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The influence of background wind direction on the roadside turbulent velocity field within a complex urban street

The turbulent velocity field within a complex urban street in the city of York, United Kingdom was measured over a one‐month period, with data coverage over a wide range of background wind directions, θref (where θref = 0° is relative to the street axis, and angles increasing clockwise). Within the street, a persistent mean‐flow cross‐street circulation exists for 15° ⩽θref < 165° in addition to possible flow convergence for 240° ⩽θref < 300° . The magnitude of the in‐street normalised turbulent kinetic energy (TKE) is dependent on the type of predominant in‐street mean‐flow structures. During conditions that correspond to mean‐flow cross‐street circulation, the TKE is approximately twice the magnitude on the windward side compared with the leeward side. For nearly all wind directions, and on both sides of the street, the TKE is approximately constant with height for 0.4 < z/H < 0.8. There is evidence that the in‐street TKE increases with background TKE when other meteorological influences are relatively constant. For background wind directions free from mean flow convergence, the least variability in the sector‐averaged turbulence data occurs when the TKE is normalised by the in‐street mean wind speed, rather than the background wind speed. The two‐point cross‐correlation of the vertical‐velocity component fluctuations on the windward side is at least 0.6 between the mid and upper anemometers. The two‐point cross‐correlation between cross‐street (same height) vertical‐velocity component fluctuations is negative and non‐negligible during mean‐flow circulation, which indicates possible cross‐street coherence in the turbulent velocity field. The turbulent Reynolds stress anisotropy tensor, which provides an indication of the level of TKE redistribution between the components, and the overall level of turbulence anisotropy, is discussed with reference to the mean‐flow structures within the street. Copyright © 2008 Royal Meteorological Society