Daytime Turbulence Statistics above a Steep Forested Slope

Abstract

Six levels of simultaneously sampled ultrasonic data are used to analyse the turbulencestructure within a mixed forest of 13 m height on a steep slope (35 ◦ ) in an alpine valley. The data setis compared to other studies carried out over forests in more ideal, flat terrain. The analysis is carriedout for 30-min mean data, joint probability distributions, length scales and spectral characteristics.Thermally induced upslope winds and cold air drainage lead to a wind speed maximum within thetrunk space. Slope winds are superimposed on valley winds and the valley-wind component becomesstronger with increasing height. Slope and valley winds are thus interacting on different spatial andtime scales leading to a quite complex pattern in momentum transport that differs significantly fromsurface-layer characteristics. Directional shear causes lateral momentum transports that are in thesame order or even larger than the longitudinal ones. In the canopy, however, a sharp attenuationof turbulence is observed. Skewed distributions of velocity components indicate that intermittentturbulent transport plays an important role in the energy distribution.Even though large-scale pressure fields lead to characteristic features in the turbulent structurethat are superimposed on the canopy flow, it is found that many statistical properties typical of bothmixing layers and canopy flow are observed in the data set.