Naslov
Turbulence spectral characteristics in the nocturnal boundary layer over heterogeneous terrain

Autori
Babić, Karmen ; Rotach, Mathias W.

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Skup
6th International Meeting on Meteorology and Climatology of the Mediterranean & Challenges in Meteorology 5

Mjesto i datum
Zagreb, Hrvatska, 20.02.2017. - 22.02.2017

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
roughness sublayer ; turbulence spectra ; stable boundary layer ; spectral models

Sažetak
The turbulence characteristics of flows over heterogeneous and patchy vegetation still present an ongoing issue, mainly due to the lack of experimental results. In this study we focus on turbulence spectral characteristics in the wintertime, nocturnal boundary layer over inhomogeneous surface. Through the spectral representation the amount of kinetic energy, variance or eddy flux is associated with each scale of motion. Measurements from a tall mast which was situated in a small deciduous forest were analyzed. Measurements at five levels above the 18 m high canopy were influenced by the upwind conditions which represent different types of surfaces. In this study we focus on the combined influence of the roughness sublayer (RSL) found above tall vegetation and the internal boundary layer (IBL). The objective is to investigate turbulence spectral characteristics and TKE dissipation rate within and above the RSL. The validity of the local isotropy hypothesis is tested for measurements above an inhomogeneous surface. Results show that the ratio of the horizontal spectral densities (Sv/Su) approaching the 4/3, while the ratio of the vertical to the longitudinal spectral density (Sw/Su) was less than 1 for all levels indicating an anisotropic turbulence above the canopy. We found that an appropriate stability function for the non-dimensional dissipation of TKE calculated from spectra in the inertial subrange differs from the classical linear function obtained for the Kansas data and a modified relationship is proposed. The appropriate scaling parameters needed to collapse spectra are obtained. Spectra obtained in this study are compared with the existing models which are valid for HHF terrain. We compared our results with the empirical models of Kaimal et al. (Q J R Meteorol Soc 98:563– 589, 1972) and Olesen et al. (Boundary-Layer Meteorol 29: 285–312, 1984). Using Kaimal's model, normalized spectra show that -2/3 slope is followed quite closely for a wide range of frequencies in the inertial subrange. Extending the analysis to the Olesen approach, normalized spectra collapse to one single curve.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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