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 JarvisStewart 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.

A land data assimilation system for sub-Saharan Africa food and water security applications

“…For example, Noah is the operational LSM in the NOAA National Centers for Environmental Prediction (NCEP) weather, climate and data assimilation systems and at the US Air Force 557th Weather Wing. There are a number of examples of Noah being used over different domains in Africa, e.g., West Africa 29 , 30 , Volta Basin 31 , East Africa 32 , 33 and coupled with the Weather Research and Forecasting (WRF) model for West Africa 34 and Kenya 35 . The FLDAS uses the Noah LSM version 3.3 (ref.…”

Surface response to rain events throughout the West African monsoon

“…The differences between LSMs and observations increase for drier soils at higher latitudes where the EF turns out to be very sensitive to evaporation and transpiration schemes. Schüttemeyer et al (2008) already pointed out the various sensitivities of simulated surface fluxes to soil moisture in comparing HTESSEL and NOAH. At Niamey, the relationship between the surface response and SWCA ranges from an almost linear relationship, as in Fig.…”

Confronting the WRF and RAMS mesoscale models with innovative observations in the Netherlands: Evaluating the boundary layer heat budget

“…[66] The NOAH land surface scheme [e.g., Ek et al, 2003, Schüttemeyer et al, 2008 the evapotranspiration is the sum of evaporation from bare soil, the wet canopy and plant evaporation. We expect that in this case the flux is dominated by the latter: E t = s f E p B c 1 À ffiffiffiffi ffi W c S q , in which B c is a function of canopy conductance of the Jarvis-Stewart approach, s f is the vegetation fraction, E p potential evaporation following the Penman approach.…”