The residual of the energy balance closure and its influence on the results of three SVAT models

Abstract

Commonly used measuring methods only allow for a closure of the energy balance at the Earth's surface except for a residual. Models, in contrast, inherently demand an entirely closed energy balance, which they achieve in different ways. This paper examines the model simulations depending on the methods used for closing the energy balance, the optimization of the parameters, and the equations used. The one-dimensional surface vegetation atmosphere transfer (SVAT) models REMO, SEWAB and TERRA, which were made available by the GKSS Institute in Geesthacht, are examined and compared with measured data from the LITFASS-2003 experiment. LITFASS-2003 took place during May and June 2003 in the experimental fields of the Meteorological Observatory Lindenberg. Parameter sets optimized with regard to the turbulent fluxes yield better results, but are often not consistent with site characteristics and differ within the models. We ran the models with the optimized parameter sets, and with a set of selected parameters which agree with the respective sites. SEWAB closes the energy balance by using an iteration of the surface temperature, through which the influence of the residual is distributed over all fluxes. In REMO and TERRA the soil heat flux is estimated as the residual of the other components of the energy balance equation. This results in an extreme overestimation of this heat flux. Parameter optimization has an impact on the partitioning of the available energy and its distribution to the turbulent heat fluxes. It does not influence the effect of the residual on the results. Currently we do not know how to close the measured energy balance at the surface appropriately because of the underestimation of the turbulent fluxes by eddy-covariance measurements and the unknown partitioning of the residual between the sensible and latent heat flux. Therefore, the method used in SEWAB yields better results than the one used in REMO and TERRA. The method used to close the energy balance has more impact on the simulated surface energy balance components than does their parameterization. For this reason it is not possible to attribute the differences in the calculated heat fluxes to particular differences in the equations and parameters.