P. Konopka

Forschungszentrum Jülich, Jülich, North Rhine-Westphalia, Germany

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Publications (116)242.5 Total impact

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    ABSTRACT: An evaluation of water vapor in the upper troposphere and lower stratosphere (UTLS) of the ERA-Interim, the global atmospheric reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ECMWF), is presented. Water vapor measurements are derived from the Fast In situ Stratospheric Hygrometer (FISH) during a large set of airborne measurement campaigns from 2001 to 2011 in the tropics, midlatitudes and polar regions, covering isentropic layers from 300 to 400K (5–18km). The comparison shows around 87% of the reanalysis data are within a factor of 2 of the FISH water vapor measurements and around 30% have a nearly perfect agreement with an over- and underestimation lower than 10%. Nevertheless, strong over- and underestimations can occur both in the UT and LS, in particularly in the extratropical LS and in the tropical UT, where severe over- and underestimations up to 10 times can occur. The analysis data from the evolving ECMWF operational system is also evaluated, and the FISH measurements are divided into time periods representing different cycles of the Integrated Forecast System (IFS). The agreement with FISH improves over the time, in particular when comparing water vapor fields for time periods before 2004 and after 2010. It appears that influences of tropical tropospheric and extratropical UTLS processes, e.g., convective and quasi-isentropic exchange processes, are particularly challenging for the simulation of the UTLS water vapor distribution. Both the reanalysis and operational analysis data show the tendency of an overestimation of low water vapor mixing ratio (⪅10ppmv) in the LS and underestimation of high water vapor mixing ratio (⪆300ppmv) in the UT.
    Atmospheric Chemistry and Physics 10/2014; 14:10803-10822. · 5.51 Impact Factor
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    ABSTRACT: Lagrangian transport schemes have proven to be useful tools for modelling stratospheric trace gas transport since they are less diffusive than classical Eulerian schemes and therefore especially well suited for maintaining steep tracer gradients. Here, we present the implementation of the full-Lagrangian transport core of the Chemical Lagrangian Model of the Stratosphere (CLaMS) into the ECHAM/MESSy Atmospheric Chemistry model (EMAC). We performed a ten-year time-slice simulation to evaluate the coupled model system EMAC/CLaMS. Simulated zonal mean age of air distributions are compared to age of air derived from airborne measurements, showing a good overall representation of the stratospheric circulation. Results from the new Lagrangian transport scheme are compared to tracer distributions calculated with the standard flux-form semi-Lagrangian (FFSL) transport scheme in EMAC. The differences in the resulting tracer distributions are most pronounced in the regions of strong transport barriers. The polar vortices are presented as an example and simulated trace gas distributions are compared to satellite measurements. The analysis of CFC-11, N2O, CH4, and age of air in the polar vortex regions shows that the CLaMS Lagrangian transport scheme produces a stronger, more realistic transport barrier at the edge of the polar vortex than the FFSL transport scheme of EMAC. Differences in simulated age of air range up to one year in the Arctic polar vortex in late winter/early spring. The new coupled model system EMAC/CLaMS thus constitutes a suitable tool for future model studies of stratospheric tracer transport.
    02/2014; 7(2).
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    ABSTRACT: We aim to reconcile the recently published, apparently contrasting results regarding the relative importance of tropical upwelling versus horizontal transport for the seasonality of ozone above the tropical tropopause. Different analysis methods in the literature (Lagrangian versus Eulerian, and isentropic versus pressure vertical coordinates) yield different perspectives of ozone transport, and the results must be carefully compared in equivalent terms to avoid misinterpretation. By examining the Lagrangian calculations in the Eulerian formulation, we show here that the results are in fact consistent with each other and with a common understanding of the ozone transport processes near and above the tropical tropopause. We further emphasize that the complementary approaches are suited for answering two different scientific questions: (1) what drives the observed seasonal cycle in ozone at a particular level above the tropical tropopause? and (2) how important is horizontal transport from mid-latitudes for ozone concentrations in the tropical lower stratosphere? Regarding the first question, the analysis of the transformed Eulerian mean (TEM) ozone budget shows that the annual cycle in tropical upwelling is the main forcing of the ozone seasonality at altitudes with large vertical gradients in the tropical lower stratosphere. To answer the second question a Lagrangian framework must be used, and the results show that a large fraction (~50%) of the ozone molecules ascending through the tropical lower stratosphere is of extra-tropical origin and has been in-mixed from mid-latitudes.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 11/2013; 13(21):10787-10794. · 5.51 Impact Factor
  • Bulletin of the American Meteorological Society. 07/2013; 94(7):1051-1058.
