground heat flux is the energy residual of net radiation minus latent plus sensible heat fluxes at the soil surface (also referred to as the soil heat flux). However, the energy residual above the canopy was used as a surrogate for the ground heat flux by the two models in the PILPS 2c study. The two energy residuals (i.e., above the canopy and at the soil surface) can differ depending on the modeled time step, the order in which the canopy and soil temperatures are computed, and whether canopy heat storage is included or neglected. As expected, reducing the areal coverage of vegetation results in an increased daytime soil heat nux, and increasing LAI leads to decreased soil heat flux and greater above-canopy latent heat flux. Both models show a strong sensitivity to LAI when LAI is small and little sensitivity when LAI is large. Allowing transmission of solar radiation through the canopy in BATS reduces the sensible heat flux above the canopy and enhances all the flux terms at the soil surface, especially when LAI is low. This model behavior is similar to that from IAP94, which uses a two-stream radiation scheme. This modification to BATS also results in a soil heat flux that lies within estimated bounds for a wide range of LAI (0.5-5.5).