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The calculation of the radiative transfer is a key component of global circulation models. In this article, we describe the most recent updates of the radiation infrastructure in the Modular Earth Submodel System (MESSy). These updates include the implementation of the PSrad radiation scheme within the RAD submodel. Furthermore, the radiation-related submodels CLOUDOPT (for the calculation of cloud optical properties) and AEROPT (for the calculation of aerosol optical properties) have been updated and are now more flexible in order to deal with different sets of shortwave and longwave bands of radiation schemes. In the wake of these updates, a new submodel (ALBEDO), which features solar-zenith-angle-dependent albedos and a new satellite-based background (white sky) albedo, was created. All of these developments are backward compatible, and previous features of the MESSy radiation infrastructure remain available. Moreover, these developments mark an important step in the use of the ECHAM/MESSy Atmospheric Chemistry (EMAC) model, as the update of the radiation scheme was a key aspect in the development of the sixth generation of the European Centre for Medium-Range Weather Forecasts – HAMburg (ECHAM6) model from ECHAM5. The developments presented here are also aimed towards using the MESSy infrastructure with the ICOsahedral Non-hydrostatic (ICON) model as a base model. The improved infrastructure will also aid in the implementation of additional radiation schemes once this should be needed. We have optimized the set of free parameters for two general circulation model-type (GCM-type) setups for pre-industrial and present-day conditions: one with the radiation scheme that was used to date (i.e. the radiation scheme of ECHAM5) and one with the newly implemented PSrad radiation scheme. After this parameter optimization, we performed four model simulations and evaluated the corresponding model results using reanalysis and observational data. The most apparent improvements related to the updated radiation scheme are the reduced cold biases in the tropical upper troposphere and lower stratosphere and the extratropical lower stratosphere and a strengthened polar vortex. The former is also related to improved stratospheric humidity and its variability if the new radiation scheme is employed. Using the multiple radiation call capability of MESSy, we have applied the two model configurations to calculate instantaneous and stratospheric-adjusted radiative forcings related to changes in greenhouse gases. Overall, we find that for many forcing experiments the simulations with the new radiation scheme show improved radiative forcing values. This is in particular the case for methane radiative forcings, which are considerably higher when assessed with the new radiation scheme and thus in better agreement with reference values. [ABSTRACT FROM AUTHOR]

The calculation of the radiative transfer is a key component of global circulation models. In this manuscript we describe the most recent updates of the radiation infrastructure in the Modular Earth Submodel System (MESSy). These updates include the implementation of the PSrad radiation scheme within the RAD submodel. Further, the radiation-related submodels 5 CLOUDOPT (for the calculation of cloud optical properties) and AEROPT (for the calculation of aerosol optical properties) have been updated and are now more flexible in order to deal with different sets of shortwave and longwave bands of radiation schemes. In the wake of these updates a new submodel (ALBEDO), which features solar zenith angle dependent albedos and a new satellite-based background (white-sky) albedo, was created. All of these developments are backward compatible and previous features of the MESSy radiation infrastructure remain available. Moreover, these developments mark an important 10 step in the use of the ECHAM/MESSy Atmospheric Chemistry (EMAC) model as the update of the radiation scheme was a key aspect in the development of sixth generation of the the European Centre for Medium-Range Weather Forecasts - HAMburg (ECHAM6) model from ECHAM5 and they also aim towards the use of MESSy with the ICOsahedral Non-hydrostatic (ICON) model. The improved infrastructure will also aid in the implementation of additional radiation schemes once this should be needed. We have optimized the set of free parameters for two dynamical model setups for pre-industrial and present-day conditions: one with the radiation scheme that was used up to date (i.e. the radiation scheme of ECHAM5) and one with the newly implemented PSrad radiation scheme. After this parameter optimization, we performed four model simulations and evaluated the corresponding model results using reanalysis and observational data. The most apparent improvements related to the updated 20 radiation scheme are the reduced cold biases in the tropical upper troposphere and lower stratosphere and the extratropical lower stratosphere, and a strengthened polar vortex. The former is also related to improved stratospheric humidity and its variability if the new radiation scheme is employed. Using the multiple radiation call capability of MESSy, we have applied the two model configurations to calculate instantaneous and stratospheric adjusted radiative forcings related to changes in greenhouse gases. Overall, we find that for many forcing experiments the simulations with the new radiation scheme show improved radiative forcing values. This is in particular the case for methane radiative forcings, which are considerably higher when asessed with the new radiation scheme and thus in better agreement with reference values. [ABSTRACT FROM AUTHOR]

Air masses within the Asian monsoon anticyclone (AMA) show anomalous signatures in various trace gases. In this study, we investigate how air masses are transported from the planetary boundary layer (PBL) to the AMA based on multiannual trajectory analyses. In particular, we focus on the climatological perspective and on the intraseasonal and interannual variability. Further, we also discuss the relation of the interannual east–west displacements of the AMA with the transport from the PBL to the AMA. To this end we employ backward trajectories, which were computed for 14 northern summer (June–August) seasons using reanalysis data. Further, we backtrack forward trajectories from a free-running chemistry–climate model (CCM) simulation, which includes parametrized Lagrangian convection. The analysis of 30 monsoon seasons of this additional model data set helps us to carve out robust or sensitive features of transport from the PBL to the AMA with respect to the employed model. Results from both the trajectory model and the Lagrangian CCM emphasize the robustness of the three-dimensional transport pathways from the top of the PBL to the AMA. Air masses are transported upwards on the south-eastern side of the AMA and subsequently recirculate within the full AMA domain, where they are lifted upwards on the eastern side and transported downwards on the western side of the AMA. The contributions of different PBL source regions to AMA air are robust across the two models for the Tibetan Plateau (TP; 17 % vs. 15 %) and the West Pacific (around 12 %). However, the contributions from the Indian subcontinent and Southeast Asia are considerably larger in the Lagrangian CCM data, which might indicate an important role of convective transport in PBL-to-AMA transport for these regions. The analysis of both model data sets highlights the interannual and intraseasonal variability of the PBL source regions of the AMA. Although there are differences in the transport pathways, the interannual east–west displacement of the AMA – which we find to be related to the monsoon Hadley index – is not connected to considerable differences in the overall transport characteristics. Our results from the trajectory model data reveal a strong intraseasonal signal in the transport from the PBL over the TP to the AMA: there is a weak contribution of TP air masses in early June (less than 4 % of the AMA air masses), whereas in August the contribution is considerable (roughly 24 %). The evolution of the contribution from the TP is consistent across the two modelling approaches and is related to the northward shift of the subtropical jet and the AMA during this period. This finding may help to reconcile previous results and further highlights the need of taking the subseasonal (and interannual) variability of the AMA and associated transport into account. [ABSTRACT FROM AUTHOR]

