Calculating advective fluxes in tall canopies – Towards better wind speed distribution using 3D vegetation scans in high resolution

Abstract

The wind speed distribution in forests is dominated by inhomogeneities like step changes in stand height and forest clearings. Thus a major limitation in the attempts to describe and model the wind field in destined tall canopies is the parameterization of plant architecture. The relationship between wind speed, drag coefficient and plant area distribution was experimentally investigated in a mixed conifer forests in the lower ranges of the Osterzgebirge. The results of this study will be applied to different kinds of flow models to investigate the influence of advective fluxes of energy and matter. From May 2008 to May 2009 intensive turbulence measurements took place on a transect over the forest clearing „Wildacker“ (Tharandter Wald, N 50°57'49", E 13°34'01"). In total 25 measurement points, at 4 towers (heights: 40m, 40m, 40m, 30m) including five at ground level position (2 m), are used to record the turbulent flow simultaneously. Terrestrial laser scanning is a fast developing tool and appears to be an efficient method to record 3D models of the vegetation. The forest stands around the clearing (500 m x 60 m) were scanned applying a Riegl LMS-Z 420i and a Faro LSHE880. Thereby scans from different ground positions and from the top of the main tower (height: 40m) were done. The scans were filtered and combined to a single 3D representation of the stands. The detection of trees was done automatically and mean tree distances were calculated. The 3D point cloud of trees in a 60m x 310m x 50m model domain was transformed into a 3D voxel space. The normalized point density of each voxel represents the plant area density PAD. A scaling of the laser derived totals per floor space was done by measurements with the LAI2000 (LICOR). The so calculated PAD and the spatial arrangement of points inside the voxel can be used to derive a parameterization for the drag coefficients. Simultaneously, the drag coefficients are calculated from turbulence measurements at the positions of anemometers. Finally the dependency between drag coefficients and PAD can be investigated with respect to stability and wind speed. Using measured wind profiles this study aims further to validate and develop estimates of parameters like mixing length, displacement height and roughness length from the plant area density profile.