Application of Doppler lidar in forestry research

Damage due to wind is a large economic risk in forest management. The interaction between the wind and the forest edge yields in turbulence structures with locally extremely high wind velocities. Up to now these structures are not well understood. Physical models, numerical simulations as well as in situ measurements were used to study the interaction between wind and forest, but lidars have not been used in this research. During winter 2009/2010 the Institute of Meteorology and Climate research (IMK) and the Institute of Hydromechanics (IfH) of Karlsruhe Institute of Technology (KIT) carried out a feasibility study to investigate the performance of Doppler lidars to measure turbulence across a forest edge. The field measurements were realized at a long straight forest edge in the Rhine Valley at Hatzenbühl, Germany, using two scanning Doppler lidars, one detecting continuously the vertical profile of the horizontal wind and one performing rapid RHI scans at small elevation angles perpendicular to the forest edge. Additionally a true to original physical model of the forest edge was build in a boundary layer wind channel, where wind velocity was measured using laser Doppler anemometry (LDA). This setup gives the opportunity to compare the new lidar measurements from real world with the well introduced physical model. Our results show, that Doppler lidar is an adequate instrument to resolve the turbulent structure across a forest edge. Flow conditions similar to the modelled situation in the wind channel yield in analogous distributions of the zero and first moment of the horizontal wind across the forest, although the lidar measurements show smaller absolute values of wind velocity variance. Additionally to the investigation of typical flow situations, we show measurements made during the winter storm Xynthia, which passed the measurement area in February 2010 with wind speeds up to 160 km per hour, as an example for changes in the turbulence structure at a forest edge due to an upcoming storm.