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    ABSTRACT: We describe the implementation of a Lagrangian transport core in a chemistry climate model (CCM). Thereby we address the common problem of properly representing trace gas distributions in a classical Eulerian framework with a fixed model grid, particularly in regions with strong trace gas gradients. A prominent example is stratospheric water vapor, which is an important driver of surface climate change on decadal scales. In this case, the transport representation is particularly important in the tropical tropopause layer (TTL), where tropospheric air enters into the stratosphere. We have coupled the Chemical Lagrangian Model of the Stratosphere (CLaMS) with the ECHAM-MESSy Atmospheric Chemistry Model (EMAC). The latter includes the ECHAM5 climate model, and the MESSy interface, which allows for flexible coupling and switching between different submodels. The chemistry transport model CLaMS provides a fully Lagrangian transport representation to calculate constituent transport for an ensemble of air parcels that move along trajectories. To facilitate the calculation of long time-series a simplified chemistry scheme was implemented. Various studies show that the CLaMS model is particularly suited to properly represent dynamics and chemistry in the UT/LS region. The analysis of mean age of stratospheric air gives insight into the different transport characteristics of the Eulerian and the Lagrangian transport schemes. Mean age of air, calculated in both frameworks, is compared regarding the representation of important processes, i.e. descent in the polar vortex, upwelling in the tropical pipe, and isentropic in-mixing in subtropical regions. We also compared the zonal mean distributions and photochemical lifetimes of CFC-11 and CFC-12 with climatologies from different satellite experiments (ACE-FTS, HIRDLS, and MIPAS). CLaMS stratospheric water vapor distributions show remarkable differences compared to the stratospheric water vapor simulated by ECHAM, especially in the Northern hemisphere in summer. The results are compared to statellite water vapor mesurements. Qualitatively, Lagrangian CLaMS data are in better agreement with the satellite climatologies. It can be expected that the more accurate representation of the UT/LS region with the CLaMS model likely yields to improved climate predictions.
    04/2013;
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    ABSTRACT: Intense vertical transport of air from the troposphere to the stratosphere occurs in the maritime continent-west Pacific in boreal winter (Fueglistaler et al., 2004). Convective uplift injects tropospheric air masses into the TTL, where strong radiative heating fosters further vertical transport to the stratosphere and the upper branch of the Brewer Dobson Circulation. Based on observations of very low tropospheric ozone made during the TransBrom-Cruise (Ridder et al., 2012), Rex et al. (2011) has hypothesized that tropospheric air in the western Pacific region should be rather depleted in OH - the main tropospheric oxidant - leading to significantly longer lifetimes of compounds carrying halogens (VSLS) and sulfur (SO2) in these air masses. We investigate this hypothesis and its possible impact on SO2 and VSLS transport to the stratosphere by looking at aircraft measurements made during the SCOUT-O3 field experiment in Darwin, Australia, in November and December 2005. Trajectory calculations show that tropospheric ozone mixing ratios below 15 ppb encountered during several flights are typically found in clean Pacific air masses that are also relatively low in CO. A slightly negative correlation between CO and SO2 in these air masses may indeed be caused by a longer lifetime due to low OH. However, the tropospheric SO2 concentrations observed during SCOUT-O3 are too low to represent a significant sulfur source to the stratosphere. Samples of several VSLS made in the TTL are also analyzed for a possible signature of enhanced tropospheric lifetimes. Fueglistaler, S., et al.: Tropical troposphere-to-stratosphere transport inferred from trajectory calculations, J. Geophys. Res., 109, 10.1029/2003jd004069, 2004. Rex, M., et al.: Is There a Hole in the Global OH Shield Over the Tropical Western Pacific Warm Pool?, NDACC symposium, Reunion Island, 2011. Ridder, T., et al.: Ship-borne FTIR measurements of CO and O3 in the Western Pacific from 43° N to 35° S: an evaluation of the sources, Atmos. Chem. Phys., 12, 815-828, 10.5194/acp-12-815-2012, 2012.
