engleski

Observations of turbulence in the stable surface layer over inhomogeneous terrain

The fate of pollutants in the boundary layer is strongly affected by turbulence which is extremely complicated in the complex terrain and over heterogeneous landscapes. The stable boundary layer (SBL) is generally favorable for establishment of air pollution episodes. Therefore, investigations of its characteristics are of the great importance. In this work we investigate turbulence characteristics in the stable, wintertime nocturnal boundary layer over inhomogeneous surface. We analyze data measured at five levels of 62 m high mast located near the industrial town of Kutina. Kutina is located at the southern slopes of the afforested Moslavacka Mountain in the hilly region of Moslavina, Croatia. The mast is located in the vicinity of a petrochemical plant, which is the main source of the pollution in the area. The measurement tower is placed over the heterogeneous surface, surrounded by approximately 20 m high trees. The closest trees are 20 to 30 m away from the tower. Wind data were collected by five WindMaster ultrasonic anemometers (Gill Instruments), which were installed at heights of 20, 32, 40, 55 and 62 m. Each of the sonic anemometers measured three wind components and sonic temperature at a sampling frequency of 20 Hz. Since our intention was to capture highly stable situations, we analyze the wintertime, nighttime data collected during December 2008 – February 2009 period. Here we address the issue of selecting the averaging time scale used for separation of turbulent perturbation from the mean flow, which is appropriate for the nocturnal, stable conditions. This scale is determined by applying several methods widely used in the literature. Preliminary results based on the case study analysis, which were obtained previously for the same location, suggest turbulence averaging time scales between 7 and 11.5 minutes and local maximums of vertical momentum flux and kinematic heat flux at the height of 55 m. Compared to the previous study, here we inspect the larger dataset. In addition, apart from the vertical profiles of vertical momentum flux and kinematic heat flux, here we also investigate vertical profiles of some other turbulence variables.

high frequency measurements; Fourier spectral analysis; turbulence averaging time scale; turbulence quantities

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