Untersuchung stratosphärischen Wasserdampfs anhand der Simulation von Wasserisotopologen

This modelling study aims to gain an improved understanding of the processes that determine the water vapour budget in the stratosphere by means of the investigation of water isotope ratios. At first, a separate hydrological cycle has been introduced into the chemistry-climate model EMAC, including the water isotopologues HDO and H218O and their physical fractionation processes. Additionally, an explicit computation of the contribution of methane oxidation to HDO has been incorporated. EMAC simulates explicit stratospheric dynamics and a highly resolved tropical tropopause layer. These model expansions, now allow detailed analyses of water vapour and its isotope ratio with respect to deuterium (deltaD(H2O)), throughout the stratosphere and in the transition region to the troposphere. In order to assure the correct representation of the water isotopologues in the model's hydrological cycle, the expanded system has been evaluated in several steps. The physical fractionation effects have been evaluated by comparison of the simulated isotopic composition of precipitation with measurements from a ground-based network (GNIP) and with the results from an isotopologue-enabled ECHAM5 general circulation model version. The model's representation of the chemical HDO precursor CH3D in the stratosphere has been confirmed by a comparison with chemical transport models (CHEM1D, CHEM2D) and measurements from radiosonde flights. Finally, the simulated HDO and deltaD(H2O) have been evaluated in the stratosphere, with respect to retrievals from three different satellite instruments (MIPAS, ACE-FTS, SMR). Discrepancies in stratospheric deltaD(H2O) between two of the three satellite retrievals can now partly be explained. The simulated seasonal cycle of tropical deltaD(H2O) in the stratosphere exhibits a weak tape recorder signal, which fades out at altitudes around 25 km. This result ranges between the pronounced tape recorder signal in the MIPAS observations and the missing upward propagation of the seasonal variations in the ACE-FTS retrieval. Revisions of different insufficencies in the respective satellite measurements, however, are expected to alter both observational datasets towards the results of the EMAC model. Extensive analyses of the water isotope ratios have revealed the driving mechanisms of the stratospheric deltaD(H2O) tape recorder signal in the EMAC simulation. A sensitivity study without the impact of methane oxidation on deltaD(H2O) demonstrates the damping effect of this chemical process on the tape recorder signal. An investigation of the origin of the enhanced deltaD(H2O) in the lower stratosphere during boreal summer, shows isotopically enriched water vapour, crossing the tropopause over the subtropical Western Pacic. A correlation analysis confirms this link, and thus the Asian Summer Monsoon could be identified to be the major contributing process for the stratospheric deltaD(H2O) tape recorder. This finding contradicts an analysis of ACE-FTS satellite data, which assigns the lower stratospheric deltaD(H2O) increase during boreal summer to the North American Monsoon. A possible explanation for this discrepancy has been found to be an underrepresentation of convective ice overshooting in the applied convection scheme.Diese Modellstudie hat ein besseres Verständnis jener Prozesse zum Ziel, die das Wasserdampfbudget in der Stratosphäre bestimmen und stützt sich auf die Untersuchung des Isotopenverhältnisses von Wasser. Zunächst wurde ein eigenständiger hydrologischer Zyklus in das Chemie-Klimamodell EMAC eingebaut, welcher die Wasserisotopologe HDO und H218O sowie deren physikalische Fraktionierungsprozesse enthält. Zusätzlich wurde eine explizite Berechnung des Beitrages der Methanoxidation zu HDO eingefügt. EMAC simuliert eine hochaufgelöste tropische Tropopausenschicht sowie explizite Stratosphärendynamik. Mit diesen Modellerweiterungen ist es nun möglich, genaue Analysen von Wasserdampf und dessen Isotopenverhältnis im Bezug auf Deuterium (deltaD(H2O)) in der gesamten Stratosphäre, sowie im Übergangsbereich zur Troposphäre durchzuführen. Um die korrekte Darstellung der Wasserisotopologe im hydrologischen Zyklus des Modells zu gewährleisten, wurde das erweiterte System in mehreren Schritten evaluiert. Die physikalischen Fraktionierungseffekte wurden in einem Vergleich der simulierten Isotopenverhältnisse im Niederschlag mit Messungen eines Netzwerkes an Bodenstationen (GNIP) und mit Ergebnissen einer, mit Wasserisotopologen ausgestatteten, ECHAM5 Modellversion evaluiert. Die Güte des simulierten chemischen HDO-Vorläufers CH3D in der Stratosphäre des Modells wurde durch einen Vergleich der Ergebnisse mit chemischen Transportmodellen (CHEM1D, CHEM2D) und Messungen von Radiosondenaufstiegen überprüft. Abschließend wurde simuliertes HDO und deltaD(H2O) anhand von Messungen drei verschiedener Satelliteninstrumente (MIPAS, ACE-FTS, SMR) evaluiert. Abweichungen im deltaD(H2O) zwischen zwei der drei satellitengestützten Beobachtungen können nun teilweise erklärt werden. Der simulierte Jahresgang von tropischem deltaD(H2O) in der Stratosphäre weist ein schwaches 'tape recorder' Signal auf, welches sich in Höhen um 25 km auflöst. Dieses Ergebnis ist zwischen das ausgeprägte 'tape recorder' Signal in MIPAS- Beobachtungen und die nicht erkennbare vertikale Ausbreitung des Jahresgangs in ACE-FTS-Messungen einzuordnen. Die Beseitigung unterschiedlicher Mängel in den jeweiligen Satellitenmessungen lässt jedoch eine Veränderung beider Beobachtungsdatensätze in Richtung der Ergebnisse des EMAC Modells erwarten. Eingehende Analysen der Wasserisotopenverhältnisse in der EMAC Simulation haben die für den stratosphärischen deltaD(H2O)-'tape recorder' verantwortlichen Prozesse aufgezeigt. Eine Sensitivitätsstudie ohne Einfluss der Methanoxidation auf deltaD(H2O) veranschaulicht den dämpfenden Einfluss dieses chemischen Prozesses auf das 'tape recorder' Signal. Eine Untersuchung des Ursprungs des erhöhten deltaD(H2O) in der unteren Stratosphäre im Nordsommer weist isotopisch angereicherten Wasserdampf nach, welcher die Tropopause über dem subtropischen Westpazifik durchquert. Eine Korrelationsanalyse bestätigt diese Verbindung und kennzeichnet damit den Asiatischen Sommermonsun als den wesentlichen beitragenden Faktor zum stratosphärischen deltaD(H2O)-'tape recorder'. Dieses Ergebnis steht im Gegensatz zu einer Auswertung von ACE-FTS-Satellitendaten, welche den deltaD(H2O) Anstieg in der unteren Stratosphäre im Nordsommer dem Nordamerikanischen Monsun zuweist. Als mögliche Erklärung für diesen Widerspruch konnte das, in dem verwendeten Konvektionsschema unzureichend auftretende, konvektive Überschießen von Wolkeneis ausgemacht werden