    04/2013;
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    ABSTRACT: We compare global water vapor observations in the lower stratosphere from MLS with global simulations with the Lagrangian chemical transport model CLaMS to investigate the pathways of water vapor into the lower stratosphere during northern hemisphere (NH) summer. Model simulations and observations both show that the Asian and American monsoons are main regions of upward transport of water vapor into the upper troposphere during summer, moistening the NH subtropics. In NH mid- and high-latitudes, a clear anticorrelation between water vapor and ozone tendencies reveals a large region influenced by frequent horizontal transport from low latitudes, extending up to about 430-450K during summer and fall. Close to the subtropics, this horizontal transport is caused by the shallow Brewer-Dobson circulation branch. In contrast, at higher latitudes polewards of about 50°, horizontal transport is caused by eddy mixing, related to Rossby-wave breaking. Additional sensitivity simulations with transport barriers in the model confirm that the entire annual cycle of water vapor mixing ratios in NH extratropics at altitudes above the subtropical jet core is caused by horizontal transport from the subtropics. Hence, NH water vapor between about 370-430K during summer and fall appears to be `subtropically controlled'. In the model, highest water vapor mixing ratios in this region are closely linked to horizontal transport from the subtropics rather than to mid-latitude convection. Further, an asymmetry exists in lower stratospheric water vapor, with a significantly moister NH than SH. This asymmetry is largely caused by processes at high latitudes, like strong dehydration within the Antarctic vortex and hemispheric differences in downwelling, and is only weakly affected by horizontal transport from low latitudes.
    04/2013;
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    ABSTRACT: Based on OSIRIS satellite observations, Bourassa et al. (2012) suggested that the June 2011 eruption of the Nabro volcano had the strongest impact on stratospheric aerosol since Pinatubo. Based on a reported visible plume height of 13 km, they claimed that no direct stratospheric injection of ash, sulfate and SO2 occurred, and that volcanic material was transported to the stratosphere exclusively via the Asian summer monsoon anticyclone. In contrast, Sawamura et al. (2012) and Vernier et al. (2012) present undisputable evidence for a direct injection contribution using back trajectory calculations from ground based lidar and space-borne CALIOP observations within the first few days after the eruption. To assess which pathway - direct injection (DI) or uplift via the Asian monsoon (AMU) - dominated transport of Nabro sulfur and aerosol to the stratosphere, we use a trajectory ensemble approach. Forward trajectories were started from Nabro at the time of eruption, and the distribution of air parcels in the stratosphere was monitored separately for trajectories initial-ized in the stratosphere (corresponding to DI) and in the troposphere (some of which reaching the stratosphere by AMU). While the path of a single trajectory tends to become rather uncertain after several days, the ensemble approach allows for a statistical analysis where random errors are expected to average out. During the first week after the eruption, only DI air parcels are found in the stratosphere, in agreement with satellite observations of SO2 (MIPAS) and aerosols (MIPAS, CALIOP). About a week after the eruption, the first trajectories initialized in the troposphere reach the stratosphere inside the Asian monsoon anticyclone. By the end of July, the pattern of the AMU air parcels resembles the observed distribution of stratospheric aerosol much more closely than the pattern of the DI air parcels does. The simulations further show that some of the air parcels that entered the stratospheric part of the TTL rise further when upwelling intensifies with the onset of boreal winter. The observation of stronger aerosol signatures in MIPAS spectra for tangent altitudes above 20 km in the tropics in winter 2011/12 compared to other years suggests that aerosol originating from Nabro may enter the upper branch of the BD-circulation. This study has implications beyond revealing the transport pathway of a stratospheric aerosol plume from the Nabro volcano. Because the aerosol signal is readily picked up by satellites, it represents an ideal case study to investigate the efficiency of the Asian monsoon as a transport pathway to the stratosphere in general, e.g. for anthropogenic SO2 and other pollutants. Bourassa, A. E., et al.: Large Volcanic Aerosol Load in the Stratosphere Linked to Asian Monsoon Transport, Science, 337, 78-81, 2012. Sawamura, P., et al.: Stratospheric AOD after the 2011 eruption of Nabro volcano measured by lidars over the Northern Hemisphere, Environ. Res. Lett., 7, 2012. Vernier, J.-P., et al.: Comment on "Large volcanic aerosol load in the stratosphere linked to Asian Monsoon Transport" by Bourassa and co-authors. Accepted for publication Science (December 2012).
