Selected Spectral Characteristics of Turbulence over an Urbanized Area in the Centre of Łódź, Poland
Streszczenie
We present the turbulence spectra and cospectra derived frommore than five years
of eddy-covariance measurements at two urban sites inŁód´z, central Poland. The fast response
wind velocity components were obtained using sonic anemometers placed on narrow masts
at 37 and 42 m above ground level. The analysis follows Kaimal et al. (Q J R Meteorol Soc
98:563–589, 1972) who established the spectral and cospectral properties of turbulent flow in
atmospheric surface layer on the basis of the Kansas experiment. Our results illustrate many
features similar to those of Kaimal et al., but some differences are also observed. The velocity
(co)spectra from Łód´z show a clear inertial subrange with −2/3 slope for spectra and −4/3
slope for cospectra. We found that an appropriate stability function for the non-dimensional
dissipation of turbulent kinetic energy calculated from spectra in the inertial subrange differs
from that of Kaimal et al., and it can be satisfactorily estimated with the assumption of local
equilibrium using standard functions for the non-dimensional shear production. A similar
function for the cospectrum corresponds well to Kaimal et al. for unstable and weakly stable
conditions. The (co)spectra normalized by their spectral values in the inertial subrange are in
general similar to those of Kaimal et al., but they peak at lower frequencies in strongly stable
conditions. Moreover, our results do not confirm the existence of a clear “excluded region”
at low frequencies for the transition from stable to unstable conditions, for longitudinal and
lateral wind components. The empirical models of Kaimal et al. with adjusted parameters fit
well to the vertical velocity spectrum and the vertical momentum flux cospectrum. The same
type of function should be used for longitudinal and lateral wind spectra because of their
sharper peak than occurs for the Kansas data. Finally, it should be stressed that the above
relationships are well-defined for averaged values. The results for individual 1-h periods are
very scattered and can be significantly different from the generalized functions.
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