Search

This work is an extended evaluation of near surface ozone as part of the global reanalysis of atmospheric composition, produced within the European Funded project MACC (Monitoring Atmospheric Composition and Climate). It includes an evaluation over the period 2003–2012 and provides an overall assessment of the modelling system performance with respect to near surface ozone for specific European subregions. Measurements at rural locations from the European Monitoring and Evaluation Program (EMEP) and the European Air Quality Database (AirBase) were used for the evaluation assessment. The annual overall error of near surface ozone reanalysis is on average 24% over Europe, the highest found over Scandinavia (27%) and the lowest over the Mediterranean marine stations (21%). Near surface ozone shows mostly a negative bias in winter and a positive bias during warm months. Assimilation reduces the bias in near surface ozone and its impact is mostly notable in winter. With respect to the seasonal cycle, the MACC reanalysis reproduces the photochemically driven broad spring-summer maximum of surface ozone of central and south Europe. However, it does not capture adequately the early spring peak and the shape of the seasonality at northern and north-eastern Europe. The diurnal range of surface ozone, which is an indication of the local photochemical production processes, is reproduced fairly well, with a tendency for a small overestimation during the warm months for most subregions (especially in central and southern Europe). Possible reasons leading to discrepancies between the MACC reanalysis and observations are discussed.

This study provides a new look at the observed and calculated long-term temperature changes since 1958 for the region extending from the lower troposphere up to the lower stratosphere of the Northern Hemisphere. The analysis is mainly based on monthly layer mean temperatures derived from geopotential height thicknesses between specific pressure levels. Layer mean temperatures from thickness improve homogeneity in both space and time and reduce uncertainties in the trend analysis. Datasets used include the NCEP/NCAR I reanalysis, the Free University of Berlin (FU-Berlin) and the RICH radiosonde datasets as well as historical simulations with the CESM1-WACCM global model participating in CMIP5. After removing the natural variability with an autoregressive multiple regression model our analysis shows that the time interval of our study 1958–2011 can be divided in two distinct sub-periods of long term temperature variability and trends; before and after 1980s. By calculating trends for the summer time to reduce interannual variability, the two periods are as follows. From 1958 until 1979, non-significant trends or slight cooling trends prevail in the lower troposphere (0.06 ± 0.06 °C decade−1 for NCEP and −0.12 ± 0.06 °C decade−1 for RICH). The second period from 1980 to the end of the records shows significant warming trends (0.25 ± 0.05 °C decade−1 for both NCEP and RICH). Above the tropopause a persistent cooling trend is clearly seen in the lower stratosphere both in the pre-1980s period (−0.58 ± 0.17 °C decade−1 for NCEP, −0.30 ± 0.16 °C decade−1 for RICH and −0.48 ± 0.20 °C decade−1 for FU-Berlin) and the post-1980s period (−0.79 ± 0.18 °C decade−1 for NCEP, −0.66 ± 0.16 °C decade−1 for RICH and −0.82 ± 0.19 °C decade−1 for FU-Berlin). The cooling in the lower stratosphere is a persistent feature from the tropics up to 60 north for all months. At polar latitudes competing dynamical and radiative processes are reducing the statistical significance of these trends. Model results are in line with re-analysis and the observations, indicating a persistent cooling in the lower stratosphere during summer before and after the 1980s by −0.33 °C decade−1; a feature that is also seen throughout the year. However, the lower stratosphere modelled trends are generally lower than re-analysis and the observations. The contrasting effects of ozone depletion at polar latitudes in winter/spring and the anticipated strengthening of the Brewer Dobson circulation from man-made global warming at polar latitudes are discussed. Our results provide additional evidence for an early greenhouse cooling signal in the lower stratosphere before the 1980s, which it appears well in advance relative to the tropospheric greenhouse warming signal. Hence it may be postulated that the stratosphere could have provided an early warning of man-made climate change. The suitability for early warning signals in the stratosphere relative to the troposphere is supported by the fact that the stratosphere is less sensitive to changes due to cloudiness, humidity and man-made aerosols. Our analysis also indicates that the relative contribution of the lower stratosphere vs. the upper troposphere low frequency variability is important for understanding the added value of the long term tropopause variability related to human induced global warming.