    04/2013;
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    ABSTRACT: We compare global water vapor observations from Microwave Limb Sounder (MLS) and simulations with the Lagrangian chemical transport model CLaMS (Chemical Lagrangian Model of the Stratosphere) to investigate the pathways of water vapor into the lower stratosphere during Northern Hemisphere (NH) summer. We find good agreement between the simulation and observations, with an effect of the satellite averaging kernel especially at high latitudes. The Asian and American monsoons emerge as regions of particularly high water vapor mixing ratios in the lower stratosphere during boreal summer. In NH midlatitudes and high latitudes, a clear anticorrelation between water vapor and ozone daily tendencies reveals a large region influenced by frequent horizontal transport from low latitudes, extending up to about 450K during summer and fall. Analysis of the zonal mean tracer continuity equation shows that close to the subtropics, this horizontal transport is mainly caused by the residual circulation. In contrast, at higher latitudes, poleward of about 50°N, eddy mixing dominates the horizontal water vapor transport. Model simulations with transport barriers confirm that almost the entire annual cycle of water vapor in NH midlatitudes above about 360K, with maximum mixing ratios during summer and fall, is caused by horizontal transport from low latitudes. In the model, highest water vapor mixing ratios in this region are clearly linked to horizontal transport from the subtropics.
    Journal of Geophysical Research 01/2013; 118:8111-812. · 3.17 Impact Factor
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    ABSTRACT: Using a combination of ozonesonde data and numerical simulations of the Chemical Lagrangian Model of the Stratosphere (CLaMS), the trend of tropospheric ozone (O3) during 2002-2010 over Beijing was investigated. Tropospheric ozone over Beijing shows a winter minimum and a broad summer maximum with a clear positive trend in the maximum summer ozone concentration over the last decade. The observed significant trend of tropospheric column ozone is mainly caused by photochemical production (3.1% yr-1 for a mean level of 52 DU). This trend is close to the significant trend of partial column ozone in the lower troposphere (0-3 km) resulting from the enhanced photochemical production during summer (3.0% yr-1 for a mean level of 23 DU). Analysis of the CLaMS simulation shows that transport rather than chemistry drives most of the seasonality of tropospheric ozone. However, dynamical processes alone cannot explain the trend of tropospheric ozone in the observational data. Clearly enhanced ozone values and a negative vertical ozone gradient in the lower troposphere in the observational data emphasize the importance of photochemistry within the troposphere during spring and summer, and suggest that the photochemistry within the troposphere significantly contributes to the tropospheric ozone trend over Beijing during the last decade.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 09/2012; 12(18):8389-8399. · 5.51 Impact Factor
  • Paul Konopka, Laura L. Pan
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    ABSTRACT: We present a case study on the formation and structure of the Extratropical Transition Layer (ExTL) using in situ observations and a Lagrangian chemical transport model. The results show that the model with mixing parameterized from the large-scale flow deformations well reconstructs the observed asymmetric structure of the ExTL with a deeper transition layer on the cyclonic side of the jet stream. Information from the model and observations are integrated using tracer-tracer correlations between ozone (O3) and carbon monoxide (CO). Transport of chemical tracers from the stratospheric or tropospheric background to the ExTL through mixing is identified by the change of the CO-O3correlation in the CO-O3 space. The ExTL formation process simulated by the model, therefore, provides a scenario to connect the mixed air parcels to the history of mixing. An estimate of timescales of ExTL formation is made using model experiments. The results show that the fastest formation of the ExTL occurs on the isentropic levels below the subtropical jet core, e.g. around 3 weeks for 310 K, whereas at 360 K level (jet core) the formation of the ExTL needs around 3 months. Overall, this result demonstrates the important role of mixing in transport of trace gases across the tropopause.
