The 11-year solar cycle in stratospheric ozone: Comparison between Umkehr and SBUVv8 and effects on surface erythemal irradiance

Ozone profiles derived from ground-based Umkehr measurements at five stations and from the merged data set of Solar Backscattered Ultra Violet (SBUVv8) satellite observations are used to estimate the seasonal influence of the 11-year solar signal in the vertical distribution of stratospheric ozone. Both data sets show a strong response (2-3% of the annual mean) in the upper stratosphere, a very small response in the middle tropical stratosphere and a secondary maximum (∼2%) in the tropical lower stratosphere, in accordance to earlier reports. Thus a sequence of high-low-high response is seen in the vertical pattern of the solar cycle effect on ozone in the tropics, with its position sensitive to season, as it appears to move across the equator in direction opposite to the location of the maximum solar insolation. Results from Umkehr and satellite observations are in good agreement at levels up to 35 km, while discrepancies (of up to 2%) found at higher levels (around 40 km) are largest in summer. Finally, the estimated erythemal irradiance response to the 11-year solar cycle, calculated for the observed ozone change during a solar cycle (minimum to maximum), showed an overall decrease in the irradiance at the surface which is maximum (-2% of annual mean) over the tropics. Copyright 2007 by the American Geophyscial Union

Optical properties of different aerosol types: Seven years of combined Raman-elastic backscatter lidar measurements in Thessaloniki, Greece

We present our combined Raman/elastic backscatter lidar observations which were carried out at the EARLINET station of Thessaloniki, Greece, during the period 2001-2007. The largest optical depths are observed for Saharan dust and smoke aerosol particles. For local and continental polluted aerosols the measurements indicate high aerosol loads. However, measurements associated with the local path indicate enhanced aerosol load within the Planetary Boundary Layer. The lowest value of aerosol optical depth is observed for continental aerosols, from West directions with less free tropospheric contribution. The largest lidar ratios, of the order of 70 sr, are found for biomass burning aerosols. A significant and distinct correlation between lidar ratio and backscatter related Ångström exponent values were estimated for different aerosol categories. Scatter plot between lidar ratio values and Ångström exponent values for local and continental polluted aerosols does not show a significant correlation, with a large variation in both parameters possibly due to variable absorption characteristics of these aerosols. Finally for continental aerosols with west and northwest directions that follow downward movement when arriving at our site constantly low lidar ratios almost independent of size are found. © Author(s) 2010

Search for man-made cirrus contrails over Southeast Asia

In this study we examine the effect of natural perturbations on cirrus cloud cover in the tropics and we look for possible signal of cirrus contrails in tropical air traffic regions focussing on Southeast Asia, a region that has received much less attention than the well investigated North Atlantic and North American air traffic corridors. The analysis is based on the latest version of the ISCCP D2 cirrus cloud dataset and covers the period 1984-2000. Four parameters were examined for their relation with cirrus cloud trends: deep convective clouds from ISCCP, vertical velocities, relative humidity and air temperature at 200 mb from ECMWF/ERA40. The results show that there is a strong correlation between cirrus clouds and dynamical parameters (deep convective clouds, vertical velocities) over Southeast Asia, explaining a significant part of the tropical cirrus cloud variability. After removing seasonality, the ENSO signal becomes dominant on cirrus, on deep convective clouds and on vertical velocities over regions of the western and the eastern tropical Pacific Ocean. Over Southeast Asia, the average decrease in cirrus during the strong 1997/98 El Nino event was about 6 % cloud cover or ∼25 % of the regional mean. In order to search for possible signal of cirrus contrails over S. E. Asia, we calculated trends in cirrus coverage over tropical regions with high air traffic after removing the ENSO effect. The results show that there is a small positive trend in cirrus clouds over the S. E. Asia air corridors during winter (+0.1 % cover/decade), which is not statistically significant and is related to small positive trends in deep convective clouds. In summer, cirrus clouds are anti-correlated with deep convective clouds over this region and the trends are opposite. The negative trends in cirrus clouds, which are observed in the summer (4.5% cover/decade), are related to trends in dynamical and thermo-dynamical parameters. It is shown that cirrus clouds are statistically significant correlated with vertical velocities and air temperature at 200 mb (correlations of -0.7 and -0.6, respectively), explaining the highest part of the long-term variability of cirrus clouds over S. E. Asia. Over the Caribbean air corridors, on the other hand, there are significant increases in cirrus cloudiness by about 2.5% per decade in winter (99% confidence level) and 2.7% cover/ decade in the summer (95% confidence level), part of which is also related to trends in dynamical and thermo-dynamical parameters. According to our findings, it is difficult to detect possible effects of regional persistent contrails on cirrus cloud trends over the S. E. Asia air traffic corridors. This is because in winter there are not statistically significant trends in cirrus clouds and in summer trends in dynamics and thermo-dynamics mask this issue. Taking also into account that flight frequencies and fuel consumption are moderate over the tropical air traffic corridors, it makes it even more difficult to detect and quantify any possible anthropogenic effects

