The composition of the upper troposphere and the stratosphere (UTS) plays a key role in the climate system. Our understanding of the interactions between dynamics, chemistry and climate in this region is rapidly increasing thanks to both observational and modelling studies. In this session we invite studies of dynamical, transport and chemical processes determining the variability at all scales, including long-term trends in the composition of the UTS. We particularly encourage studies bringing together recent in situ and/or remote sensing observations and model simulations.
vPICO presentations: Wed, 28 Apr
We report on measurements of total bromine (Brtot) 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. Brtot is calculated from measured total organic bromine (Brorg) (i.e., the sum of bromine contained in CH3Br, the halons and the major very short-lived brominated substances) added to inorganic bromine (Bryinorg), evaluated from measured BrO and photochemical modelling. Combining these data, the weighted mean [Brtot] is 19.2 ± 1.2 ppt in the extratropical lower stratosphere (Ex-LS) of the northern hemisphere. The inferred average Brtot 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 Brtot in the Ex-LS due to influxes of shorter lived brominated source and product gases from different regions of entry. A closer look into Brorg and Bryinorg 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 Brtot 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 [Brtot] = 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 Brtot 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 (VSLBr) 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 VSLBr sources and chemistry. Model output of VSLBr, long-lived bromine and chlorine (LLBr and LLCl) 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 VSLBr as well as for LLCl 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 LLBr 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.
Biogenic very short-lived bromocarbons (VSLBr) currently represent ∼25 % of the total stratospheric bromine loading. Owing to their much shorter lifetime compared to anthropogenic long-lived bromine (e.g. halons) and chlorine (e.g. chlorofluorocarbons), the impact of VSLBr on ozone peaks in the lowermost stratosphere, which is a key climatic and radiative atmospheric region. Here we present a modelling study of the evolution of stratospheric ozone and its chemical loss within the tropics and at mid-latitudes during the 21st century. Two different experiments are explored: considering and neglecting the additional stratospheric injection of 5 ppt biogenic bromine naturally released from the ocean. Our analysis shows that the inclusion of VSLBr results in a realistic stratospheric bromine loading and improves the agreement between the model and satellite observations of the total ozone column (TOC) for the 1980–2015 period at mid-latitudes. We show that the overall ozone response to VSLBr at mid-latitudes follows the stratospheric evolution of long-lived inorganic chlorine and bromine throughout the 21st century. Moreover, the seasonal VSLBr impact on lowermost stratospheric ozone at mid-latitude is influenced by the seasonality of the heterogeneous inorganic-chlorine reactivation processes on ice crystals. Indeed, due to the more efficient reactivation of chlorine reservoirs (mainly ClONO2 and HCl) within the colder SH-ML lowermost stratosphere, the seasonal VSLBr impact shows a small but persistent hemispheric asymmetry through the whole modelled period. We conclude that the link between biogenic bromine sources and seasonal changes in heterogeneous chlorine reactivation is a key feature for future projections of mid-latitude lowermost stratospheric ozone during the 21st century.
How to cite: Barrera, J., Fernandez, R., Iglesias-Suarez, F., Cuevas, C. A., Lamarque, J.-F., and Saiz-Lopez, A.: Seasonal impact of biogenic very short-lived bromocarbons on lowermost stratospheric ozone between 60◦ N and 60◦ S during the 21st century, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14360, https://doi.org/10.5194/egusphere-egu21-14360, 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.
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.
There are distinct types of aerosol particles in the lower stratosphere. Stratospheric sulfuric acid particles with and without meteoric metals coexist with mixed organic-sulfate particles that originated in the troposphere. That these particles remain distinct has important implications for aerosol chemistry and the concentrations of several gas-phase species. Neither semi-volatile organics nor ammonia can be in equilibrium with the gas phase. The gas-phase concentrations of semi-volatile organics and ammonia must be very low, or else the sulfuric acid particles would not stay so pure. The upper concentration limits are around a pptv. Yet the sulfuric acid particles in the Northern Hemisphere show a very small but measurable uptake of organics and ammonia, indicating non-zero gas-phase concentrations of those species. Finally, the organic-sulfate particles must be resistant to photochemical loss, or else they would no longer retain their organic content.