This study analyses the variability and trends of ultraviolet-B (UV-B, wavelength 280–320 nm) radiation that can cause DNA damage. The variability and trends caused by climate change due to enhanced greenhouse gas (GHG) concentrations. The analysis is based on DNA-active irradiance, total ozone, total cloud cover, and surface albedo calculations with the European Centre for Medium-Range Weather Forecasts – Hamburg (ECHAM)/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) chemistry–climate model (CCM) free-running simulations following the RCP 6.0 climate scenario for the period 1960–2100. The model output is evaluated with DNA-active irradiance ground-based measurements, satellite SBUV (v8.7) total-ozone measurements, and satellite MODerate-resolution Imaging Spectroradiometer (MODIS) Terra cloud cover data. The results show that the model reproduces the observed variability and change in total ozone, DNA-active irradiance, and cloud cover for the period 2000–2018 quite well according to the statistical comparisons. Between 50 ∘ N–50 ∘ S, the DNA-damaging UV radiation is expected to decrease until 2050 and to increase thereafter, as was shown previously by Eleftheratos et al. (2020). This change is associated with decreases in the model total cloud cover and negative trends in total ozone after about 2050 due to increasing GHGs. The new study confirms the previous work by adding more stations over low latitudes and mid-latitudes (13 instead of 5 stations). In addition, we include estimates from high-latitude stations with long-term measurements of UV irradiance (three stations in the northern high latitudes and four stations in the southern high latitudes greater than 55 ∘). In contrast to the predictions for 50 ∘ N–50 ∘ S, it is shown that DNA-active irradiance will continue to decrease after the year 2050 over high latitudes because of upward ozone trends. At latitudes poleward of 55 ∘ N, we estimate that DNA-active irradiance will decrease by 8.2%±3.8 % from 2050 to 2100. Similarly, at latitudes poleward of 55 ∘ S, DNA-active irradiance will decrease by 4.8 % ± 2.9 % after 2050. The results for the high latitudes refer to the summer period and not to the seasons when ozone depletion occurs, i.e. in late winter and spring. The contributions of ozone, cloud, and albedo trends to the DNA-active irradiance trends are estimated and discussed. [ABSTRACT FROM AUTHOR]

This study analyses the variability and trends of ultraviolet-B (UV-B, wavelength 280-320 nm) radiation that can cause DNA damage, which are caused by climate change due to enhanced greenhouse gas (GHG) concentrations. The analysis is based on DNA active irradiance, total ozone, total cloud cover, and surface albedo calculations with the EMAC Chemistry-Climate Model (CCM) free running simulations following the RCP-6.0 climate scenario for the period 1960-2100. The model output is evaluated with DNA active irradiance ground-based measurements, satellite SBUV (v8.7) total ozone measurements and satellite MODIS/Terra cloud cover data. The results show that the model reproduces the observed variability and change of total ozone, DNA active irradiance, and cloud cover for the period 2000-2018 quite well. Between 50° N-50° S, the DNA-damaging UV radiation is expected to decrease until 2050 and to increase thereafter, as it was shown previously by Eleftheratos et al. (2020). This change is associated with decreases in the model total cloud cover and insignificant trends in total ozone after about 2050. The new study confirms the previous work by adding more stations over low and mid-latitudes (13 instead of 5 stations). In addition, we include estimates from high latitude stations with long-term measurements of UV irradiance (2 stations in the northern high latitudes and 4 stations in the southern high latitudes greater than 55°). In contrast to the predictions for 50° N-50° S, it is shown that DNA active irradiance will continue to decrease after the year 2050 over high latitudes because of upward ozone trends. At latitudes poleward of 55° N, we estimate that DNA active irradiance will decrease by 10.6 ± 3.7 % from 2050 to 2100. Similarly, at latitudes poleward of 55° S, DNA active irradiance will decrease by 4.8 ± 2.9 % after 2050. The results for the high latitudes refer to the summer period and not to the seasons when ozone depletion occurs, i.e., in late winter and spring. The contributions of ozone, cloud and albedo trends on the DNA active irradiance trends are estimated and discussed. [ABSTRACT FROM AUTHOR]

Air masses within the Asian monsoon anticyclone (AMA) show anomalous signatures in various trace gases. In this study, we analyze how air masses are transported from the planetary boundary layer (PBL) to the AMA via multiannual trajectory anlyses. While previous studies analyzed the PBL to AMA transport mainly for individual monsoon seasons or particular periods, we focus on the climatological perspective and on the interannual and intraseasonal variability. To this end we employ backward trajectories, which were computed using reanalysis data. Based on these trajectories, we analyze air mass transport from the PBL to the AMA during northern summer (June–August) for 14 summer seasons. Further, we backtrack forward trajectories from a free-running chemistry-climate model (CCM) simulation, which includes parametrized Lagrangian convection. The analysis of this additional model data set helps us to carve out robust or sensitive features of PBL to AMA transport with respect to the employed model. Results from both the trajectory model and the Lagrangian CCM emphasize the robustness of the three-dimensional transport pathways from the PBL to the AMA. Air masses are transported upwards on the eastern side of the AMA and are uplifted within the full AMA domain above. While this is in agreement with previous modelling studies, we refine the picture of the so-called "conduit" (Bergman et al., 2013). The contributions from the Tibetan Plateau (TP; 17% vs. 15%) and the West Pacific (around 12%) are similar in both model results. However, the contributions from the Indian subcontinent and South-East Asia are considerably larger in the Lagrangian CCM data, which might point towards the importance of convective transport for PBL to AMA transport for these regions. The analysis of both model data sets highlights the interannual and intraseasonal variability with respect to PBL source regions of the AMA. Additionally, we analyze the relation of the interannual east–west displacement of the AMA - which we find to be related to the monsoon Hadley index - to the transport behaviour and find that there are differences for "east" and "west years", the main transport characteristics, however, are comparable. Regarding the intraseasonal variability our trajectory model results show that transport from the PBL over the Tibetan Plateau (TP) to the AMA is weak in early June (less than 4% of the AMA air masses), whereas in August TP air masses contribute considerably (roughly 24%). The evolution of the contribution from the TP is supported by data from the Lagrangian CCM and is related to the northward shift of the subtropical jet and the AMA during this period. This result may help to reconcile previous results and further highlights the need of taking the subseasonal (and interannual) variability of the AMA and associated transport into account. [ABSTRACT FROM AUTHOR]

