Flux measurements from a tall tower in a complex landscape

Flux measurements of momentum, heat, water vapor and carbon dioxide at 30m 60m and 304m were analyzed from a tower in rolling terrain above a complex rural landscape of forest, agricultural land, and residential areas. The purpose of the analysis was to determine the magnitude and significance of departures from an ideal homogeneous landscape.

Heterogeneity was measured in terms of the vertical and sector variation of the friction velocity and the streamwise velocity variance, which were associated with variations in upwind fetch. Aerodynamic effecs near the tower were assessed by comparing mean vertical advection with eddy covariances determined in the planar fit coordinate system. Representativeness of the fluxes was studied by comparing the vertical eddy flux convergence to the the time rate of change in the 30-60m and the 30-300 meter layers.

It was found that the friction velocity was approximately constant with sector at 30m and above which implies good blending of fluxes from the various landscape types. The heat and moisture flux convergence were in reasonable agreement with the changes in the layer mean quantities except near sunrise, which implies that the tower fluxes are representative of the area around the tower for most of the day. However,the changes in the layer average carbon dioxide was signicantly different from the layer flux convergences which suggests significant horizontal advection of CO2.