The UBL/CLU (urban boundary layer/couche limite urbaine) observation and modelling campaign is a side-project of the regional photochemistry campaign ESCOMPTE. UBL/CLU focuses on the dynamics and thermodynamics of the urban boundary layer of Marseille, on the Mediterranean coast of France. The objective of UBL/CLU is to document the four-dimensional structure of the urban boundary layer and its relation to the heat and moisture exchanges between the urban canopy and the atmosphere during periods of low wind conditions, from June 4 to July 16, 2001. The project took advantage of the comprehensive observational set-up of the ESCOMPTE campaign over the Berre-Marseille area, especially the ground-based remote sensing, airborne measurements, and the intensive documentation of the regional meteorology. Additional instrumentation was installed as part of UBL/CLU. Analysis objectives focus on (i) validation of several energy balance computational schemes such as LUMPS, TEB and SM2-U, (ii) ground truth and urban canopy signatures suitable for the estimation of urban albedos and aerodynamic surface temperatures from satellite data, (iii) high resolution mapping of urban land cover, land-use and aerodynamic parameters used in UBL models, and (iv) testing the ability of high resolution atmospheric models to simulate the structure of the UBL during land and sea breezes, and the related transport and diffusion of pollutants over different districts of the city. This paper presents initial results from such analyses and details of the overall experimental set-up.Ã
High-frequency fluctuations of temperature and of longitudinal and vertical velocity components have been measured with high-resolution probes in order to test the local-isotropy assumption. The simultaneous measurements of u’, w’, θ’ and the measurements in two space points with various separations in either the longitudinal or transverse directions were made in the large boundary layer (Rλ = 616) of the I.M.S.T. Air-Sea Interaction Simulation Tunnel. There is consistent evidence that the local-isotropy assumption is satisfied by the velocity field at all scales smaller than twenty times the Kolmogorov microscale (η ≈ 0.27 × 10−3 m), i.e. in the dissipative range of scales but not in the expected inertial subrange. The direct comparisons of the lateral and longitudinal temperature autocorrelation and structure functions show that the temperature field does not verify the isotropy assumption at all scales greater thanor equal to 37 and presumably at even smaller scales. This is confirmed by the study of the temperature-increment skewness and flatness factors. The spectral distribution -of the non-zero derivative skewness (S(θ) = +0.9) shows that it is essentially contributed by those scales for which the dynamic field satisfies isotropy.
The inertial‐dissipation method has long been used to estimate air‐sea fluxes from ships because it does not require correction for ship motion. A detailed comparison of the inertial‐dissipation fluxes with the direct covariance method is given, using data from the Humidity Exchange Over the Sea (HEXOS) main experiment, HEXMAX. In this experiment, inertial‐dissipation packages were deployed at the end of a 17 m boom, in a region relatively free of flow distortion; and on a mast 7 m above the platform (26 m above the sea surface) in a region of considerable flow distortion. An error analysis of the inertial‐dissipation method indicates that stress is most accurately measured in near‐neutral conditions, whereas scalar fluxes are most accurately measured in near‐neutral and unstable conditions. It is also shown that the inertial‐dissipation stress estimates are much less affected by the flow distortion caused by the platform as well as by the boom itself. The inertial‐dissipation (boom and mast) and boom covariance estimates of stress agree within ±20%. The latent heat flux estimates agree within approximately ±45%. The sensible heat flux estimates agree within ±26% after correction for velocity contamination of the sonic temperature spectra. The larger uncertainty in the latent heat fluxes is due to poor performance of our Lyman‐α hygrometers in the sea spray environment. Improved parameterizations for the stability dependence of the dimensionless humidity and temperature structure functions are given. These functions are used to find a best fit for effective Kolmogorov constants of 0.55 for velocity (assuming a balance of production and dissipation of turbulent kinetic energy) and 0.79 for temperature and humidity. A Kolmogorov constant of 0.51 implies a production‐dissipation imbalance of approximately 12% in unstable conditions.
Abstract.A comparison between numerical simulations and wind tunnel modelling has been performed to examine the variation with streamwise aspect ratio (width/height, W/H) of the mean flow patterns in a street canyon. For this purpose a two-dimensional (2-D) cavity was subjected to a thick turbulent boundary layer flow perpendicular to its principal axis. Five different test cases, W/H = 0.3, 0.5, 0.7, 1.0 and 2.0, have been studied experimentally with flow measurements taken using pulsedwire anemometry. The results show that the skimming flow regime, with a large vortex in the canyon, occurred for all the cases investigated. For the cavities with W/H≤0.7 a weaker secondary circulation developed beneath the main vortex. The narrower the canyon, the smaller the wind speed close to the cavity ground, giving increasingly poor ventilation qualities. The corresponding numerical results were obtained with the Computational Fluid Dynamics (CFD) code CHENSI that uses the standard k-ε model. The intercomparison showed good agreement in terms of the gross features of the mean flow for all the geometries examined, although some detailed differences were observed.
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