In a previous study the quasi-instantaneous chemical impacts (rapid adjustments) of strongly enhanced methane (CH 4) mixing ratios have been analysed. However, to quantify the influence of the respective slow climate feedbacks on the chemical composition it is necessary to include the radiation-driven temperature feedback. Therefore, we perform sensitivity simulations with doubled and quintupled present-day (year 2010) CH 4 mixing ratios with the chemistry–climate model EMAC (European Centre for Medium-Range Weather Forecasts, Hamburg version – Modular Earth Submodel System (ECHAM/MESSy) Atmospheric Chemistry) and include in a novel set-up a mixed-layer ocean model to account for tropospheric warming. Strong increases in CH 4 lead to a reduction in the hydroxyl radical in the troposphere, thereby extending the CH 4 lifetime. Slow climate feedbacks counteract this reduction in the hydroxyl radical through increases in tropospheric water vapour and ozone, thereby dampening the extension of CH 4 lifetime in comparison with the quasi-instantaneous response. Changes in the stratospheric circulation evolve clearly with the warming of the troposphere. The Brewer–Dobson circulation strengthens, affecting the response of trace gases, such as ozone, water vapour and CH 4 in the stratosphere, and also causing stratospheric temperature changes. In the middle and upper stratosphere, the increase in stratospheric water vapour is reduced with respect to the quasi-instantaneous response. We find that this difference cannot be explained by the response of the cold point and the associated water vapour entry values but by a weaker strengthening of the in situ source of water vapour through CH 4 oxidation. However, in the lower stratosphere water vapour increases more strongly when tropospheric warming is accounted for, enlarging its overall radiative impact. The response of the stratosphere adjusted temperatures driven by slow climate feedbacks is dominated by these increases in stratospheric water vapour as well as strongly decreased ozone mixing ratios above the tropical tropopause, which result from enhanced tropical upwelling. While rapid radiative adjustments from ozone and stratospheric water vapour make an essential contribution to the effective CH 4 radiative forcing, the radiative impact of the respective slow feedbacks is rather moderate. In line with this, the climate sensitivity from CH 4 changes in this chemistry–climate model set-up is not significantly different from the climate sensitivity in carbon-dioxide-driven simulations, provided that the CH 4 effective radiative forcing includes the rapid adjustments from ozone and stratospheric water vapour changes. [ABSTRACT FROM AUTHOR]

Ozone data derived from the Tropospheric Monitoring Instrument (TROPOMI) sensor on board the Sentinel-5 Precursor satellite show exceptionally low total ozone columns in the polar region of the Northern Hemisphere (Arctic) in spring 2020. Minimum total ozone column values around or below 220 Dobson units (DU) were seen over the Arctic for 5 weeks in March and early April 2020. Usually the persistence of such low total ozone column values in spring is only observed in the polar Southern Hemisphere (Antarctic) and not over the Arctic. These record low total ozone columns were caused by a particularly strong polar vortex in the stratosphere with a persistent cold stratosphere at higher latitudes, a prerequisite for ozone depletion through heterogeneous chemistry. Based on the ERA5, which is the fifth generation of the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis, the Northern Hemisphere winter 2019/2020 (from December to March) showed minimum polar cap temperatures consistently below 195 K around 20 km altitude, which enabled enhanced formation of polar stratospheric clouds. The special situation in spring 2020 is compared and discussed in context with two other Northern Hemisphere spring seasons, namely those in 1997 and 2011, which also displayed relatively low total ozone column values. However, during these years, total ozone columns below 220 DU over several consecutive days were not observed in spring. The similarities and differences of the atmospheric conditions of these three events and possible explanations for the observed features are presented and discussed. It becomes apparent that the monthly mean of the minimum total ozone column value for March 2020 (221 DU) was clearly below the respective values found in March 1997 (267 DU) and 2011 (252 DU), which highlights the special evolution of the polar stratospheric ozone layer in the Northern Hemisphere in spring 2020. A comparison with a typical ozone hole over the Antarctic (e.g., in 2016) indicates that although the Arctic spring 2020 situation is remarkable, with total ozone column values around or below 220 DU observed over a considerable area (up to 0.9 million km 2), the Antarctic ozone hole shows total ozone columns typically below 150 DU over a much larger area (of the order of 20 million km 2). Furthermore, total ozone columns below 220 DU are typically observed over the Antarctic for about 4 months. [ABSTRACT FROM AUTHOR]

Ozone data derived from the TROPOMI sensor onboard the Sentinel-5 Precursor satellite are showing an atypical ozone hole feature in the polar region of the Northern hemisphere (Arctic) in spring 2020. A persistent ozone hole pattern with minimum total ozone column values around or below 220 Dobson units (DU) was seen for the first time over the Arctic for about 5 weeks in March and early April 2020. Usually an ozone hole with such low total ozone column values has only been observed in the polar Southern hemisphere (Antarctic) in spring over the last 4 decades, but not over the Arctic. The ozone hole pattern was caused by a particularly stable polar vortex in the stratosphere, enabling a persistent cold stratosphere at higher latitudes, a prerequisite for ozone depletion through heterogeneous chemistry. Based on the ERA5 reanalysis from ECMWF, the Northern winter 2019/2020 (from December to March) showed minimum polar cap temperatures consistently below 195 K around 20 km altitude, which enabled enhanced formation of polar stratospheric clouds. The special situation in spring 2020 is compared and discussed in context with two other ozone hole-like features in spring 1997 and 2011 that were showing comparable dynamical conditions in the stratosphere in combination with low total ozone column values. However, during these years total ozone columns below 220 DU over larger areas and over several consecutive days have not been observed. The similarities and differences of the atmospheric conditions of these three events and possible explanations are presented and discussed. It becomes apparent that the monthly mean of the minimum total ozone column value for March 2020 (i.e. 221 DU) was clearly below the respective values found in March 1997 (i.e. 267 DU) and 2011 (i.e. 252 DU), which emphasizes the noteworthiness of the evolution of the polar stratospheric ozone layer in Northern hemisphere spring 2020. These results provide a first description and classification of the development of the Arctic ozone hole in boreal spring 2020 and highlight its peculiarity. [ABSTRACT FROM AUTHOR]

