During hemispheric winter a vortex forms on poles resulting in a very sharp drop in stratospheric temperatures. When the temperature drops below a particular threshold TNAT that allows nitric acid nucleation, polar stratospheric clouds (PSCs) can start to form. PSCs are responsible for the thinning of the ozone layer. According to climate models, the ozone layer is expected to return to 1960 levels around 2060. However this progress could be slowed down by enhanced PSC formation due to the cooler and wetter stratosphere that could be caused by climate change. 

The spaceborne lidar from CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) has helped us understand better the spatial and temporal occurrence of PSCs. The CALIPSO PSC product, derived from CALIPSO level 1B measurements, describes the spatial distribution, optical properties, and composition of PSCs along CALIPSO orbits. The product also includes reanalysis temperatures from MERRA2 and complementary information from the Microwave Limb Sounder (MLS), such as HNO3/H2O mixing ratios which are essential to PSCs formation and to TNAT calculation.

In this study we will present a statistical model based on the analysis of the CALIPSO PSC product from 2006 to 2020. It establishes a relationship between the PSC density observed by CALIOP and the density of stratospheric temperatures colder than TNAT. This model allows the prediction of PSC density by pressure level derived from stratospheric temperature. We will show that this model allows us to tell if there is a PSC or not in (2°x4°) boxes over monthly periods, even in places where the satellite CALIPSO doesn't overpass. We will discuss its application on temperatures predicted by Shared Socio-economic Pathways (SSP) scenarios to know the evolution of PSCs over this century. One of our eventual goals would be to investigate if observed PSC densities can constrain stratospheric temperatures predicted by GCMs. 

How to cite: Leroux, M. and Noel, V.: Can we predict the expected evolution of polar stratospheric clouds on climatic time scales ?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-941, https://doi.org/10.5194/egusphere-egu23-941, 2023.

The aim of this poster is to compare the discontinuity occurrence in the ozone data from the following reanalyses: ERA-5, MERRA-2 and JRA-55. We use the ozone concentration data from all layers between 500 hPa and 1 hPa in the period 1980-2020 in January. We also compute the total ozone content between these layers by vertical integration of ozone profile. We search for discontinuities also in this content.    This is important topic, because the presence of discontinuities influences the values of trends and their significance. Discontinuities arise from the changing in the assimilation procedure, introducing new observation to the reanalyse, and changing of data quality.  There are dates which the occurrence of discontinuities is expected in: 2004- transition from SBUV to EOS Aura data and 2015-  the 4.2 MLS data were started to use instead of version 2.2. We search for discontinuities in the following classes of extremity: 1^st, 10^th, 25^th, 50^th,75^th,90^th and 99^th percentile as well as the mean. Ozone data with high occurrence of the discontinuities is not suitable for trend analyses.

How to cite: Krizan, P.: Comparison of discontinuity occurrence in ozone data in selected reanalyses, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1900, https://doi.org/10.5194/egusphere-egu23-1900, 2023.

Polar stratospheric clouds (PSCs) play a key role in the polar chemistry of the stratosphere. Nitric acid trihydrate (NAT) particles have been shown to lead to denitrification of the lower stratosphere. While the existence of large NAT particles (NAT "rocks") has been verified by many measurements, especially in the Northern Hemisphere (NH), most current chemistry-climate models use simplified parametrizations, often based on evaluations in the Southern Hemisphere where the polar vortex is stable enough that accounting for NAT rocks is not as important as in the NH. Here, we evaluate the probability density functions of various gaseous species in the polar vortex using one such model, the Whole Atmosphere Community Climate Model (WACCM), and compare these with measurements by the Michelson Interferometer for Passive Atmospheric Sounding onboard the Environmental Satellite (MIPAS/Envisat) and two ozonesonde stations for a range of years and in both hemispheres. Using the maximum difference between the distributions of MIPAS and WACCM as a measure of coherence, we find better agreement for HNO₃ when reducing the NAT number density from the standard value of 1 x 10⁻² cm^-3 used in this model to 5 × 10⁻⁴ cm⁻³ for almost all spring seasons during the MIPAS period in both hemispheres. The distributions of ClONO₂ and O₃ are not greatly affected by the choice of NAT density. The average difference of WACCM to ozonesondes supports the need to reduce the NAT number density in the model. Therefore, this study suggests using a NAT number density of 5 × 10⁻⁴ cm⁻³ for future simulations with WACCM. 

