Detection and analysis of coherent structures in urban turbulence

Abstract

SummaryThe continuous wavelet transform provides a suitable tool to visualize the vertical structure of turbulence and to detect coherent structures in turbulent time series. This is demonstrated with a simple example of an artificially ramp structured time series. In this study turbulence data, i. , sampled with 20.83 Hz and measured simultaneously at three levels (z=h ¼ 1.5, 2.1 and 3.2, with z as the sensor height and h the height of the roughness elements) over an urban canopy in the inner city of Basel, Switzerland, are analyzed. The detection of the coherent structures was performed using the Mexican hat wavelet and the zero-crossing method. The analysis for unstable conditions shows that organized structures (ejection-sweep cycles) cover about 45% of the total run time. A conditional average from a total of 116 detected ejection-sweep sequences during 7 hours was calculated over a time window of 100 seconds. This dominating time scale was derived from peak frequencies of the wavelet spectra as well as from the Fourier spectra. It is shown that the normalized amplitudes of fluctuations of temperature and longitudinal wind speed during the events are largest at the lowest measurement level just above the canopy and decrease with increasing distance from the roughness elements. A comparison of related studies over different non-urban surfaces (mainly forests) shows that the shape of conditionally averaged ejectionsweep sequences is very similar for all canopies, however, the dominating time scale in general increases the rougher the surface is and the higher the roughness elements are.

Analysis of coherent structures and atmosphere-canopy coupling strength during the CABINEX field campaign

“…Multiple methods are available for wavelet detection of coherent structures (Barthlott et al, 2007;Collineau and Brunet, 1993a;Feigenwinter and Vogt, 2005;Lu and Fitzjarrald, 1994;Thomas and Foken, 2005). Here we employ the method of Barthlott et al (2007), which uses temperature fluctuations to detect ramp structures under stable and unstable conditions.…”

“…We select this method because the use of temperature ramps provides a physical basis and easy visualization for the selection of coherent structure events. We apply wavelet analysis to the 10 Hz sonic anemometer temperature records for each 30-min period and use a "Mexican Hat" wavelet, which has been shown to effectively detect coherent structures (e.g., Collineau and Brunet, 1993a;Feigenwinter and Vogt, 2005). For each 30-min time period throughout the field campaign (1152 total periods after pre-processing and filtering), we detect coherent structures according to the following techniques defined in Barthlott et al (2007).…”

“…This result is consistent with Barthlott et al (2007), who found that the coherent structures were slightly more efficient in their transport of heat than momentum. The relative contribution of coherent structures to heat or momentum transport is still unresolved in the literature; for example, some studies show that the contributions are roughly equal (Gao et al, 1989;Lu and Fitzjarrald, 1994), others indicate that momentum fluxes are higher (Bergstrom and Hogstrom, 1989), and others suggest that the heat flux contribution is greater (Barthlott et al, 2007;Collineau and Brunet, 1993b;Feigenwinter and Vogt, 2005). Reasons why coherent structures may differ in their transport of heat and momentum are uncertain, yet have implications for atmospheric chemistry.…”

Unsteady dynamics and organized structures from DNS over an idealized building canopy

“…the review by Britter and Hanna, 2003), but little is currently known about the unsteady flow dynamics. It is now believed that unsteady effects due to organized or coherent structures in urban turbulence must be at least as important, in an order of magnitude sense, as the mean flow (Oikawa and Meng, 1995;Feigenwinter and Vogt, 2005). Yet we still do not have a conceptual model of the unsteady flow, let alone a detailed quantitative understanding that might lend itself to future parameterizations.…”

Analysis of phase coherence in fully developed atmospheric turbulence: Amazon forest canopy

“…In the present paper, we have demonstrated that both kurtosis and phase coherence index techniques are capable of characterizing the degree of departure from Gaussianity, due to phase coherence, of atmospheric intermittent turbulence. The nonlinear techniques discussed in this paper can be applied to investigate the role played by coherent structures in experimental (Gao et al, 1989;Barthlott et al, 2007), theoretical (Raupach, 1996;Harman and Finnigan, 2007), and large-eddy simulation (Su et al, 1998;Qiu et al, 2008) studies of atmospheric turbulence in forest canopy, as well as in orchard canopy (Wang et al, 1992;Stoughton et al, 2002), rice canopy (Gao et al, 2003), corn canopy (Yue et al, 2007;Zhu et al, 2007), cotton and grape canopies (Mitic et al, 1999), coral canopy (Reidenbach et al, 2007), and urban canopy (Feigenwinter and Vogt, 2005;Salmond et al, 2005).…”