Regional surface fluxes from a coupled mesoscale atmosphere/vegetative canopy model

The UCDavis UCD Advanced Canopy-Atmosphere-Surface Algorithm (ACASA) has been coupled to a version of the Penn State NCAR MM5 V2 regional atmospheric model for the Western United States. ACASA is a multi-layer canopy-surface-layer model that solves the steady-state Reynolds-averaged fluid flow equations to the third order. It includes a fourth-order, near-exact technique to calculate temperatures and surface energy fluxes at various levels within the canopy, and plant physiological responses using the Ball-Berry/von Caemmerer- Farquhar formulations. The output from ACASA compares well with observed energy fluxes and microenvironmental conditions from a groomed grass field in the Netherlands, deciduous and coniferous forests in Canada, tropical pasture and forest in Brazil, and an ancient temperate rainforest in the United States.

The coupled model results replicate meteorological conditions in good agreement with standard meteorological analyses. They also emphasize the general importance of net surface radiation and soil moisture on the surface fluxes of latent and sensible heat and carbon dioxide. On the other hand each surface flux has a unique complex spatial pattern, which is partially related to terrain and precipitation. Results also show that the typical observational strategy of excluding measurements near rain events or during times of high wind speeds could have important consequences on mean fluxes.