Impact of gravity waves on the turbulent exchange above a forest site

Gravity waves are an essential part of the dynamics of the atmosphere over a wide range of meteorological scales. Their importance as a source for energy and momentum transport is widely accepted. Due to their extensive range of wavelengths and periods, phase and group velocities, amount of energy and momentum involved, gravity waves affect atmospheric phenomena from the synoptic to micrometeorological scale. The propagation of gravity waves over a forest has significant effects on the turbulence structure and turbulent exchange conditions within and above the forest canopy.

During the intensive observation period carried out in June and July 2008 during the EGER (ExchanGE processes in mountainous Regions) project, a gravity wave event was detected by a SODAR-RASS system installed at the Waldstein site in the Fichtelgebirge Mountains in North-Eastern Bavaria, Germany. The gravity wave was associated with wind shear above the coniferous canopy due to a low-level jet. Eddy-covariance measurements were used to investigate the impact of the wave on the generation of coherent structures and turbulent transport above the forest site. Turbulence structure and turbulent fluxes of meteorological quantities were observed using a vertical profile of sonic anemometers covering all parts of the forest up to the lower part of the roughness sub layer. All heights were additionally equipped with fast response CO₂ and H₂O analysers for assessing the carbon dioxide and water vapour budgets. The relevant data for in- and above canopy profiles of wind, temperature and humidity as well as for radiation were provided by the long term observing programme.

Applying the wavelet transform to the vertical wind measurements, the observed period and vertical wavelength of the detected gravity wave were estimated and separated from the mean wind flow. A comprehensive study of gravity wave parameters is done using the linear wave theory. The analysis of the wind perturbation profiles indicates a downward wave energy propagation above the canopy level. The eddy-covariance measurements show that coherent structures have smaller temporal scales when the gravity wave is present. A significant impact of the gravity wave on the momentum exchange was found. The conditional sampling analysis of the turbulent time series during the wave occurrence indicated increased transport of the momentum by ejection motions of coherent structures; however, in the absence of the wave the sweep motions were mostly responsible for the momentum transfer. The obtained results are essential to study effects of the wave propagation close to the ground surface and their impact on coherent structures and turbulent exchange in the vicinity of a tall canopy.