K. Eleftheratos - Academia.edu (original) (raw)

Papers by K. Eleftheratos

Clean Air Journal, 2014

Using the red-to-green (R/G) ratio in paintings of sunsets from 1500-2000 with an updated techniq... more Using the red-to-green (R/G) ratio in paintings of sunsets from 1500-2000 with an updated technique, Zerefos et al. (2014) create a historical record of aerosol optical depth (AOD) values that are well-aligned with other techniques. This current paper builds off of an earlier paper (Zerefos et al., 2007) where an analysis of sunsets was performed on digital images of 554 photos that were downloaded from several art galleries and museums. This previous study found that despite the style or the school of the painter, the R/G ratios at low solar zenith angles correlated well with modelled AOD values following large volcanic eruptions. In their 2014 paper, the authors were able to analyse 124 digital images from the Tate gallery and compare with the analysis of the pictures from the website, and found very similar results. In addition, the team ran an experiment where an artist painted a sunset during and after the Greek island of Hydra was impacted by a Saharan dust storm, and the AOD was measured simultaneously. During the high dust event there was a 30% difference between the AOD derived from the painting and the measured, however there was only a 0.02 difference in the AOD between the painting and the measured values during the low dust event. For both events, there was strong agreement with AOD from the painting and from a high resolution picture of the painting. This paper presents a very novel way to create a historical record of AOD, which can be used to identify when the global atmosphere as impacted by volcanoes, as well as the impact of the industrial revolution. The authors th th estimate that the AOD increases from 0.15 in the middle of the 19 century to about 0.20 by the end of the 20 century. As they conclude, "The main conclusion of the paper is that nature speaks to the hearts and souls of artists. When colouring sunsets the R/G ratios perceived by the brain contain important environmental information."

This study examines the climatology of cirrus cloud cover on regional and global scales based on ... more This study examines the climatology of cirrus cloud cover on regional and global scales based on observations by satellites from space. The analysis is based on the ISCCP D2 cloud dataset and covers the period 1984-1998. The natural variability of cirrus cloud cover is examined in relation to known large-scale natural phenomena such as ENSO, QBO and the NAO as well as in combination with dynamical proxies. As an index of dynamics in the UT/LS we use the vertical velocities at 300 hPa from the NCEP/NCAR reanalysis dataset. While QBO was found to have a negligible effect on cirrus clouds, the effect of ENSO and NAO is more significant and can be very important regionally. Cirrus clouds and ENSO are significantly correlated mainly in the tropics while the correlations between cirrus cloud cover and NAO were found to be statistical significant over regions extending from the eastern part of the North Atlantic - to the Mediterranean - to the Middle East. Changes in cirrus cloud cover in ...

Geophysical Research Letters, 2011

This work describes observational and modelling results of the ozone depletion which took place d... more This work describes observational and modelling results of the ozone depletion which took place during the winter/ spring of 2011 in the Arctic stratosphere. Assimilated total ozone data from GOME-2 were used to estimate the integrated ozone mass deficit at polar latitudes and the Oslo CTM2 model calculated low winter/spring ozone values over the Arctic, which compare well with the satellite observations. Model runs with and without chemistry in the Arctic during the winter/spring of 2011 show that the very low Arctic stratospheric air temperatures led to significant chemical ozone loss. The calculated winter/spring ozone mass deficit (O 3 MD) reached extreme high values in 2011 (2700 Mt) and the seasonal zonal mean total ozone extreme low values of 333DU. Dynamics have set up the conditions for cold temperatures in the lower stratosphere in winter/ spring of 2011. Comparison of ozone columns with the previous 13 years shows record low ozone column values during winter/spring in the Arctic in 2011. A comparison is also given with similar model studies for the overall warmer winter/spring of 2010 which show higher ozone column values and significantly less chemical ozone loss. The interannual variability of column ozone over the northern polar region is, as expected, highly correlated with the corresponding year-to-year variability of the seasonally-averaged temperatures in the lower stratosphere. Citation: Balis, D., et al.

