AS3.17

We report on measurements of total bromine (Br^tot) in the upper troposphere and lower stratosphere (UTLS) taken from the German High Altitude and LOng range research aircraft (HALO) over the North Atlantic, Norwegian Sea and north-western Europe in September/ October 2017 during the WISE (Wave-driven ISentropic Exchange) research campaign. Br^tot is calculated from measured total organic bromine (Br^org) (i.e., the sum of bromine contained in CH₃Br, the halons and the major very short-lived brominated substances) added to inorganic bromine (Br_y^inorg), evaluated from measured BrO and photochemical modelling. Combining these data, the weighted mean [Br^tot] is 19.2 ± 1.2 ppt in the extratropical lower stratosphere (Ex-LS) of the northern hemisphere. The inferred average Br^tot for the Ex-LS is slightly smaller than expected for the middle stratosphere in 2016 (~19.6 ppt (ranging from 19-20 ppt) as reported by the WMO/UNEP Assessment (2018)). However, it reflects the expected variability in Br^tot in the Ex-LS due to influxes of shorter lived brominated source and product gases from different regions of entry. A closer look into Br^org and Br_y^inorg as well as simultaneously measured transport tracers (CO, N₂O, ...) and an air mass lag-time tracer (SF₆), suggests that a filament of air with elevated Br^tot protruded into the extratropical lowermost stratosphere (Ex-LMS) from 350-385 K and between equivalent latitudes of 55-80˚N (high bromine filament – HBrF). Lagrangian transport modelling shows the multi-pathway contributions to Ex-LMS bromine. According to CLaMS air mass origin simulations, contributions to the HBrF consist of predominantly isentropic transport from the tropical troposphere (also with elevated [Br^tot] = 21.6 ± 0.7 ppt) as well as a smaller contribution from an exchange across the extratropical tropopause which are mixed into the stratospheric background air. In contrast, the surrounding LS above and below the HBrF has less tropical tropospheric air, but instead additional stratospheric background air. Of the tropical tropospheric air in the HBrF, the majority is from the outflow of the Asian monsoon anticyclone and the adjacent tropical regions, which greatly influences concentrations of trace gases transported into the Ex-LMS in boreal summer and fall. The resulting increase of Br^tot in the Ex-LMS and its consequences for ozone is investigated through the TOMCAT/SLIMCAT model simulations. However, more extensive monitoring of total stratospheric bromine in more aged air (i.e., in the middle stratosphere) as well as globally and seasonally is required in addition to model simulations to fully understand its impact on Ex-LMS ozone and the radiative forcing of climate.

How to cite: Rotermund, M., Bense, V., Chipperfield, M., Engel, A., Grooß, J.-U., Hoor, P., Hüneke, T., Keber, T., Kluge, F., Schreiner, B., Schuck, T., Vogel, B., Zahn, A., and Pfeilsticker, K.: Organic and inorganic bromine measurements around the extratropical tropopause and lowermost stratosphere (Ex-LMS): Insights into transport pathways and total bromine, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10873, https://doi.org/10.5194/egusphere-egu21-10873, 2021.

In the framework of the SouthTRAC Campaign (Transport and Composition of the Southern Hemisphere Upper Troposphere and Lower Stratosphere) based on Rio Grande, Argentina, a local research group from CONICET (Argentine National Research Council)  joined the German consortium maintaining the HALO research aircraft (High-Altitude and LOng-range aircraft)  to help with the flight planning and evaluation of the chemical composition of the upper troposphere and lower stratosphere within the ozone hole periphery. The SouthTRAC aircraft campaign was carried out in two phases which took place in September and November 2019, respectively. With the purpose of providing additional information of the atmospheric composition of brominated Very Short-Lived (VSL^Br) species and compare with HALO observations during the transfer and campaign flights, a CAM-Chem (Community Atmosphere Model with Chemistry) global chemistry-climate simulation was conducted. The model setup used in the halogenated CAM-Chem simulation had a 1° x 1.25° lat-lon resolution, 56 hybrid vertical levels from the surface to the middle stratosphere and considered assimilated meteorology from MERRA, including an explicit treatment of VSL^Br sources and chemistry. Model output of VSL^Br, long-lived bromine and chlorine (LL^Br and LL^Cl) species and ozone mixing ratios, as well as the main inorganic halogen reactive and reservoir species and gas/heterogeneous phase reaction rates affecting lowermost stratospheric ozone were analyzed in horizontal domains and vertical cross-sections across each flightpath. The model performance with respect to the HALO observations has a general good agreement, presenting better results for mid latitudes (between 30º S and 50º S) than for southern latitudes (>50º S). In particular, CAM-Chem timeseries consistently reproduced the spatio-temporal variation of the main VSLBr species (CH₂Br₂ and CHBr₃), including the sharp variations observed across the tropopause. For both VSL^Br as well as for LL^Cl compounds such as CFC-12, the Pearson correlation coefficient r obtained during each of the flights ranged between 0.7 and 0.9, while the Normalized Mean Bias (NMB) was smaller than 8% for almost every flight. Regarding LL^Br CH₃Br, the correlation with the aircraft observations is high (r>0.9) but the inter-hemispheric variability during transfer flights is not fully captured. For Ozone, the model presents mid to high correlation with respect to measures (0.5<r<0.95) with a variable overestimation ranging from 10% to at most 40% in some flights.

How to cite: Berná, L., Lopez-Noreña, A. I., Puliafito, E., Barrera, J. A., Engel, A., Jesswein, M., Cuevas, C. A., Saiz-Lopez, A., and Fernandez, R. P.: Evaluation of CAM-Chem VSLBr model performance during SouthTRAC campaign, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2298, https://doi.org/10.5194/egusphere-egu21-2298, 2021.

