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

Abstract

Abstract. 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 µm 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. For each radiosonde profile we 5 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 K and −2 K. Inspection of individual profiles for large, medium and small values of ∆T 12 pointed to the role of the middle 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 middle troposphere. This allowed us to construct a correction 10 for the HIRS/3 T 12 , a correction that leads to a pseudochannel 12 brightness temperature such 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 pseudoHIRS/2 T 12 data compared well with that calculated from intersatellite-calibrated data, confirming that the two intercalibrated time series (HIRS/2 and HIRS/3) can be fairly combined to form a continuous HIRS time series for long term UTH analyses.