Turbulence measurements above forests

This project is a cooperation together with:

Barry Gardiner, Northern Research Station (NRS)
Alan Coulson, Vattenfall Wind Power

Background

Atmospheric turbulence is influenced by the roughness of the underlying surface and forests are the roughest possible natural land cover. In comparison to less rough surfaces this causes an increase in turbulence and wind shear, while the horizontal wind speed is reduced, due to an elevated zero plane displacement height.

At the moment air flow models are not able to calculate reliable predictions for situations where a forest canopy is present.

Objectives

The objectives of this experiment are to quantify the impact of the forest canopy on atmospheric turbulence, by analysing the data for different wind sectors, which represent varying types of land cover.

Setup

Two 70 m masts were erected in summer 2007 and the data logging of the cup anemometers started shortly after. Turbulence measurements started in spring 2009 and will be carried out for several month until enough data have been collected. Unfortunately, one of the sonics was not working, when we tried to connect it to the data logger. The reason for this remains unclear until the mast will be dismantled, since there is no possibility to access the sonic.

Turbulence (u,v,w) is measured at 10 Hz using ultrasonic anemometers (METEK USA-1), which are mounted near the top of the masts (see photos). The sonics permanently stream the data as RS422 signal to a GIGALOG, where the data are stored on compact flash cards as plain text files.

Photos