Monday, 30 July 2001
A Photosynthesis-based Gas-Exchange Evapotranspiration Model (GEM) coupled with a Land Surface Scheme for Mesoscale Applications
Accurate representation of land surface processes (LSPs) is a pivotal component of mesoscale models. LSPs affect not only the surface energy balance but also the entire boundary layer structure, mesoscale circulations, cloud formation, and precipitation patterns. In the majority of mesoscale models a diagnostic Jarvis-type LSP scheme is employed. This scheme, though robust, needs significant tuning to simulate surface energy balances over different natural surfaces, and requires specification of arbitrary constants that cannot be measured. Recent advances in vegetation models have included explicit relations for modeling photosynthesis and stomatal conductance and hence the variations in the surface evapotranspiration. These photosynthesis- or carbon-assimilation-based stomatal models have been successfully employed in models ranging from leaf scale to climate scale. In this study, we developed, coupled, and validated a gas-exchange-based surface evapotranspiration model (GEM) as a land surface/soil-vegetation-atmosphere transfer (SVAT) scheme for mesoscale models. The GEM was dynamically coupled with a prognostic soil moisture, soil temperature model in an atmospheric boundary layer (ABL) model. This coupled system was then validated over various natural surfaces: a C4 grass prairie, a C4 corn field, a C3 soybean field, a C3 fallow site, a C3 hardwood forest site, and a tropical field site. For each of the surfaces, except the fallow site, two case studies were performed under contrasting surface conditions (such as different soil moisture or leaf area index). In all, 11 case studies were conducted and the model simulations were compared with actual field measurements of surface sensible and latent heat fluxes. For some of the observations, measurements of vertical boundary layer profiles or direct measurements of stomatal resistance and photosynthesis rates were available and were compared with the GEM-based coupled SVAT model output. Results indicate that the model is able to simulate the various surface and boundary layer characteristics quite successfully. Generally the surface energy fluxes, particularly the latent heat flux, were within 10%-20% of the observations without any tuning of the biophysical-vegetation characteristics. The model also satisfactorily simulated the day-to-day variations in the heat fluxes. The model response to the changes in the surface characteristics has been consistent with observations and theory. Thus, the coupled GEM-ABL model can be efficiently applied for a range of environmental applications at different scales. We conclude that the photosynthesis-based SVAT approaches are superior to Jarvis-based approaches and can be applied for mesoscale environmental and weather models at various scales.