Untersuchung stratosphärischen Wasserdampfs anhand der Simulation von Wasserisotopologen

This modelling study aims to gain an improved understanding of the processes that determine the water vapour budget in the stratosphere by means of the investigation of water isotope ratios. At first, a separate hydrological cycle has been introduced into the chemistry-climate model EMAC, including the water isotopologues HDO and H218O and their physical fractionation processes. Additionally, an explicit computation of the contribution of methane oxidation to HDO has been incorporated. EMAC simulates explicit stratospheric dynamics and a highly resolved tropical tropopause layer. These model expansions, now allow detailed analyses of water vapour and its isotope ratio with respect to deuterium (deltaD(H2O)), throughout the stratosphere and in the transition region to the troposphere. In order to assure the correct representation of the water isotopologues in the model's hydrological cycle, the expanded system has been evaluated in several steps. The physical fractionation effects have been evaluated by comparison of the simulated isotopic composition of precipitation with measurements from a ground-based network (GNIP) and with the results from an isotopologue-enabled ECHAM5 general circulation model version. The model's representation of the chemical HDO precursor CH3D in the stratosphere has been confirmed by a comparison with chemical transport models (CHEM1D, CHEM2D) and measurements from radiosonde flights. Finally, the simulated HDO and deltaD(H2O) have been evaluated in the stratosphere, with respect to retrievals from three different satellite instruments (MIPAS, ACE-FTS, SMR). Discrepancies in stratospheric deltaD(H2O) between two of the three satellite retrievals can now partly be explained. The simulated seasonal cycle of tropical deltaD(H2O) in the stratosphere exhibits a weak tape recorder signal, which fades out at altitudes around 25 km. This result ranges between the pronounced tape recorder signal in the MIPAS observations and the missing upward propagation of the seasonal variations in the ACE-FTS retrieval. Revisions of different insufficencies in the respective satellite measurements, however, are expected to alter both observational datasets towards the results of the EMAC model. Extensive analyses of the water isotope ratios have revealed the driving mechanisms of the stratospheric deltaD(H2O) tape recorder signal in the EMAC simulation. A sensitivity study without the impact of methane oxidation on deltaD(H2O) demonstrates the damping effect of this chemical process on the tape recorder signal. An investigation of the origin of the enhanced deltaD(H2O) in the lower stratosphere during boreal summer, shows isotopically enriched water vapour, crossing the tropopause over the subtropical Western Pacic. A correlation analysis confirms this link, and thus the Asian Summer Monsoon could be identified to be the major contributing process for the stratospheric deltaD(H2O) tape recorder. This finding contradicts an analysis of ACE-FTS satellite data, which assigns the lower stratospheric deltaD(H2O) increase during boreal summer to the North American Monsoon. A possible explanation for this discrepancy has been found to be an underrepresentation of convective ice overshooting in the applied convection scheme.Diese Modellstudie hat ein besseres Verständnis