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Report on the Regional SPARC Science Workshop17-18 September 2007, Bremen, GermanyB.-M. Sinnhuber, Institute of Environmental Physics, University of Bremen
(bms@iup.physik.uni-bremen.de) A two-day workshop on SPARC related activities in Germany and near-by Europe took place in Bremen, Germany from 17 to 18 September 2007, back-to-back with the SSG meeting.. The idea was to foster regional collaborations in SPARC related research. The concept was well received, and at a following discussion with the SSG it was suggested that this idea of regional meetings might be expanded, although it was realised that this would not work equally well in different parts of the world. The workshop consisted of a series of invited and contributed talks, loosely arranged according to the three SPARC Initiatives. Session 1: Chemistry Climate Interactions including TTL Cornelius Schiller presented airborne measurements of total water in the tropical tropopause layer (TTL) from three tropical missions in Brazil (TROCCINOX), Australia (SCOUT-O3) and West Africa (AMMA/SCOUT-O3). During all three experiments, convection penetrating the tropopause and moistening the stratosphere up to 420 K was observed. However, extrapolation of these events does not imply a major impact on the stratospheric water vapour budget. Although the averaged H2O abundances and relative humidities at the cold point varied substantially for the different campaigns, depending on season and geographical locaion, the observed mixing ratios were all consistent with recent saturation history (i.e. over the preceeding 10 days) as demonstrated in a backward trajectory analysis. Michael Volk presented recent in situ observations with the Geophysica aircraft to identify key transport processess in the TTL. A large amount of in situ trace gas observations in the TTL and the lower stratosphere (up to 20 km) has been obtained during recent deployments of the Geophysica over Brazil (TROCCINOX), the Maritime Continent (SOCUT-O3) and West Africa (AMMA/SCOUT-O3). Measurements were made by the University of Frankfurt’s High Altitude Gas Analyzer (HAGAR) (long-lived tracers N2O, CH4, CO2, H2, F12, F11, H-1211, SF6), the Cryogenically Operated Laser Diode (COLD) (CO), and the Fast Ozone ANalyzer (FOZAN) (O3). The three campaigns comprise over 30 tropical flights, and include flights aimed at improving our understanding of large-scale transport, and flights aimed at investigating the impact of mesoscale convective system (under both continental and marine conditions) on the tropical upper troposphere/lower stratosphere (UTLS). Measurements were used to i) contrast observations of the background TTL and convectively influenced air, ii) diagnose irreversible mixing of convectively overshooting air with the background TTL, iii) detect isentropic mixing across the subtropical tropopause and the subtropical transport barrier, and iv) to assess slow up-welling in the TTL and the lower stratosphere.
To study the composition of air entering the stratosphere, Paul Konopka presented multi-annual simulations (2001-2006) with the Chemical Lagrangian Model of the Stratosphere (CLaMS). In addition to the convective and radiative transport, the composition of air within the TTL is strongly influenced by mixing on a time scale of weeks or even months. Based on the CLaMS transport studies, in which mixing can be completely switched off, it was deduced that vertical mixing, driven mainly by the vertical shear in the tropical flanks of the subtropical jets and, to some extent in the outflow regions of the large-scale convection, offers an explanation for the upward transport of trace species from the main convective outflow layer, around 350 K, up to 380 K. Furthermore, the seasonal dependence of the composition of the TTL is controlled by the isentropic mixing across the subtropical jets with a strong influence of the Asian monsoon during the boreal summer. Kirstin Krüger showed results from a Lagrangian study using diabatic heating rates to calculate vertical ascent of the mass transport through the TTL, instead of vertical winds from assimilation systems, which tend to be too strong and very noisy. With this alternative method, much slower and more realistic diabatic ascent rates in the upper part of the TTL were obtained, in contrast to previous published results. The mean residence time was determined to be approximately 40 days for the 360 to 380 K layer during the NH winter 2000/2001. Klaus Pfeilsticker reported on the contribution of very short-lived species (VSLS) to the burden of stratospheric halogen as inferred from recent balloon soundings in the TTL and UTLS over north eastern Brazil. For all three halogens potentially relevant for stratospheric ozone (chlorine, bromine, iodine), the quasi-simultaneous detection of VSL organic and inorganic halogens species across the TTL reveals the following contributions to their total stratospheric budgets: for chlorine 100 – 150 ppt or 3 -4 %, for bromine 4.0 ± 2.5 ppt or 20 % and for iodine < 0.3 ppt. These results are in reasonably good agreement with the respective assessments provided by the recent UNEP WMO (2007) report. With respect to present errors in