How to cite: Murphy, D., Froyd, K., Schill, G., Brock, C., Kupc, A., and Williamson, C.: Distinct particle modes in the lower stratosphere constrain secondary aerosol chemistry and gas-phase concentrations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1365, https://doi.org/10.5194/egusphere-egu21-1365, 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.
Tropical cyclones (TCs) containing widespread and intense convection, play a dominant role in stratosphere-troposphere exchange (STE) processes in the upper troposphere and lower stratosphere (UTLS) region. Here we examine the variation of meteorological and chemical fields associated with two different pre-monsoon tropical cyclones: MORA and FANI, by combining satellite-based observations from AIRS (The Atmospheric Infrared Sounder ) and different model reanalysis datasets from ERA5 (fifth generation of ECMWF atmospheric reanalyses), CAMS (Copernicus Atmosphere Monitoring Service), MERRA-2 (The Modern-Era Retrospective analysis for Research and Applications, Version 2), and NCEP (National Centers for Environmental Prediction). An increase in the upper-tropospheric ozone (O3) by 15– 30 ppbv is observed over the Bay of Bengal during the high phase of MORA cyclone. Intrusion of O3 from lower stratosphere to upper troposphere is clearly observed from 50 to 300 hPa during the cyclonic period, contributing enhancement in the upper tropospheric O3. There are no such indication of enhanced O3 values before and after the dissipation of MORA cyclone. General behavior of intrusion associated with severe MORA cyclone is well captured by all the models and satellite, however some differences are seen in the intensity and structure of the STE events. Strong updrafts and downdrafts present in the vicinity of tropopause during TC passage weakened the stability of tropopause layer. The low tropopause temperature with enhanced potential vorticity (PV) feature extended vertically downward from lower stratosphere to troposphere confirms the stratosphere to tropospheric intrusion during the cyclonic period. Concurrently, low relative humidity (RH) along with negative RH-O3 correlation during the overhead cyclone further supports the intrusion. Contrarily, satellite and model results revealed no significant variation in O3 mixing ratio in the lower stratosphere down to the tropopause level during the high phase of extremely severe FANI cyclone. Strong convective activity during the passage of FANI confirms the upward propagation of CO rich (O3 poor) air masses from surface to the mid/upper troposphere. The air masses are then trapped by anticyclone around the tropopause levels. This study clearly reveals that tropical cyclones play major role in exchanges of mass and energy between the stratosphere and troposphere (and vice versa) besides being general weather phenomena.
How to cite: Chutia, L., Bhuyan, P., Pathak, B., and Bharali, C.: Investigating the Effect of Tropical Cyclones on Atmospheric Chemistry in the Upper Troposphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11136, https://doi.org/10.5194/egusphere-egu21-11136, 2021.
Chlorinated very short-lived substances (Cl-VSLS) are not controlled by the Montreal Protocol but the recent emission increase of the Cl-VSLS CH2Cl2 (Dichloromethane) and CHCl3 (Chloroform) is believed to significantly increase the stratospheric chlorine loading from VSLS. Provided efficient transport of Cl-VSLS from the source region into the stratosphere further emission increases could ultimately even cause a significant delay of the predicted recovery date of the ozone layer to pre-1980 values. During the WISE (Wave-driven ISentropic Exchange) campaign in autumn 2017 excessive probing of the UTLS (upper troposphere lower stratosphere) region above Western Europe and the Atlantic Ocean was conducted from aboard the HALO (High Altitude and Long range) research aircraft. We use real-time in situ WISE measurements of CH2Cl2 and CHCl3 from HAGAR-V (High Altitude Gas AnalyzeR – 5 channel version) in correlation with N2O from UMAQS (University of Mainz Airborne QCL Spectrometer), as well as CLaMS (Chemical Lagrangian Model of the Stratosphere) global 3-dimensional simulations of air mass origin tracers and backward trajectories to identify the most efficient transport mechanisms for Cl-VSLS entering the LS region in northern hemispheric summer.