jener Prozesse zum Ziel, die das Wasserdampfbudget in der Stratosphäre bestimmen und stützt sich auf die Untersuchung des Isotopenverhältnisses von Wasser. Zunächst wurde ein eigenständiger hydrologischer Zyklus in das Chemie-Klimamodell EMAC eingebaut, welcher die Wasserisotopologe HDO und H218O sowie deren physikalische Fraktionierungsprozesse enthält. Zusätzlich wurde eine explizite Berechnung des Beitrages der Methanoxidation zu HDO eingefügt. EMAC simuliert eine hochaufgelöste tropische Tropopausenschicht sowie explizite Stratosphärendynamik. Mit diesen Modellerweiterungen ist es nun möglich, genaue Analysen von Wasserdampf und dessen Isotopenverhältnis im Bezug auf Deuterium (deltaD(H2O)) in der gesamten Stratosphäre, sowie im Übergangsbereich zur Troposphäre durchzuführen. Um die korrekte Darstellung der Wasserisotopologe im hydrologischen Zyklus des Modells zu gewährleisten, wurde das erweiterte System in mehreren Schritten evaluiert. Die physikalischen Fraktionierungseffekte wurden in einem Vergleich der simulierten Isotopenverhältnisse im Niederschlag mit Messungen eines Netzwerkes an Bodenstationen (GNIP) und mit Ergebnissen einer, mit Wasserisotopologen ausgestatteten, ECHAM5 Modellversion evaluiert. Die Güte des simulierten chemischen HDO-Vorläufers CH3D in der Stratosphäre des Modells wurde durch einen Vergleich der Ergebnisse mit chemischen Transportmodellen (CHEM1D, CHEM2D) und Messungen von Radiosondenaufstiegen überprüft. Abschließend wurde simuliertes HDO und deltaD(H2O) anhand von Messungen drei verschiedener Satelliteninstrumente (MIPAS, ACE-FTS, SMR) evaluiert. Abweichungen im deltaD(H2O) zwischen zwei der drei satellitengestützten Beobachtungen können nun teilweise erklärt werden. Der simulierte Jahresgang von tropischem deltaD(H2O) in der Stratosphäre weist ein schwaches 'tape recorder' Signal auf, welches sich in Höhen um 25 km auflöst. Dieses Ergebnis ist zwischen das ausgeprägte 'tape recorder' Signal in MIPAS- Beobachtungen und die nicht erkennbare vertikale Ausbreitung des Jahresgangs in ACE-FTS-Messungen einzuordnen. Die Beseitigung unterschiedlicher Mängel in den jeweiligen Satellitenmessungen lässt jedoch eine Veränderung beider Beobachtungsdatensätze in Richtung der Ergebnisse des EMAC Modells erwarten. Eingehende Analysen der Wasserisotopenverhältnisse in der EMAC Simulation haben die für den stratosphärischen deltaD(H2O)-'tape recorder' verantwortlichen Prozesse aufgezeigt. Eine Sensitivitätsstudie ohne Einfluss der Methanoxidation auf deltaD(H2O) veranschaulicht den dämpfenden Einfluss dieses chemischen Prozesses auf das 'tape recorder' Signal. Eine Untersuchung des Ursprungs des erhöhten deltaD(H2O) in der unteren Stratosphäre im Nordsommer weist isotopisch angereicherten Wasserdampf nach, welcher die Tropopause über dem subtropischen Westpazifik durchquert. Eine Korrelationsanalyse bestätigt diese Verbindung und kennzeichnet damit den Asiatischen Sommermonsun als den wesentlichen beitragenden Faktor zum stratosphärischen deltaD(H2O)-'tape recorder'. Dieses Ergebnis steht im Gegensatz zu einer Auswertung von ACE-FTS-Satellitendaten, welche den deltaD(H2O) Anstieg in der unteren Stratosphäre im Nordsommer dem Nordamerikanischen Monsun zuweist. Als mögliche Erklärung für diesen Widerspruch konnte das, in dem verwendeten Konvektionsschema unzureichend auftretende, konvektive Überschießen von Wolkeneis ausgemacht werden

