Intercalibration between HIRS/2 and HIRS/3 channel 12 based on physical considerations

Abstract

Abstract. High-resolution Infrared Radiation Sounder (HIRS) brightness temperatures at channel 12 (T 12 ) 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 T 12 . 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 T 12 (denoted as T 12 := T 12/15 − T 12/14 ) ranging between −12 and −2 K. Inspection of individual profiles for large, medium and small values of T 12 pointed to the role of the mid-tropospheric humidity. This guided us to investigate the relation between T 12 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 T 12 , 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 T 12 and the HIRS/3 data inferred from the correction method with R = 0.986. Upper-tropospheric humidity (UTH) derived from the pseudo HIRS/2 T 12 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.

Error Correlations in High-Resolution Infrared Radiation Sounder (HIRS) Radiances

“…The filters for channels 1-12 are located close to the central axis of the filter wheel, whereas the filters for the remaining channels are located close to the outer edge. The order in which the channels are measured is: 1,17,2,3,13,4,18,11,19,7,8,20,10,14,6,5,15,12,16, and 9. HIRS detects radiation using three different detectors: a Mercury Cadmium Telluride (HgCdTe) detector receives radiation for longwave (wavelength λ > 6 µm) channels 1 to 12, an Indium Antimonide (InSb) detector for shortwave (λ < 5 µm) channels 13 to 19, and a silicon photodiode for the visible channel 20.…”

“…Other sensors with a clear temporal evolution are NOAA-6 and -7 (inter-detector and InSb), NOAA-8 (inter-detector), NOAA-9 (HgCdTe), and NOAA-12 (inter-detector). The behaviour in recent sensors (NOAA-18 onward) appears more stable, although the error correlation in Metop-A and -B increases somewhat for the InSb detector (channels [13][14][15][16][17][18][19], and in MetOp-B decreases somewhat for the HgCdTe detector (channels 1-12). Pearson product-moment correlation between the error in different channels for all HIRS satellites.…”

“…As for early, except that peak at 3.7 pixels has disappeared. Peak at 2.3 pixels (channels [13][14][15][16][17]. Peak at 6.9 pixels.…”

“…In [13,14], the authors optimised HIRS spectral response functions to reduce inter-satellite biases. More recently, the authors of [15] used physical considerations to intercalibrate HIRS channel 12 between HIRS/2 and HIRS/3. The aforementioned studies all relate to improvements of HIRS for the purpose of climate studies but do not include per-datum traceable uncertainties, and improvements are limited to subsets of channels and mostly to subsets satellites.…”

Determination and significance of upper-tropospheric humidity

“…The authors came up with a second statistical correction procedure that brings HIRS 3 levels down to HIRS 2 levels, which takes effect at cold temperatures when T 12 < 235 K. The method appeared to correct satisfactorily the low T 12 values, such that jumps in the time series from HIRS 2 to HIRS 3 where no longer apparent at the low T 12 temperature range. Very recently, Gierens et al (2018) tested the physics behind the statistical inter-calibration of Shi and Bates (2011), wondering whether it is right from a physical point of view to combine measurements by two instruments which sense different layers of the upper atmosphere. They compared T 12 data calculated by radiative transfer modelling of large sets of temperature and relative humidity profiles, using the HIRS 2 and HIRS 3 spectral response functions.…”