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SUMMARYEstimates of annual-mean stratospheric temperature trends over the past twenty years, from a wide variety of models, are compared both with each other and with the observed cooling seen in trend analyses using radiosonde and satellite observations. The modelled temperature trends are driven by changes in ozone (either imposed from observations or calculated by the model), carbon dioxide and other relatively well-mixed greenhouse gases, and stratospheric water vapour.The comparison shows that whilst models generally simulate similar patterns in the vertical pro le of annualand global-mean temperature trends, there is a signi cant divergence in the size of the modelled trends, even when similar trace gas perturbations are imposed. Coupled-chemistry models are in as good agreement as models using imposed observed ozone trends, despite the extra degree of freedom that the coupled models possess.The modelled annual-and global-mean cooling of the upper stratosphere (near 1 hPa) is dominated by ozone and carbon dioxide changes, and is in reasonable agreement with observations. At about 5 hPa, the mean cooling from the models is systematically greater than that seen in the satellite data; however, for some models, depending on the size of the temperature trend due to stratospheric water vapour changes, the uncertainty estimates of the model and observations just overlap. Near 10 hPa there is good agreement with observations. In the lower stratosphere (20-70 hPa), ozone appears to be the dominant contributor to the observed cooling, although it does not, on its own, seem to explain the entire cooling.Annual-and zonal-mean temperature trends at 100 hPa and 50 hPa are also examined. At 100 hPa, the modelled cooling due to ozone depletion alone is in reasonable agreement with the observed cooling at all latitudes. At 50 hPa, however, the observed cooling at midlatitudes of the northern hemisphere signi cantly exceeds the modelled cooling due to ozone depletion alone. There is an indication of a similar effect in high northern latitudes, but the greater variability in both models and observations precludes a rm conclusion.The discrepancies between modelled and observed temperature trends in the lower stratosphere are reduced if the cooling effects of increased stratospheric water vapour concentration are included, and could be largely removed if certain assumptions were made regarding the size and distribution of the water vapour increase. However, given the uncertainties in the geographical extent of water vapour changes in the lower stratosphere, and the time period over which such changes have been sustained, other reasons for the discrepancy between modelled and observed temperature trends cannot be ruled out.

[1] A Lagrangian methodology is applied to operational European Centre for Medium-Range Weather Forecasts analyses to study upward cross-tropopause exchange (troposphere to stratosphere exchange (TSE)) and downward cross-tropopause exchange (stratosphere to troposphere exchange (STE)) in the extratropical Northern Hemisphere for the period from May 1995 to April 1996. A residence time criterion serves to distinguish between short-(<1 -2 days) and long-lasting exchange events. The adopted approach enables identification of a range of novel aspects of extratropical cross-tropopause transport which are of primary importance when assessing its chemical impact. For the considered year the annual cycle of the hemispherically integrated net cross-tropopause mass flux compares well with estimates from previous studies. The part of STE and TSE which occurs with equal amplitude in both directions (referred to as ''symmetric two-way exchange'') has only a weak annual cycle and, for short residence times, a larger amplitude than the net exchange. The meridional distribution of the net flux reveals an upward branch in the subtropics, pronounced downward exchange in the midlatitudes and weak upward fluxes in the Arctic region. Detailed geographical distributions show significant zonal asymmetries with maximum exchange in the Atlantic and Pacific storm track regions. It is further shown that STE (TSE) events occur typically below (slightly above) the climatological tropopause position. Deep exchange (that is, rapid vertical transport between the stratosphere and the lower troposphere) is strongest during winter and confined to the midlatitude regions of baroclinic wave activity. The localized source regions for deep TSE indicate that pollutants emitted in eastern North America and Asia have an enhanced potential for being rapidly transported into the lowermost stratosphere.

Consideration of the time scales and vertical extent of stratosphere-troposphere exchange events reveals new insight into this phenomenon and its impact on surface ozone. However, STE is still poorly understood and inadequately quantified, due to the involvement of physical and dynamical processes on local to global scales (Holton et al. 1995), and conceptual problems.On a long-term and global scale, and in the zonally averaged sense (Brewer 1949), there is slow ascent from the troposphere to the stratosphere in the Tropics (Plumb 1996;Mote et al. 1996), quasi-isentropic transport to the extratropics in the stratosphere (Waugh 1996), and downward flow from the stratosphere to the troposphere in middle and higher latitudes (Fig. 1). This circulation is related to the dissi-

Two distinct dynamical processes near the dynamical tropopause (2-PVU surface) and their relation are discussed in this study: stratosphere–troposphere exchange (STE) and the formation of distinct potential vorticity (PV) structures in the form of stratospheric and tropospheric streamers and cutoffs on isentropic surfaces. Two previously compiled climatologies based upon the 15-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-15) dataset (from 1979 to 1993) are used to establish and quantify the link between STE and these PV structures. An event-based analysis reveals a strong relation between the two processes. For instance, on isentropes below 320 K, 30%–50% of the stratospheric streamers are associated with downward STE. In the reverse perspective, between 60% and 80% of all STE events between 290 and 350 K are found in the vicinity of at least one PV structure. On different isentropes, the averaged downward (STT) and upward (TST) mass fluxes associated with PV structures are quantified. As a novel quantity, the activity of a particular PV structure is measured as the STT/TST flux per unit length of its boundary on the considered isentropic level. The STT activity for stratospheric streamers and the TST activity of tropospheric streamers reach similar values of 3 × 109 kg km−1 h−1. Thereby, the flux is not uniformly distributed along a streamer’s boundary. STT (TST) is found preferentially on the upstream (downstream) side of stratospheric streamers, and vice versa for tropospheric streamers. This asymmetry is lost for cutoffs, for which an essentially uniform distribution results along the boundaries. Finally, the link between STE and PV structures shows considerable geographical variability. Particularly striking is the fact that nearly all deep STT events (reaching levels below 700 hPa) over central Europe and the North American west coast are associated with a stratospheric streamer.

