Influence of Low-Level Jets and Gravity Waves on Turbulent Fluxes
Atmospheric waves and local wind phenomena in the atmospheric boundary layer are common forms of air motions observed above the forest canopy at night. Low-level jets with duration times of several hours and the gravity wave event were detected by SODAR-RASS and miniSODAR systems installed in the Fichtelgebirge Mountains in Germany.
Varying wind directions with low turbulence and wind speed are observed at times of sunrise and sunset. At midday, secondary circulations due to convection over a big clear-cut are possible. The existence of a low-level jet seems to be independent of the general weather situation. At nighttimes and during the morning hours the profile of the wind vector often shows a strong turn of the wind direction with increasing height.
As a result, gravity wave generation was connected to the wind shear effect and change of the wind direction observed in the ascending low-level jet. The observed period and vertical wavelength were obtained by application of the wavelet transform, allowing the gravity wave to be filtered from the mean wind flow. A comprehensive study of gravity wave parameters was 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 are used to investigate the impact of the gravity wave on the generation of coherent structures and turbulent transport. It was shown that coherent structures have smaller temporal scales when the gravity wave occurs, in contrast to the period before the wave was detected. It was found that there was a significant impact of the gravity wave on the momentum exchange, and that this led to the higher transport of the momentum during the ejection phases of coherent structures, whereas the sweep phases were mostly responsible for transport in the absence of the gravity wave in the mean flow.
The project was funded by the German Science Foundation (FO 226/16-1, ME2100/4-1 and FO 226/21-1). The authors wish to acknowledge the technical support given by the staff of the Bayreuth Center for Ecology and Environmental Research (BayCEER) of the University of Bayreuth, and the German Weather Service for providing the windprofiler data. The authors also wish to gratefully acknowledge Stephanie Schier (now Dix) for her support during work on her Master’s Thesis and all those who supported the field measurements, especially Lukas Siebicke, Katharina Staudt, Johannes Lüers, and Johannes Olesch for advice, comments, and technical assistance.
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