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

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 5 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 10 geometry of CRISTA-NF. NAT particles can cause different spectral features in the region 810 – 820 cm−1. The simulation results show that the appearance 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 15 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. 1 https://doi.org/10.5194/amt-2020-144 Preprint. Discussion started: 27 July 2020 c © Author(s) 2020. CC BY 4.0 License.


Comments
This paper demonstrates the clear capability of infrared FTS limb sounders to provide detection, discrimination of particle types and particle sizing in polar stratospheric clouds and is an advance on the current state of the art. The paper is acceptable for publication following some minor corrections. General comments: 1. I strongly suggest that an attempt is made to make an additional plot that shows the optical depth vs CI for some samples of different PSC cloud types. Likewise the CI vertical gradient is related to the optical thickness gradient.
Unfortunately, there is no clear relationship between the optical depth and the CI. Different parameters can influence the CI that leads to different values although the optical depth is the same. Spang et al. (2008) already showed that the CI depends on the altitude and that additionally the background atmosphere (e.g. polar winter vs. tropics) can have a large influence. Griessbach et al. (2014) showed that the observed radiance, and thus the CI, also depends on the radius of the particles. In Griessbach et al. (2020) the authors showed that there is some correlation between CI and extinction for ice and volcanic aerosol but a distinct relationship could not be determined. CIs of large particles (r > 5 µm) are more related / show a quite good correlation with the integrated surface area densities along the line of sight or for ice with the ice water path divided by effective radius (Spang et al. (2012(Spang et al. ( , 2015). We also did some studies with the new simulations and found similar results. Furthermore, the particle type also plays a role as the spectral slope of the extinction is different for the different particle types. Thus, the radiance enhancement in the regions used for the CI can be different although the total extinction and thus the optical depth are the same. As a consequence the CI values can only be related to an optical depth when all influencing parameters (altitude and thickness of cloud, particle type and radius, background atmosphere) are known, which is typically not the case. Therefore, we cannot give numbers in this direction in the paper as there are too many unknowns.
Specific comments and typos The paper has not been rejected, but the reply to the reviewer comments and the upload of a revised manuscript have not been done. However, it is the best paper describing the atmosphere and it has been cited far more than 100 times.
16. Page 6 L177: median radius varied in steps of ? For small particles it has been varied in steps of 0.5 µm, then in steps of 1 µm, and at the end there is one step of 2 µm. Because of the different step sizes all used radii are summarised in Tab. 3. 17. Page 8 L215: imaginary part/s/ done 18. L234-249 and everywhere else including figure captions and tables: Is it possible to give all these spectral regions a distinct short name? e.g. R1, R2, R3 etc Otherwise the reader has to scan the characters and check to see which regions are the same thing rather than seeing that immediately from the short name.
We added a table for all indices and we marked the regions in Fig. 1  to the old method? e.g. in terms of the minimum volume density um3/cm3. The detection level for the small particles is the same as for the old method, as they are detected with the same method. The improvement of our method is that the detection is expanded to larger NAT particle sizes that are not detectable with the old method.
25. L501: "minimisation" means "reduction"? yes, we changed this 26. L507: "safely" means "always"? we removed safely 27. Figures 3, 6 and 7: Can an approximate optical depth scale be put on the x-axis? Unfortunately, there is no distinctive relationship between optical depth and CI (see General comments).
28. Figure 8: What are the actual optical depths corresponding to these CI values?
Unfortunately, there is no clear relationship between optical depth and CI (see General comments). As clouds with many different parameters enter this plot, the answer cannot be given.