Synthetic halocarbons are used for a wide range of applications, for example air conditioning or foam blowing. Many of them are long-lived greenhouse gases contributing to climate change and, in addition, may contribute to stratospheric ozone depletion if containing chlorine or bromine. Therefore, their production and use are regulated by the Montreal Protocol and its amendments. These long-lived halocarbons are increasingly replaced by a fourth generation of unsaturated short-lived halocarbons, the hydrochlorofluoroolefines (HCFOs) and hydrofluoroolefines (HFOs). The main removal process of these compounds in the atmosphere is reaction with OH radicals, and their average lifetimes are of the order of up to a few tens of days.

As part of the IAGOS-CARIBIC instrument package we operate an automated air sample collection system during regular flights in the upper troposphere and lowermost stratosphere. At altitudes around 10-12 km, samples are collected in stainless steel and glass flasks at predefined times. Post-flight laboratory analyses include gas chromatography - mass spectrometry measurements of a wide range of halocarbons. The short-lived compounds HFO-1234ze(E) and HCFO-1233zd(E) were detected in a small number of samples, indicating that these compounds are sufficiently long lived for transport into the upper troposphere. There were not found in stratospheric samples.

At this altitude, low abundance of OH and low temperatures may slow down chemical decay, and tracer lifetimes may increase significantly. Based on average temperatures and OH abundance, we estimate local lifetimes of HFO-1234ze(E) and HCFO-1233zd(E)  in the mid-latitudes of up to 75 days and 200 days, respectively. Short-lived H(C)FOs reaching the upper troposphere could thus be transported over large distances and their degradation products may be deposited  far from their emission sources.

How to cite: Schuck, T., Meixner, K., van Velthoven, P., O’Doherty, S., Vollmer, M., Engel, A., and Zahn, A.: Detection of Fourth Generation Synthetic Halocarbons in the Upper Troposphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4203, https://doi.org/10.5194/egusphere-egu21-4203, 2021.

Knowing the stratospheric lifetime of an Ozone Depleting Substance (ODS), and its potential depletion of ozone during that time, is vital to reliably monitor and control the use of ODSs. Here, we present improved policy-relevant parameters: Fractional Release Factors (FRFs), Ozone Depletion Potentials (ODPs), and stratospheric lifetimes, for four understudied long-lived CFCs: CFC-13 (CClF₃), CFC-114 (CClF₂CCCLF₂), CFC-114a (CCl₂FCF₃), and CFC-115 (C₂ClF₅). Previously derived lifetime estimates for CFC-114 and CFC-115 have substantial uncertainties, while lifetime uncertainties for CFC-13 and CFC-114a are absent from the peer-reviewed literature (Carpenter & Danie et al, 2018).

This study used both observational and model data to investigate these compounds and this work derives, for the first time, observation-based lifetimes utilising measurements of air samples collected in the stratosphere. We also used a version of the NASA Goddard Space Flight Center (GSFC) 2-D atmospheric model driven by temperature and transport fields derived from MERRA/MERRA-2 reanalysis.

FRFs for these compounds, which had been lacking until now, were derived using stratospheric air samples collected from several research flights with the high-altitude aircraft M55-Geophysica, and the background trend from archived surface air samples from Cape Grim, Tasmania.

 By using a previously-published correlation between lifetime and FRF for seven well-characterised compounds (CF₄, C₂F₆, C₃F₈, CHF₃, HFC-125, HFC-227ea and SF₆), we were able to derive lifetimes for these four new species. Lifetime estimates for CFC-114a agreed within the uncertainties (agreement to one sigma) with the lifetime estimates compiled in Burkholder et al. (2018), while the one for CFC-114 agreed within 2 sigma (measurement-related uncertainties) with those cited in Burkholder et al. (2018). However, observation-based lifetimes for CFC-13 and CFC-115 only agreed with those in Burkholder et al. (2018) within 3 sigma. The lifetime uncertainties in this study were reduced compared to those in Carpenter & Danie et al (2018).

As our lifetime estimates for these latter two compounds are notably lower than previous estimates, this suggests that these two compounds may have had greater emissions than previously thought, in order to account for their abundance. It also implies that they will be removed from the atmosphere more quickly than previously thought.

New ODPs were derived for these compounds from their new lifetimes and FRFs. Since for two of these compounds (CFC-13 and CFC-114a), there is an absence of observation-derived ODPs in the peer-reviewed literature, this is new and relevant information. The ODPs for CFC-114 and CFC-115 are comparable with estimates from the most recent Scientific Assessment of Ozone Depletion (Burkholder et al., 2018). Providing new and updated lifetimes, FRFs and ODPs for these compounds will help improve future estimates of their tropospheric emissions and their resulting damage to the stratospheric ozone layer.

References

Burkholder et al. (2018). Appendix A, Table A-1 in Scientific Assessment of Ozone Depletion: 2018, Global Ozone Research and Monitoring Project, Report No. 58, World Meteorological Organization, Geneva, Switzerland,  http://ozone.unep.org/science/assessment/sap.

Carpenter, L.J., Danie, J.S. et al (2018). Scenarios and Information for Policymakers Chapter 6, Table 6-1 in Scientific Assessment of Ozone Depletion: 2018, Global Ozone Research and Monitoring Project, Report No. 58, World Meteorological Organization, Geneva, Switzerland.

