Impact of atmospheric motions on CO; fluxes in non-ideal terrain

P.I./Institution: Monique Y. Leclerc, The University of Georgia

Co-PI/Institution: Timothy Martin, University of Florida

Summarize in two to four sentences what activities will be carried out in the project: We will deepen and refine our knowledge of net ecosystem exchange (NEE) and narrow the information gap in the area of integrated NEE assessment 1) by documenting the presence of typically undetected physical mechanisms responsible for the creation of significant systematic CO2 flux bias resulting from single point flux tower systems, with particular attention to nighttime conditions 2) by evaluating the impact of atmospheric flow features observed in real-terrain terrestrial ecosystems on CO2 flux measurements, to provide a more robust assessment of carbon sources and sinks, and their link to environmental change in the SouthEast and 3) by providing spatially integrated measurements of CO2 fluxes using the nocturnal boundary layer budget method (NBL), and by providing boundary layer information required to improve our regional scale modeling of CO2 exchange. This project augments the Florida AmeriFlux site and cross-links with the WESGEC Center's proposal 'Regional Scale Modeling and Turbulence Measurements of Long-Term Carbon and Energy Exchange'.

Project Abstract: This project builds upon recent findings from our current NIGEC project in which initial forays into investigations of regular organized flow motions (Prabha et al., 2003; Leclerc et al., 2003) were initiated at the Florida site. This proposal explores both differences in NEE attributed to spatial heterogeneities in sources/ sinks and attributed to the site associated with land use and topography. This project also brings a contribution to our integration into regional assessment of NEE because: 1. it characterizes the contribution — and impact — of common 'anomalous' flow properties to CO2 flux measurements, with particular attention to spatial inhomogeneities 2. it provides spatial information of carbon sources and sinks strengths using the nocturnal boundary layer budget method 3. it characterizes small-scale and linked regional flow to improve regional carbon exchange modeling. This project examines large, systematic, most often undetectable flux towers bias resulting from several sources including density gradients arising from the presence of two adjoining dissimilar natural surfaces which can generate a flow organization of large magnitude; plantations of interspersed stands of mixed ages and species typically create CO2 gradients leading to advection, a feature characterizing the Florida site and many others; furthermore, initial observations within our present NIGEC project (Prabha et al., 2003) suggest that nocturnal flows have the potential to alter the measured NEE in a drastic manner. Jets, waves and other flow features observed at night in the lower layers of the atmosphere, contribute to CO2 fluxes. How those impact the exchange of carbon between the forest and the atmosphere must be evaluated. Small-scale flow and linked regional flow that appears to be present in non-ideal terrain will be characterized to improve the accuracy of carbon exchange at the regional scale. The latter element of the proposal is cross-linked with a concurrent project in the WESTGEC region by Paw U et al. entitled 'Regional scale modeling and turbulence measurements of long-term carbon and energy exchange' allows a cost-effective pooling of resources. Insight into the sources of potential bias will be important to all AmeriFlux sites because all sites have mean flows capable of significant mean advective exchange.

Summary in "layman's" terms of what will be done and why: The amount of carbon taken up by terrestrial ecosystems varies by several orders of magnitude. This is in part because many measurements are crippled with theoretical and technical challenges, often requiring additional spatial measurements to judiciously interpret CO2 exchange measurements. For the experimentalist at a real, non-ideal site representative of terrestrial ecosystems including those of the SouthEast, a thorough knowledge of the "site's personality" is required in order to derive true NEE estimates. Understanding these 'personality traits' is a must if we want to develop better methods of measurement sampling and analysis leading to true landscape-wide NEE integrated assessments. This project provides improved carbon sources and sinks determination, a spatially integrated NEE assessment, and the input to carbon exchange regionalization efforts thanks to a cross-linkage with WESTGEC, thus benefiting both regions.