Atmospheric Chemistry and Physics, 2014

We examine sunsets painted by famous artists as proxy information for the aerosol optical depth a... more We examine sunsets painted by famous artists as proxy information for the aerosol optical depth after major volcanic eruptions. Images derived from precision colour protocols applied to the paintings were compared to online images, and found that the latter, previously analysed, provide accurate information. Aerosol optical depths (AODs) at 550 nm, corresponding to Northern Hemisphere middle latitudes, calculated by introducing red-to-green (R / G) ratios from a large number of paintings to a radiative transfer model, were significantly correlated with independent proxies from stratospheric AOD and optical extinction data, the dust veil index, and ice core volcanic indices. AODs calculated from paintings were grouped into 50-year intervals from 1500 to 2000. The year of each eruption and the 3 following years were defined as "volcanic". The remaining "non-volcanic" years were used to provide additional evidence of a multidecadal increase in the atmospheric optical depths during the industrial "revolution". The increase of AOD at 550 nm calculated from the paintings grows from 0.15 in the middle 19th century to about 0.20 by the end of the 20th century. To corroborate our findings, an experiment was designed in which a master painter/colourist painted successive sunsets during and after the passage of Saharan

Abstract. This work examines changes in cirrus cloud cover (CCC) in possible association with avi... more Abstract. 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%

Atmospheric Measurement Techniques Discussions, 2016

A MkIV Brewer spectrophotometer has been operating in Athens since 2004. Direct sun measurements ... more A MkIV Brewer spectrophotometer has been operating in Athens since 2004. Direct sun measurements originally scheduled for nitrogen dioxide retrievals were reprocessed to provide aerosol optical depths (AODs) at a wavelength of about 440 nm. A novel retrieval algorithm was specifically developed and the resulting AODs were compared to those obtained from a colocated Cimel filter radiometer belonging to the Aerosol Robotic Network (AERONET). The series are perfectly correlated, with Pearson’s correlation coefficients being as large as 0.996 and with 90% of AOD deviations between the two instruments being within the World Meteorological Organisation (WMO) traceability limits. In order to reach such a high agreement, several instrumental factors impacting the quality of the Brewer retrievals must be taken into account, including sensitivity to the internal temperature, and the state of the external optics and pointing accuracy must be carefully checked. Furthermore, the long-term radiom...

Till recently the ground based measurements of the total columnar ozone in Greece were based on D... more Till recently the ground based measurements of the total columnar ozone in Greece were based on Dobson and Brewer instruments. In the frame of the national UV monitoring network, UVNET, the Laboratory of Atmospheric Physics of AUTH has installed nine NILU-UV multi-filter radiometers at different locations in Greece and Cyprus, starting in 2005. The stations of Thessaloniki and Athens are also equipped with Brewer spectrophotometers. The NILU-UV instrument provides irradiance measurements at 5 spectral bands in the ultraviolet, which are used to derive various products, among them the total ozone column. The measurements are recorded in 1-minute intervals and can provide sufficient details about the daily variation of irradiance. In this study, the accuracy of the total ozone estimations by the NILU-UV instruments is examined. Detailed comparisons between the measurements of the Brewer spectrophotometers at the two stations and the corresponding estimations of the NILU-UV are present...

Atmospheric Chemistry and Physics Discussions, 2015

Theoretical derivations are given on the change of upper-tropospheric humidity (UTH) in a warming... more Theoretical derivations are given on the change of upper-tropospheric humidity (UTH) in a warming climate. Considered view is that the atmosphere, getting moister with increasing temperatures, will retain a constant relative humidity. In the present study we show that the upper-tropospheric humidity, a weighted mean over a relative humidity profile, will change in spite of constant relative humidity. The simple reason for this is that the weighting function, that defines UTH, changes in a moister atmosphere. Through analytical calculations using observations and through radiative transfer calculations we demonstrate that two quantities that define the weighting function of UTH can change: the water vapour scale height and the peak emission altitude. Applying these changes to real profiles of relative humidity shows that absolute UTH changes typically do not exceed 1 %. If larger changes would be observed they would be an indication of climatological changes of relative humidity. As ...

