The effect of radiometer placement and view on inferred directional and hemispheric radiometric temperatures of an urban canopy

[摘要] Any radiometer at a fixed location has a biased view when observing aconvoluted, three-dimensional surface such as an urban canopy. The goal ofthis contribution is to determine the bias of various sensors views observinga simple urban residential neighbourhood (nadir, oblique, hemispherical) overa 24 hour cycle under clear weather conditions. The error in measuring alongwave radiation flux density (L) and/or inferring surface temperatures(T₀) is quantified for different times over a diurnal cycle. Panoramictime-sequential thermography (PTST) data were recorded by a thermal camera ona hydraulic mast above a residential canyon in Vancouver, BC. The data setresolved sub-facet temperature variability of all representative urban facetsin a 360° swath repetitively over a 24-hour cycle. This data set isused along with computer graphics and vision techniques to project measuredfields of L for a given time and pixel onto texture sheets of athree-dimensional urban surface model at a resolution of centimetres. Theresulting data set attributes L of each pixel on the texture sheets todifferent urban facets and associates facet location, azimuth, slope,material, and sky view factor. The texture sheets of L are used tocalculate the complete surface temperature (T_0,C) and to simulatethe radiation in the field of view (FOV) of narrow and hemisphericradiometers observing the same urban surface (in absence of emissivity andatmospheric effects). The simulated directional (T_0,d) and hemispheric(T_0,h) radiometric temperatures inferred from various biased views arecompared to T_0,C. For a range of simulated off-nadir (φ) andazimuth (Ω) angles, T_0,d(φ,Ω) and T_0,C differbetween −2.6 and +2.9 K over the course of the day. The effects of effectiveanisotropy are highest in the daytime, particularly around sunrise and sunsetwhen different views can lead to differences in T_0,d(φ,Ω) thatare as high as 3.5 K. For a sensor with a narrow FOV in the nadir of theurban surface, T_0,d(φ=0) differs from T_0,C by +1.9 K (day)and by −1.6 K (night).

Simulations of the FOV of hemispherical, downward-facing pyrgeometers at 270positions show considerable variations in the measured L and inferredhemispherical radiometeric temperature T_0,h as a function of bothhorizontal placement and height. The root mean squared error (RMSE) betweendifferent horizontal positions in retrieving outgoing longwave emittanceL_↑ decreased exponentially with height, and was 11.2, 6.3 and 2.0 W m⁻² at 2, 3, and 5 times the mean building height z_b.Generally, above 3.5z_b the horizontal positional error is less than thetypical accuracy of common pyrgeometers. The average T_0,h over 24 hdetermined from the hemispherical radiometer sufficiently above an urbansurface is in close agreement with the average T_0,C. However, overthe course of the day, the difference between T_0,h and T_0,Cshows an RMSE of 1.7 K (9.4 W m⁻²) because the relativecontributions of facets within the projected FOV of a pyrgeometer do notcorrespond to their fractions of the complete urban surface.

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