A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Atmospheric Responses to Land and Water: Simulations and Observations ofMesoscale Circulations and CO 2 Concentrations in the Santarém MesoscaleCampaignA. Scott Denning 1 , Lixin Lu 1 , Elicia Inazawa 1 , Maria Assuncao Silva Dias 2 , Pedro SilvaDias 2 , Raymond Desjardins 3 , Jeffrey Richey 4 , Marek Uliasz 1 , and Peter Bakwin 51 Department of Atmospheric Science, Colorado State University, USA2 Universidade de São Paulo, IAG, São Paulo, Brazil3 Agriculture and Agri-Food Canada, Ottawa, Canada4 School of Oceanography, University of Washington, USA5 Climate Monitoring and Diagnostics Laboratory, National Oceanic and AtmosphericAdministration, Boulder, CO, USAVariations of the concentrations and stable isotope ratios of atmospheric CO 2 containinformation about sources and sinks at the underlying surface. We have investigatedmesoscale variations of atmospheric CO 2 over a heterogeneous landscape of forests,pastures, and large rivers during the Santarém Mesoscale Campaign (SMC) duringAugust, 2001. We simulated the variations of surface fluxes and atmosphericconcentrations of CO 2 using the CSU Regional Atmospheric Modeling System (RAMS)on a multiply-nested grid which included a 1-km inner grid centered on the FlonaTapajos. Surface fluxes of CO 2 were prescribed in the model using idealized diurnalcycles over forested and pasture vegetation, and over surface water using a valuesuggested by in-situ measurements in the Amazôn River. Land vegetation cover wasprescribed using AVHRR NDVI data. Atmospheric winds and structure and boundarylayerdepth were compared to observations made by radiosondes at Belterra and bySODAR at Santarém.Mesoscale circulations were simulated in the vicinity of both the Amazôn andTapajos Rivers on most days, with magnitudes of 1-2 m s -1 near the surface. These“riverbreeze” circulations were also present in observations made in the field. SimulatedCO 2 concentrations were perturbed by over 10 ppm in the immediate vicinity of therivers, with the strongest effect in the early morning. By midafternoon, the effect of theriver evasion fluxes on simulated concentrations was mixed through a deeper layer andinfluenced by the riverbreeze, but still easily measurable.In-situ measurements of atmospheric CO 2 and its stable isotopic ratios duringtransects flown in a small aircraft at midmorning were consistent with the river evasionflux hypothesis, though the magnitude was weaker than simulated. This suggests that theprescribed evasion flux in the model was too strong.Corresponding Author: Scott Denning, Atmospheric Science, Colorado State UniversityFort Collins, CO 80523-1371 USA denning@atmos.colostate.edu