Radiative transfer simulations and observations of airborne infrared emission spectra in the presence of PSCs: Detection of clouds and discrimination of cloud types

Polar stratospheric clouds (PSCs) have an important influence on the spatial and temporal
evolution of different trace gases, (e.g. ozone, HNO₃) in the polar vortex in winter due to direct
and indirect processes (e.g. activation of chlorine, redistribution of HNO₃). Thus, the detection
of PSCs and a detailed distinction between the different PSCs types Nitric Acid Trihydrade
(NAT), Supercooled Ternary Solution (STS), and ice are important as they build a basis for
model comparisons to reduce uncertainties in the representation of PSCs in models. Infrared
limb sounder are well suited for this purpose as they enable both, the detection of clouds and
the discrimination between the different types.
The CRISTA-NF instrument, an airborne infrared limb sounder, observed a new spectral fea-
ture during measurements inside PSCs within the RECONCILE aircraft campaign. In contrast
to the previously known feature at 820 cm^-1, which has been used in former studies for the
detection of NAT PSCs, the new feature was detected at about 816 cm^-1. We performed a
large set of radiative transfer simulations for different PSC situations (varying PSC altitude
and thickness, PSC type, number density and median radius of the particle size distribution)
for the airborne viewing geometry of CRISTA-NF. The simulation results show that under the
assumption of spherical NAT particles the spectral feature transforms from the original feature
at 820 cm^-1 to a shifted version (peak shifts to smaller wavenumbers) and finally to a step-like
feature with increasing median radius. Based on this behaviour we defined different colour ra-
tios to detect PSCs containing NAT particles and to subgroup them into three sizes regimes:
small NAT, medium size NAT, and large NAT. In addition, we used the simulation results to
adopt a method, which has been used to detect ice in MIPAS-ENV observations, to the airborne
geometry and to refine the corresponding threshold values.
We applied all methods of cloud detection and type discrimination to the CRISTA-NF observa-
tions during the RECONCILE campaign. The new defined NAT detection method is capable
to detect the shifted NAT feature, which is clearly visible in the radiance spectra.