Modeling Heat, Water Vapor, and Carbon Dioxide Flux Distribution Inside Canopies Using Turbulent Transport Theories

all participating in a world-wide flux monitoring initiative known as FluxNet (Baldocchi et al., 2001). This study reports recent developments in mulitlayer turbulent Within FluxNet, many of the flux measurements are transport methods to compute distributions of strengths of scalar conducted within the canopy sublayer of tall forests, sources and sinks Sc as well as turbulent fluxes Fc within the plant– thereby prohibiting the use of single-layer (or big leaf) atmosphere continuum. In particular, we focus on the so-called “inapproaches to link turbulent fluxes with their microcliverse methods” that estimate Sc from measured mean scalar concentration (or temperature) distribution within the canopy without resorting mate. Here we consider mulitlayer theories (Finnigan to any ecophysiologically based input. These approaches are able to and Raupach, 1987; Raupach, 1988) which explicitly reproduce measured turbulent fluxes above and within the canopy consider the nonuniform vertical structure of the canopy without relying on gradient diffusion formulation. Comparison beand can resolve the subsequent feedbacks of such nontween measured and modeled sensible heat flux vertical attenuation uniformity on the microclimate via complex turbulent within the canopy suggests that all three methods provided comparatransfer processes. The objective is to present and evaluble root-mean squared error (RMSE) ( 50 W m 2 ). Furthermore, ate recent advances in methods that estimate scalar correcting for local atmospheric stability significantly improved the source–sink distribution within the canopy using multiagreement between model calculations and measurements. Comparilayer transport theories and over a broad range of time sons between measured and modeled land surface fluxes of sensible scales, ranging from minutes to years. heat, latent heat, and CO2 above the canopy are conducted using wavelet spectral methods applied to a wide range of temporal scales Multilayer theories are commonly classified as either (30 min–2 yr). This study is the first to rigorously assess the perfor“forward” or “inverse” depending on the types of meamance of several inverse methods for such a broad range of time surements performed (Raupach, 1989a,b). In forward scales. We found that the three inverse modeled flux spectra bound approaches, the source strength and location is specified the measured one. Hence, spectral agreement among the three models such that knowledge of the turbulent transport mechanprovides the necessary confidence in calculated fluxes and scalar ics permits the estimation of downwind mean scalar sources. Conversely, large disagreement between the inverse models concentration and flux distribution. The inverse prob“flags” large uncertainties at those particular time scales. lem utilizes measured mean scalar concentration distribution downwind from the source in concert with knowlO of the major challenges in quantifying mass edge of the turbulent transport mechanics to infer the source–sink strength distribution. The interest in inverse (water vapor, CO2 ) and energy exchanges between models is driven, in part, by the fact that mean concenthe land surface and the atmosphere is that vegetation tration can be more easily measured (or monitored) is a biologically active source or sink of matter and within the canopy than can the actual sources and sinks, energy and can modify its microclimate through comor fluxes. Furthermore, these methods do not require plex turbulent exchange processes. How vegetated surany ecological or ecophysiological input, making them faces interact with their local microclimate, which in suitable for long-term source–sink calculations. To date, turn influences the exchange of CO2, water vapor, and no one study evaluated inverse models over a broad heat with the atmosphere has received considerable atrange of time scales ranging from hour to years. tention. In fact, variants on this question form key reWhile we focus on inferring sources and sinks within search themes in the Carbon Cycle Science Plan (Sarthe canopy volume, we stress that both their interpretamiento and Wofsy, 1999) and the Water Cycle Science tion and controls cannot be divorced from their counterPlan (Hornberger et al., 2001) of the United States Glopart in the vadose zone. For example, water vapor sources bal Change Research Program (USGCRP). This is not in the canopy volume are strongly impacted by the hysurprising given the modulating role of the biosphere draulics of root water uptake; forest floor respiration is on global atmospheric CO2 concentration (Wofsy et al., intimately linked to root and microbial respiration, 1993) and on the general spatial and temporal characterwhich in turn are influenced by the amount of C sequesistics of the water cycle (Chen et al., 2001; Maurer et tered by the leaves. Hence, it is constructive to probe al., 2001). A direct outcome of such attention is the some analogies between the laminar fluid flow comemergence and exponential growth of four continental monly encountered in the vadose zone and turbulent flux-monitoring networks: EuroFlux, AmeriFlux, Asiflows inside canopies before presenting the inverse modaFlux, and OzFlux (Valentini et al., 2000; Kaiser, 1997), els for the canopy volume. Canopy Turbulence vs. Vadose Zone Transport G. Katul, Nicholas School of the Environment and Earth Sciences Biological sources (or sinks) and fluxes are directly and Department of Civil and Environmental Engineering, Box 90328, Duke University, Durham, NC 27708-0328 and M. Siqueira, Departrelated by a scalar continuity equation (in the vertical ment of Civil and Environmental Engineering, Duke University, Durham, NC 27708. Received 28 Nov. 2001. *Corresponding author Abbreviations: ASL, atmospheric surface layer; EUL, Eulerian clo([email protected]). sure “inverse” model; HEL, hybrid Eulerian–Lagrangian model; LNF, localized near field theory; ODE, ordinary differential equation. Published in Vadose Zone Journal 1:58–67 (2002).