Case studies from a field campaign to measure canopy air flows over complex terrain

Dynamic interactions between forest canopies and the boundary layer

are important in understanding and parameterising the boundary

conditions in the atmosphere. These interactions also play a key role in

transporting gases between the forest and the boundary layer.

It is widely recognised that such canopy flows are

further complicated by the presence of complex terrain. Recent

theoretical and computational work has begun to look specifically at

canopy flow processes over complex terrain and it has been shown that

the presence of complex terrain within a forested area can change the

balance between terms in the momentum equation within and above the

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 can have important implications for scalar transport between

the forest canopy and the boundary layer above. 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 the mean flow

and 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. A brief

introduction to the project will be presented before discussing

several case studies from the field campaign. Where possible

comparisons will be made with results obtained from the latest

theoretical models. The key differences from flow over flat forest

canopies will be highlighted.