    Journal of Geophysical Research 09/2012; 117(D18):18301-. · 3.17 Impact Factor
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    ABSTRACT: The upper troposphere/lower stratosphere (UTLS) region plays an important role in the climate system. Changes in the structure and chemical composition of this region result in particularly large changes in radiative forcings of the atmosphere. Quantifying the processes that control UTLS composition (e.g., stratosphere-troposphere exchange) therefore represents a crucial task. We assess the influence of uncertainties in the atmospheric mixing strength on global UTLS distributions of greenhouse gases (water vapor, ozone, methane, and nitrous oxide) and associated radiative effects. The study is based on multiannual simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by ERA-Interim meteorological data and on a state-of-the-art radiance code. Mixing, the irreversible part of transport, is controlled by the local horizontal strain and vertical shear of the atmospheric flow. We find that simulated radiative effects of water vapor and ozone, both characterized by steep gradients in the UTLS, are particularly sensitive to uncertainties of the atmospheric mixing strength. Globally averaged radiative effects are about 0.72 and 0.17 W/m2for water vapor and ozone, respectively. For ozone, the largest impact of mixing uncertainties is observed in the extra-tropical lower stratosphere.
    Journal of Geophysical Research 08/2012; 117(D16305). · 3.17 Impact Factor
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    ABSTRACT: The composition and dynamic structure of the upper troposphere/lower stratosphere (UTLS) have a significant impact on surface climate and its variability, through radiative and dynamic coupling. Changes in the chemical composition of this region result in particularly large changes in the radiative forcing of the atmosphere. In addition, there is growing evidence that dynamic coupling between the troposphere and stratosphere has a significant impact on regional weather and climate. Improvements of forecasts by chemistry-climate models (CCMs) therefore rely on a quantitative representation of radiative and dynamic couplings and the underlying physical and chemical processes. We will give a brief overview on physical and chemical processes, determining UTLS composition (e.g., stratosphere-troposphere exchange) and influencing the dynamical coupling with the troposphere (e.g., gravity waves). In the second part, we present results of an analysis of the influence of uncertainties of one particular process, mixing of air masses, on global UTLS distributions of greenhouse gases (water vapor, ozone, methane, and nitrous oxide) and associated radiative effects. The study is based on multi-annual simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by ERA-Interim meteorological data and on a state-of-the-art radiance code. It is shown that radiative effects of of water vapor and ozone, both characterized by steep gradients in the UTLS, are particularly sensitive to uncertainties of the atmospheric mixing strength. Globally averaged radiative effects are about 0.72 and 0.17 W/m2 for water vapor and ozone, respectively.
    07/2012;
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    ABSTRACT: We analyze horizontal transport from midlatitudes into the tropics (in-mixing) and its impact on seasonal variations of ozone, carbon monoxide and water vapor in the Tropical Tropopause Layer (TTL). For this purpose, we use three-dimensional backward trajectories, driven by ECMWF ERA-Interim winds, and a conceptual one-dimensional model of the chemical composition of the TTL. We find that the fraction of in-mixed midlatitude air shows an annual cycle with maximum during NH summer, resulting from the superposition of two inversely phased annual cycles for in-mixing from the NH and SH, respectively. In-mixing is driven by the monsoonal upper-level anticyclonic circulations. This circulation pattern is dominated by the Southeast Asian summer monsoon and, correspondingly, in-mixing shows an annual cycle. The impact of in-mixing on TTL mixing ratios depends on the in-mixed fraction of midlatitude air and on the meridional gradient of the particular species. For CO the meridional gradient and consequently the effect of in-mixing is weak. For water vapor, in-mixing effects are negligible. For ozone, the meridional gradient is large and the contribution of in-mixing to the ozone maximum during NH summer is about 50%. This in-mixing contribution is not sensitive to the tropical ascent velocity, which is about 40% too fast in ERA-Interim. As photochemically produced ozone in the TTL shows no distinct summer maximum, the ozone annual anomaly in the upper TTL turns out to be mainly forced by in-mixing of ozone-rich extratropical air during NH summer.
