Flux measurements in complex, forested terrain

The planar fit method is often used for long term eddy covariance flux measurements since it offers a number of advantages over rotating each 15-minute sample into streamwise coordinates. For sites over complex, forested terrain a single planar fit may not be appropriate since there can be significant variations in the slope and in the forest canopy cover with wind direction. We present an alternative method where the tilt angle is fitted as a continuous function of the wind direction. This retains many of the benefits of the planar fit method, while at the same time better representing local variations in tilt with wind direction. The method is tested through application to sonic anemometer measurements from several flux towers at a site on the Isle of Arran, Scotland as part of a project studying forest canopy-atmosphere interactions over complex terrain. The towers are on a ridge, and there is significant variability in the forest canopy around some of the sites. In this case we only have measurements to compare momentum and heat fluxes between the different methods. In most cases the new continuous planar fit method demonstrates better agreement in terms of momentum and heat fluxes with the values obtained from rotating into streamwise coordinates when compared to the traditional planar fit method. The new method also compares favourably with the sector planar fit method and has the advantage of giving fluxes which vary smoothly with wind direction and avoids any discontinuities where different planar fits are used for different wind sectors.