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Eleftheratos K.,National and Kapodistrian University of Athens | Eleftheratos K.,Biomedical Research Foundation of the Academy of Athens | Isaksen I.,University of Oslo | Zerefos C.,National and Kapodistrian University of Athens | And 6 more authors.
Water, Air, and Soil Pollution | Year: 2013

The results of a comparison between monthly mean ozone column variations calculated from the chemical transport model Oslo CTM2 and those derived from solar backscatter ultraviolet (SBUV) satellite observations are presented for the period 1998-2009. Monthly mean total ozone derived from improved model simulations were used to compute monthly zonal means over 10 latitude zones over the northern and southern hemispheres. Ozone column variations from Oslo CTM2 are highly correlated with SBUV retrievals at all latitude zones. Equatorial zonal winds at 30 hPa were used as index to study the impact of quasi-biennial oscillation (QBO) on ozone. Correlations between modeled ozone and the QBO were found to be the order of +0.8 in the tropics. The impact of QBO was most pronounced at equatorial latitudes with amplitudes of +4 to -4 %. Seasonal variations in surface ozone and tropospheric ozone column calculated by the model are also presented. © 2013 Springer Science+Business Media Dordrecht.

Zerefos C.S.,Academy of Athens | Zerefos C.S.,Navarino Environmental Observatory Neo | Tourpali K.,Aristotle University of Thessaloniki | Zanis P.,Aristotle University of Thessaloniki | And 7 more authors.
Atmospheric Chemistry and Physics | Year: 2014

This study provides a new look at the observed and calculated long-term temperature changes from the lower troposphere to the lower stratosphere since 1958 over the Northern Hemisphere. The data sets include the NCEP/NCAR reanalysis, the Free University of Berlin (FU-Berlin) and the RICH radiosonde data sets as well as historical simulations with the CESM1-WACCM global model participating in CMIP5. The analysis is mainly based on monthly layer mean temperatures derived from geopotential height thicknesses in order to take advantage of the use of the independent FU-Berlin stratospheric data set of geopotential height data since 1957. This approach was followed to extend the records for the investigation of the stratospheric temperature trends to the earliest possible time. After removing the natural variability with an autoregressive multiple regression model our analysis shows that the period 1958-2011 can be divided into two distinct sub-periods of long-term temperature variability and trends: before and after 1980. By calculating trends for the summer time to reduce interannual variability, the two periods are as follows. From 1958 until 1979, a non-significant trend (0.06 ± 0.06 °C decade-1 for NCEP) and slightly cooling trends (-0.12 ± 0.06 °C decade-1 for RICH) are found in the lower troposphere. The second period from 1980 to the end of the records shows significant warming (0.25 ± 0.05 °C decade-1 for both NCEP and RICH). Above the tropopause a significant cooling trend is clearly seen in the lower stratosphere both in the pre-1980 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-1980 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 persists throughout the year from the tropics up to 60° N. At polar latitudes competing dynamical and radiative processes reduce the statistical significance of these trends. Model results are in line with reanalysis and the observations, indicating a persistent cooling (-0.33 °C decade-1) in the lower stratosphere during summer before and after 1980; a feature that is also seen throughout the year. However, the lower stratosphere CESM1-WACCM modelled trends are generally lower than reanalysis 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 1980, which appears well in advance relative to the tropospheric greenhouse warming signal. The suitability of 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 versus 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. © 2014 Author(s).

Kalabokas P.D.,Academy of Athens | Kalabokas P.D.,European Commission - Joint Research Center Ispra | Thouret V.,CNRS Laboratory for Aerology | Cammas J.-P.,CNRS Laboratory for Aerology | And 4 more authors.
Tellus, Series B: Chemical and Physical Meteorology | Year: 2015

In continuation of previous research for evaluation of the high ozone levels observed during summer time over the eastern Mediterranean, MOZAIC profiles collected at the airport of Cairo from 1994 to 2008 are analysed. Average profiles corresponding, respectively, to the highest and the lowest ozone mixing ratios for the 0-1.5km layer over Cairo in summer (JJA) (94 profiles) are examined along with their corresponding composite maps of geopotential height (and anomalies), vertical velocity (and anomalies), specific humidity anomalies, precipitable water anomalies, air temperature anomalies and wind speed at 850 hPa. In addition, backward trajectories arriving in the boundary layer over Cairo during the days with highest or lowest ozone mixing ratios are examined. During the 7% highest ozone days at the 0-1500m layer over Cairo, very high ozone concentrations of about 80 ppb on average are observed from the surface up to 4-5 km altitude. The difference in ozone concentrations between the 7% highest and the 7% lowest ozone days reaches maximum values around 60 ppb close to the ground. During the highest ozone days for both 1.5-5 and 0-1.5km layer, there are extended regions of strong subsidence in the eastern Mediterranean but also in eastern and northern Europe and over these regions the atmosphere is dryer than average. In addition, characteristic profiles with the highest ozone concentrations in the 0-1500m layer are examined in order to assess the influence of atmospheric transport and photochemistry on the ozone concentrations over the area. © 2015 P. D. Kalabokas et al.