assessing such budgets, the detection of any change in the influx of VSLS into the stratosphere due to climate change is likely to require a decade-long monitoring of the tropical UTLS by modern high-precision measurement techniques. Peter Hoor presented an analysis of transport pathways and time scales in the lowermost stratosphere using the relationship between N2O and CO2 , which is interpreted as mixing lines between tropospheric and stratospheric air. The CO2 intercept of the CO2-N2O relation in the lower stratosphere evaluated for N2O at the tropical tropopause can be regarded as the CO2 mixing ratio at the tropical tropopause, when the air was mixed into the stratosphere. The relation between this CO2-mixing ratio and the well known tropospheric seasonal CO2 cycle provides information on the time elapsed since last contact with the tropopause. Whereas mean age describes only the mean of a transit time distribution, the new method helps to constrain the younger part of the age spectrum, and therefore the range of very short-lived compounds that can enter the stratosphere. Session 2: Detection, Attribution and Prediction of Stratospheric Change Wolfgang Steinbrecht discussed the evolution of ozone in recent years. In the upper stratosphere (around 40 km) lidar and microwave measurements from various stations of the Network for the Detection of Atmospheric Composition Change (NDACC), as well as satellite data, indicate that the ozone decline of the 1980s and 1990s has not continued after 2000. At all NDACC stations outside polar regions, upper stratospheric ozone has in fact been increasing in recent years. This is attributed to the decline of stratospheric chlorine, and indicates success of the Montreal Protocol. At more northern stations, the recent increase is modulated substantially by temperature variations. While this effect is expected to continue in the future, evidence for the beginning of ozone recovery can be seen in the upper stratosphere. However, the same cannot be said for total global ozone. Several factors have contributed substantially (on the order of 5 to 15 DU) to the higher ozone columns observed in recent years at northern mid-latitudes, e.g. above Hohenpeissenberg: the slow removal of stratospheric aerosol in the years after the 1991 Pinatubo eruption, the decline in the winter North-Atlantic-Oscillation Index after its peak around 1990, and the recent solar maximum. Compared to these factors, the expected ozone recovery due to chlorine turnaround is much smaller (only about 2 DU), and currently cannot be identified with statistical significance. Gabi Stiller presented calculations of stratospheric age-of-air diagnosed from MIPAS/ENVISAT observations of SF6. The global data set of the mean age of stratospheric air was derived from MIPAS SF6 observations, covering the period September 2002 to March 2004.. This data set demonstrates high seasonal and interannual variability of the stratospheric mean age in middle and high latitudes as well as inter-hemispheric differences; frequent intrusions of mesospheric air into the polar winter vortices during all polar winters are observed. The data set will be used to validate CCMs and GCMs and there are plans to be extend it for the complete MIPAS mission lifetime. Mark Weber talked about the role of the Brewer-Dobson (BD) circulation and solar activity on stratospheric ozone. Using SCIAMACHY and GOME data up to 2007, he showed an update from the last WMO ozone assessment on the compact relationship between the strength of the BD circulation (here expressed by the integrated absolute winter eddy heat flux) and the spring-to-fall ratio of total ozone confirming the close coupling of dynamics and polar chemistry in the interannual variability of polar ozone. The time series of the monthly mean absolute eddy heat flux added from both hemispheres show a clear step-like rise that correlates with the drop in tropical lower stratospheric water vapour after 2000 observed by SAGE and HALOE up to 2005. Both the 11-year solar cycle and the enhancement of the BD circulation are main drivers for the rather rapid increase in NH total ozone after the middle 1990s, as derived from a regression analysis of 27 years of SBUV total ozone data. Markus Rex analysed the impact of recently published new laboratory measurements of the absorption cross sections of ClOOCl on our understanding of polar stratospheric chlorine and ozone chemistry. He presented comparisons between model calculations and in situ measurements of ClO, ClOOCl and ozone loss rates and concluded that if the new cross sections are correct, a fundamental lack of understanding of stratospheric chlorine chemistry limits our understanding of observed ozone loss rates, and that there must exist an unknown process that leads to the breakdown of ClOOCl in the stratosphere and accounts for most of the observed ozone loss. Session 3: Stratosphere-Troposphere Dynamical Coupling The introduction to this session was given in an invited overview talk on stratosphere-troposphere dynamical coupling by Mark Baldwin. The talk presented recent evidence for the impact of the stratosphere on weather and climate at the Earth's surface. For example, springtime stratospheric ozone