How to cite: Kinnison, D., Weimer, M., Wilka, C., and Solomon, S.: Effects of denitrification on the distributions of trace gas abundances in the polar regions: Comparison of the Whole Atmosphere Community Climate Model with observations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3650, https://doi.org/10.5194/egusphere-egu23-3650, 2023.

After more than three decades of research, the roles of polar stratospheric clouds (PSCs) in stratospheric ozone depletion are well established. Heterogeneous reactions on PSCs convert the stable chlorine reservoirs HCl and ClONO₂ to chlorine radicals that destroy ozone catalytically.  PSCs also prolong ozone depletion by delaying chlorine deactivation through the removal of gas-phase HNO₃ and H₂O by sedimentation of large nitric acid trihydrate (NAT) and ice particles. A substantial recovery of the ozone layer is expected by the middle of this century with reduced global production of ozone depleting substances in accordance with the Montreal Protocol and subsequent amendments.  But as climate changes, leading to a colder and perhaps wetter stratosphere and upper troposphere, reliable model predictions of recovery of the Antarctic ozone hole and of potentially more severe ozone depletion in the Arctic are challenging.  This is due both to a lack of detailed understanding of the underlying physics and the fact that many global models use simple parameterizations that do not accurately represent PSC processes.

A more complete picture of PSC processes on vortex-wide scales has emerged from the CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) instrument on the CALIPSO satellite that has been observing PSCs at latitudes up to 82 degrees in both hemispheres since June 2006.  The CALIOP Version 2.0 (v2) PSC algorithm was recently developed to address known deficiencies in previous algorithms and includes additional refinements to increase the robustness of the inferred PSC composition. In this paper, we present an updated PSC reference data record and comprehensive climatology constructed by applying the v2 algorithm to the more than 17-year CALIOP spaceborne lidar dataset.  In addition, we will examine the potential impact of aerosol and water vapor injections into the stratosphere from the January 2022 Hunga Tonga eruption on PSC occurrence in both the Arctic and Antarctic regions. 

How to cite: Pitts, M. and Poole, L.: Updated PSC climatology based on CALIOP measurements from 2006-2023, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3669, https://doi.org/10.5194/egusphere-egu23-3669, 2023.

The Arctic winter 2015/2016 was characterized by extremely low temperatures in the stratosphere and by a very strong polar vortex, accompanied by extended fields of Polar Stratospheric Clouds.  During this winter, aircraft-based measurements were carried out with the research aircraft HALO (High Altitude and Long-Range Research Aircraft) from Kiruna/Sweden and Oberpfaffenhofen/Germany.

Total reactive nitrogen and its distribution between the gas and particle phases are key parameters for understanding processes controlling the ozone budget in the polar winter stratosphere. Tracer-tracer correlations were applied to study the vertical redistribution of gas-phase total reactive nitrogen.  The extended observation period from December to March provided the opportunity to study the changing distribution of reactive nitrogen in the lowermost Arctic stratosphere during the course of the winter. In early winter, during December, the lowermost Arctic stratosphere did not show any indications for disturbed conditions.

The situation changed during the observational period in January and February. Tracer-tracer correlations showed elevated levels of total reactive nitrogen of up to 6 nmol/mol. These observations could be interpreted by evaporation of polar stratospheric particles falling down from the stratosphere above and leading to a nitrification of the lowermost stratosphere. During some periods up to 60 % of the observed total reactive nitrogen can be attributed to evaporating particles. The observation of gas phase nitrification was accompanied by the occurrence of particulate nitrate in extended regions at altitudes between about 10 and 14 km. Usually, the occurrence of particulate nitrate is rare at such altitudes. The diameter of these particles was estimated to range between about 9 and 18 µm.

