Intercalibration between HIRS/2 and HIRS/3 channel 12 based on physical considerations
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Published:2018-02-16
Issue:2
Volume:11
Page:939-948
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ISSN:1867-8548
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Container-title:Atmospheric Measurement Techniques
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language:en
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Short-container-title:Atmos. Meas. Tech.
Author:
Gierens KlausORCID, Eleftheratos KostasORCID, Sausen RobertORCID
Abstract
Abstract. High-resolution Infrared Radiation Sounder (HIRS) brightness temperatures at
channel 12 (T12) can be used to assess the water vapour content of the
upper troposphere. The transition from HIRS/2 to HIRS/3 in 1999 involved a
shift in the central wavelength of channel 12 from 6.7 to 6.5 µm, causing a discontinuity in the time series of T12. To understand the
impact of this change in the measured brightness temperatures, we have
performed radiative transfer calculations for channel 12 of HIRS/2 and HIRS/3
instruments, using a large set of radiosonde profiles of temperature and
relative humidity from three different sites. Other possible changes within
the instrument, apart from the changed spectral response function, have been
assumed to be of minor importance, and in fact, it was necessary to assume as
a working hypothesis that the spectral and radiometric calibration of the two
instruments did not change during the relatively short period of their common
operation. For each radiosonde profile we performed two radiative transfer
calculations, one using the HIRS/2 channel response function of NOAA 14 and
one using the HIRS/3 channel response function of NOAA 15, resulting in
negative differences of T12 (denoted as ΔT12:=T12/15-T12/14) ranging between −12 and −2 K.
Inspection of individual profiles for large, medium and small values of
ΔT12 pointed to the role of the mid-tropospheric humidity. This
guided us to investigate the relation between ΔT12 and the channel
11 brightness temperatures which are typically used to detect signals from
the mid-troposphere. This allowed us to construct a correction for the HIRS/3
T12, which leads to a pseudo-channel 12 brightness temperature as if a
HIRS/2 instrument had measured it. By applying this correction we find an
excellent agreement between the original HIRS/2 T12 and the HIRS/3 data
inferred from the correction method with R=0.986. Upper-tropospheric
humidity (UTH) derived from the pseudo HIRS/2 T12 data compared well with that
calculated from intersatellite-calibrated data, providing independent
justification for using the two intercalibrated time series (HIRS/2 and
HIRS/3) as a continuous HIRS time series for long-term UTH analyses.
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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