Handout (326.0 kB)
Weiguo Wang, Kenneth J Davis, Daniel M Ricciuto, Martha P. Butler
Department of Meteorology, The Penn State university, State College, PA16802
The location and size of the footprint for eddy-covariance flux measurements over a heterogeneous surface are critical because the measured flux depends largely on which parts of the surface flux are sampled. There are a number of studies on the flux footprint for measurements within the surface layer. The flux footprint for measurements above the surface layer, however, has not been extensively studied. In this study, an empirical model is derived by combining an analytical footprint solution under an ideal convective boundary layer with the results from a Lagrangian stochastic model driven by more realistic atmospheric variables. As an application, footprints for eddy-covariance CO2 fluxes measured at 30m, 122m, and 396m of the WLEF tower over a mixed forest are simulated under unstable conditions. A surface layer footprint model and the empirical convective boundary layer footprint model are applied within and above the surface layer, respectively, using the meteorological data collected in 1998 and 1999. Boundary layer depths are estimated from data of a 915-MHz boundary layer profiling radar. Stability parameters and wind velocities are measured directly at the tower. Potential measurement biases at the lowest level are found under strongly unstable conditions, resulting from the significant contribution of flux from the clear-cut grass area surrounding the tower base. Long-term footprint modeling indicates that the sampling areas of the top two levels in the day consist mainly of wetland and upland forest areas while the lowest level measurements sample the areas of grass, wetland, and forest, and hence measurements at the top two levels better represent the daytime wetland and forest fluxes in this region.
- Indicates paper has been withdrawn from meeting
- Indicates an Award Winner