Abstract : 3N.13
Measuring turbulent flows over a forested hill

Rosey Grant, Jennifer Hutton, Ian Brooks, Barry Gardiner, Kevin Jones, Stephen Mobbs, Andrew Ross
earrg@env.leeds.ac.uk
University of Leeds

It is widely recognized that turbulent flows are complicated by both the presence of forest canopies and topography. To date most field campaigns and modeling efforts have been designed to address these complications individually. Although a significant portion of the world's mountainous terrain is forested, current orographic flow models tend to parameterise terrain surfaces with a roughness length, effectively ignoring the flow processes that take place within the forest canopy.

The presence of complex terrain can change the balance between terms in the momentum equation within and above a forest canopy. On smaller hills in particular, this can lead to flow into the forest over the upwind slope and flow out of the forest as well as enhanced separation over the lee slope. This difference in flow can also lead to an increase in the pressure drag over a forested hill and have important implications for scalar transport between the forest canopy and the boundary layer above.

Recent theoretical and computational work has begun to look specifically at canopy flow processes over complex terrain (see for example Finnigan and Belcher, 2004; Ross and Vosper, 2005). However, there are currently very few field measurements to validate the latest model developments.

This work presents data from an intensive measurement campaign undertaken on a forested hillside on the Scottish island of Arran beginning in November 2006. Detailed profiles of turbulence were measured from three tall masts erected at selected points across the ridge. In addition, a large network of automated weather stations was used to reveal spatial variation in the pressure field and to identify other significant features of the flow over a forested hill. An introduction to the project, including a summary of the theory, motivation, methodology and some preliminary results from the field measurements will be discussed.