One of the key questions in the air quality and climate sciences is how tropospheric ozone concentrations will change in the future. This will depend on two factors: changes in stratosphere-to-troposphere transport (STT) and changes in tropospheric chemistry. Here we aim to identify robust changes in STT using simulations from the Chemistry Climate Model Initiative (CCMI) under a common climate change scenario (RCP6.0). We use two idealized stratospheric tracers to isolate changes in transport: stratospheric ozone (O3S), which is exactly like ozone but has no chemical sources in the troposphere, and st80, a passive tracer with fixed volume mixing ratio in the stratosphere. We find a robust increase in the tropospheric columns of these two tracers across the models. In particular, stratospheric ozone in the troposphere is projected to increase 10 %–16 % by the end of the 21st century in the RCP6.0 scenario. Future STT is enhanced in the subtropics due to the strengthening of the shallow branch of the Brewer–Dobson circulation (BDC) in the lower stratosphere and of the upper part of the Hadley cell in the upper troposphere. The acceleration of the deep branch of the BDC in the Northern Hemisphere (NH) and changes in eddy transport contribute to increased STT at high latitudes. These STT trends are caused by greenhouse gas (GHG) increases, while phasing out of ozone-depleting substances (ODS) does not lead to robust transport changes. Nevertheless, the decline of ODS increases the reservoir of ozone in the lower stratosphere, which results in enhanced STT of O3S at middle and high latitudes. A higher emission scenario (RCP8.5) produces stronger STT trends, with increases in tropospheric column O3S more than 3 times larger than those in the RCP6.0 scenario by the end of the 21st century. [ABSTRACT FROM AUTHOR]

This numerical model study is motivated by the observed global deviation from assumed emissions of chlorofluorocarbon-11 (CFC-11, CFCl3) in recent years. Montzka et al. (2018) discussed a strong deviation of the assumed emissions of CFC-11 over the past 15 years, which indicates a violation of the Montreal Protocol for the protection of the ozone layer. An investigation is performed which is based on chemistry–climate model (CCM) simulations that analyze the consequences of an enhanced CFC-11 surface mixing ratio. In comparison to a reference simulation (REF-C2), where a decrease of the CFC-11 surface mixing ratio of about 50 % is assumed from the early 2000s to the middle of the century (i.e., a mixing ratio in full compliance with the Montreal Protocol agreement), two sensitivity simulations are carried out. In the first simulation the CFC-11 surface mixing ratio is kept constant after the year 2002 until 2050 (SEN-C2-fCFC11_2050); this allows a qualitative estimate of possible consequences of a high-level stable CFC-11 surface mixing ratio on the ozone layer. In the second sensitivity simulation, which is branched off from the first sensitivity simulation, it is assumed that the Montreal Protocol is fully implemented again starting in the year 2020, which leads to a delayed decrease of CFC-11 in this simulation (SEN-C2-fCFC11_2020) compared with the reference simulation; this enables a rough and most likely upper-limit assessment of how much the unexpected CFC-11 emissions to date have already affected ozone. In all three simulations, the climate evolves under the same greenhouse gas scenario (i.e., RCP6.0) and all other ozone-depleting substances decline (according to this scenario). Differences between the reference (REF-C2) and the two sensitivity simulations (SEN-C2-fCFC11_2050 and SEN-C2-fCFC11_2020) are discussed. In the SEN-C2-fCFC11_2050 simulation, the total column ozone (TCO) in the 2040s (i.e., the years 2041–2050) is particularly affected in both polar regions in winter and spring. Maximum discrepancies in the TCO values are identified with reduced ozone values of up to around 30 Dobson units in the Southern Hemisphere (SH) polar region during SH spring (in the order of 15 %). An analysis of the respective partial column ozone (PCO) for the stratosphere indicates that the strongest ozone changes are calculated for the polar lower stratosphere, where they are mainly driven by the enhanced stratospheric chlorine content and associated heterogeneous chemical processes. Furthermore, it was found that the calculated ozone changes, especially in the upper stratosphere, are surprisingly small. For the first time in such a scenario, we perform a complete ozone budget analysis regarding the production and loss cycles. In the upper stratosphere, the budget analysis shows that the additional ozone depletion due to the catalysis by reactive chlorine is partly compensated for by other processes related to enhanced ozone production or reduced ozone loss, for instance from nitrous oxide (NOx). Based on the analysis of the SEN-C2-fCFC11_2020 simulation, it was found that no major ozone changes can be expected after the year 2050, and that these changes are related to the enhanced CFC-11 emissions in recent years. [ABSTRACT FROM AUTHOR]

One of the key questions in the air quality and climate sciences is how will tropospheric ozone concentrations change in the future. This will depend on two factors: changes in stratosphere-to-troposphere transport (STT) and changes in tropospheric chemistry. Here we aim to identify robust changes in STT using simulations from the Chemistry Climate Model Initiative (CCMI) under a common climate change scenario (RCP6.0). We use two idealized stratospheric tracers to isolate changes in transport: stratospheric ozone (O3S), which is exactly like ozone but has no chemical sources in the troposphere, and st80, a passive tracer with fixed volume mixing ratio in the stratosphere. We find a robust increase in the tropospheric columns of these two tracers across the models. In particular, stratospheric ozone in the troposphere is projected to increase 10–16 % by the end of the 21st century in the RCP6.0 scenario. Future STT is enhanced in the subtropics due to the strengthening of the shallow branch of the Brewer-Dobson circulation (BDC) in the lower stratosphere and of the upper part of the Hadley cell in the upper troposphere. The acceleration of the deep branch of the BDC and changes in eddy transport contribute to increase STT at high latitudes. The idealized tracer st80 shows that these STT changes are dominated by greenhouse gas (GHG) increases, while phasing out of ozone depleting substances (ODS) does not lead to robust STT changes. Nevertheless, the increase of O3S concentrations in the troposphere is attributed to GHG only in the subtropics. At middle and high latitudes it is due to stratospheric ozone recovery linked to ODS decline. A higher emission scenario (RCP8.5) produces qualitatively similar but stronger STT trends, with changes in tropospheric column O3S more than three times larger than those in the RCP6.0 scenario by the end of the 21st century. [ABSTRACT FROM AUTHOR]

