Abstract. We present a novel retrieval for upper-tropospheric humidity (UTH)
from High-resolution Infrared Radiation
Sounder (HIRS) channel 12 radiances that successfully bridges the
wavelength change from 6.7 to 6.5 µm that occurred from
HIRS/2 on National Oceanic and
Atmospheric Administration satellite NOAA-14 to HIRS/3 on satellite NOAA-15.
The jump in average brightness temperature (in the water vapour channel;
T12) that this change had caused (about
−7 K) could be fixed with a statistical inter-calibration method
(Shi and Bates, 2011). Unfortunately, the retrieval of UTHi
(upper-tropospheric humidity with respect to ice)
based on the inter-calibrated data was not satisfying at the high tail of the
distribution of UTHi. Attempts to construct a better
inter-calibration in the low T12 range (equivalent to the high
UTHi range) were either not successful (Gierens et al., 2018) or
required additional statistically determined corrections to the
measured brightness temperatures (Gierens and Eleftheratos, 2017). The new method presented here is based on the original one
(Soden and Bretherton, 1993; Stephens et al., 1996; Jackson and Bates, 2001), but it extends
linearisations in the formulation of water vapour saturation
pressure and in the temperature dependence of the Planck function to
second order. To achieve the second-order formulation we derive the
retrieval from the beginning, and we find that the most influential
ingredient is the use of different optical constants for the two
involved channel wavelengths (6.7 and 6.5 µm). The result of
adapting the optical constant is an almost perfect match between UTH
data measured by HIRS/2 on NOAA-14 and HIRS/3 on NOAA-15 on 1004
common days of operation. The method is applied to both UTH and
UTHi. For each case retrieval coefficients are derived. We present a number of test applications, e.g. on computed
brightness temperatures based on high-resolution radiosonde
profiles, on the brightness temperatures measured by the satellites
on the mentioned 1004 common days of operation. Further, we present
time series of the occurrence frequency of high UTHi cases, and we
show the overall probability distribution of UTHi. The two latter
applications expose indications of moistening of the upper
troposphere over the last 35 years. Finally, we discuss the significance of UTH. We state that UTH
algorithms cannot be judged for their correctness or incorrectness,
since there is no true UTH. Instead, UTH algorithms should fulfill a
number of usefulness postulates, which we suggest and discuss.
On the interpretation of upper-tropospheric humidity based on a second-order retrieval from infrared radiances