The generic MESSy submodel TENDENCY (v1.0) for process-based analyses in Earth system models

The tendencies of prognostic variables in Earth system models are usually only accessible, e.g. for output, as a sum over all physical, dynamical and chemical processes at the end of one time integration step. Information about the contribution of individual processes to the total tendency is lost, if no special precautions are implemented. The knowledge on individual contributions, however, can be of importance to track down specific mechanisms in the model system. We present the new MESSy (Modular Earth Submodel System) infrastructure submodel TENDENCY and use it exemplarily within the EMAC (ECHAM/MESSy Atmospheric Chemistry) model to trace process-based tendencies of prognostic variables. The main idea is the outsourcing of the tendency accounting for the state variables from the process operators (submodels) to the TENDENCY submodel itself. In this way, a record of the tendencies of all process–prognostic variable pairs can be stored. The selection of these pairs can be specified by the user, tailor-made for the desired application, in order to minimise memory requirements. Moreover, a standard interface allows the access to the individual process tendencies by other submodels, e.g. for on-line diagnostics or for additional parameterisations, which depend on individual process tendencies. An optional closure test assures the correct treatment of tendency accounting in all submodels and thus serves to reduce the model's susceptibility. TENDENCY is independent of the time integration scheme and therefore the concept is applicable to other model systems as well. Test simulations with TENDENCY show an increase of computing time for the EMAC model (in a setup without atmospheric chemistry) of 1.8 ± 1% due to the additional subroutine calls when using TENDENCY. Exemplary results reveal the dissolving mechanisms of the stratospheric tape recorder signal in height over time. The separation of the tendency of the specific humidity into the respective processes (large-scale clouds, convective clouds, large-scale advection, vertical diffusion and methane oxidation) show that the upward propagating water vapour signal dissolves mainly because of the chemical and the advective contribution. The TENDENCY submodel is part of version 2.42 or later of MESSy

The impact of sulfur hexafluoride (SF₆) sinks on age of air climatologies and trends

Mean age of air (AoA) is a common diagnostic for the strength of the stratospheric overturning circulation in both climate models and observations. AoA climatologies and AoA trends over the recent decades of model simulations and proxies derived from observations of long-lived tracers do not agree. Satellite observations show much older air than climate models, and while most models compute a clear decrease in AoA over the last decades, a 30-year time series from measurements shows a statistically nonsignificant positive trend in the Northern Hemisphere extratropical middle stratosphere. Measurement-based AoA derivations are often founded on observations of the trace gas sulfur hexafluoride (SF$_{6}$), a fairly long-lived gas with a near-linear increase in emissions during recent decades. However, SF$_{6}$ has chemical sinks in the mesosphere that are not considered in most model studies. In this study, we explicitly compute the chemical SF$_{6}$ sinks based on chemical processes in the global chemistry climate model EMAC (ECHAM/MESSy Atmospheric Chemistry). We show that good agreement between stratospheric AoA in EMAC and MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is reached through the inclusion of chemical SF$_{6}$ sinks, as these sinks lead to a strong increase in the stratospheric AoA and, therefore, to a better agreement with MIPAS satellite observations. Remaining larger differences at high latitudes are addressed, and possible reasons for these differences are discussed. Subsequently, we demonstrate that the AoA trends are also strongly influenced by the chemical SF6 sinks. Under consideration of the SF$_{6}$ sinks, the AoA trends over the recent decades reverse sign from negative to positive. We conduct sensitivity simulations which reveal that this sign reversal does not result from trends in the stratospheric circulation strength nor from changes in the strength of the SF$_{6}$ sinks. We illustrate that even a constant SF$_{6}$ destruction rate causes a positive trend in the derived AoA, as the amount of depleted SF$_{6}$ scales with increasing SF$_{6}$ abundance itself. In our simulations, this effect overcompensates for the impact of the accelerating stratospheric circulation which naturally decreases AoA. Although various sources of uncertainties cannot be quantified in detail in this study, our results suggest that the inclusion of SF$_{6}$ depletion in models has the potential to reconcile the AoA trends of models and observations. We conclude the study with a first approach towards a correction to account for SF$_{6}$ loss and deduce that a linear correction might be applicable to values of AoA of up to 4 years

Correction of stratospheric age-of-air derived from SF 6 for the effect of chemical sinks