Methane (CH4) is the second-most important directly emitted greenhouse gas, the atmospheric concentration of which is influenced by human activities. In this study, numerical simulations with the chemistry–climate model (CCM) EMAC are performed, aiming to assess possible consequences of significantly enhanced CH4 concentrations in the Earth's atmosphere for the climate. We analyse experiments with 2×CH4 and 5×CH4 present-day (2010) mixing ratio and its quasi-instantaneous chemical impact on the atmosphere. The massive increase in CH4 strongly influences the tropospheric chemistry by reducing the OH abundance and thereby extending the CH4 lifetime as well as the residence time of other chemical substances. The region above the tropopause is impacted by a substantial rise in stratospheric water vapour (SWV). The stratospheric ozone (O3) column increases overall, but SWV-induced stratospheric cooling also leads to a enhanced ozone depletion in the Antarctic lower stratosphere. Regional patterns of ozone change are affected by modification of stratospheric dynamics, i.e. increased tropical upwelling and stronger meridional transport towards the polar regions. We calculate the net radiative impact (RI) of the 2×CH4 experiment to be 0.69 W m -2 , and for the 5×CH4 experiment to be 1.79 W m -2. A substantial part of the RH is contributed by chemically induced O3 and SWV changes, in line with previous radiative forcing estimates. To our knowledge this is the first numerical study using a CCM with respect to 2- and 5-fold CH4 concentrations and it is therefore an overdue analysis as it emphasizes the impact of possible strong future CH4 emissions on atmospheric chemistry and its feedback on climate. [ABSTRACT FROM AUTHOR]

This investigation is motivated by the results presented by Montzka et al. (2018). They discussed a strong deviation of the assumed emissions of chlorofluorocarbon-11 (CFC-11, CFCl3) in the past 15 years, which indicates a violation of the Montreal Protocol for the protection of the ozone layer. A Chemistry-Climate Model (CCM) study is performed, investigating the consequences of a constant CFC-11 surface mixing ratio for stratospheric ozone: In comparison to a reference simulation (REF-C2), where a decrease of the CFC-11 surface mixing ratio of about 50 % is assumed from the early 2000s to the middle of the century, a sensitivity simulation (SEN-C2-fCFC11) is carried out where after the year 2002 the CFC-11 surface mixing ratio is kept constant until 2050. Differences between these two simulations are shown. These illustrate possible effects on stratospheric ozone. The total column ozone (TCO) in the 2040s (i.e. the years 2041–2050) is in particular affected in both polar regions in winter and spring. At the end of the 2040s maximum discrepancies of TCO are identified with reduced ozone values of up to around 30 Dobson Units (in the order of 10 %) in the SEN simulation. An analysis of the respective partial column ozone (PCO) for the stratosphere indicates that strongest ozone changes are calculated for the polar lower stratosphere, where they are mainly driven by the enhanced stratospheric chlorine content and associated heterogeneous chemical processes. Furthermore, it turns out that the calculated ozone changes, especially in the upper stratosphere, are smaller than expected. In this altitude region the additional ozone depletion due to the catalysis by reactive chlorine is compensated partly by other processes related to enhanced ozone production or reduced ozone loss, for instance from nitrous oxide (NOx). [ABSTRACT FROM AUTHOR]

Methane (CH4) is the second most important greenhouse gas, which atmospheric concentration is influenced by human activities. In this study, numerical simulations with a chemistry-climate model (CCM) are performed aiming to assess possible consequences of significantly enhanced CH4 concentrations in the Earth's atmosphere for the climate. We analyze experiments with 2xCH4 and 5xCH4 present day (2010) mixing ratio and its quasi-instantaneous chemical impact on the atmosphere. The massive increase in CH4 strongly influences the tropospheric chemistry by reducing the hydroxyl radical (OH) abundance and thereby extending the CH4 lifetime as well as the residence time of other chemical pollutants. The region above the tropopause is impacted by a substantial rise in stratospheric water vapor (SWV). The stratospheric ozone (O3) column increases overall, but SWV induced stratospheric cooling also leads to a enhanced ozone depletion in the Antarctic lower stratosphere. Regional patterns of ozone change are affected by modification of stratospheric dynamics, i.e. increased tropical up-welling and stronger meridional transport towards the polar regions. We calculate the net radiative impact (RI) of the 2xCH4 experiment to be 0.69W/m2 and for the 5xCH4 experiment to be 1.79W/m2. A substantial part of the RI is contributed by chemically induced O3 and SWV changes, in line with previous radiative forcing estimates. To our knowledge this is the first numerical study using a CCM with respect to two/fivefold CH4 concentrations and it is therefore an overdue analysis as it emphasizes the impact of possible strong future CH4 emissions on atmospheric chemistry and its feedback on climate. [ABSTRACT FROM AUTHOR]

An updated and improved version of a global, vertically resolved, monthly mean zonal mean ozone database has been calculated - hereafter referred to as the BSVertOzone (Bodeker Scientific Vertical Ozone) database. Like its predecessor, it combines measurements from several satellite-based instruments and ozone profile measurements from the global ozonesonde network. Monthly mean zonal mean ozone concentrations in mixing ratio and number density are provided in 5° latitude bins, spanning 70 altitude levels (1 to 70 km), or 70 pressure levels that are approximately 1 km apart (878.4 to 0.046 hPa). Different data sets or "tiers" are provided: Tier 0 is based only on the available measurements and therefore does not completely cover the whole globe or the full vertical range uniformly; the Tier 0.5 monthly mean zonal means are calculated as a filled version of the Tier 0 database where missing monthly mean zonal mean values are estimated from correlations against a total column ozone (TCO) database. The Tier 0.5 data set includes the full range of measurement variability and is created as an intermediate step for the calculation of the Tier 1 data where a least squares regression model is used to attribute variability to various known forcing factors for ozone. Regression model fit coefficients are expanded in Fourier series and Legendre polynomials (to account for seasonality and latitudinal structure, respectively). Four different combinations of contributions from selected regression model basis functions result in four different Tier 1 data sets that can be used for comparisons with chemistry-climate model (CCM) simulations that do not exhibit the same unforced variability as reality (unless they are nudged towards reanalyses). Compared to previous versions of the database, this update includes additional satellite data sources and ozonesonde measurements to extend the database period to 2016. Additional improvements over the previous version of the database include the following: (i) adjustments of measurements to account for biases and drifts between different data sources (using a chemistry-transport model, CTM, simulation as a transfer standard), (ii) a more objective way to determine the optimum number of Fourier and Legendre expansions for the basis function fit coefficients, and (iii) the derivation of methodological and measurement uncertainties on each database value are traced through all data modification steps. Comparisons with the ozone database from SWOOSH (Stratospheric Water and OzOne Satellite Homogenized data set) show good agreement in many regions of the globe. Minor differences are caused by different bias adjustment procedures for the two databases. [ABSTRACT FROM AUTHOR]

