Large-eddy simulation of forest edge flow with ARPS

Forest landscapes are often characterized by a large spatial variability due to the presence of clearings, roads, forest patches of different heights, etc. This fragmentation induces significant spatial variability in wind velocity or turbulence, with occasional regions of high level of turbulence that may be responsible for damages during windstorms. In order to better understand the turbulent flow behavior over sharp heterogeneities such as forest edges, the Advanced Regional Prediction System (ARPS) has been modified in order to simulate turbulent flow at very fine scale (2 m) within and above heterogeneous vegetation canopies with a large-eddy simulation (LES) approach. This new version of ARPS is first validated on a homogeneous forest canopy and on a simple forest-clearing-forest pattern, using field and wind-tunnel measurements. Both validations show that the model is able to reproduce accurately the main features of the turbulent flow observed in both configurations, especially the enhanced gust zone around treetop at a few canopy heights downwind from the edge, characterized by a peak value of the streamwise velocity skewness. This indicates the presence of intense intermittent wind gusts. A sensitivity study shows to what extent the development and intensity of this enhanced gust zone depend on the vertical leaf area distribution, as well as on the subcanopy density at the leading edge, that modulates the development of a wind jet within the trunk space. The position and magnitude of the enhanced gust zone seems correlated positively with the maximum average upward motion at treetop, downwind from the leading edge.