Abstract. An important part of extra-tropical stratosphere-to-troposphere transport occurs in association with baroclinic wave breaking and cut-off decay at the tropopause. In the last decade many studies have attempted to estimate stratosphere-troposphere exchange (STE) in such synoptic events with various methods, and more recently efforts have been made to inter-compare these methods. These inter-comparisons show large variations between estimates from different methods. This large uncertainty points to a need to thoroughly evaluate such methods, assess the realism of the resulting STE estimates and determine the sensitivities to intrinsic parameters of the methods. The present study focuses on a trajectory-based Lagrangian method which has been applied in the past to climatological studies. This method is applied here to the quantification of STE in the context of a typical baroclinic wave breaking event. The analysis sheds light on (i) the complex three-dimensional temporal and spatial structures that are associated with the rapid inflow of stratospheric air into the troposphere, (ii) the variation of STE mass flux with the choice of the dynamical tropopause definition within 1.5 to 5 PVU, (iii) the sensitivity of the results to resolution, and in particular the minimum spatial resolution of 1°×1° required to reasonably capture STE fluxes in this wave breaking event, (iv) the effective removal of spurious exchange events using a threshold residence time larger than 8 h.

[1] Deep stratosphere-to-troposphere transport (STT) conveying ozone-rich stratospheric air to the lower troposphere in the extratropics can episodically increase ozone concentrations in the lower troposphere. However, dynamical aspects of the descent, including dispersion and mixing with the surrounding tropospheric air and necessary conditions for reaching the lower troposphere, are not clearly understood yet. This study focuses on August 2006, as daily balloon sonde measurements were made from many sites covering North America within the Intercontinental Chemical Transport Experiment Ozonesonde Network Study campaign. During this period, four profiles were found with clear signs of deep STT. A mesoscale model was used together with trajectory calculation to represent these events. Over 10 days, 20 distinct clusters of trajectories were identified as significant deep STT events, including three observed. The four largest clusters carried 41, 35, 25, and 16 × 10 12 kg of mass of air, respectively. A dynamical analysis was performed on the three observed events that were captured numerically. The descents showed three distinct phases: (1) crossing of the tropopause, (2) free descent, and (3) quasi-horizontal dispersion in the lower troposphere. Clusters are rapidly sliding down sloping isentropes while being slowly diabatically cooled (approximately −1 K d −1 ). The tilt in the isentropes along the descent is due to an approximately equal combination of a negative potential temperature anomaly at the tropopause during phase 1 and a nearby baroclinic zone at the ground. The combination of these two conditions appears to be necessary for reaching the lower troposphere. In the three cases, the clusters stayed compact until they reach the lower troposphere, and it is estimated that approximately 80% of the ozone of stratospheric origin is released directly in the lower troposphere.

A new stratospheric chemical-dynamical data assimilation system was developed, based upon an ensemble Kalman filter coupled with a Chemistry-Climate Model [i.e., the intermediate-complexity general circulation model Fast Stratospheric Ozone Chemistry (IGCM-FASTOC)], with the aim to explore the potential of chemical-dynamical coupling in stratospheric data assimilation. The system is introduced here in a context of a perfect-model, Observing System Simulation Experiment. The system is found to be sensitive to localization parameters, and in the case of temperature (ozone), assimilation yields its best performance with horizontal and vertical decorrelation lengths of 14 000 km (5600 km) and 70 km (14 km). With these localization parameters, the observation space background-error covariance matrix is underinflated by only 5.9% (overinflated by 2.1%) and the observation-error covariance matrix by only 1.6% (0.5%), which makes artificial inflation unnecessary. Using optimal localization parameters, the skills of the system in constraining the ensemble-average analysis error with respect to the true state is tested when assimilating synthetic Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) retrievals of temperature alone and ozone alone. It is found that in most cases background-error covariances produced from ensemble statistics are able to usefully propagate information from the observed variable to other ones. Chemical-dynamical covariances, and in particular ozone-wind covariances, are essential in constraining the dynamical fields when assimilating ozone only, as the radiation in the stratosphere is too slow to transfer ozone analysis increments to the temperature field over the 24-h forecast window. Conversely, when assimilating temperature, the chemicaldynamical covariances are also found to help constrain the ozone field, though to a much lower extent. The uncertainty in forecast/analysis, as defined by the variability in the ensemble, is large compared to the analysis error, which likely indicates some amount of noise in the covariance terms, while also reducing the risk of filter divergence.