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The energy balance of an urban canyon Nunez, Manuel

Abstract

A review of the literature indicates that there are deficiencies in numerical models which describe the sensible heat flux at the complex urban interface. Similarly, experimental techniques are not applicable to the urban area (e.g., Bowen's ratio, aerodynamic methods) or can only be applied at a sufficient height above the roughness elements (eddy correlation). This study investigates an experimental approach to calculating the sensible heat flux for cloudless skies and light winds. Using the energy balance at the surface, this term is obtained as a residual if the other terms are measured. The experiment was conducted in what was considered to be the essential unit of the urban structure: the 'urban canyon1 consisting of .the combination of walls and ground (street), and the air volume contained between two adjacent buildings. If the fluxes of sensible heat are known at the three canyon surfaces, then the sensible heat released to or from the atmosphere is obtained if proper consideration is given to advection and heat storage change in the canyon-air volume. The canyon chosen was located in central Vancouver, B.C., was aligned North-South and consisted of two concrete food processing plants, painted white and separated by a clay-base floor covered by gravel. The canyon was approximately 8 m in width, 6 m in height and 79 m in length. A boom was in- stalled across the canyon top and served as a trackway for a movable carriage and a vertical mast. Eight net radiometers and thermocouple air temperature sensors were mounted on the mast, and seventeen subsurface flux plates were installed on the canyon walls and ground. A lysimeter measured the surface water loss. In addition to these measurements, special experiments were conducted to investigate the individual roles of advection, solar radiation and subsurface heat storage in the canyon. Results indicate that the solar radiation received along the walls is a function of the solar zenith and azimuth angle. The average solar flux received by each canyon surface is characterized by a peak; with added secondary maxima on each wall corresponding to periods of maximum irradiance on the opposite wall. The albedo of the canyon system shows two distinct peaks at the times of maximum irradiance of the white walls. At noon the albedo of the canyon system approximates that of the canyon floor. Both the net radiation and subsurface heat flux respond to solar radiation during the day and thus a similar dependence on solar zenith and azimuth angle is observed for these quantities. At night the net long-wave radiation at locations in the canyon varies linearly with the sky view factor. The transport of heat by advection into or out of the canyon air volume depends to a large extent on the wind field. With winds ~2 m s⁻¹ on cloudless days, the net advection gives rise to an upward flux of approximately 70 W m⁻². With winds of 1 m s⁻¹ the corresponding advection is 15 W m⁻². In the absence of strong advective fluxes, the partitioning of the radiant energy, across the canyon top, and at solar noon on a clear summer day is as follows: net all wave radiation flux: 510 W m⁻², sensible heat flux: 320 W m⁻², latent heat flux: 50 W m⁻², subsurface heat flux; 140 W m⁻². At night the wind field drops below 1 m s⁻¹ and a balance is established between the net radiation deficit and the subsurface heat flux which flows towards the canyon-air volume. 'Typical values for both are 60 W m⁻² at the canyon top. Both the sensible and latent heat fluxes are close to zero. Measurements indicate that under these nocturnal conditions, the volume divergence of net radiation is the main cooling mechanism for the canyon-air volume. The cooling may also be approximated by the Brunt formula curve which employs a mean canyon surface admittance and a net radiation across the canyon top. A modelling scheme is devised which calculates solar radiation as the main energy source during periods of sunny skies and light winds. Linkages are then sought, via canyon measurements, between the net solar radiation, the net all-wave radiation and the subsurface heat flux. The sensible heat flux is the end result. The results appear promising, although more measurements are needed over a variety of urban surfaces.

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