The WISE measurements reveal two distinct transport pathways into the UTLS region of particularly CH2Cl2-rich and CH2Cl2-poor air. CH2Cl2-rich air could be identified to be transported by the Asian summer monsoon within about 4-10 weeks from its source regions in Asia into the stratosphere above the Atlantic Ocean at around 380 K and above. CH2Cl2-poor air could be identified to be mainly uplifted to potential temperatures of about 365 K by the North American monsoon above the region of Central America with transport times of only 2-5 weeks. In addition, we could link backward trajectories of CH2Cl2-poor air in the LS region to be uplifted by the category 5 hurricane Maria in September 2017. Based on all analyzed WISE measurements, we found that almost all young (transport time < 4 months) air masses were uplifted either above Asia or above Central America, emphasizing not only the impact of the Asian summer monsoon on the stratospheric tracer distribution but also that of the North American monsoon and hurricanes.
The measurements of both CH2Cl2 and CHCl3 show the lowest stratospheric mixing ratios originating in the region of Central America and enhanced mixing ratios from Asia (enhancements > 100 % and > 50 %, respectively). However, the source distribution of CHCl3 is much less clear than that of CH2Cl2 and inconspicuous CH2Cl2 measurements can also contain enhanced CHCl3 mixing ratios. Nevertheless, the anthropogenic impact on CHCl3 -rich air from Asia is clearly visible in the measurements and we believe it is likely that a future increase of Asian CHCl3 emissions could lead to similarly large stratospheric enhancements as already observed for CH2Cl2. Consequently, this would further increase ozone depletion from stratospheric chlorine deposition of VSLS.
How to cite: Lauther, V., Wintel, J., Gerhardt, E., Rau, A., Hoor, P., Kluschat, B., Vogel, B., Müller, R., and Volk, C. M.: Investigating the roles of the Asian monsoon, the North American monsoon, and Hurricanes for efficient transport of chlorinated short-lived species to the UTLS based on in situ observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6136, https://doi.org/10.5194/egusphere-egu21-6136, 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.
Air Transport has for a long time been linked to environmental issues like pollution, noise and climate change. Aviation emissions, such as carbon dioxide (CO2), water vapour (H2O), nitrogen oxides (NOx), soot and sulphate aerosols, alter the concentration of atmospheric Greenhouse gases and trigger the formation of contrails and cirrus clouds. The ClimOP collaboration, an Horizon 2020 project, aims to identify, evaluate and support the implementation of mitigation strategies to initiate and foster operational improvements which reduce the climate impact of the aviation sector. To this end, we present a study that assesses the likelihood of contrail formation as a function of key atmospheric variables, at different altitudes.
How to cite: Caria, G. and Dal Gesso, S.: An assessment of the likelihood of contrail formation , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15289, https://doi.org/10.5194/egusphere-egu21-15289, 2021.
STRATEOLE 2 is a French-American project based on superpressure balloon borne measurements to study dynamics and processes in the TTL and the lower stratosphere of equatorial regions. One single flight of these balloons (of a duration of about 80 days) can make several turns of the Earth.