How to cite: Tuffnell, E., Laube, J., Leedham Elvidge, E., Sturges, B., Adcock, K., Fraser, P., Krummel, P., Langenfelds, R., Oram, D., Fleming, E., Liang, Q., and Roeckmann, T.: New Fractional Release Factors, Ozone Depletion Potentials, and Lifetimes for Four Long-Lived CFCs: CFC-13, CFC-114, CFC-114a, and CFC-115, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4995, https://doi.org/10.5194/egusphere-egu21-4995, 2021.

Carbonyl sulfide (OCS or COS) is the longest lived and the most abundant reduced sulfur gas in the atmosphere. As chemical loss of OCS in the troposphere is slow, it can reach the stratosphere, where it is  photochemically oxidized and converted to stratospheric sulfate aerosol, being the largest source thereof in times of volcanic quiescence. Chemistry transport models show that OCS conversion occurs mainly in the ‘tropical pipe’ region, while along the lower branch of Brewer-Dobson circulation (BDC), OCS is passively transported without significant chemical loss. The OCS depleted air is transported along the upper branch of BDC and descends again at high latitudes. Using the distinct characteristics of  ‘age of air’ in the upper and lower branches of the BDC, this picture of OCS transport and especially the role of the ‘tropical pipe’ as the main region of OCS conversion can be supported by looking at the relationship between age spectra and OCS mixing ratios.

In this study, we will investigate the relation of OCS mixing ratios and mean age of air as well as mass fractions of air with different transit times using satellite-based measurements from MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) and ACE-FTS (Atmospheric Chemistry Experiment - infrared Fourier Transform Spectrometer), and age spectra of air from CLaMS (Chemical Lagrangian Model of the Stratosphere).

In addition to satellite data analysis, we will investigate the distribution of OCS in the UTLS (upper troposphere and lower stratosphere) region and its relation to the age spectra using high-resolution in-situ observations of OCS. This unique dataset was obtained during the SOUTHTRAC mission in autumn 2019 by AMICA (Airborne Mid-Infrared Cavity enhanced Absorption spectrometer) on board the HALO (High Altitude Long Range) research aircraft. Flights from the main  campaign base in Río Grande, Argentina (53.8S, 67.7W) covered a wide latitude range from 48° N to 70° S, even reaching the southern polar vortex where aged air masses having descended from high altitudes are typically found.

Our analysis of both satellite and in-situ data generally supports the established picture of OCS conversion in the ‘tropical pipe’.

How to cite: Qiu, C., Ploeger, F., Grooß, J.-U., and von Hobe, M.: Study on Stratospheric Carbonyl Sulfide Transport and Chemistry using ‘Age of Air’, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5550, https://doi.org/10.5194/egusphere-egu21-5550, 2021.

On the NASA Atmospheric Tomography Mission (ATom), we observed a sharp hemispheric contrast in the concentration of ultrafine aerosols (3-12 nm diameter) in the lowermost stratosphere that persisted through all four seasons. Exploring possible causes, we show that this is likely caused by aircraft, which emit both ultrafine aerosol and precursor gases for new particle formation (NPF) in quantities that agree well with our observations. While aircraft may emit a range of NPF precursors, we focus here on sulphur dioxide (a major source of atmospheric sulphuric acid), of which we have observations from the same mission.  We observe the same hemispheric contrast in sulphur dioxide as ultrafine aerosol, and find that the observed concentrations are in alignment with inventoried aircraft emissions. We present box modeling and thermodynamic calculations that support the plausibility of NPF under the conditions and sulphur dioxide concentrations observed on ATom.

While the direct climate impact of ultrafine aerosol in the lowermost stratosphere (LMS) may currently be small, our observations show a definitive size distribution shift of the background aerosol distribution in the northern hemisphere. This is important for assessing aviation impacts, and the expected impacts of increased air-traffic. Furthermore, climate intervention via injection of sulphate or aerosols into the stratosphere is a current subject of research. Our study shows that NPF is possible and likely already happening in the LMS, which must be accounted for in models for stratospheric modification, and points out that we must consider that any intentional stratospheric modification will be applied to two very different hemispheres: a largely pristine southern hemisphere; and an already anthropogenically modified northern hemisphere.

How to cite: Williamson, C. J., Kupc, A., Rollins, A., Kazil, J., Froyd, K. D., Ray, E. A., Murphy, D. M., Schill, G. P., Peischl, J., Thompson, C., Bourgeois, I., Ryerson, T., Diskin, G. S., DiGangi, J. P., Blake, D. R., Bui, T. V., Dollner, M., Weinzierl, B., and Brock, C. A.: Large hemispheric difference in ultrafine aerosol concentrations in the lowermost stratosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6674, https://doi.org/10.5194/egusphere-egu21-6674, 2021.

We present trace gas measurements obtained by the airborne imaging limb sounder GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) that has been operated onboard the German HALO (High Altitude and Long Range) research aircraft above the South Atlantic during the SouthTRAC campaign between September and November 2019. We show retrieval results as two-dimensional trace-gas distributions derived from GLORIA observations in the UTLS (Upper Troposphere Lower Stratosphere) region above South America and the Atlantic Ocean. Targeted gases are, amongst others, O₃, HNO₃, PAN, C₂H₆, and HCOOH. Using trajectories from the HYSPLIT model, measured pollution trace gas plumes are linked to possible regions of origin. Emission sources are connected to large scale biomass burning events in central Africa, South America and Australia. In our contribution, we compare these GLORIA measurements with results of the CAMS (Copernicus Atmosphere Monitoring Service) reanalysis model. We show that there are very delicate structures of pollutant trace gas distributions in the South Atlantic UTLS, and that CAMS generally is able to reproduce measured distributions of pollutants. Quantitatively, PAN volume mixing ratios are captured quite well by the model, which however underestimates the concentrations of C₂H₆ and in particular of HCOOH. Furthermore, biomass burning emissions from the beginning of the intensive Australian fires in November 2019, which are measured by GLORIA in thin filaments are not reproduced by the model.

