Turbulent transport above a mixed Northern forest

This study is motivated by the need to observe NEE (net ecosystem-atmosphere exchange) of CO2 in forest ecosystems. Data from an above-canopy tower (Willow Creek) and a very tall tower (WLEF) are compared. The towers are located within a mixed forest-wetland landscape in northern Wisconsin, and are part of the Chequamegon ecosystem-Atmosphere Study (ChEAS, http://cheas.umn.edu). We compare the diurnal and seasonal patterns of NEE of CO2 and the rate of CO2 storage from the two towers. WLEF data represents a wetland-forest mix, while Willow Creek observations isolate the contribution of mature hardwood forests to the NEE of the region. In order to understand the mechanisms of turbulent transport and to reduce the uncertainty in measurements of NEE of CO2 under light wind conditions, statistical properties of winds, temperature, water vapor, carbon dioxide and eddy fluxes are studied at the Willow Creek tower. Light wind conditions often happen at night (low turbulence, light winds) and during the day (convective turbulence, light mean winds). The time scales dominating the transport of scalars such as temperature, vapor and carbon dioxide under these conditions are analyzed using wavelet and spectral methods. The profiles of CO2 within the forest are analyzed as a function of time of day and turbulent intensity. The data are also used to test similarity theory and flux-gradient relationships during light winds. Transport processes not captured by traditional one-dimensional NEE measurements are discussed, and the impact on NEE of CO2 at our sites is analyzed.