Here we present water vapour measurements by the Pico-SDLA infrared laser spectrometer on-board the TTL 2 gondola. The float altitude was of about 19 km during the technical campaign of STRATEOLE 2, providing measurements at the top of the TTL or the lower stratosphere. In this presentation, we analyse the tape recorder signal at a constant altitude during the 80 days of flight. We compute an anomaly of the in situ water vapour measurements with respect to a regional/temporal satellite-borne mean climatology from Aura MLS. It allows to analyse the local measurements by Pico-SDLA with respect to what is expected at a given position and a given time. The obtained contrast allows the positioning of observations with respect to local climatology and therefore, the identification of singular events responsible for modulation of the local water vapour content. Our analysis shows that a long wet anomaly above the Pacific Ocean is explained by the balloon “surfing” on a warm perturbation of a Kelvin wave. Concurrently, a dry anomaly is put to the fore over the Indian Ocean, associated to a packet of gravity waves cold perturbations. The balloon has flown twice above the Maritime Continent. For each passage, a short scale succession of dry and wet anomalies is shown, indicating a possible influence of local deep convection. This influence is studied further using satellite borne cloud top data.
How to cite: Riviere, E., Ghysels, M., Durry, G., Burgalat, J., Amarouche, N., Carbone, S., Podglajen, A., and Capitaine, C.: Equatorial belt vapour measurements in the upper TTL under superpressure balloon during STRATEOLE 2 pre-campaign: tape recorder effect, role of waves and deep convection., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2832, https://doi.org/10.5194/egusphere-egu21-2832, 2021.
Water repartition in the stratosphere is a key compound in the atmospheric chemical and
radiative equilibrium. Since the 80’s, an increase of the water concentration in the
stratosphere has been observed.This presence in the stratosphere can be explained by the
slow ascent of air mass above convective clouds in tropical regions. The amount of water
vapor entering in the stratosphere depends on the coldest temperature and countered
during this slow ascent because it can lead to ice cristal formation that sediment and
dehydrate the air masses. But some other processes may contribute to the stratospheric
water budget, especially to explain the increase of water vapor. Stratospheric overshoots
phenomenon can take part in the stratospheric hydratation, by injecting directly water ice in
the stratosphere. Injected ice water, by sublimation, will hydrate stratosphere locally. The
local role of overshoots is better known but their contributions at the global scale steal need
to be quantified. In order to estimate this contribution, previous studies have used the 3D
simulation mesoscale model BRAMS to show overshoot impact in the upper Tropical
Tropopause Layer (TTL). These studies are the starting point of our study.
The aim of this paper is to present the new development inside BRAMS to nudge
stratospheric ice injection by overshoots. It uses an overshoot occurrence climatology from
MHS (Microwave Humidity Sounder) satellite measurement. Ice injection in the model is
made according to ice model categories previously shown to be present in the overshoot
plumes with ratios already diagnosed in previous studies. Ice injection is made between two
layers of TTL’s stratospheric part: between 380 and 385K and between 385 et 400K. Nudging
is triggered only if, in the grid mesh (20 x 20 km) where MHS has detected an overshoot,
BRAMS computes a cumulonimbus with a top above 13.5km. For the layer above 385 K
isentrope, a subgrid box of 2 km x 2 km is considered for the computation of ice injection.
Sensibility test of this nudging scheme will be presented in this presentation.
How to cite: Pichon, J., Riviere, E., Behera, A., and Burgalat, J.: Towards a nudging of stratospheric overshoots in the 3D mesoscale BRAMS model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1323, https://doi.org/10.5194/egusphere-egu21-1323, 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.
Observations show robust near-surface trends in Southern Hemisphere tropospheric circulation towards the end of the twentieth century, including a poleward shift in the mid-latitude jet, a positive trend in the Southern Annular Mode and an expansion of the Hadley cell. It has been established that these trends were driven by ozone depletion in the Antarctic stratosphere due to emissions of ozone-depleting substances. Here we show that these widely reported circulation trends paused, or slightly reversed, around the year 2000. Using a pattern-based detection and attribution analysis of atmospheric zonal wind, we show that the pause in circulation trends is forced by human activities, and has not occurred owing only to internal or natural variability of the climate system. Furthermore, we demonstrate that stratospheric ozone recovery, resulting from the Montreal Protocol, is the key driver of the pause. Because pre-2000 circulation trends have affected precipitation, and potentially ocean circulation and salinity, we anticipate that a pause in these trends will have wider impacts on the Earth system. Signatures of the effects of the Montreal Protocol and the associated stratospheric ozone recovery might therefore manifest, or have already manifested, in other aspects of the Earth system.
