Evaluation of Two Land Surface Schemes Used in Terrains of Increasing Aridity in West Africa

Abstract

In this study different parameterizations for land surface models currently employed in meteorological models at ECMWF [Tiled ECMWF Surface Scheme for Exchange Processes over Land (TESSEL)] and NCEP (Noah) are evaluated for a semiarid region in Ghana, West Africa. Both schemes utilize the Jarvis?Stewart approach to calculate canopy conductance as the critical variable for partitioning the available energy into sensible and latent heat flux. Additionally, an approach within Noah is tested to calculate canopy conductance based on plant physiology (A-gs method), where the photosynthetic assimilation is coupled to the leaf stomatal conductance. All parameterizations were run offline for a seasonal cycle in 2002/03 using observations as forcings at two test sites. The two locations are in the humid tropical southern region and in the drier northern region. For the purpose of forcing and evaluation, a new set of data has been utilized to include surface fluxes obtained by scintillometry. The measurements include the rapid wet-to-dry transition after the wet season at both sites. As a general trend, it has been found that during the wet period of a season net radiation is described well by all parameterizations. During the drying process the errors in modeled net radiation increased at both sites. The models perform poorly in simulating soil heat fluxes with larger errors for TESSEL for both sites. The evolution in time for sensible heat flux and latent heat flux was tackled in different ways by the utilized parameterizations and sites with enhanced model performance for the more southern site. Soil moisture in the upper soil layers is modeled with small errors for the different parameterizations. Key adjustments for reducing net radiation during the dry period of a season are discussed. In particular, the ratio of roughness length of momentum and heat was found to be an important parameter, but will require seasonal adjustments.