Optical properties of Saharan dust layers as detected by a Raman lidar at Thessaloniki, Greece

Measurements during Saharan dust transport events were performed at several stations of the European Aerosol Research Lidar Network. During the period 2001-2002, 12 cases were captured at Thessaloniki, Greece (40.5°N, 22.9°E) with a 355 nm Raman lidar. For these cases the vertical profiles of the extinction and backscatter coefficients were determined and examined, as well as profiles of extinction-to-backscatter ratio (lidar ratio) and the backscatter-related Ångström exponent between 355 and 532 nm (color index). Model calculations from the DREAM model confirmed the existence of Saharan dust in all cases examined. Within the dust plumes the lidar ratios ranged from 20 to 100 sr and the color index ranged from -1 to 3. An anticorrelation was found between the lidar ratio and the color index during the Saharan dust events. In certain cases the results suggest that the mixing of dust particles with boundary layer aerosols reduces the dust 'signature' and thus, makes difficult the separation of the dust particles from those of mostly urban origin. Copyright 2004 by the American Geophysical Union

Sensitivity of solar UV radiation to ozone and temperature profiles at Thessaloniki (40.5°N, 23°E), Greece

Measured ozone and temperature vertical profiles from Thessaloniki, Greece, were used together with the mid-latitude standard profiles as input data in a radiative transfer model. Calculations of direct and global solar irradiance, actinic flux, UV-B and CIE weighted integrals for solar zenith angles of 30°, 70° and 85° were performed and analyzed. Variable temperature values and ozone redistribution may change UV radiation reaching the surface significantly more than the proposed measurement uncertainties for high solar zenith angles. A specific measured profile corresponding to air masses of polar origin probed over Thessaloniki was selected and the differences in vertical distribution of UV-B radiation were discussed. Obtained results revealed that the use of an inappropriate temperature and ozone profile may lead to significant changes at small UV-B wavelengths and high solar zenith angles. In this case, the use of seasonal average vertical profiles of ozone and temperature for a given area may be carefully utilized when accurate model spectral calculations are needed and comparison with measurements in the troposphere performed. © 2005 Elsevier Ltd. All rights reserved

On changes of spectral UV-B in the 90's in Europe

The variability of solar irradiance in the UV-B spectral region is studied at two European stations, Thessaloniki, Greece (40.5°N) and Uccle, Belgium (50.8°N), operating well calibrated spectroradiometers during the period 1990-97, which is the longest available clear sky time series of spectral UV-B data. Solar spectral irradiance at two distinct wavelenghts is used in the presented analysis, the strongly ozone dependent 305nm and the weakly dependent 325nm. It is quantified in this study to what extend total ozone is controlling long-term changes of solar irradiance below 315nm at ground level. This is supported by the similar long-term variability under clear sky and all sky conditions during the past seven years. It was found that the increase in UV-B solar irradiance, which can be attributed solely to the observed ozone decrease of 4.5% per decade, is of the order of 10% per decade at 305nm, 63° SZA. The increase found at 325 nm is associated with the observed decreases of various air pollutants in the area, as it appears from the long-term columnar SO2 measurements. The observed changes in solar irradiance (in % per year) are verified with model calculations performed with a radiative transfer model

Evidence of impact of aviation on cirrus cloud formation

This work examines changes in cirrus cloud cover (CCC) in possible association with aviation activities at congested air corridors. The analysis is based on the latest version of the International Satellite Cloud Climatology Project D2 data set and covers the period 1984-1998. Over the studied areas, the effect of large-scale modes of natural climate variability such as ENSO, QBO and NAO as well as the possible influence of the tropopause variability, were first removed from the cloud data set in order to calculate long-term changes of observed cirrus cloudiness. The results show increasing trends in (CCC) between 1984 and 1998 over the high air traffic corridors of North America, North Atlantic and Europe. Of these upward trends, only in the summertime over the North Atlantic and only in the wintertime over North America are statistically significant (exceeding +2.0% per decade). Over adjacent locations with low air traffic, the calculated trends are statistically insignificant and in most cases negative both during winter and summer in the regions studied. These negative trends, over low air traffic regions, are consistent with the observed large scale negative trends seen in (CCC) over most of the northern middle latitudes and over the tropics. Moreover, further investigation of vertical velocities over high and low air traffic regions provide evidence that the trends of opposite signs in (CCC) over these regions, do not seem to be caused by different trends in dynamics. It is also shown that the longitudinal distribution of decadal changes in (CCC) along the latitude belt centered at the North Atlantic air corridor, parallels the spatial distribution of fuel consumption from highflying air traffic, providing an independent test of possible impact of aviation on contrail cirrus formation. The correlation between the fuel consumption and the longitudinal variability of (CCC) is significant (+0.7) over the middle latitudes but not over the tropics. This could be explained by the fact that over the tropics the variability of (CCC) is dominated by dynamics while at middle latitudes microphysics explain most of its variability. Results from this study are compared with other studies and for different periods of records and it appears that there exists general agreement as to the evidence of a possible aviation effect on high cloud positive trends over regions with congested air traffic. © 2003 European Geosciences Union