Observational monitoring of the stratospheric transport circulation, the Brewer-Dobson-Circulation (BDC), is crucial to estimate any decadal to long-term changes therein, a prerequisite to interpret trends in stratospheric composition and to constrain the consequential impacts on climate. The transport time along the BDC (i.e., the mean age of stratospheric air, AoA) can best be deduced from trace gas measurements of tracers which increase linearly in time and are chemically passive. The gas SF6 is often used to deduce AoA, because it has been increasing monotonically since the ~1950s, and previously its chemical sinks in the mesosphere have been assumed to be negligible for AoA estimates. However, recent studies have shown that the chemical sinks of SF6 are stronger than assumed, and become increasingly relevant with rising SF6 concentrations. To adjust biases in AoA that result from the chemical SF6 sinks, we here propose a simple correction scheme for SF6-based AoA estimates accounting for the time-dependent effects of chemical sinks. The correction scheme is based on theoretical considerations with idealized assumptions, resulting in a relation between ideal AoA and apparent AoA which is a function of the tropospheric reference time-series of SF6 and of the AoA-dependent effective lifetime of SF6. The correction method is thoroughly tested within a self-consistent data set from a climate model that includes explicit calculation of chemical SF6 sinks. It is shown within the model that the correction successfully reduces biases in SF6-based AoA to less than 5 % for mean ages below 5 years. Tests with using only sub-sampled data for deriving the fit coefficients show that applying the correction scheme even with imperfect knowledge of the sink is far superior to not applying a sink correction. Further, we show that based on currently available measurements, we are not able to constrain the fit parameters of the correction scheme based on observational data alone. However, the model-based correction curve lies within the observational uncertainty, and we thus recommend to use the model-derived fit coefficients until more high-quality measurements will be able to further constrain the correction scheme. The application of the correction scheme to AoA from satellites and in-situ data suggests that it is highly beneficial to reconcile different observational estimates of mean AoA

The Influence of Ozone Changes on the Stratospheric Dynamics in 4xCO2 Climate Simulations

An increase in atmospheric CO2 concentration changes the temperatures and dynamics of the stratosphere and thus also the ozone concentrations. Since ozone is sensitive to longwave as well as shortwave radiation, the altered ozone distribution in turn affects temperature and thus also modifies the CO2-induced change in stratospheric dynamics. To study this in detail, we performed model simulations using the EMAC climate-chemistry model in which CO2 concentrations were quadrupled compared to pre-industrial times. For the 4xCO2 simulations, this was done by prescribing once an unchanged (pre-industrial) and once a changed ozone distribution from a previous 4xCO2 simulation. It is shown here, that the change in ozone leads to a strengthening of the stratospheric easterly winds in summer, as well as a weakening of the polar vortex in both hemispheres. While the high variability in the northern hemisphere does not lead to clear results, they are statistically significant in the southern hemisphere. In addition, the duration of the polar vortex westerlies is shorter due to the CO2-induced change in the ozone field. Furthermore, the acceleration of the Brewer-Dobson circulation, which is caused by an increase in CO2, is damped in the summer hemisphere by the ozone influence. This in turn affects the transport from the tropics to the extra-tropics. The ozone-induced changes in the stratospheric circulation also affect the tropospheric circulation and have an effect on the tropospheric polar front jet. It experiences a systematically weaker shift toward the pole due to a changed ozone distribution in the southern hemisphere than with a constant ozone distribution. We discuss the results, presented here, and place them in the context of the influence of stratospheric ozone on climate change dynamics

Stratospheric Ozone Changes Damp the CO2-Induced Acceleration of the Brewer-Dobson Circulation

The increase of atmospheric CO2 concentrations changes the atmospheric temperature distribution, which in turn affects the circulation. A robust circulation response to CO2 forcing is the strengthening of the stratospheric Brewer–Dobson circulation (BDC), with associated consequences for transport of trace gases such as ozone. Ozone is further affected by the CO2-induced stratospheric cooling via the temperature dependency of ozone chemistry. These ozone changes in turn influence stratospheric temperatures and thereby modify the CO2-induced circulation changes. In this study, we perform dedicated model simulations to quantify the modification of the circulation response to CO2 forcing by stratospheric ozone. Specifically, we compare simulations of the atmosphere with preindustrial and with quadrupled CO2 climate conditions, in which stratospheric ozone is held fixed or is adapted to the new climate state. The results of the residual circulation and mean age of air show that ozone changes damp the CO2-induced BDC increase by up to 20%. This damping of the BDC strengthening is linked to an ozone-induced relative enhancement of the meridional temperature gradient in the lower stratosphere in summer, thereby leading to stronger stratospheric easterlies that suppress wave propagation. Additionally, we find a systematic weakening of the polar vortices in winter and spring. In the Southern Hemisphere, ozone reduces the CO2-induced delay of the final warming date by 50%