How to cite: Johansson, S., Höpfner, M., Friedl-Vallon, F., Glatthor, N., Ungermann, J., and Wetzel, G.: Biomass burning in the southern hemisphere UTLS: GLORIA trace gas observations during SouthTRAC 2019 to evaluate the CAMS model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4717, https://doi.org/10.5194/egusphere-egu21-4717, 2021.

Airborne measurements of upper troposphere and lower stratosphere biomass burning smoke show a large size mode at 350nm radius. Furthermore,  very thickly coated black carbon (300-400nm radius) is observed in 2 month aged Pyro-cumulonimbus (PyroCb) smoke in the lower stratosphere. Finally, the stratospheric aerosol mass injections from the 2017 British Columbia (BC17) PyroCbs are much larger than fuel loading predicts.  We propose a secondary organic aerosol (SOA) production mechanism where volatile organic compounds (VOCs) emitted by fires condense in the cold convective PyroCb updrafts to explain the aforementioned data. Observations supporting this mechanism present in FIREX-AQ, ATOM and CARIBEC airborne data are synthesized. The condensation, evaporation and coagulation mechanisms are implemented into LANL’s large eddy cloud resolving model called HIGRAD.  Our simulations provide insights into the vertical distribution of SOA in the BC17 PyroCb and the role of warm and ice clouds in lofting it into the lower stratosphere. We show that SOA formation can increase aerosols by a factor of 2-3 and latent heat from warm and ice clouds adds 5 km to the injection height of BC17 fire smoke. The fate, transport and impacts  of smoke from BC17 and 2020 Australian fires are examined using climate model (CESM) simulations.  

How to cite: Dubey, M. K., Gorkowski, K., Reisner, J., Benedict, K., Josephson, A., Koo, E., D'Angelo, G., Peterson, D., and Guimond, S.: Organic Vapor Condensation in Pyro-cumulonimbus Outflow Explains Large Stratospheric Smoke Mass Injection and Thickly Coated Black Carbon, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12961, https://doi.org/10.5194/egusphere-egu21-12961, 2021.

Wildfire-driven pyro-convection (PyroCb) is capable of lofting combustion products into the stratosphere, polluting it with smoke aerosols at hemispheric and yearly scales. This realization has emerged after the record-breaking British Columbia PyroCb event in August 2017 that approached moderate volcanic eruptions in terms of stratospheric aerosol load perturbation. The Australian “Black Summer” bushfires in 2019/20 have surpassed the previous record by a factor of 3 and rivaled the strongest volcanic eruptions in the XXI century. Here we exploit a synergy of various satellite observations, ECMWF meteorological analysis and radiative transfer modeling to quantify the perturbation of stratospheric particulate and gaseous composition, dynamical circulation and radiative balance caused by the Australian New Year’s PyroCb outbreak. One of the most striking repercussions of this event was the generation of several persistent anticyclonic vortices that provided confinement to the PyroCb plumes and preserved them from rapid dilution in the environment. The most intense vortex measured 1000 km in diameter, persisted in the stratosphere for over 13 weeks and lifted a confined bubble of combustion gases, aerosols and moisture to 35 km altitude. It was accompanied by a synoptic-scale ozone hole with the total column reduction by about 30%. The startling consequences of the Australian event provide new insights into climate-altering potential of the wildfires, that have increased in frequency and strength over the recent years.

How to cite: Khaykin, S., Legras, B., Bucci, S., Sellitto, P., Isaksen, L., Tencé, F., Bekki, S., Bourassa, A., Rieger, L., Zawada, D., Jumelet, J., and Godin-Beekmann, S.: Striking repercussions of the Australian "Black Summer" bushfires on the stratospheric composition and dynamical circulation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9645, https://doi.org/10.5194/egusphere-egu21-9645, 2021.

Extreme convective events in the troposphere have not only immediate destructive impact on the surface, but can also influence the dynamic and composition of the lower stratosphere (LS). One of the impacts is the moistening of the LS. This effect plays a crucial role in climate feedback as water vapor in the UTLS (Upper Troposphere/Lower Stratosphere) has a major impact on the radiation budget of the atmosphere. 
In this case study we investigate water vapor injection into the LS by convective events in mid-latitudes. In the framework of the MOSES (Modular Observation Solutions for Earth Systems) measurement campaign during the early summer of 2019, balloon borne measurements were performed to capture the water vapor injected into the stratosphere by convective events. On two consecutive days the balloon profiles showed clear evidence of water vapor transported above the tropopause by convection. The magnitude of the water vapor enhancement is comparable to other studies which show measurements above North America. At the altitude of the measured injection a sharp cut-off in a local ozone enhancement peak verifies the tropospheric origin of the water vapor injection. Back trajectories of the measured air masses reveal that the moistening took place multiple hours before the balloon launch and correlate well with ERA5 data showing a strong change in the structure of isotherms and a sudden and short lived increase in potential vorticity at the altitude of the trajectory. A comparison with MLS data shows that this process can barely be recognized by satellite measurements due to the low vertical and horizontal resolution. It is hence desirable to increase the number of in-situ measurements focusing on the impacts of convective events on the lower stratosphere over Europe and to assess its impact on UTLS water vapor.