How to cite: Banerjee, A., Fyfe, J. C., Polvani, L. M., Waugh, D., and Chang, K.-L.: A pause in Southern Hemisphere circulation trends due to the Montreal Protocol, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-611, https://doi.org/10.5194/egusphere-egu21-611, 2021.
Future trends in isentropic mixing in the lower stratosphere remain largely unexplored, in contrast with the advective component of the Brewer-Dobson circulation. This study examines trends in effective diffusivity (κeff ), a measure of the potential of the flow to produce isentropic mixing, in recent chemistry-climate model simulations. The results highlight substantial reduction of κeff in the upper flanks of the subtropical jets from fall to spring, which are strengthened in response to greenhouse gas increases. This contrasts with stronger eddy transport, associated with increased wave drag in the region, peaking in summer near the critical lines. The projected ozone recovery leads to enhanced κeff in polar austral spring and summer, associated with a weaker and shorter-lived austral polar vortex by the end of the 21st century.
How to cite: Abalos, M. and de la Cámara, A.: Twenty-First Century Trends in Mixing Barriers and Eddy Transport in the Lower Stratosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2678, https://doi.org/10.5194/egusphere-egu21-2678, 2021.
The Brewer-Dobson circulation (BDC) is a key feature of the stratosphere that models need to accurately represent in order to improve the representation of surface climate variability. For the first time, the Climate Model Intercomparison Project includes in its phase 6 (CMIP6) a set of diagnostics that allow for careful evaluation of the BDC. Here, the BDC is evaluated against observations and reanalyses using historical simulations. CMIP6 results confirm the well-known inconsistency in BDC trends between observations and models in the middle and upper stratosphere. The increasing CO2 simulations feature a robust acceleration of the BDC but also reveal large uncertainties in the deep branch trends. The very close connection between the shallow branch and surface temperature is highlighted, which is absent in the deep branch. The trends in mean age of air are shown to be more robust throughout the stratosphere than those in the residual circulation.
How to cite: Calvo, N., Abalos, M., Benito-Barca, S., Garny, H., Hardiman, S., and Lin, P.: The Brewer-Dobson circulation in CMIP6, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15246, https://doi.org/10.5194/egusphere-egu21-15246, 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.
Inter-hemispheric transport may strongly affect the trace gas composition of the atmosphere, especially in relation to anthropogenic emissions which originate mainly in the Northern Hemisphere. This study investigates the transport from the boundary surface layer of the Northern Hemispheric (NH) extratropics (30-90oN), Southern Hemispheric (SH) extratropics (30-90oS), and tropics (30oS-30oN) into the global upper troposphere and lower stratosphere (UTLS) using simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS). In particular, we diagnose inter-hemispheric transport in terms of the air mass fractions (AMF), age spectra, and the mean age of air (AoA) calculated for these three source regions. We find that the AMFs from the NH extratropics to the UTLS are about five times larger than the corresponding contributions from the SH extratropics and almost twenty times smaller than those from the tropics. The amplitude of the AMF seasonal variability originating from the NH extratropics is comparable to that from the tropics. The NH and SH extratropics age spectra show much stronger seasonality compared to the seasonality of the tropical age spectra. The transit time of NH extratropical origin air to the SH extratropics is longer than vice versa. The asymmetry of the inter-hemispheric transport is mainly driven by the Asian summer monsoon (ASM). We confirm the important role of ASM and westerly ducts in the inter-hemispheric transport from the NH extratropics to the SH. However, we find that it is an interplay between the ASM and westerly ducts which triggers such cross-equator transport from boreal summer to fall, mainly westerly ducts over the eastern Atlantic.
