IMK/IAA MIPAS temperature retrieval version 8: nominal measurements
Abstract. A new global set of atmospheric temperature profiles is retrieved from
recalibrated radiance spectra recorded with the Michelson Interferometer
for Passive Atmospheric Sounding (MIPAS). Changes with respect to previous
data versions include a new radiometric calibration considering the
time dependency of the detector nonlinearity and a more robust frequency
calibration scheme. Temperature is retrieved using a smoothing constraint,
while tangent altitude pointing information is constrained using optimal
estimation. ECMWF ERA-Interim is used as a priori temperature below 43 km.
Above, a priori data are based on data from the Whole Atmosphere Community Climate
Model Version 4 (WACCM4).
Bias-corrected fields from specified dynamics runs, sampled at the MIPAS
times and locations, are used, blended with ERA-Interim between 43 and 53 km.
Horizontal variability of temperature is considered
by scaling an a priori 3D temperature field in the orbit plane in a way that
the horizontal structure is provided by the a priori while the vertical
structure comes from the measurements.
Additional microwindows with better
sensitivity at higher altitudes are used. The background continuum is
jointly fitted with the target parameters up to 58 km altitude. The radiance
offset correction is strongly regularized towards an empirically determined
vertical offset profile. In order to avoid the propagation of uncertainties of
O 3 and H 2 O a priori assumptions, the abundances of these species are
retrieved jointly with temperature. The retrieval is based on HITRAN 2016
spectroscopic data, with a few amendments.
Temperature-adjusted climatologies of vibrational populations of CO 2 states
emitting in the 15  µ m region are used in the radiative transfer
modeling in order to account for non-local thermodynamic equilibrium.
Numerical integration in the radiative transfer model is now performed at
higher accuracy. The random component of the temperature uncertainty typically
varies between 0.4 and 1 K, with occasional excursions up to 1.3 K above 60 km
altitude.
The leading sources of the random component of the temperature error are
measurement noise,
gain calibration uncertainty, spectral shift, and uncertain CO 2 mixing
ratios. The systematic error is caused by uncertainties in spectroscopic data
and line shape uncertainties. It ranges from 0.2 K at 20 km altitude for
northern midlatitude summer conditions to 2.3 K at 12 km for tropical
conditions. The estimated total uncertainty amounts to values between
0.6 K at 20 km for midlatitude summer conditions to 2.5 K at 12–15 km for
tropical conditions. The vertical resolution varies
around 3 km for altitudes below 50 km. The long-term drift encountered in the
previous temperature product has been largely reduced.
The consistency between
high spectral resolution results from 2002 to 2004 and the reduced spectral
resolution results from 2005 to 2012 has been largely improved.
As expected, most
pronounced temperature differences between version 8 and previous data versions are
found in elevated stratopause situations.
The fact that the phase of temperature waves seen
by MIPAS is not locked to the wave phase found in ECMWF analyses demonstrates
that our retrieval provides independent information and does not merely reproduce
the prior information.