How to cite: Khordakova, D., Rolf, C., Grooß, J.-U., Konopka, P., Müller, R., Krämer, M., and Riese, M.: A case study on the impact of severe convective storms on the water vapor mixing ratio in the lower mid-latitude stratosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13130, https://doi.org/10.5194/egusphere-egu21-13130, 2021.

A cold bias in the extratropical lowermost stratosphere in forecasts is one of the most prominent systematic temperature errors in numerical weather prediction models. Hypothesized causes of this bias include radiative effects from a collocated moist bias in model analyses. Such biases would be expected to affect extratropical dynamics and result in the misrepresentation of wave propagation at tropopause level. Here the extent to which these biases are connected is quantified. Observations from radiosondes are compared to operational analyses and forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS) and Met Office Unified Model (MetUM) to determine the magnitude and vertical structure of these biases. Both operational models over-estimate lowermost stratospheric specific humidity by around 70% of the observed values on average, around 1km above the tropopause. This moist bias is already present in the initial conditions and changes little in forecasts over the first five days. Though temperatures are represented well in the analyses, the IFS forecasts anomalously cool in the lower stratosphere, relative to verifying radiosonde observations, by 0.2K per day. The IFS single column model is used to show this temperature change can be attributed to increased long-wave radiative cooling due to the lowermost stratospheric moist bias in the initial conditions. However, the MetUM temperature biases cannot be entirely attributed to the moist bias, and another significant factor must be present. These results highlight the importance of improving the humidity analysis to reduce the extratropical lowermost stratospheric cold bias in forecast models and the need to understand and mitigate the causes of the moist bias in these models.

How to cite: Bland, J., Gray, S., Methven, J., and Forbes, R.: Characterising extratropical near tropopause analysis humidity biases and their radiative effects on temperature forecasts, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8834, https://doi.org/10.5194/egusphere-egu21-8834, 2021.

The Brewer-Dobson Circulation (BDC) is a wintertime stratospheric circulation characterized by upwelling of tropospheric air in the tropics, poleward flow in the stratosphere, and downwelling at mid and high latitudes, with important implications for chemical tracer distributions, stratospheric heat and momentum budgets, and mass exchange with the troposphere. 
Nitrous oxide (N2O) is continuously emitted in the troposphere, where has no sinks, and transported into the stratosphere, where is destroyed by photodissociaiton. The lifetime of N2O is approximately 100 years, which makes it an excellent long-lived tracer for transport studies in the stratosphere. 
In this study, we investigate the long-term N2O changes in the stratosphere using a number a different datasets. We analyze the simulation from the state-of-the-art Chemistry-Climate Model WACCM (period: 1990-2014), together with the BASCOE Chemistry-Transport Model driven by five dynamical reanalyses (ERA5, ERA-Interim, JRA-55, MERRA, MERRA-2, period: 1996-2014), and the chemical reanalysis of Aura Microwave Limb Sounder version 3 (BRAM3, period: 2004-2013). We will also compare those gridded data to ground-based observations from Fourier transform infrared spectrometer at the Jungfraujoch station in the Swiss Alps. 
The long-term trends of the N2O concentration are investigated using the Dynamic Linear Model (DLM). The DLM is a regression model based on the Bayesian inference, which allow fitting atmospheric data with four main components: a linear trend, a seasonal cycle, a number of proxies (solar cycle, ENSO, QBO ?) and an autoregressive process. DLM has the advantage that the trend and the seasonal and regression coefficients depend on time; DLM can therefore detect changes in the recovered trend, and modulations of the amplitude of the regressors with time. 
Early results show that the datasets exhibit hemispheric differences in the long-term N2O changes in the lower stratosphere. In the Southern Hemisphere, the DLM fit of the N2O concentrations increases across the datasets, but the resulting trend is statistically significant only in limited regions of the stratosphere. In the Northern Hemisphere, the N2O fit does not change significantly in the considered period, resulting in a near-zero trend. These hemispheric differences are in line with previous studies of transport that identify different long-term trends of tracers and mean age of air between the hemispheres. 
The fit through the DLM allows the amplitude of the seasonal cycle component to vary in time. Preliminary results indicate that the time variations depend on the hemisphere in the extra-tropical regions. In the Southern Hemisphere, the datasets generally show a constant amplitude of the seasonal cycle throughout the considered periods, with the largest values in the high latitudes in response to the polar vortex. In the Northern Hemisphere, the inter-annual variations of the seasonal cycle amplitude are stronger, with BRAM3 showing the largest modulations. In addition, larger differences arise in the amplitude of the seasonal component. WACCM simulates large amplitudes of the seasonal cycle, while the reanalyses show smaller values. 
A more detailed analysis of the results will include ground-based observations, and the extension of the CTM runs to a longer period that matches the length of the WACCM run.

How to cite: Minganti, D., Chabrillat, S., Errera, Q., Prignon, M., and Mahieu, E.: Preliminary investigation of long-term changes in the stratospheric N2O abundances as a proxy for the Brewer-Dobson Circulation in a climate model, dynamical and chemical reanalyses and observations., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11790, https://doi.org/10.5194/egusphere-egu21-11790, 2021.