loss in the Southern Hemisphere (SH) has driven changes in surface climate over Antarctica. In the Northern Hemisphere (NH), circulation changes in the lower stratosphere during winter precede similar changes at the surface with substantial changes to surface weather and the likelihood of extreme weather events. The mechanisms for the coupling between stratosphere and troposphere are not well understood at present. Predicting how the stratosphere will affect climate change will require coupled chemistry climate models. Ideas were presented on how the fidelity of current models, with respect to stratosphere-troposphere coupling, could be tested. Dieter Peters discussed the impact of zonally asymmetric ozone anomalies on stratospheric temperatures, the strength of the polar vortex and planetary wave propagation. For NH winter, decadal means of the zonally asymmetric ozone components are derived from ERA-40 and used in MAECHAM5 to investigate their effects on temperature and planetary wave propagation in the troposphere, stratosphere and lower mesosphere. The analysed stratospheric ozone from ERA-40 shows a strong increase in wave 1 structure during the last decades, with amplitudes of about 10% of the zonal mean ozone during the 1990’s. Based on model calculations, it was found that the related radiation perturbations induce significant changes in temperature, increasing with height due to an increase in amplitude and shift in phase of wave 1, i.e. a shift of the polar vortex further from the pole. Furthermore, the accompanying changes in the three-dimensional wave activity flux vector reveal that regions of strong vertically propagating wave trains become much weaker over the Asian/North Pacific region and much stronger over the North America/North Atlantic region. This suggests that the decadal change in zonally asymmetric ozone may have contributed largely to observed temperature trends in the stratosphere and lower mesosphere by efficiently altering the balance between large-scale dynamics and planetary wave propagation. Björn-Martin Sinnhuber presented observational evidence of a correlation between stratospheric ozone anomalies at high latitudes in summer and autumn with total ozone anomalies in the following spring. Not only is springtime total ozone correlated with mid-stratospheric ozone several months before, but there exists a statistically significant correlation between ozone anomalies in autumn and the wave activity as expressed by the Eliassen-Palm flux during mid-winter. This unexpected finding raises the question of what controls the interannual variability of Arctic total ozone in spring, and at the same time offers an approach from predicting total ozone several months in advance. It is currently still unclear what the underlying mechanisms for this observed correlation are. Peter Preusse discussed global gravity wave modelling constrained by satellite measurements. A typical annual cycle of gravity wave temperature amplitudes retrieved from infrared emission limb sounding measurements by SABER has been compared to global ray tracing simulations of gravity waves based on a homogeneous and isotropic source at 5 km altitude tuned to match the zonal mean distributions in July. Salient features of global maps for the various seasons, as well as the overall annual cycle are matched, though some structures due to localized wave forcing are missing. In contrast to the assumption generally made in gravity wave parameterization schemes, even average gravity wave propagation can exceed 20 degrees in latitude. The simulations will also be used to test whether horizontal refraction is an important process for gravity wave-mean flow interaction. Thomas Reddmann presented simulations with the 3D model KASIMA using NOx enhancements in the lower mesosphere derived from observations from MIPAS. Covering the period from mid 2002 to early 2004, these observations represent one of the most complete data sets, and include the strong solar proton event in fall 2003 and intrusions connected to auroral activity during the Arctic and Antarctic winters. The comparison of the disturbed run with a control run reveals persistent reduction of ozone concentration for several months in the middle stratosphere but which is restricted to high latitudes. By including ion cluster chemistry in the model, the HNO3 build-up observed in the upper stratosphere by MIPAS/ENVISAT in Antarctic winter 2003 and the subsequent Arctic winter can be reproduced qualitatively.Katja Matthes reported on solar cycle studies at the Freie Universität Berlin. Model simulations with two GCMs, MAECHAM5-Messy and WACCM, show results comparable to observational estimates in the annual mean as well as during NH winter. During solar maximum, higher temperatures exist in the tropical upper stratosphere that lead to dynamical changes throughout the atmosphere. Certain aspects of the observed modulation of the polar night jet and the BD circulation, as well as the dependence of the solar signal on the phase of the QBO, can be reproduced in the simulations, e.g. a significant positive AO signal during NH winter in the stratosphere and troposphere.
The workshop concluded with a joint poster session together with the SSG. |