Atmosphere-canopy interaction over complex terrain

Land surface fluxes of momentum, heat, moisture and constituents are factors of crucial importance in numerical weather prediction and climate models. They are all strongly influenced by vegetation and forests in particular, where flows and exchanges within the canopy determine the sources and sinks. The understanding of these canopy flows is now quite well developed for extensive areas of flat ground, but many hilly and mountainous areas are either partially or fully forested. To date, models for orographic flow have generally ignored the processes within the forest and parametrize the hill surface using a roughness length. Very recently, theoretical and computational developments have begun to set out a framework for understanding the mean flow within these forest canopies. These suggest that correctly modelling the interaction between the canopy and the atmosphere can have important consequences on mountain and larger scales. There is an urgent need to validate the latest model developments using field measurements, but to date none are available. This project will provide such a validation dataset by collecting several months of measurements within and above a forest covering a small but steep hill. There is as yet little theoretical framework for understanding the turbulence structure within canopies on hills and yet this is crucial for wind damage and dispersal applications. High resolution turbulence measurements within and above the canopy will provide new insight into the structure of turbulence within the canopy and this will lead to improved turbulence closure schemes for canopy flows. Further numerical modelling incorporating these schemes will extend the range of the predictions to more complex terrain.