The connection between the dominant mode of interannual variability in the tropical troposphere, El Niño Southern
Oscillation (ENSO), and entry of stratospheric water vapor, is analyzed in a set of the model simulations archived for the
Chemistry-Climate Model Initiative (CCMI) project and for phase 6 of the Coupled Model Intercomparison Project. While the
models agree on the temperature response to ENSO in the tropical troposphere and lower stratosphere, and all models also agree
 on the zonal structure of the response in the tropical tropopause layer, the only aspect of the entry water vapor with consensus
is that La Niña leads to moistening in winter relative to neutral ENSO. For El Niño and for other seasons there are significant
differences among the models. For example, some models find that the enhanced water vapor for La Niña in the winter of the
event reverses in spring and summer, other models find that this moistening persists, while some show a nonlinear response
with both El Niño and La Niña leading to enhanced water vapor in both winter, spring, and summer. A moistening in the spring
 following El Niño events, perhaps the strongest signal in observations, is simulated by only half of the models. Focusing on
Central Pacific ENSO versus East Pacific ENSO, or temperatures in the mid-troposphere as compared to temperatures near the
surface, does not narrow the inter-model discrepancies. Despite this diversity in response, the temperature response near the
cold point can explain the response of water vapor when each model is considered separately. While the observational record is
too short to fully constrain the response to ENSO, it is clear that most models suffer from biases in the magnitude of interannual
variability of entry water vapor. This bias could be due to biased cold point temperatures in some models, but others appear to
be missing forcing processes that contribute to observed variability near the cold point

How to cite: Harari, O., garfinkel, C., and Ziskin, S.: Influence of ENSO on entry stratospheric water vapor in coupled chemistry-ocean CCMI and CMIP6 models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-619, https://doi.org/10.5194/egusphere-egu21-619, 2021.

El Niño‐Southern Oscillation (ENSO) is the main source of interannual variability in the global climate. Previous studies have shown ENSO has impacts on stratospheric ozone concentrations through changes in stratospheric circulation. The aim of this study is to extend these analysis by examining the anomalies in residual circulation and mixing associated with different El Niño flavors (Eastern Pacific (EP) and Central Pacific (CP)) and La Niña in boreal winter. For this purpose, we use four 60-year ensemble members of the Whole Atmospheric Community Climate Model version 4, reanalysis and satellite data.

Significant ozone anomalies are identified in both tropics and extratropics. In the northern high-latitudes (70-90N), significant positive ozone anomalies appear in the middle stratosphere in early winter during both CP and EP El Niño, which propagates downward during winter to the lower stratosphere only during EP-El Niño events. Anomalies during La Niña events are opposite to EP-El Niño. The analysis of the different terms in the continuity equation for zonal-mean ozone concentration reveals that Arctic ozone changes during ENSO events  are mainly driven by advection due to residual circulation, although contributions of mixing and chemistry are not negligible, especially in upper stratosphere.

The ENSO impact on total ozone column (TOC) is also investigated. During EP-El Niño, a significant reduction of TOC appears in the tropics and an increase in the middle latitudes. During La Niña the response is the opposite. The TOC response to CP El Niño events is not as robust. In the Northern Hemisphere polar region the TOC anomalies are not significant, probably due to its large variability associated with sudden stratospheric warmings in this region.

How to cite: Benito-Barca, S., Calvo, N., and Abalos, M.: Impact of ENSO on stratospheric ozone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7665, https://doi.org/10.5194/egusphere-egu21-7665, 2021.

Links between springtime Arctic stratospheric ozone anomalies and anomalous surface weather in the Northern Hemisphere have been found recently. Stratospheric ozone thus provides valuable information which may help to improve seasonal predictability. However, the extent and causality of the ozone-surface climate coupling remain unclear and many state-of-the-art forecast models lack any representation of ozone feedbacks on planetary circulation.

We investigate the importance of the ozone-surface climate coupling with two Chemistry Climate Models, contrasting simulations with fully interactive ozone against prescribed zonally averaged climatological ozone under fixed present-day boundary conditions. We focus on springtime Arctic ozone minima and compare subsequent surface patterns in runs with and without interactive ozone, thus rendering a detailed and physically-based quantification of the stratospheric ozone impact on surface climate possible.  

All model simulations show a connection between Arctic ozone minima and a positive phase of the Arctic Oscillation in the month after the depletion in spring. Runs with interactive ozone chemistry show an amplified surface response and a 40% stronger Arctic Oscillation index after ozone depletion. This amplified Arctic Oscillation goes along with enhanced positive surface temperature anomalies over Eurasia. Moreover, composite surface patterns after spring ozone minima in model simulations with interactive ozone show a better agreement with composites in reanalysis data compared to runs with prescribed ozone.

Mechanisms whereby stratospheric ozone affects both the stratospheric and tropospheric circulation are explored. These include the reduction of short-wave heating over the pole due to ozone loss, thus amplifying stratospheric temperature anomalies and allowing for an intensification of the polar vortex with subsequent impacts on wave propagation and the stratospheric meridional circulation. This suggests that ozone is not only passively responding to stratospheric dynamics, but actively feeds back into the circulation. Following these results, stratospheric ozone anomalies actively contribute to anomalous surface weather in spring, emphasizing the potential importance of interactive ozone chemistry for seasonal predictions.

How to cite: Friedel, M., Chiodo, G., Stenke, A., Domeisen, D., Muthers, S., Anet, J., and Peter, T.: The Influence of Ozone Feedbacks on Surface Climate following Spring Arctic Ozone Depletion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12668, https://doi.org/10.5194/egusphere-egu21-12668, 2021.

