Diurnal bias in calibration of broad-band radiance measurementsfrom space
Thomas, D.; Duvel, J.P.; Kandel, R.
Geoscience and Remote Sensing, IEEE Transactions on
Volume 33, Issue 3, May 1995 Page(s):670 - 683
Digital Object Identifier 10.1109/36.387582
Summary:Examines the problem of determining the separate shortwave (SW)
and longwave (LW) components of the Earth radiation budget from space.
Because true broad-band longwave filters do not exist, daytime LW
radiance determinations can depend entirely or in part on subtraction of
the measured SW radiance from the “Total” (TW) radiance
involving integration over the entire electromagnetic pectrum. Examining
radiances measured in the three channels (SW, imperfectly filtered
broad-band LW, TW) of the Earth Radiation Budget Experiment (ERBE)
scanners on board the NOAA-9, ERBS and NOAA-10 satellites, the authors
find small discrepancies in the daytime estimates of broad-band
(“unfiltered”) LW radiances using the ERBE “spectral
correction” procedure. The authors show that these result from
errors (of order 2.5%) in the calibration of the SW channel and possibly
in the spectral characterization of the SW and/or TW channel of the ERBE
scanners on NOAA-9 and NOAA-10. Nighttime estimates show no such bias,
and there appears to be no such error in the data from ERBS. Considering
the LW radiant exitances determined from ERBE scanner data from the
three satellites, the authors find systematic differences in individual
satellite estimates of simultaneous instantaneous regional means and of
regional monthly means, consistent with the radiance discrepancies,
instantaneous daytime LW estimates can be in error by 20% in the extreme
case of very bright cold cloud, and LW cloud radiative forcing may be
significantly biased. They consider the implications of these small
SW-dependent errors on the determination of diurnal variation and of
cloud radiative forcing in the longwave domain. They show how the ERBE
estimates can be corrected, and consider how procedures can be used to
validate results of future experiments (ScaRaB and CERES)
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