How to cite: Yan, X., Konopka, P., Hauck, M., Podglajen, A., and Ploeger, F.: Asymmetry and pathways of inter-hemispheric transport in the upper troposphere and lower stratosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6842, https://doi.org/10.5194/egusphere-egu21-6842, 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 stratospheric residual circulation cannot be directly measured, hence various observable proxies have been used to indirectly quantify changes in the residual circulation on various timescales. However, the extent to which these proxies are successful in capturing the behaviour of the residual circulation is an open question. Here, we use an ensemble of Chemistry-Climate Model Initiative (CCMI) hindcast simulations from the Community Earth System Model version 1 Whole Atmosphere Community Climate Model (CESM1-WACCM) to compare observation-based proxies with direct measures of the residual circulation in a self-consistent manner. The three proxies studied are measures of the contrast in lower stratospheric temperatures between the tropics and poles, and ozone and water vapour concentrations in the tropical lower stratosphere. The temperature-based measure exhibits robust correlations with tropical lower stratospheric upwelling on interannual timescales, and a good year-round correlation (r = 0.73) between their monthly trends during the post-1998 ozone recovery era. We find that tropical mean ozone at 50 hPa has a maximum correlation with tropical upwelling at 70 hPa with a lag of 2 months. After accounting for this lag, ozone closely mirrors tropical upwelling variability on seasonal and interannual timescales as well as for long-term trends, especially for the ozone recovery period. On interannual timescales particularly, both the tropical mean ozone and temperature-based indices are strongly (anti-)correlated with tropical upwelling (r ~ 0.9), indicating these are suitable proxies for the residual circulation in CESM1-WACCM on this timescale. In terms of multi-year trends, tropical ozone shows the highest anti-correlation across months with tropical upwelling (r = -0.82) followed by the temperature-based index. The correlations of monthly trends are consistently smaller during the ozone depletion era (1979−1997) than during the era of ozone recovery (post 1998). The results indicate that both temperature and ozone based measures are suitable proxies for the residual circulation when tested in a self-consistent model framework.
How to cite: Chrysanthou, A., Maycock, A., Chipperfield, M., and Kinnison, D.: The utility of indirect measures of the lower stratospheric residual circulation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6384, https://doi.org/10.5194/egusphere-egu21-6384, 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.
The Madden-Julian oscillation (MJO) is a major source of intraseasonal variability in the tropical troposphere. It refers to a recurring pattern of strong convection, which travels from the Indian ocean over the Maritime Continent to the Pacific ocean with time scales of 30 to 90 days.
Although some studies have recently indicated that the occurrence of tropospheric MJO events could also affect stratospheric parameters, the MJO is not very much recognized as a source of stratospheric variability. This bears the risk of mixing it up with other sources of variability on this time scale, e.g., with signatures of the solar 27-day variations. Many of the studies that have found MJO signatures in the stratosphere are, however, based on either modelled or reanalyzed data. Particularly, we are not aware of any purely observational studies related to the temperature response in the middle atmosphere.
To fill this gap, we analyze the signature of the MJO in stratospheric temperatures measured by the Microwave Limb Sounder (MLS) satellite instrument aboard the Aura satellite. Analyzing the period from about 2004 to 2018, we indeed identify corresponding temperature variations in various altitudes and locations with many of them being significant according to Monte Carlo tests. The amplitudes of these signatures are on the order of 0.5 K. Moreover, basic characteristics of signatures, which have been identified in the preceding publications, are confirmed in this study based on purely observational data.
Hence, our study supports the coupling of parts of the stratospheric variability on the intraseasonal time scale to anomalous tropospheric convection represented by the MJO.
How to cite: Hoffmann, C., Buth, L., and von Savigny, C.: Signatures of the Madden-Julian Oscillation in Stratospheric Temperature from Aura MLS, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3032, https://doi.org/10.5194/egusphere-egu21-3032, 2021.