This paper investigates the global stratospheric Brewer-Dobson circulation (BDC) in the ERA5 meteorological reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF). The analysis is based on simulations of stratospheric mean age of air, including the full age spectrum, with the Lagrangian transport model CLaMS, driven by winds and total diabatic heating rates from the reanalysis. ERA5-based results are compared to those of the preceding ERA-Interim reanalysis. Our results show a significantly slower BDC for ERA5 than for ERA-Interim, manifesting in weaker diabatic heating rates and larger age of air. In the tropical lower stratosphere, heating rates are 30-40% weaker in ERA5, likely correcting a known bias in ERA-Interim. Above, ERA5 age of air appears slightly high-biased and the BDC slightly slow compared to tracer observations. The age trend in ERA5 over 1989-2018 is negative throughout the stratosphere, as climate models predict in response to global warming. However, the age decrease is not linear over the period but exhibits steplike changes which could be caused by muti-annual variability or changes in the assimilation system. Over the 2002-2012 period, ERA5 age shows a similar hemispheric dipole trend pattern as ERA-Interim, with age increasing in the NH and decreasing in the SH. Shifts in the age spectrum peak and residual circulation transit times indicate that reanalysis differences in age are likely caused by differences in the residual circulation. In particular, the shallow BDC branch accelerates similarly in both reanalyses while the deep branch accelerates in ERA5 and decelerates in ERA-Interim.

How to cite: Ploeger, F., Diallo, M., Charlesworth, E., Konopka, P., Legras, B., Laube, J., Grooß, J.-U., Günther, G., Engel, A., and Riese, M.: The stratospheric Brewer-Dobson circulation inferred from age of air in the ERA5 reanalysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4344, https://doi.org/10.5194/egusphere-egu21-4344, 2021.

The stratospheric transport circulation, or Brewer-Dobson Circulation (BDC), is often conceptually seperated into advection along the residual circulation and two-way mixing. In particular the latter part has recently been found to exert a strong influence on inter-model differences of mean age of Air (AoA), a common measure of the BDC. However, the precise reason for model differences in two-way mixing remains unknown, as many model
components in multi-model projects differ. One component that likely plays an important role is model resolution, both vertically and horizontally. To analyse this aspect, we carried out a set of simulations with identical and constant year 2000 climate forcing varying the spectral horizontal
resolution (T31,T42,T63,T85) and the number of vertical levels (L31,L47,L90). We find that increasing the vertical resolution leads to an increase in mean AoA. Most of this change can be attributed to aging by mixing. The mixing efficiency, defined as the ratio of isentropic mixing strength and the diabatic circulation, shows the same dependency on vertical resolution. While horizontal resolution changes do not systematically change mean AoA, we do
find a systematic decrease in the mixing efficiency with increasing horizontal resolution. Non-systematic changes in the residual circulation partly compensate the mixing efficiency changes, leading to the non-systematic mean AoA changes. The mixing efficiency changes with vertical and horizontal resolution are consistent with expectations on the effects of numerical dispersion on mean AoA. To further investigate the most relevant regions of mixing differences, we analyse height-resolved mixing efficiency differences. Overall, this work will help to shed light on the underlying reasons for the large biases of climate models in simulating stratospheric transport.

How to cite: Garny, H., Dietmüller, S., Eichinger, R., Gupta, A., and Linz, M.: On the role of vertical and horizontal model resolution for the simulation of stratospheric transport, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2635, https://doi.org/10.5194/egusphere-egu21-2635, 2021.

The mean age of air is a powerful diagnostic tool to investigate stratospheric transport processes. It can be derived from suitable trace gas measurements and from model calculations. In contrast to the Northern Hemisphere (NH), data coverage of in situ measurements of such trace gases in the Southern Hemisphere (SH) is sparse. Due to its tropospheric trend and its very long atmospheric lifetime, SF₆ is such a suitable trace gas. SF₆ mixing ratios were measured with an airborne in situ GC-ECD system during several HALO aircraft campaigns, including locations in the SH polar vortex.

Here we present the mean age derived from in situ SF₆ measurements during the POLSTRACC campaign (Polar Stratosphere in a Changing Climate) in NH winter/spring 2015/2016 and during the SouthTRAC campaign (Transport and Composition of the Southern Hemisphere UTLS) in SH winter/spring 2019. Mean age values over 4 years were observed in both polar vortices. On average, higher mean age values were observed at lower levels of potential temperature during SouthTRAC 2019 than during POLSTRACC 2015/2016. The findings will be discussed in context of the Brewer-Dobson circulation.

How to cite: Wagenhäuser, T., Jesswein, M., Keber, T., Schuck, T., and Engel, A.: Comparison of northern hemispheric and southern hemispheric Mean Age derived from in situ tracer measurements during POLSTRACC and SouthTRAC, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7566, https://doi.org/10.5194/egusphere-egu21-7566, 2021.

Previous studies have suggested that the recent increase in tropical extreme deep convection, in particular over Asia and Africa during the boreal summer, has occurred in association with a cooling in the tropical lower stratosphere. The present study is focused on the Sahel region of West Africa, where an increased occurrence of extreme precipitation events has been reported over recent decades. The results show that the changes since the 1980s involve a cooling trend in the tropical lower stratosphere and tropopause layer, combined with a warming in the troposphere. This feature is similar to that which might result from increased greenhouse gas levels. It is suggested that the decrease in the vertical temperature gradient in the tropical tropopause region enhances extreme deep convection where penetrating convection is frequent, whereas tropospheric warming suppresses the shallower convection. The essential feature of the recent changes over the tropics is therefore the depth of convection, rather than the total amount of surface precipitation. This could enhance cooling in the lower stratosphere through decrease in ozone concentration.