Major mid-winter stratospheric sudden warmings (SSWs) are the largest instance of wintertime variability in the Arctic stratosphere. Because SSWs are able to cause significant surface weather anomalies on intra-seasonal timescales, several previous studies have focused on their potential future change, as might be induced by anthropogenic forcings. However, a wide range of results have been reported, from a future increase in the frequency of SSWs to an actual decrease. Several factors might explain these contradictory results, notably the use of different metrics for the identification of SSWs and the impact of large climatological biases in single-model studies. To bring some clarity, we here revisit the question of future SSW changes, using an identical set of metrics applied consistently across 12 different models participating in the Chemistry--Climate Model Initiative. Our analysis reveals that no statistically significant change in the frequency of SSWs will occur over the 21st century, irrespective of the metric used for the identification of the event. Changes in other SSW characteristics -- such as their duration, deceleration of the polar night jet, and the tropospheric forcing -- are also assessed: again, we find no evidence of future changes over the 21st century. [ABSTRACT FROM AUTHOR]

An important driver of climate change is stratospheric water vapor (SWV), which in turn is influenced by the oxidation of atmospheric methane (CH4). In order to parameterize the production of water vapor (H2O) from CH4 oxidation, it is often assumed that the oxidation of one CH4 molecule yields exactly two molecules of H2O. However, this assumption is based on an early study, which also gives evidence that this is not true at all altitudes. In the current study, we re-evaluate this assumption with a comprehensive systematic analysis using a state-of-the-art chemistry-climate model (CCM), namely the ECHAM/MESSy Atmospheric Chemistry (EMAC) model, and present three approaches to investigate the yield of H2O and hydrogen gas (H2) from CH4 oxidation. We thereby make use of the Module Efficiently Calculating the Chemistry of the Atmosphere (MECCA) in a box model and global model configuration. Furthermore, we use the kinetic chemistry tagging technique (MECCA-TAG) to investigate the chemical pathways between CH4, H2O and H2, by being able to distinguish hydrogen atoms produced by CH4 from H2 from other sources. We apply three approaches, which all agree that assuming a yield of 2 overestimates the production of H2O in the lower stratosphere (calculated as 1.5-1.7). Additionally, transport and subsequent photochemical processing of longer-lived intermediates (mostly H2) raise the local yield values in the upper stratosphere and lower mesosphere above 2 (maximum > 2.2). In the middle and upper mesosphere, the influence of loss and recycling of H2O increases, making it a crucial factor in the parameterization of the yield of H2O from CH4 oxidation. An additional sensitivity study with the Chemistry As A Boxmodel Application (CAABA) shows a dependence of the yield on the hydroxyl radical (OH) abundance. No significant temperature dependence is found. We focus representatively on the tropical zone between 23°S and 23°N. It is found in the global approach that presented results are mostly valid for midlatitudes as well. During the polar night, the method is not applicable. Our conclusions question the use of a constant yield of H2O from CH4 oxidation in climate modeling and encourage to apply comprehensive parameterizations that follow the vertical profiles of the H2O yield derived here and take the chemical H2O loss into account. [ABSTRACT FROM AUTHOR]

We analyse simulations performed for the Chemistry-Climate Model Initiative (CCMI) to estimate the return dates of the stratospheric ozone layer from depletion caused by anthropogenic stratospheric chlorine and bromine. We consider a total of 155 simulations from 20 models, including a range of sensitivity studies which examine the impact of climate change on ozone recovery. For the control simulations (unconstrained by nudging towards analysed meteorology) there is a large spread (±20 DU in the global average) in the predictions of the absolute ozone column. Therefore, the model results need to be adjusted for biases against historical data. Also, the interannual variability in the model results need to be smoothed in order to provide a reasonably narrow estimate of the range of ozone return dates. Consistent with previous studies, but here for a Representative Concentration Pathway (RCP) of 6.0, these new CCMI simulations project that global total column ozone will return to 1980 values in 2049 (with a 1σ uncertainty of 2043–2055). At Southern Hemisphere mid-latitudes column ozone is projected to return to 1980 values in 2045 (2039–2050), and at Northern Hemisphere mid-latitudes in 2032 (2020–2044). In the polar regions, the return dates are 2060 (2055–2066) in the Antarctic in October and 2034 (2025–2043) in the Arctic in March. The earlier return dates in the Northern Hemisphere reflect the larger sensitivity to dynamical changes. Our estimates of return dates are later than those presented in the 2014 Ozone Assessment by approximately 5–17 years, depending on the region, with the previous best estimates often falling outside of our uncertainty range. In the tropics only around half the models predict a return of ozone to 1980 values, around 2040, while the other half do not reach the 1980 value. All models show a negative trend in tropical total column ozone towards the end of the 21st century. The CCMI models generally agree in their simulation of the time evolution of stratospheric chlorine and bromine, which are the main drivers of ozone loss and recovery. However, there are a few outliers which show that the multi-model mean results for ozone recovery are not as tightly constrained as possible. Throughout the stratosphere the spread of ozone return dates to 1980 values between models tends to correlate with the spread of the return of inorganic chlorine to 1980 values. In the upper stratosphere, greenhouse gas-induced cooling speeds up the return by about 10–20 years. In the lower stratosphere, and for the column, there is a more direct link in the timing of the return dates of ozone and chlorine, especially for the large Antarctic depletion. Comparisons of total column ozone between the models is affected by different predictions of the evolution of tropospheric ozone within the same scenario, presumably due to differing treatment of tropospheric chemistry. Therefore, for many scenarios, clear conclusions can only be drawn for stratospheric ozone columns rather than the total column. As noted by previous studies, the timing of ozone recovery is affected by the evolution of N2O and CH4. However, quantifying the effect in the simulations analysed here is limited by the few realisations available for these experiments compared to internal model variability. The large increase in N2O given in RCP 6.0 extends the ozone return globally by ∼ 15 years relative to N2O fixed at 1960 abundances, mainly because it allows tropical column ozone to be depleted. The effect in extratropical latitudes is much smaller. The large increase in CH4 given in the RCP 8.5 scenario compared to RCP 6.0 also lengthens ozone return by ∼ 15 years, again mainly through its impact in the tropics. Overall, our estimates of ozone return dates are uncertain due to both uncertainties in future scenarios, in particular those of greenhouse gases, and uncertainties in models. The scenario uncertainty is small in the short term but increases with time, and becomes large by the end of the century. There are still some model–model differences related to well-known processes which affect ozone recovery. Efforts need to continue to ensure that models used for assessment purposes accurately represent stratospheric chemistry and the prescribed scenarios of ozone-depleting substances, and only those models are used to calculate return dates. For future assessments of single forcing or combined effects of CO2, CH4, and N2O on the stratospheric column ozone return dates, this work suggests that it is more important to have multi-member (at least three) ensembles for each scenario from every established participating model, rather than a large number of individual models. [ABSTRACT FROM AUTHOR]

