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| Titel |
The effect of radiometer placement and view on inferred directional and hemispheric radiometric temperatures of an urban canopy |
| VerfasserIn |
C. Adderley, A. Christen, J. A. Voogt |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 8, no. 7 ; Nr. 8, no. 7 (2015-07-03), S.2699-2714 |
| Datensatznummer |
250116474
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| Publikation (Nr.) |
 copernicus.org/amt-8-2699-2015.pdf |
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| Zusammenfassung |
Any radiometer at a fixed location has a biased view when observing a
convoluted, three-dimensional surface such as an urban canopy. The goal of
this contribution is to determine the bias of various sensors views observing
a simple urban residential neighbourhood (nadir, oblique, hemispherical) over
a 24 hour cycle under clear weather conditions. The error in measuring a
longwave radiation flux density (L) and/or inferring surface temperatures
(T0) is quantified for different times over a diurnal cycle. Panoramic
time-sequential thermography (PTST) data were recorded by a thermal camera on
a hydraulic mast above a residential canyon in Vancouver, BC. The data set
resolved sub-facet temperature variability of all representative urban facets
in a 360° swath repetitively over a 24-hour cycle. This data set is
used along with computer graphics and vision techniques to project measured
fields of L for a given time and pixel onto texture sheets of a
three-dimensional urban surface model at a resolution of centimetres. The
resulting data set attributes L of each pixel on the texture sheets to
different urban facets and associates facet location, azimuth, slope,
material, and sky view factor. The texture sheets of L are used to
calculate the complete surface temperature (T0,C) and to simulate
the radiation in the field of view (FOV) of narrow and hemispheric
radiometers observing the same urban surface (in absence of emissivity and
atmospheric effects). The simulated directional (T0,d) and hemispheric
(T0,h) radiometric temperatures inferred from various biased views are
compared to T0,C. For a range of simulated off-nadir (φ) and
azimuth (Ω) angles, T0,d(φ,Ω) and T0,C differ
between −2.6 and +2.9 K over the course of the day. The effects of effective
anisotropy are highest in the daytime, particularly around sunrise and sunset
when different views can lead to differences in T0,d(φ,Ω) that
are as high as 3.5 K. For a sensor with a narrow FOV in the nadir of the
urban surface, T0,d(φ=0) differs from T0,C by +1.9 K (day)
and by −1.6 K (night).
Simulations of the FOV of hemispherical, downward-facing pyrgeometers at 270
positions show considerable variations in the measured L and inferred
hemispherical radiometeric temperature T0,h as a function of both
horizontal placement and height. The root mean squared error (RMSE) between
different horizontal positions in retrieving outgoing longwave emittance
L↑ 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 zb.
Generally, above 3.5zb the horizontal positional error is less than the
typical accuracy of common pyrgeometers. The average T0,h over 24 h
determined from the hemispherical radiometer sufficiently above an urban
surface is in close agreement with the average T0,C. However, over
the course of the day, the difference between T0,h and T0,C
shows an RMSE of 1.7 K (9.4 W m−2) because the relative
contributions of facets within the projected FOV of a pyrgeometer do not
correspond to their fractions of the complete urban surface. |
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