Observations show that the Mediterranean troposphere is characterized by a marked enhancement in summertime ozone with a maximum over the Eastern Mediterranean. This has been linked to enhanced ozone photochemical production and subsidence under cloud-free anticyclonic conditions. The Eastern Mediterranean region has among the highest levels of background tropospheric ozone around the globe and it can be considered as a global air pollution hotspot. A 12 yr climatological analysis (1998–2009) of free tropospheric ozone was carried out over the region based on ECMWF (European Centre for Medium-Range Weather Forecasts) ERA-interim reanalysis data and simulations with the EMAC (ECHAM5-MESSy for Atmospheric Chemistry) atmospheric chemistry climate model. EMAC is nudged towards the ECMWF analysis data and includes a stratospheric ozone tracer. A characteristic summertime pool with high ozone concentrations is found in the middle troposphere over the Eastern Mediterranean/Middle East (EMME) by ERA-interim ozone data, which is supported by Tropospheric Emission Spectrometer (TES) satellite ozone data and simulations with EMAC. The enhanced ozone over the EMME is a robust feature, propagating down to lower free tropospheric levels. The investigation of ozone in relation to potential vorticity and water vapour and the stratospheric ozone tracer indicates that the dominant mechanism causing the free tropospheric ozone pool is downward transport from the upper troposphere and lower stratosphere associated with the enhanced subsidence and the limited outflow transport that dominates the summertime EMME circulation. The implications of these summertime high free tropospheric ozone values on the seasonal cycle of near surface ozone over the Mediterranean are discussed.

Regional air quality decadal simulations were carried out using the air quality model CAMx driven off-line by the regional climate model RegCM3 for the time slice 1991–2000 using two different datasets of external meteorological forcing to constrain RegCM3: the ERA40 global atmospheric reanalysis dataset and the output from the GCM ECHAM5. The focus of this work is to compare the perfect lateral boundary conditions experiment with the GCM driven control experiment and to investigate how this external meteorological forcing affects near surface ozone. The different RegCM3 meteorological forcings resulted in changes of near surface ozone over Europe ranging between ±5 ppb for winter and summer, while all model parameterizations and anthropogenic emissions remained unchanged. Changes in near surface ozone are induced by changes in meteorological fields and biogenic emissions, which are on-line calculated and meteorology-dependent. The model simulations suggest that the change in solar radiation is the factor that mostly modulates the ozone changes in summer. During winter season it is found that the induced changes in NOx explain about 40% of the ozone variability. The meteorological induced changes in biogenic emissions are quite low for winter with rather small impact on ozone while they are more temperature than radiation dependent. Using multiple regression analysis to associate the changes in near surface ozone with the respective changes in selected meteorological parameters and ozone precursors, an explained variance of 70% in summer and 60% in winter is reproduced.

A wide range of measurements was carried out in central and southeastern Europe within the framework of the EU-project STACCATO (Influence of Stratosphere-Troposphere Exchange in a Changing Climate on Atmospheric Transport and Oxidation Capacity) with the principle goal to create a comprehensive data set on stratospheric air intrusions into the troposphere along a rather frequently observed pathway over central Europe from the North Sea to the Mediterranean Sea. The measurements were based on predictions by suitable quasi-operational trajectory calculations using ECMWF forecast data. A predicted deep Stratosphere to Troposphere Transport (STT) event, encountered during the STACCATO period on 20–21 June 2001, could be followed by the measurements network almost from its inception. Observations provide evidence that the intrusion affected large parts of central and southeastern Europe. Especially, the ozone lidar observations on 20–21 June 2001 at Garmisch-Partenkirchen, Germany captured the evolution of two marked tongues of high ozone with the first one reaching almost a height of 2 km, thus providing an excellent data set for model intercomparisons and validation. In addition, for the first time to our knowledge concurrent measurements of the cosmogenic radionuclides 10Be and 7Be and their ratio 10Be/7Be are presented together as stratospheric tracers in a case study of a stratospheric intrusion. The ozone tracer columns calculated with the FLEXPART model were found to be in good agreement with water vapour satellite images, capturing the evolution of the observed dry streamers of stratospheric origin. Furthermore, the time-height cross section of ozone tracer simulated with FLEXPART over Garmisch-Partenkirchen captures with many details the evolution of the two observed high-ozone filaments measured with the IFU lidar, thus demonstrating the considerable progress in model simulations. Finally, the modelled ozone (operationally available since October 1999) from the ECMWF (European Centre for Medium-Range Weather Forecasts) atmospheric model is shown to be in very good agreement with the observations during this case study, which provides the first successful validation of a chemical tracer that is used operationally in a weather forecast model. This suggests that coupling chemistry and weather forecast models may significantly improve both weather and chemical forecasts in the future.