International Journal of Remote Sensing, 2015

Atmospheric Chemistry and Physics Discussions, 2014

This study provides a new look at the observed and calculated long-term temperature changes from ... more 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 Published by Copernicus Publications on behalf of the European Geosciences Union.

Environmental Science and Technology Library, 2005

Springer Atmospheric Sciences, 2012

Atmospheric Chemistry and Physics, 2014

We use 30 years of intercalibrated HIRS (High-Resolution Infrared Radiation Sounder) data to prod... more We use 30 years of intercalibrated HIRS (High-Resolution Infrared Radiation Sounder) data to produce a 30-year data set of upper tropospheric humidity with respect to ice (UTH_i). Since the required brightness temperatures (channels 12 and 6, <i>T</i>₁₂ and…

Atmospheric Measurement Techniques Discussions, 2015

The three GOME-2 instruments will provide unique and long data sets for atmospheric research and ... more The three GOME-2 instruments will provide unique and long data sets for atmospheric research and applications. The complete time period will be 2007–2022, including the period of ozone depletion as well as the beginning of ozone layer recovery. Besides ozone chemistry, the GOME-2 products are important e.g. for air quality studies, climate modeling, policy monitoring and hazard warnings. The heritage for GOME-2 is in the ERS/GOME and Envisat/SCIAMACHY instruments. The current Level 2 (L2) data cover a wide range of products such as trace gas columns (NO₂, BrO, H₂CO, H₂O, SO₂), tropospheric columns of NO₂, total ozone columns and vertical ozone profiles in high and low spatial resolution, absorbing aerosol indices from the main science channels as well as from the polarization channels (AAI, AAI-PMD), Lambertian-equivalent reflectivity database, clear-sky and cloud-corre...

Atmospheric Chemistry and Physics Discussions, 2011

Global Environmental Change, 2014

Advances in the Research of Aquatic Environment, 2011

Clean Air Journal, 2014

Using the red-to-green (R/G) ratio in paintings of sunsets from 1500-2000 with an updated techniq... more Using the red-to-green (R/G) ratio in paintings of sunsets from 1500-2000 with an updated technique, Zerefos et al. (2014) create a historical record of aerosol optical depth (AOD) values that are well-aligned with other techniques. This current paper builds off of an earlier paper (Zerefos et al., 2007) where an analysis of sunsets was performed on digital images of 554 photos that were downloaded from several art galleries and museums. This previous study found that despite the style or the school of the painter, the R/G ratios at low solar zenith angles correlated well with modelled AOD values following large volcanic eruptions. In their 2014 paper, the authors were able to analyse 124 digital images from the Tate gallery and compare with the analysis of the pictures from the website, and found very similar results. In addition, the team ran an experiment where an artist painted a sunset during and after the Greek island of Hydra was impacted by a Saharan dust storm, and the AOD was measured simultaneously. During the high dust event there was a 30% difference between the AOD derived from the painting and the measured, however there was only a 0.02 difference in the AOD between the painting and the measured values during the low dust event. For both events, there was strong agreement with AOD from the painting and from a high resolution picture of the painting. This paper presents a very novel way to create a historical record of AOD, which can be used to identify when the global atmosphere as impacted by volcanoes, as well as the impact of the industrial revolution. The authors th th estimate that the AOD increases from 0.15 in the middle of the 19 century to about 0.20 by the end of the 20 century. As they conclude, "The main conclusion of the paper is that nature speaks to the hearts and souls of artists. When colouring sunsets the R/G ratios perceived by the brain contain important environmental information."

