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A multi-instrument approach for studying complex patterns of turbulent mixing

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Title: A multi-instrument approach for studying complex patterns of turbulent mixing
Author(s): Pentikäinen, Pyry
Contributor: University of Helsinki, Faculty of Science, Department of Physics
Discipline: Meteorology
Language: English
Acceptance year: 2017
Turbulent mixing in the atmospheric boundary layer above the Hyytiälä forestry field station in southern Finland was studied with a combination of Doppler lidar and in-situ measurements on a 125 m tall mast. The intensity of turbulent mixing was derived from measurements of the vertical and horizontal wind speeds. Other meteorological data was included in the analysis to aid in the interpretation process. The methods applied to the data performed robustly under standard weather conditions, and thus can be used with high confidence to study more complex patterns of turbulent mixing. This includes two case studies of turbulent mixing under complex circumstances, one of which strongly implied a causal relationship between sudden changes in heat fluxes and the initiation of a nocturnal jet. The turbulent data from Doppler lidar Vertical Azimuth Display scans were separated into directional components to study the spatial variability of turbulent mixing. No significant spatial variability was observed during the daytime when strong turbulence consisting of large–scale turbulent eddies encompassing the whole boundary layer dominate. However, significant spatial differences were sometimes seen in the growth of the mixing layer during the morning, and stark spatial variability in turbulent mixing was detected on several summer nights. No single mechanism was conclusively shown to be responsible for the observed distribution of turbulence, but the night–time variability seemed to be connected to the presence of nocturnal jets. The area of the most intense nocturnal mixing is located in the vicinity of the nearby Station for Measuring Ecosystem - Atmosphere Relations II where comprehensive aerosol and canopy exchange research is performed. The observed nocturnal mixing may have implications for the conclusions resulting from the measurements performed at the station. The thermodynamic stability of the near–surface boundary layer was investigated using scaled potential temperature profiles measured at various altitudes on the 125 m measurement mast. There was good agreement with Doppler lidar observations, but due to calibration issues in the thermometers on the mast, quantitative results lack accuracy even after corrections were applied.

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