Trend estimates with different signs are reported in the literature for lower stratospheric water vapour considering the time period between the late 1980s and 2010. The NOAA (National Oceanic and Atmospheric Administration) frost point hygrometer (FPH) observations at Boulder (Colorado, 40.0° N, 105.2° W) indicate positive trends (about 0.1 to 0.45 ppmvdecade-1). On the contrary, negative trends (approximately -0.2 to -0.1 ppmvdecade-1) are derived from a merged zonal mean satellite data set for a latitude band around the Boulder latitude. Overall, the trend differences between the two data sets range from about 0.3 to 0.5 ppmvdecade-1, depending on altitude. It has been proposed that a possible explanation for these discrepancies is a different temporal behaviour at Boulder and the zonal mean. In this work we investigate trend differences between Boulder and the zonal mean using primarily simulations from ECHAM/MESSy (European Centre for Medium-Range Weather Forecasts Hamburg/Modular Earth Submodel System) Atmospheric Chemistry (EMAC), WACCM (Whole Atmosphere Community Climate Model), CMAM (Canadian Middle Atmosphere Model) and CLaMS (Chemical Lagrangian Model of the Stratosphere). On shorter timescales we address this aspect also based on satellite observations from UARS/HALOE (Upper Atmosphere Research Satellite/ Halogen Occultation Experiment), Envisat/MIPAS (Environmental Satellite/Michelson Interferometer for Passive Atmospheric Sounding) and Aura/MLS (Microwave Limb Sounder). Overall, both the simulations and observations exhibit trend differences between Boulder and the zonal mean. The differences are dependent on altitude and the time period considered. The model simulations indicate only small trend differences between Boulder and the zonal mean for the time period between the late 1980s and 2010. These are clearly not sufficient to explain the discrepancies between the trend estimates derived from the FPH observations and the merged zonal mean satellite data set. Unless the simulations underrepresent variability or the trend differences originate from smaller spatial and temporal scales than resolved by the model simulations, trends at Boulder for this time period should also be quite representative for the zonal mean and even other latitude bands. Trend differences for a decade of data are larger and need to be kept in mind when comparing results for Boulder and the zonal mean on this timescale. Beyond that, we find that the trend estimates for the time period between the late 1980s and 2010 also significantly differ among the simulations. They are larger than those derived from the merged satellite data set and smaller than the trend estimates derived from the FPH observations. [ABSTRACT FROM AUTHOR]

Trend estimates with different signs are reported in the literature for lower stratospheric water vapour considering the time period between the late 1980s and 2010. The NOAA (National Oceanic and Atmospheric Administration) frost point hygrometer (FPH) observations at Boulder (Colorado, 40.0° N, 105.2° W) indicate positive trends (about 0.12 ppmv decade−1–0.45 ppmv decade−1). Contrary, negative trends (approximately −0.15 ppmv decade−1–−0.05  ppmv decade−1) are derived from a merged zonal mean satellite data set for a latitude band around the Boulder latitude. Overall, the trend differences between the two data sets range from about 0.25 ppmv decade−1 to 0.45 ppmv decade−1, depending on altitude. A possible explanation for these discrepancies is a different temporal behaviour at Boulder and the zonal mean, which simply indicates a sampling bias. In this work we investigate trend differences between Boulder and the zonal mean using primarily simulations from ECHAM/MESSy (European Centre for Medium-Range Weather Forecasts Hamburg/Modular Earth Submodel System) Atmospheric Chemistry (EMAC), WACCM (Whole Atmosphere Community Climate Model), CMAM (Canadian Middle Atmosphere Model) and CLaMS (Chemical Lagrangian Model of the Stratosphere). On shorter time scales we address this aspect also based on satellite observations from UARS/HALOE (Upper Atmosphere Research Satellite/Halogen Occultation Experiment), Envisat/MIPAS (Environmental Satellite/Michelson Interferometer for Passive Atmospheric Sounding) and Aura/MLS (Microwave Limb Sounder). Overall, both the simulations and observations exhibit trend differences between Boulder and the zonal mean. The differences are dependent on altitude and the time period considered. The model simulations indicate only small trend differences between Boulder and the zonal mean for the time period between the late 1980s and 2010. These are clearly not sufficient to explain the discrepancies between the trend estimates derived from the FPH observations and the merged zonal mean satellite data set. Unless the simulations underrepresent variability or the trend differences originate from smaller spatial and temporal scales than resolved by the model simulations, trends at Boulder for this time period should be quite representative also for the zonal mean and even other latitude bands. Trend differences for a decade of data are larger and need to be kept in mind when comparing results for Boulder and the zonal mean on this time scale. Beyond that, we find that the trend estimates for the time period between the late 1980s and 2010 also significantly differ among the simulations. They are larger than those derived from the merged satellite data set and smaller than the trend estimates derived from the FPH observations. [ABSTRACT FROM AUTHOR]

