Atmosphere-canopy interaction over complex terrain

Lead Research Organisation: University of Leeds
Department Name: School of Earth and Environment

Abstract

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.

Publications

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Grant E (2016) Modelling Canopy Flows over Complex Terrain in Boundary-Layer Meteorology

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Grant E (2015) Field Observations of Canopy Flows over Complex Terrain in Boundary-Layer Meteorology

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Ross A (2008) Large-eddy Simulations of Flow Over Forested Ridges in Boundary-Layer Meteorology

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Ross A (2011) Scalar Transport over Forested Hills in Boundary-Layer Meteorology

 
Description This project has conducted the first detailed field measurements of the impact of heterogenity and topography on the interactions between forest canopies and the atmosphere. The observations have provided a reference dataset for developing and testing new modelling approaches.

The results highlight that theoretical ideas developed in much simpler, idealised laboratory experiments and numerical simulation can be used to understand more complicated real-world situation with 3-D heterogeneous forest canopies and topography. The results highlight the importance of directional wind shear in such situations and this may be important for interpreting field observations and also in refining turbulence models for canopy flow.
Exploitation Route This project is part of a larger ongoing programme of work on atmosphere-forest interactions over heterogeneous terrain and has led to follow-on collaborations and publications.

We have had significant interest in the results from the wind energy industry, where the impact of forests on both mean wind speeds and turbulence is a big issue for wind turbine energy production and the lifetime of the turbines.

The work is also being followed up by our project partners, Forest Research, in terms of understanding wind damage to trees, which areas of a forest are most susceptible, and how this may be alleviated through planning plantations.
Sectors Agriculture, Food and Drink,Energy,Environment

 
Title Field observations of forest - atmosphere interactions over Arran. 
Description This dataset provides the first detailed measurements of forest-atmosphere interactions over complex, heterogeneous terrain and will be a valuable resource in developing and testing new models for such flow. The dataset includes a network of 10 automatic weather stations recording wind speed, direction, temperature, humidity and pressure at sites within and outside the forest. There are also 3 instrumented towers in a transect across the ridge including profiles of mean wind speed and temperature at 6 heights and sonic anemometers at 4 heights. 
Type Of Material Database/Collection of data 
Provided To Others? No  
Impact This dataset has led to a number of publications on the results, and is also being used to validate numerical simulations of canopy flows, and