Campbell Scientific CNR1 Net Radiometer - Manual (Page 11)

CNR1 Net Radiometer
4.4 Calculation of the Albedo for Solar Radiation
The albedo is the ratio of incoming and reflected Solar radiation. It is a figure
somewhere between 0 and 1. Typical values are 0.9 for snow, and 0.3 for
grassland. To determine albedo, the measured values of the two CM3s can be
used. The CG3s are not involved, as they do not measure Solar radiation. Do
not use measured values when solar elevation is lower than 10 degrees above
the horizon. Errors in measurement at these elevations are likely and thus
yielding unreliable results. This is due to deviations in the directional response
of the CM3s.
Albedo = (E lower CM3) / (E upper CM3) (4.3)
In the above formula, E is calculated according to Equation 4.1.
Albedo will always be smaller than 1. Checking this can be used as a tool for
quality assurance of your data. If you know the approximate albedo at your
site, the calculation of albedo can also serve as a tool for quality control of your
measured data at this specific site.
4.5 Calculation of the Net Solar Radiation (Net Short-Wave)
Net Solar radiation is equal to the incoming solar radiation minus the reflected
solar radiation.
Net Solar radiation = (E upper CM3) - (E lower CM3) (4.4)
In this formula E is calculated according to Equation 4.1.
Net Solar radiation will always be positive. Checking this can be used as a tool
for quality assurance of your measured data.
4.6 Calculation of the Net Infrared Radiation (Net Long-Wave)
Net Far Infrared radiation is, like Net Solar radiation, the part that contributes
to heating or cooling of the earth's surface. In practice, most of the time Net
Far Infrared radiation will be negative.
Net Far Infrared radiation = (E upper CG3) - (E lower CG3) (4.5)
In this formula E is calculated according to Equation 4.2. From this equation
the term with T cancels.
The E measured with the CG3 actually represents the irradiance of the sky (for
the upward- facing CG3) or the ground (for the downward-facing CG3).
Assuming that these two, ground and sky, behave like perfect blackbodies
(actually this is only in theory), one can calculate an effective "Sky
temperature" and an effective "Ground temperature".
Sky temperature
E upper CG3
=
⋅
⎡
⎣⎢ ⎤
⎦⎥
−
5 67 10 8
1 4
.
/
(4.6)
Ground Temperature
E lower CG3
=
⋅
⎡
⎣⎢ ⎤
⎦⎥
−
5 67 10 8
1 4
.
/
(4.7)
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