Flux footprints over complex terrain covered by heterogeneous forest

An innovative model was developed for the canopy–planetary boundary layer flow and scalar transport. It was applied to a forested surface with varying topography. First, we considered the 2D flow over a bell-shaped ridge. The model predicted the most significant flow features such as upwind deceleration of the flow at the foot of the ridge, acceleration at the crest and the downhill flow separation with a reverse flow pattern. These features are more pronounced if the ridge is covered by rough vegetation, such as a forest. Second, the model was applied to a real surface flux tower site located in Hyytiälä, Southern Finland taking into account the complex topography and surface heterogeneities. Both studies revealed the effects of topography on scalar concentration and flux fields within the atmospheric surface layer. The fluxes at a fixed height vary as a function of position in respect of topography. The fluxes tend to be larger at the upwind foot of the ridge and at the downwind side of the ridge crest, being smaller downhill. Correspondingly, the flux contribution and footprint functions depend on the location of the flux measurement point and may significantly deviate from those for a flat terrain. Caution should be exercised when analytical footprint models are used in the interpretation of flux measurements over complex sites.