This study examines the climatology of cirrus cloud cover on regional and global scales based on ... more This study examines the climatology of cirrus cloud cover on regional and global scales based on observations by satellites from space. The analysis is based on the ISCCP D2 cloud dataset and covers the period 1984-1998. The natural variability of cirrus cloud cover is examined in relation to known large-scale natural phenomena such as ENSO, QBO and the NAO as well as in combination with dynamical proxies. As an index of dynamics in the UT/LS we use the vertical velocities at 300 hPa from the NCEP/NCAR reanalysis dataset. While QBO was found to have a negligible effect on cirrus clouds, the effect of ENSO and NAO is more significant and can be very important regionally. Cirrus clouds and ENSO are significantly correlated mainly in the tropics while the correlations between cirrus cloud cover and NAO were found to be statistical significant over regions extending from the eastern part of the North Atlantic - to the Mediterranean - to the Middle East. Changes in cirrus cloud cover in ...

Geophysical Research Letters, 2011

This work describes observational and modelling results of the ozone depletion which took place d... more This work describes observational and modelling results of the ozone depletion which took place during the winter/ spring of 2011 in the Arctic stratosphere. Assimilated total ozone data from GOME-2 were used to estimate the integrated ozone mass deficit at polar latitudes and the Oslo CTM2 model calculated low winter/spring ozone values over the Arctic, which compare well with the satellite observations. Model runs with and without chemistry in the Arctic during the winter/spring of 2011 show that the very low Arctic stratospheric air temperatures led to significant chemical ozone loss. The calculated winter/spring ozone mass deficit (O 3 MD) reached extreme high values in 2011 (2700 Mt) and the seasonal zonal mean total ozone extreme low values of 333DU. Dynamics have set up the conditions for cold temperatures in the lower stratosphere in winter/ spring of 2011. Comparison of ozone columns with the previous 13 years shows record low ozone column values during winter/spring in the Arctic in 2011. A comparison is also given with similar model studies for the overall warmer winter/spring of 2010 which show higher ozone column values and significantly less chemical ozone loss. The interannual variability of column ozone over the northern polar region is, as expected, highly correlated with the corresponding year-to-year variability of the seasonally-averaged temperatures in the lower stratosphere. Citation: Balis, D., et al.

Atmospheric Chemistry and Physics, 2014

We examine sunsets painted by famous artists as proxy information for the aerosol optical depth a... more We examine sunsets painted by famous artists as proxy information for the aerosol optical depth after major volcanic eruptions. Images derived from precision colour protocols applied to the paintings were compared to online images, and found that the latter, previously analysed, provide accurate information. Aerosol optical depths (AODs) at 550 nm, corresponding to Northern Hemisphere middle latitudes, calculated by introducing red-to-green (R / G) ratios from a large number of paintings to a radiative transfer model, were significantly correlated with independent proxies from stratospheric AOD and optical extinction data, the dust veil index, and ice core volcanic indices. AODs calculated from paintings were grouped into 50-year intervals from 1500 to 2000. The year of each eruption and the 3 following years were defined as "volcanic". The remaining "non-volcanic" years were used to provide additional evidence of a multidecadal increase in the atmospheric optical depths during the industrial "revolution". The increase of AOD at 550 nm calculated from the paintings grows from 0.15 in the middle 19th century to about 0.20 by the end of the 20th century. To corroborate our findings, an experiment was designed in which a master painter/colourist painted successive sunsets during and after the passage of Saharan

Abstract. This work examines changes in cirrus cloud cover (CCC) in possible association with avi... more Abstract. 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%

Atmospheric Measurement Techniques Discussions, 2016

A MkIV Brewer spectrophotometer has been operating in Athens since 2004. Direct sun measurements ... more A MkIV Brewer spectrophotometer has been operating in Athens since 2004. Direct sun measurements originally scheduled for nitrogen dioxide retrievals were reprocessed to provide aerosol optical depths (AODs) at a wavelength of about 440 nm. A novel retrieval algorithm was specifically developed and the resulting AODs were compared to those obtained from a colocated Cimel filter radiometer belonging to the Aerosol Robotic Network (AERONET). The series are perfectly correlated, with Pearson’s correlation coefficients being as large as 0.996 and with 90% of AOD deviations between the two instruments being within the World Meteorological Organisation (WMO) traceability limits. In order to reach such a high agreement, several instrumental factors impacting the quality of the Brewer retrievals must be taken into account, including sensitivity to the internal temperature, and the state of the external optics and pointing accuracy must be carefully checked. Furthermore, the long-term radiom...