During the late-winter observation period, no indications for polar stratospheric cloud particles at flight altitude were found. However, extended regions with elevated gas-phase concentrations of total reactive nitrogen were still observed. In late winter, the subsidence of air masses from the polar vortex became increasingly important for the distribution of total reactive nitrogen in the lowermost stratosphere. Air masses with substantial denitrification of up to 5 nmol/mol were observed. In these cases, up to 50 % of the undisturbed total reactive nitrogen was missing. Concurrently lower ozone concentrations were observed, indicating destruction of ozone at higher altitudes.

Nitrification and denitrification of the lowermost stratosphere during the course of the winter are linked by heterogeneous processes in the above-lying stratosphere. Simulations with the CLaMS model confirm and complement the findings of the in-situ observations. They also suggest that the observations have been representative of the vortex-wide redistribution of total reactive nitrogen. The aircraft-based in-situ measurements provided a comprehensive picture of the temporal evolution of the distribution of total reactive nitrogen over the entire winter period 2015/2016.

How to cite: Ziereis, H., Hoor, P., Grooß, J.-U., Zahn, A., Stratmann, G., Stock, P., Lichtenstern, M., Krause, J., Bense, V., Afchine, A., Rolf, C., Woiwode, W., Braun, M., Ungermann, J., Marsing, A., Voigt, C., Engel, A., Sinnhuber, B.-M., and Oelhaf, H.: Redistribution of total reactive nitrogen in the lowermost Arctic stratosphere in winter 2015/2016: In-situ observations of nitrification, denitrification and particulate nitrate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6721, https://doi.org/10.5194/egusphere-egu23-6721, 2023.

Scattering codes are used to study the optical properties of Polar Stratospheric Clouds (PSC). Particle backscattering and depolarization coefficients can be computed with available scattering codes once the particle size distribution (PSD) is known and a suitable refractive index is assumed. However, PSCs often appear as external mixtures of Supercooled Ternary Solution (STS) droplets, solid Nitric Acid Trihydrate (NAT) and possibly ice particles, making questionable the assumption of a single refractive index and a single morphology to model the scatterers.
Here we consider a set of fifteen coincident measurements of PSCs above McMurdo Station, Antarctica, by ground-based lidar and balloon-borne Optical Particle Counter (OPC), and in situ observations taken by a laser backscattersonde and OPC during four balloon stratospheric flights from Kiruna, Sweden. This unique dataset of microphysical and optical observations allows to test the performances of optical scattering models when both spherical and aspherical scatterers of different composition and, possibly, shapes are present.
We consider particles as STS if their radius is below a certain threshold value Rth and NAT or possibly ice if above it. The refractive indices are assumed known from the literature. Mie scattering is used for the STS, assumed spherical, while scattering from NAT particles, considered as spheroids of different Aspect Ratio (AR), is treated with T-Matrix results where applicable, and of geometric-optics-integral-equation approach where the particle size parameter is too large to allow for a convergence of the T-matrix method.
The parameters Rth and AR of our model have been varied between 0.1 and 2 micrometers and between 0.3 and 3, respectively, and the calculated backscattering coefficient and depolarization were compared with the observed ones. The best agreement was found for Rth between 0.5 and 0.8 micrometers, and for AR less than 0.55 and greater than 1.5.
To further constrain the variability of AR within the identified intervals we have sought an agreement with the experimental data by varying AR on a case-by-case basis, and further optimizing the agreement by a proper choice of AR smaller than 0.55 and greater than 1.5, and Rth within the interval 0.5 and 0.8 micrometers. The ARs identified in this way cluster around the values 0.5 and 2.5.
The comparison of the calculations with the measurements is presented and discussed. The results of this work help to set limits to the variability of the dimensions and asphericity of PSC solid particles, within the limits of applicability of our model based on the T-matrix theory of scattering and on assumptions on a common particle shape in a PSD and a common threshold radius for all the PSDs. 

How to cite: Cairo, F., Deshler, T., Di Liberto, L., Scoccione, A., and Snels, M.: A study of T- matrix optical scattering modelling for mixed phase Polar Stratospheric Clouds, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17448, https://doi.org/10.5194/egusphere-egu23-17448, 2023.