Strateole-2 is a project aimed at studying the coupling between the troposphere and the stratosphere in the deep tropics. The project originality pertains to the use of long-duration ballons, which can fly for several months at 18 or 20 km altitude. The first Strateole-2 campaign took place from November 2019 to February 2020: 8 balloons with various instrumental configurations were released in the lower stratosphere from Seychelles Islands, in the Indian Ocean.
This first campaign was primarily devoted to testing all systems (balloons, gondolas, and instruments) developed for the project, and was very successful: the balloons flew for 85 days onaverage over the whole tropical band, and most instruments performed nominally. In-situ meteorological measurements performed every 30-s on each flight provide a unique description of gravity-wave activity in the tropics and its relation to deep convection. The first observations of aerosols and water vapor onboard long-duration balloons were also achieved, which e.g. highlighted the tape recorder signal in the tropical lower stratosphere. Very innovative instruments also premiered during the campaign: RACHuTS, a light reeled payload, for instance performed 50 high-resolution vertical profiles of temperature, aerosols and water vapor down to 2km below the balloon, crossing several times the cold-point tropopause. ROC collected hundreds of temperature profiles down to the middle troposphere through GPS radio-occultations. Last, one balloon also carried a nadir-pointing backscatter lidar, which has described the underlying convection at unprecedented temporal resolution. An overview of the flights and first results will be presented.
Two forthcoming balloon campaigns are planned within Strateole-2, in 2021-22 and 2024-25. Each will release 20 balloons.
How to cite: Hertzog, A. and Plougonven, R. and the Stratéole-2: Strateole-2: High-resolution observations of the tropical tropopause layer with long-duration balloons, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7109, https://doi.org/10.5194/egusphere-egu21-7109, 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: Söder, 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.
We present measurements of ozone, water vapour and nitric acid in the upper troposphere/lower stratosphere (UTLS) over North Atlantic and Europe. The measurements were acquired with the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) during the Wave Driven Isentropic Exchange (WISE) campaign in October 2017. GLORIA is an airborne limb imager capable of acquiring both 2-D data sets (curtains along the flight path) and, when the carrier aircraft is flying around the observed air mass, spatially highly resolved 3-D tomographic data. We show a case study of a Rossby wave (RW) breaking event observed during two subsequent flights two days apart. RW breaking is known to steepen tracer gradients and facilitate stratosphere-troposphere exchange (STE). Our measurements reveal complex spatial structures in stratospheric tracers (ozone and nitric acid) with multiple vertically stacked filaments. Backward trajectory analysis is used to demonstrate that these features are related to several previous Rossby wave breaking events and that the small-scale structure of the UTLS in the Rossby wave breaking region, which is otherwise very hard to observe, can be understood as stirring and mixing of air masses of tropospheric and stratospheric origin. It is also shown that a strong nitric acid enhancement observed just above the tropopause is likely a result of NOx production by lightning activity. The measurements showed signatures of enhanced mixing between stratospheric and tropospheric air near the polar jet with some transport of water vapour into the stratosphere. Some of the air masses seen in 3-D data were encountered again two days later, stretched to very thin filament (horizontal thickness down to 30 km at some altitudes) rich in stratospheric tracers. This repeated measurement allowed us to directly observe and analyse the progress of mixing processes in a thin filament over two days. Our results provide direct insight into small-scale dynamics of the UTLS in the Rossby wave breaking region, witch is of great importance to understanding STE and poleward transport in the UTLS.
How to cite: Krasauskas, L., Ungermann, J., Preusse, P., Friedl-Vallon, F., Zahn, A., Ziereis, H., Rolf, C., Plöger, F., Konopka, P., Vogel, B., and Riese, M.: 3-D tomographic observations of Rossby wave breaking over the Northern Atlantic during the WISE aircraft campaign in 2017, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13777, https://doi.org/10.5194/egusphere-egu21-13777, 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.
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