    Journal of Geophysical Research 05/2012; 117(D09303). · 3.17 Impact Factor
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    ABSTRACT: Using a combination of ozonesonde data and numerical simulations of the Chemical Lagrangian Model of the Stratosphere (CLaMS), the trend of tropospheric ozone (O3) during 2002-2010 over Beijing was investigated. Tropospheric ozone over Beijing shows a winter minimum and a broad summer maximum with a clear positive trend in the maximum summer ozone concentration over the last decade. The observed significant trend of tropospheric column ozone for the entire time series is 4.6% yr-1 for a mean level of 52 DU. This trend is close to the significant trend of partial column ozone in the lower troposphere (0-3 km) during summer (3.4% yr-1 for a mean level of 23 DU). Analysis of the CLaMS simulation shows that transport rather than chemistry drives most of the seasonality of tropospheric ozone. However, dynamical processes alone cannot explain the trend of tropospheric ozone in the observational data. Clearly enhanced ozone values and a negative vertical ozone gradient in the lower troposphere in the observational data emphasize the importance of photochemistry within the troposphere during spring and summer, and suggest that the photochemistry within the troposphere significantly contributed to the tropospheric ozone trend over Beijing during the last decade.
    Atmospheric Chemistry and Physics 05/2012; 12(5):11175-11199. · 4.88 Impact Factor
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    ABSTRACT: Variations in the mixing ratio of trace gases of tropospheric origin entering the stratosphere in the tropics are of interest for assessing both troposphere to stratosphere transport fluxes in the tropics and the impact on the composition of the tropical lower stratosphere of quasi-horizontal in-mixing into the tropical tropopause layer from the mid-latitude stratosphere. Here, we present a simplified chemistry scheme for the Chemical Lagrangian Model of the Stratosphere (CLaMS) for the simulation, at comparatively low numerical cost, of CO, ozone, and long-lived trace substances (CH4, N2O, CCl3F, and CO2) in the lower tropical stratosphere. The boundary conditions at the ground are represented for the long-lived trace substances CH4, N2O, CCl3F, and CO2 based on ground-based measurements. The boundary condition for CO in the free troposphere is deduced from MOPITT measurements. We find that the zonally averaged tropical CO anomaly patterns simulated by this model version of CLaMS are in good agreement with observations. The introduction of a new scheme in the ECMWF integrated forecast system (Tompkins et al., 2007) for the ice supersaturation after September 2006, results in a somewhat less good agreement between observed and simulated CO patterns in the tropical lower stratosphere after this date.
    Geoscientific Model Development Discussions 05/2011; 4:1185-1211.
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    ABSTRACT: We explore the potential of ozone observations to constrain transport processes in the tropical tropopause layer (TTL), and contrast it with insights that can be obtained from water vapour. Global fields from Halogen Occultation Experiment (HALOE) and in-situ observations are predicted using a backtrajectory approach that captures advection, instantaneous freeze-drying and photolytical ozone production. Two different representations of transport (kinematic and diabatic 3-month backtrajectories based on ERA-Interim data) are used to evaluate the sensitivity to differences in transport. Results show that mean profiles and seasonality of both tracers can be reasonably reconstructed. Water vapour predictions are similar for both transport representations, but predictions for ozone are systematically higher for kinematic transport. Compared to global HALOE observations, the diabatic model prediction underestimates the vertical ozone gradient. Comparison of the kinematic prediction with observations obtained during the tropical SCOUT-O3 campaign shows a large high bias above 390 K potential temperature. We show that ozone predictions and vertical dispersion of the trajectories are highly correlated, rendering ozone an interesting tracer for aspects of transport to which water vapour is not sensitive. We show that dispersion and mean upwelling have similar effects on ozone profiles, with slower upwelling and larger dispersion both leading to higher ozone concentrations. Analyses of tropical upwelling based on mean transport characteristics, and model validation have to take into account this ambiguity between tropical ozone production and in-mixing from the stratosphere. In turn, ozone provides constraints on transport in the TTL and lower stratosphere that cannot be obtained from water vapour.
