Radiative transfer simulations and observations of infrared spectra in the presence of polar stratospheric clouds: Detection and discrimination of cloud types

Abstract. Polar stratospheric clouds (PSCs) play an important role for the spatial and temporal evolution of trace gases inside the polar vortex due to different processes, such as chlorine activation and NOy redistribution. As there are still uncertainties in the representation of PSCs in model simulations, detailed observations of PSCs and information on their type (nitric acid trihydrate (NAT), supercooled ternary solution (STS), and ice) are desirable. The measurements inside PSCs by the airborne infrared limb sounder CRISTA-NF (CRyogenic Infrared Spectrometers and Telescope for the Atmosphere – New Frontiers) during the RECONCILE (Reconciliation of essential process parameters for an enhanced predictability of Arctic stratospheric ozone loss and its climate interactions) aircraft campaign showed a spectral peak at about 816 cm−1. This peak is shifted compared to the peak at about 820 cm−1, which is known to be caused by small NAT particles. To investigate the reason for this spectral difference we performed a large set of radiative transfer simulations of infrared limb emission spectra in the presence of various PSCs (NAT, STS, ice, and mixtures) for the airborne viewing geometry of CRISTA-NF. NAT particles can cause different spectral features in the region 810–820 cm−1. The simulation results show that the appereance of the feature changes with increasing median radius of the NAT particle size distribution from a peak at 820 cm−1 to a shifted peak and, finally, to a step-like feature in the spectrum. Based on this behaviour we defined different colour indices to detect PSCs containing NAT particles and to subgroup them into three size regimes: small NAT (≤ 1.0 μm), medium NAT (1.5–4.0 μm), and large NAT (≥ 3.5 μm). Furthermore, we developed a method to detect the bottom altitude of a cloud by using the cloud index (CI), a colour ratio indicating the optical thickness, and the gradient of the CI. Finally, we applied the methods to observations of the CRISTA-NF instrument during one local flight of the RECONCILE aircraft campaign and found STS and medium sized NAT.