Surface heating in relation to air temperature,wind and turbulence in an urban street canyon |
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Authors: | B Offerle I Eliasson C S B Grimmond B Holmer |
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Institution: | 1.Urban Climate Group,Physical Geography, Earth Sciences Centre,G?teborg,Sweden;2.Department of Geography,Indiana University,Bloomington,USA;3.Environmental Monitoring and Modelling Group,Geography, King’s College London,London,UK |
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Abstract: | Wind and temperature measurements from within and above a deep urban canyon (height/width = 2.1) were used to examine the
thermal structure of air within the canyon, exchange of heat with the overlying atmosphere, and the possible impacts of surface
heating on within-canyon air flow. Measurements were made over a range of seasons and primarily analysed for sunny days. This
allowed the study of temperature differences between opposing canyon walls and between wall and air of more than 15°C in summer.
The wall temperature patterns follow those of incoming solar radiation loading with a secondary daytime effect from the longwave
exchange between the walls. In winter, the canyon walls receive little direct solar radiation, and temperature differences
are largely due to anthropogenic heating of the building interiors. Cool air from aloft and heated air from canyon walls is
shown to circulate within the canyon under cross-canyon flow. Roofs and some portions of walls heat up rapidly on clear days
and have a large influence on heat fluxes and the temperature field. The magnitude and direction of the measured turbulent
heat flux also depend strongly on the direction of flow relative to surface heating. However, these spatial differences are
smoothed by the shear layer at the canyon top. Buoyancy effects from the heated walls were not seen to have as large an impact
on the measured flow field as has been shown in numerical experiments. At night canyon walls are shown to be the source of
positive sensible heat fluxes. The measurements show that materials and their location, as well as geometry, play a role in
regulating the heat exchange between the urban surface and atmosphere. |
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Keywords: | Buoyancy Heat flux Surface temperature Urban canyon Wind field |
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