How to cite: Kodera, K., Eguchi, N., Ueyama, R., Funatsu, B., Gaetani, M., and Taylor, C.: Impact of tropical tropopause layer cooling trend on extreme deep convection in Asian-African sector, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9359, https://doi.org/10.5194/egusphere-egu21-9359, 2021.

Eastward eddy shedding of the Asian summer monsoon (ASM) anticyclone has a large impact on the chemical composition of the upper troposphere and lower stratosphere (UTLS) over the western Pacific. Here we investigate the dynamical mechanism of eastward eddy shedding in July and August using 41 years of the ERA5 6-hourly reanalysis data. We perform composite analyses of meteorological variables focusing on the eastward eddy shedding events with the presence of anticyclonic centers falling between 135^•-140^•E. The composited outgoing longwave radiation anomalies suggest enhanced convection near the Philippines Sea and the East China Sea one week beforehand. In the tropopause level, we see evident eastward propagating geopotential and meridional wind anomalies from the North Atlantic jet exit toward the western Pacific embedded along the extratropical westerly jet during day -10 to day 0. In the lower troposphere, we find that the geopotential anomalies aligned meridionally from the east Asian coast to the North Pacific to the northern North America during day -7 to day 0. The wave-activity flux is evaluated to identify the origin and propagation of the energy of the Rossby wave–like perturbation. In the UTLS we find a strong southeastward-pointing flux along 40^•-50^•N, resembling the Silk Road pattern. While in the lower troposphere, we also see a northeastward-pointing flux originating from tropical Philippine Sea across Japan to North America, resembling the Pacific-Japan pattern. Additional analysis is needed to study the relationship between the Silk Road pattern and the Pacific-Japan pattern.

How to cite: Wang, X., Randel, W., and Wu, Y.: Eastward Eddy Shedding of the Asian Summer Monsoon Anticyclone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6567, https://doi.org/10.5194/egusphere-egu21-6567, 2021.

Tropopause folds are known as areas of enhanced stratosphere-troposphere exchange. These exchange processes are governed by turbulent mixing in the upper-tropospheric and lower-stratospheric shear zones around the tropopause jet. Since the 1970s, turbulence is also predicted to enhance the ageostrophic circulation around the jet, which leads to the formation of the tropopause fold in an upper-level jet-front system. This claim was recently confirmed by a numerical weather prediction study using the ECMWF-IFS.

With our balloon-borne turbulence measuring instrument LITOS, we recently sounded a deep and a medium tropopause fold with astonishing results: in both cases, the strength of turbulence in the lower stratospheric shear layer was three orders of magnitude higher compared to the upper tropospheric shear layer, reaching severe turbulence strengths in the deep-fold case. This has not been reported before, potentially because hardly any observational turbulence study covering both shear layers exists in the literature. In our study, we also quantitatively compare turbulence induced PV changes with PV profiles from the IFS and assess the meteorological situation using further IFS data. Additionally, we investigate mixing processes from tracer-tracer correlations of ozone and water vapour along the flight track of our instrument.

How to cite: Faber, J., Zülicke, C., Gerding, M., and Lübken, F.-J.: Case studies on turbulence in different tropopause folds, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12479, https://doi.org/10.5194/egusphere-egu21-12479, 2021.

An exceptionally strong sudden stratospheric warming (SSW) in the Southern Hemisphere (SH) during September 2019 was observed. Because SSW in the SH is very rare, comparison with the only recorded major SH SSW is done. According to World Meteorological Organization (WMO) definition, the SSW in 2019 has to be classified as minor. The cause of SSW in 2002 was very strong activity of stationary planetary wave with zonal wave-number (ZW) 2, which reached its maximum when the polar vortex split into two circulations with polar temperature enhancement by 30 K/week and it penetrated deeply to the lower stratosphere and upper troposphere. On the other hand, the minor SSW in 2019 involved an exceptionally strong wave-1 planetary wave and a large polar temperature enhancement by 50.8 K/week, but it affected mainly the middle and upper stratosphere. The strongest SSW in the Northern Hemisphere was observed in 2009. This study provides comparison of two strongest SSW in the SH and the strongest SSW in the NH to show difference between two hemispheres and possible impact to the lower or higher layers.

How to cite: Kozubek, M. and Krizan, P.: Comparison of Key Characteristics of Remarkable SSW Events in the Southern and Northern Hemisphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7702, https://doi.org/10.5194/egusphere-egu21-7702, 2021.

The potential temperature is a widely used quantity in atmospheric science since it corresponds to the entropy and is conserved for adiabatic changes of dry air. As such, it is routinely employed in applications ranging from atmospheric dynamics to transport modeling. The common formula to compute the potential temperature is based on the assumption of a constant specific heat capacity for the dry air, even though the latter is known to vary with temperature.

We re-derive the (dry air) potential temperature for a recent temperature-dependent formulation of the specific heat capacity of dry air. The result is expected to provide values which are much closer at the true entropy value (expressed as a temperature) and hence serves as the reference potential temperature. However, its computation is less straightforward compared to the classical one, motivating the development of efficient approximations. Moreover, similarities and differences are discussed between the newly derived reference potential temperature and the classical one based on a constant specific heat capacity. The new reference shows different values and vertical gradients, in particular in the stratosphere and above. Applications of the new reference potential temperature are discussed in the context of common computations in the atmospheric sciences, including the potential vorticity or diabatic heating rates.

How to cite: Spichtinger, P., Baumgartner, M., Weigel, R., Plöger, F., and Achatz, U.: Reappraising the appropriate calculation of the potential temperature, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14744, https://doi.org/10.5194/egusphere-egu21-14744, 2021.