    Atmospheric Chemistry and Physics 01/2011; · 4.88 Impact Factor
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    ABSTRACT: 1] Using isentropic trace gas gradients of O 3 and CO, the discontinuity in the chemical composition of the upper troposphere (UT) and lower stratosphere (LS) is examined on middle world isentropes from 300 to 380 K. The analysis is a follow‐up study of the dynamical discontinuity as represented by the potential vorticity (PV) gradient‐based tropopause, which is based on the product of isentropic PV gradients and wind speed. Overall, there is fairly good consistency between the chemical discontinuity in trace gas distributions and the PV gradient‐based tropopause. Trace gas gradients at the PV gradient‐based tropopause are stronger in winter than in summer, revealing the seasonal cycle of the tropopause transport barrier. The analysis of the trace gas gradients also identifies atmospheric transport pathways in the upper troposphere–lower stratosphere (UTLS). Several regions where trace gas gradients are found to be decoupled from the dynamical field indicate preferred transport pathways between the UT and LS. In particular, anomalous CO and O 3 gradients above eastern Africa, eastern Asia, and the West Pacific are likely related to convective transport, and anomalous O 3 gradients over the North Atlantic and North Pacific are related to isentropic transport connected to frequent wave breaking. The results indicate that the PV gradient‐based tropopause definition provides a good identification of the dynamical and chemical discontinuity and is therefore effective in locating the physical boundary in the UTLS.
    Journal of Geophysical Research 01/2011; 116. · 3.17 Impact Factor
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    A Kunz, P Konopka, R Müller, L L Pan
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    ABSTRACT: 1] Since its inception, the dynamical tropopause based on potential vorticity (PV) is identified by the PV gradient on isentropes. Conceptually, significant isentropic gradients shown on the middle world PV maps reflect the underlying transport barrier associated with the tropopause, formed by jet streams that separate tropospheric air masses at low latitudes and stratospheric air masses at high latitudes. Largely owing to the lack of a general method, the dynamical tropopause has often been represented by a PV value chosen ad hoc without any temporal or spatial differentiation. In this work, we present a method for determining the PV isoline of the dynamical tropopause based on the isentropic PV gradients. Using 1 year of data from the European Centre for Medium–Range Weather Forecasts, the spatial and temporal variability of this PV gradient‐based dynamical tropopause is examined. The results show that in general there is a broad distribution of PV values at the dynamical tropopause, ranging from 1.5 to 5 potential vorticity units. Therefore, a fixed PV surface for all isentropes and seasons does not accurately represent the location of the "tropopause barrier." The PV at the dynamical tropopause increases with increasing potential temperature. This increase is more pronounced in the Southern Hemisphere than in the Northern Hemisphere. The seasonal cycle shows higher PV values at the dynamical tropopause during summer than during winter. This seasonal cycle is larger on higher isentropes. The dispersion of the PV at the dynamical tropopause about its mean is twofold larger during summer and autumn than during winter and spring in both hemispheres.
    Journal of Geophysical Research 01/2011; 116. · 3.17 Impact Factor
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    ABSTRACT: Model simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by wind fields of the National Center for Environmental Prediction (NCEP) were performed in the midlatitude tropopause region in April 2008 to study two research flights conducted during the START08 campaign. One flight targeted a deep tropospheric intrusion and another flight targeted a deep stratospheric intrusion event, both of them in the vicinity of the subtropical and polar jet. Air masses with strong signatures of mixing between stratospheric and tropospheric air masses were identified from measured CO-O3 correlations, and the characteristics were reproduced by CLaMS model simulations. CLaMS simulations in turn complement the observations and provide a broader view of the mixed region in physical space. Using artificial tracers of air mass origin within CLaMS yields unique information about the transport pathways and their contribution to the composition in the mixed region from different transport origins. Three different regions are examined to categorize dominant transport processes: (1) on the cyclonic side of the polar jet within tropopause folds where air from the lowermost stratosphere and the cyclonic side of the jet is transported downward into the troposphere, (2) on the anticyclonic side of the polar jet around the 2 PVU surface air masses, where signatures of mixing between the troposphere and lowermost stratosphere were found with large contributions of air masses from low latitudes, and (3) in the lower stratosphere associated with a deep tropospheric intrusion originating in the tropical tropopause layer (TTL). Moreover, the time scale of transport from the TTL into the lowermost stratosphere is in the range of weeks whereas the stratospheric intrusions occur on a time scale of days.
    Journal of Geophysical Research 01/2011; 116. · 3.17 Impact Factor