Kalabokas P.D.,Academy of Athens | Cammas J.-P.,CNRS Laboratory for Aerology | Thouret V.,CNRS Laboratory for Aerology | Volz-Thomas A.,Institute fuer Chemie and Dynamik der Geosphaere | And 3 more authors.
Atmospheric Chemistry and Physics | Year: 2013

In order to evaluate the observed high rural ozone levels in the eastern Mediterranean area during summertime, vertical profiles of ozone measured in the period 1994–2008 in the framework of the MOZAIC project (Measurement of Ozone and Water Vapor by Airbus in Service Aircraft) over the eastern Mediterranean basin (Cairo, Tel Aviv, Heraklion, Rhodes, Antalya) were analyzed, focusing in the lower troposphere (1.5–5 km). At first, vertical profiles collected during extreme days with very high or very low tropospheric ozone mixing ratios have been examined together with the corresponding back-trajectories. Also, the average profiles of ozone, relative humidity, carbon monoxide, temperature gradient and wind speed corresponding to the 7% highest and the 7% lowest ozone mixing ratios for the 1500–5000 m height layer for Cairo and Tel Aviv have been examined and the corresponding composite maps of geopotential heights at 850 hPa have been plotted. Based on the above analysis, it turns out that the lower-tropospheric ozone variability over the eastern Mediterranean area is controlled mainly by the synoptic meteorological conditions, combined with local topographical and meteorological features. In particular, the highest ozone concentrations in the lower troposphere and subsequently in the boundary layer are associated with large-scale subsidence of ozone-rich air masses from the upper troposphere under anticyclonic conditions while the lowest ozone concentrations are associated with low pressure conditions inducing uplifting of boundary-layer air, poor in ozone and rich in relative humidity, to the lower troposphere. © Author(s) 2013. CC Attribution 3.0 License.

Philandras C.M.,Academy of Athens | Nastos P.T.,National and Kapodistrian University of Athens | Kapsomenakis I.N.,Academy of Athens | Repapis C.C.,Academy of Athens | Repapis C.C.,Mariolopoulos Kanaginis Foundation for the Environmental science
Atmospheric Research | Year: 2015

The goal of this study is to contribute to the climatology of upper air temperature in the Mediterranean region, during the period 1965-2011. For this purpose, both radiosonde recordings and gridded reanalysis datasets of upper air temperature from National Center for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) were used for seven barometric levels at 850. hPa, 700. hPa, 500. hPa, 300. hPa, 200. hPa, 150. hPa and 100. hPa. Trends and variability of upper air temperature were analyzed on annual and seasonal basis. Further, the impact of atmospheric circulation, by means of correlation between upper air temperature at different barometric levels and specific climatic indices such as Mediterranean Oscillation Index (MOI), North Sea Caspian Pattern Index (NCPI) and North Atlantic Oscillation Index (NAOI), was also quantified. Our findings have given evidence that air temperature is increasing at a higher rate in lower/middle troposphere against upper, and this is very likely due to increasing greenhouse gas concentrations. © 2013 Elsevier B.V.

Tatsis G.,University of Ioannina | Votis C.,University of Ioannina | Christofilakis V.,University of Ioannina | Kostarakis P.,University of Ioannina | And 2 more authors.
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2015

In this paper, a cost-effective prototype data acquisition system specifically designed for Schumann resonance measurements and an adequate signal processing method are described in detail. The implemented system captures the magnetic component of the Schumann resonance signal, using a magnetic antenna, at much higher sampling rates than the Nyquist rate for efficient signal improvement. In order to obtain the characteristics of the individual resonances of the SR spectrum a new and efficient software was developed. The processing techniques used in this software are analyzed thoroughly in the following. Evaluation of system's performance and operation is realized using preliminary measurements taken in the region of Northwest Greece. © 2015 Elsevier Ltd.

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