Atmospheric turbulence structure above urban heterogeneous surface

The main exchange of energy and mass between the atmosphere and the surface occurs through the means of turbulent processes in the boundary layer of the atmosphere. A modern atmospheric dynamics models use simplified schemes for calculating energy exchange with the surface, based on the Monin-Obukhov similarity theory (MOST). The main requirement of MOST is the uniformity of the underlying surface. This simplification reduces the accuracy of the forecast, especially in regions with complex (heterogeneous) orography, such as in urban conditions. The obtained regularities in the future may allow us to choose the most accurate parameterization for better use of MOST in conditions of a geometrically complex surface and increasing the accuracy of the forecast for urbanized territories.

This paper presents the result of the analysis of the data series for period from 2020 to 2022 obtained from the eddy covariance tower installed in the Meteorological observatory of Moscow State University. Acoustic anemometers with a frequency of 20 Hz record three components of wind speed  and acoustic temperature at altitudes of 2.2 m, 11.1 m and 18.8 m. For processing high-frequency mast data and calculating the static characteristics of turbulence, a set of programs implemented by the authors is used. Gaps are filled by a new algorithm proposed in [3].

Based on a long series of measurements, the seasonal and daily variability of heat and momentum fluxes over an urbanized surface was analyzed. The detailed statistical analysis of the influence of eddy structures was carried out on the formation of turbulent fluxes in the city. The method proposed in [2] was used to identify coherent vortices. The method is based on the hypothesis of the relationship of third and second moments, described by the ratio:

where C∼1 is non-dimensional constant, and S_w is skewness of vertical velocity [1]. The compliance of third moments with theoretical values was established for various stratification conditions in 80% of cases. The result indicates a significant contribution of coherent structures to the formation of vertical fluxes over a geometrically complex surface, which is consistent with the estimates received earlier [2, 4].

References

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• [2] Barskov K.V. et al. Two regimes of turbulent fluxes above a frozen small lake surrounded by forest //Boundary-Layer Meteorology. – 2019. – Т. 173. – №. 3. – С. 311-320.
• [3] Drozd I.D. et al. Comparative characteristics of gap filling methods in high-frequency data of micrometeorological measurements //IOP Conference Series: Earth and Environmental Science. – IOP Publishing, 2022. – Т. 1023. – №. 1. – С. 012009.
• [4] Pashkin A.D. et al. An experimental study of atmospheric turbulence characteristics in an urban canyon //IOP Conference Series: Earth and Environmental Science. – IOP Publishing, 2019. – Т. 386. – №. 1. – С. 012035.