engleski

Evaluation of turbulence fluxes in the nighttime stable boundary layer in Kutina, Croatia

Evaluation of turbulent fluxes in the surface layer is important for characterization of the atmospheric boundary layer (ABL). The turbulent heat flux in the surface layer is significant for the surface energy balance, while momentum turbulent flux is responsible for production of turbulence in stably stratified boundary layer. Additionally, turbulence, and specifically, turbulent fluxes strongly affect the fate of pollutants in the boundary layer. In order to investigate turbulent characteristics, it is necessary to separate turbulent perturbations from the mean flow. Turbulent and synoptic scales are typically separated by the spectral energy gap, which is often found at the mesoscale. Here we focus on nighttime conditions, which are generally favorable for the establishment of pollution episodes. In order to determine turbulent quantities from fine-resolution wind measurements, it is necessary to assess averaging time appropriate for the turbulent timescales. Therefore, here we also address to the problem of proper selection of averaging time interval.Wind data were measured by five ultrasonic anemometers with a sampling frequency of 20 Hz. Anemometers were placed at five heights, namely, 20, 32, 40, 55 and 62 m above the ground. The meteorological mast is located in a suburban, industrial area of Kutina, Croatia. The mast is above the grassy surface, and it is surrounded by approximately 20 m high trees, where the closest trees are about 20 – 30 meters distant.Lengths of averaging interval are determined by two methods based on Fourier analysis. The first method takes the length of interval as an inverse of frequency at which a minimum in the velocity spectrum is found. The second method (ogive) uses cumulative integral of cospectrum of perturbances of any two turbulent quantities. Here we employ perturbations of wind components and/or sonic temperatures. Afterwards, the averaging interval is taken as a value at which ogive converges to a constant. The time series of 5-min vertical momentum flux shows decreasing values with height up to 55 m where it starts to increase. At two uppermost levels in the nighttime period between 20 and 2 LST upward directed momentum flux exists. Kinematic heat flux has large values which decrease with height up to 55 m where it has a local maximal value.

Fourier analysis; high frequency measurements; turbulence averaging scale

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