The influence of rain sensible heat and subsurface energy transport on the energy balance at the land surface

In land surface models, which account for the energy balance at the land surface, subsurface heat transport is an important component that reciprocally influences ground, sensible, and latent heat fluxes and net radiation. In most models, subsurface heat transport parameterizations are commonly simplified for computational efficiency. A simplification made in all models is to disregard the sensible heat of rain, H_l, and convective subsurface heat flow, q_cv, i.e., the convective transport of heat through moisture redistribution. These simplifications act to decouple heat transport from moisture transport at the land surface and in the subsurface, which is not realistic. The influence of H_l and q_cv on the energy balance was studied using a coupled model that integrates a subsurface moisture and energy transport model with a land surface model of the land surface energy balance, showing that all components of the land surface energy balance depend on H_l. The strength of the dependence is related to the rainfall rate and the temperature difference between the rain water and the soil surface. The rain water temperature is a parameter rarely measured in the field that introduces uncertainty in the calculations and was approximated using the either air or wet bulb temperatures in different simulations. In addition, it was shown that the lower boundary condition for closing the problem of subsurface heat transport, including convection, has strong implications on the energy balance under dynamic equilibrium conditions. Comparison with measured data from the Meteostation Haarweg, Wageningen, the Netherlands, shows good agreement and further underscores the importance of a more tightly coupled subsurface hydrology-energy balance formulation in land surface models.