Till recently the ground based measurements of the total columnar ozone in Greece were based on D... more Till recently the ground based measurements of the total columnar ozone in Greece were based on Dobson and Brewer instruments. In the frame of the national UV monitoring network, UVNET, the Laboratory of Atmospheric Physics of AUTH has installed nine NILU-UV multi-filter radiometers at different locations in Greece and Cyprus, starting in 2005. The stations of Thessaloniki and Athens are also equipped with Brewer spectrophotometers. The NILU-UV instrument provides irradiance measurements at 5 spectral bands in the ultraviolet, which are used to derive various products, among them the total ozone column. The measurements are recorded in 1-minute intervals and can provide sufficient details about the daily variation of irradiance. In this study, the accuracy of the total ozone estimations by the NILU-UV instruments is examined. Detailed comparisons between the measurements of the Brewer spectrophotometers at the two stations and the corresponding estimations of the NILU-UV are present...

Atmospheric Chemistry and Physics Discussions, 2015

Theoretical derivations are given on the change of upper-tropospheric humidity (UTH) in a warming... more Theoretical derivations are given on the change of upper-tropospheric humidity (UTH) in a warming climate. Considered view is that the atmosphere, getting moister with increasing temperatures, will retain a constant relative humidity. In the present study we show that the upper-tropospheric humidity, a weighted mean over a relative humidity profile, will change in spite of constant relative humidity. The simple reason for this is that the weighting function, that defines UTH, changes in a moister atmosphere. Through analytical calculations using observations and through radiative transfer calculations we demonstrate that two quantities that define the weighting function of UTH can change: the water vapour scale height and the peak emission altitude. Applying these changes to real profiles of relative humidity shows that absolute UTH changes typically do not exceed 1 %. If larger changes would be observed they would be an indication of climatological changes of relative humidity. As ...

International Journal of Remote Sensing, 2015

Atmospheric Chemistry and Physics Discussions, 2014

This study provides a new look at the observed and calculated long-term temperature changes from ... more 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 Published by Copernicus Publications on behalf of the European Geosciences Union.

Environmental Science and Technology Library, 2005

Springer Atmospheric Sciences, 2012

Atmospheric Chemistry and Physics, 2014

We use 30 years of intercalibrated HIRS (High-Resolution Infrared Radiation Sounder) data to prod... more We use 30 years of intercalibrated HIRS (High-Resolution Infrared Radiation Sounder) data to produce a 30-year data set of upper tropospheric humidity with respect to ice (UTH_i). Since the required brightness temperatures (channels 12 and 6, <i>T</i>₁₂ and…

Atmospheric Measurement Techniques Discussions, 2015

The three GOME-2 instruments will provide unique and long data sets for atmospheric research and ... more The three GOME-2 instruments will provide unique and long data sets for atmospheric research and applications. The complete time period will be 2007–2022, including the period of ozone depletion as well as the beginning of ozone layer recovery. Besides ozone chemistry, the GOME-2 products are important e.g. for air quality studies, climate modeling, policy monitoring and hazard warnings. The heritage for GOME-2 is in the ERS/GOME and Envisat/SCIAMACHY instruments. The current Level 2 (L2) data cover a wide range of products such as trace gas columns (NO₂, BrO, H₂CO, H₂O, SO₂), tropospheric columns of NO₂, total ozone columns and vertical ozone profiles in high and low spatial resolution, absorbing aerosol indices from the main science channels as well as from the polarization channels (AAI, AAI-PMD), Lambertian-equivalent reflectivity database, clear-sky and cloud-corre...

Atmospheric Chemistry and Physics Discussions, 2011

Global Environmental Change, 2014

Advances in the Research of Aquatic Environment, 2011