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Regional estimates of net ecosystem-atmosphere exchange of carbon dioxide over a heterogeneous ecosystem

by Wang, Weiguo.

Abstract (Summary)
iii The net ecosystem-atmosphere exchange of CO2 (NEE) is estimated over a mixed forest ecosystem in the 40×40km 2 region centered at the WLEF tall tower in northern Wisconsin. Flux aggregation and the atmospheric boundary layer (ABL) budget methods are used. CO2 fluxes measured at three levels of the WLEF tower are decomposed in the growing season to infer the CO2 fluxes for six stand types based on a stand level classification scheme in the tower footprint area. The flux footprint models, vegetation map and eddy-covariance data are combined to estimate the optimal solutions for the parameters in two ecosystem models for each stand type. The results show differences in the parameters and fluxes among the different vegetation classes that are consistent with general expectations for the respective stand types. The inferred fluxes are compared to observations from two stand-level eddy-covariance flux towers, implying that the six stand classification scheme is insufficient to capture all variability in stand characteristics relevant to CO2 exchange. Significant uncertainty in aggregated fluxes exists due to the classification schemes used. In the calculation, a new approximate analytical model is introduced to predict the footprint for flux measurements of passive scalars in the lower part of the mixed layer. An analytical footprint model under the idealized convective boundary layer is adjusted by comparing its solutions to those from a Lagrangian stochastic model with more realistic meteorological conditions. The ABL budget method is used to infer regional NEE by using the vertical profile of CO2 mixing ratios measured at the WLEF tall tower. The estimated regional NEE is close to that measured at the highest level of the WLEF tower in the day. In the iv nocturnal boundary layer, the cases when the atmosphere is very stable or nearly-neutral are screened out due to possible systematic errors. Regional NEE is generally bounded by the current tower measurements. To estimate the effects of horizontal advection on daytime NEE estimates, a water vapor constraint is introduced, where the range of the effects of horizontal advection of CO2 is estimated by the measurements of the spatial distribution of water vapor. The horizontal advective flux of CO2, a function of wind direction, is estimated to be smaller than 10-20% of the surface flux in our budget calculation on temporal and spatial scales of one hour and 40km. Regional NEE estimates from the two methods are compared with each other and with those reported in the literature. In the dormant season, differences among all estimates are relatively small so that NEE measured at the WLEF tower might be an acceptable approximation of NEE in the region (103 km2) and even in a larger region (104 to 105 km2) on annual or longer time scales. This, however, is not true in the growing season. The cumulative regional NEE values are -103±50 and -175±60gC m-2 in 2000 and 2003, respectively, suggesting that the region is a net sink of CO2. Those values are significantly different from those measured at individual towers. It is inappropriate simply to extend fluxes measured at individual towers to a larger region. Key words: footprint modeling, atmospheric boundary layer budget, regional carbon flux estimates, aggregation, flux footprint decomposition
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School:Pennsylvania State University

School Location:USA - Pennsylvania

Source Type:Master's Thesis

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