The South Asian High (SAH) is an important component of the summer monsoon system in Asia. In this study we investigate the location and drivers of the SAH at 100 hPa during the boreal summers of 1979 to 2014 on interannual, seasonal and synoptic timescales using seven reanalyses and observational data. Our comparison of the different reanalyses focuses especially on the bimodality of the SAH, i.e. the two preferred modes of the SAH centre location: the Iranian Plateau to the west and the Tibetan Plateau to the east. We find that only the National Centers for Environmental Prediction-National Center of Atmospheric Research (NCEP-NCAR) reanalysis shows a clear bimodal structure of the SAH centre distribution with respect to daily and pentad (5 day) mean data. Furthermore, the distribution of the SAH centre location is highly variable from year to year. As in simple model studies, which connect the SAH to heating in the tropics, we find that the mean seasonal cycle of the SAH and its centre are dominated by the expansion of convection in the South Asian region (70-130° E × 15-30° N) on the south-eastern border of the SAH. A composite analysis of precipitation and outgoing long-wave radiation data with respect to the location of the SAH centre reveals that a more westward (eastward) location of the SAH is related to stronger (weaker) convection and rainfall over India and weaker (stronger) precipitation over the western Pacific. [ABSTRACT FROM AUTHOR]

This study investigates the abrupt and severe water vapour decline in the stratosphere beginning in the year 2000 (the "millennium water vapour drop") and other similarly strong stratospheric water vapour reductions by means of various simulations with the state-of-the-art Chemistry- Climate Model (CCM) EMAC (ECHAM/MESSy Atmospheric Chemistry Model). The model simulations differ with respect to the prescribed sea surface temperatures (SSTs) and whether nudging is applied or not. The CCM EMAC is able to most closely reproduce the signature and pattern of the water vapour drop in agreement with those derived from satellite observations if the model is nudged. Model results confirm that this extraordinary water vapour decline is particularly obvious in the tropical lower stratosphere and is related to a large decrease in cold point temperature. The drop signal propagates under dilution to the higher stratosphere and to the poles via the Brewer-Dobson circulation (BDC). We found that the driving forces for this significant decline in water vapour mixing ratios are tropical sea surface temperature (SST) changes due to a coincidence with a preceding strong El Niño-Southern Oscillation event (1997/1998) followed by a strong La Niña event (1999/2000) and supported by the change of the westerly to the easterly phase of the equatorial stratospheric quasi-biennial oscillation (QBO) in 2000. Correct (observed) SSTs are important for triggering the strong decline in water vapour. There are indications that, at least partly, SSTs contribute to the long period of low water vapour values from 2001 to 2006. For this period, the specific dynamical state of the atmosphere (overall atmospheric large-scale wind and temperature distribution) is important as well, as it causes the observed persistent low cold point temperatures. These are induced by a period of increased upwelling, which, however, has no corresponding pronounced signature in SSTs anomalies in the tropics. Our free-running simulations do not capture the drop as observed, because a) the cold point temperature has a low bias and thus the water vapour variability is reduced and b) because they do not simulate the appropriate dynamical state. Large negative water vapour declines are also found in other years and seem to be a feature which can be found after strong combined El Niño/La Niña events if the QBO west phase during La Niña changes to the east phase. [ABSTRACT FROM AUTHOR]

The South Asian High (SAH) is an important component of the summer monsoon system in Asia. In this study we investigate the location and drivers of the SAH at 100 hPa during the boreal summers of 1979 to 2014 on interannual, seasonal and synoptic time scales using six reanalyses. Special focus lies on the bimodality of the SAH, i.e. the two preferred modes of the SAH centre location: the Iranian Plateau to the west and the Tibetan Plateau to the east. We find that only the National Centers for Environmental Prediction - National Center of Atmospheric Research (NCEP/NCAR) reanalysis shows a clear bimodal structure of the SAH centre distribution with respect to daily and pentad (5-day mean) data. Furthermore, the distribution of the SAH centre location is highly variable from year-to-year. As in simple model studies, which connect the SAH to heating in the tropics, we find that the mean seasonal cycle of the SAH and its centre are dominated by the expansion of convection in the South Asian region (70° E-130° E × 15° N-30° N) on the southeastern border of the SAH. A composite analysis of precipitation and OLR data with respect to the location of the SAH centre reveals that a more westward (eastward) location of the SAH is related to stronger (weaker) convection and rainfall over India and stronger (weaker) precipitation over the West Pacific. [ABSTRACT FROM AUTHOR]

In this study backward trajectories from the tropical lower stratosphere were calculated for the Northern Hemisphere (NH) winters 1995-1996, 1997-1998 (El Niño) and 1998-1999 (La Niña) and summers 1996, 1997 and 1999 using both ERA-40 reanalysis data of the European Centre for Medium-RangeWeather Forecast (ECMWF) and coupled Chemistry-Climate Model (CCM) data. The calculated trajectories were analysed to determine the distribution of points where individual air masses encounter the minimum temperature and thus minimum water vapour mixing ratio during their ascent through the tropical tropopause layer (TTL) into the stratosphere. The geographical distribution of these dehydration points and the local conditions there determine the overall water vapour entry into the stratosphere. Results of two CCMs are presented: the ECHAM4.L39(DLR)/CHEM (hereafter: E39/C) from the German Aerospace Center (DLR) and the Freie Universität Berlin Climate Middle Atmosphere Model with interactive chemistry (hereafter: FUB-CMAM-CHEM). In the FUBCMAM- CHEM model the minimum temperatures are overestimated by about 9K in NH winter and about 3K in NH summer, resulting in too high water vapour entry values compared to ERA-40. However, the geographical distribution of dehydration points is fairly similar to ERA-40 for NH winter 1995-1996 and 1998-1999. The distribution of dehydration points in the boreal summer 1996 suggests an influence of the Indian monsoon upon the water vapour transport. The E39/C model displays a temperature bias of about +5 K. Hence, the minimum water vapour mixing ratios are higher relative to ERA-40. The geographical distribution of dehydration points is fairly well in NH winter 1995-1996 and 1997-1998 with respect to ERA-40. The distribution is not reproduced for the NH winter 1998-1999 (La Niña event) compared to ERA- 40. There is an excessive water vapour flux through warm regions e.g. Africa in the NH winter and summer. The possible influence of the Indian monsoon on the transport is not seen in the boreal summer 1996. Further, the residence times of air parcels in the TTL were derived from the trajectory calculations. The analysis of the residence times reveals that in both CCMs residence times in the TTL are lower compared to ERA-40 and the seasonal variation is hardly present. [ABSTRACT FROM AUTHOR]