[Show abstract][Hide abstract] ABSTRACT: A two-wavelength ultraviolet (289?316nm) ozone Differential Absorption Lidar (DIAL) system is used to perform ozone measurements in the free troposphere in the Eastern Mediterranean (Northern Greece). The ozone DIAL profiles obtained during a Stratosphere-to-Troposphere Transport (STT) event are compared to that acquired by an electrochemical ozonesonde, in the altitude range between 2 and 10 km. The measurement accuracy of these two instruments is also discussed. The mean difference between the ozone profiles obtained by the two techniques is of the order of 1.11 ppbv (1.86%), while the corresponding standard deviation is 4.69 ppbv (8.16%). A case study of an STT event which occurred on 29 November 2000 is presented and analyzed, using ozone lidar, satellite and meteorological data, as well as air mass back-trajectory analysis. During this STT event ozone mixing ratios of 55?65 ppbv were observed between 5 and 7 km height above sea level (a.s.l.). Stratospheric air was mixed with tropospheric air masses, leading to potential vorticity ( PV ) losses due to diabatic processes. The ozone DIAL system can be used for following STT events and small-scale mixing phenomena in the free troposphere, and for providing sequences of vertical ozone profiles in the free troposphere.
Keywords. Atmospheric composition and structure (Evolution of the atmosphere; Instruments and techniques) ? Meteorology and atmospheric dynamics (Middle atmosphere dynamics; Turbulence)
[Show abstract][Hide abstract] ABSTRACT: In order to cover a substantial amount of stratospheric intrusions during the project Influence of Stratosphere-Troposphere Exchange in a Changing Climate on Atmospheric Transport and Oxidation Capacity (STACCATO), coordinated measurements were carried out at several locations in central and southern Europe, based on quasi-operational stratosphere-troposphere exchange forecasts. In this context, lidar measurements of tropospheric ozone were performed at Thessaloniki (23°E, 40.5°N), Greece, for the investigation of stratosphere-to-troposphere transport (STT) events over the southeastern Mediterranean region, during 2000–2002. The study of STT in this area is of particular interest not only because of its geographic location, which is more southern than the typical position of the polar front jet, but also because of the sparseness of detailed studies in this area. A summary of the main characteristics of the STT events that were detected by lidar during the investigation period reveals a tropospheric ozone increase of the order of 10% between 4.5 and 6.5 km above sea level, a coincident decrease in relative humidity, and elevated values of potential vorticity, thus providing a direct indication of the occurrence of stratospheric air in the middle troposphere. No direct effect on surface ozone was recorded. Furthermore, the majority of the STT cases detected reveal a common pathway of the stratospheric air masses that reached Thessaloniki originating from the North Sea. In addition, the event of 9 January 2001, during which the clearest and strongest descent of stratospheric air occurred, is further analyzed. An ozone-rich layer of the order of 60–90 ppbv between 5 and 6.5 km above sea level is clearly depicted, which was the result of the intermixture of originally stratospheric and boundary layer air originating from North America.
[Show abstract][Hide abstract] ABSTRACT: We present the new re-analysis of Satellite Laser Ranging (SLR) data to LAGEOS and LAGEOS 2 for the definition of the Terrestrial Reference Frame (TRF) and its crust- fixed orientation (Earth Orientation Parameters - EOP). The TRF plays an important role in the multi-technique monitoring of temporal variations in the gravitational field and its very low degree and order components. This area is becoming extremely im- portant with the launch of recent and future geopotential mapping missions for the referencing and calibration of the data and products from these missions. Satellite laser ranging (SLR) has for a long time monitored the continuous redistribution of mass within the Earth system through concomitant changes in the Stokes' coefficients of the terrestrial gravity field. Secular changes in J2 due to post-glacial relaxation have been observed since many years and similar changes in J3, J4 J5, etc. are attributed to changes in the ice sheets of Greenland and Antarctica. Seasonal changes in these coefficients have also been closely correlated with mass transfer in the atmosphere and oceans. The hydrological cycle contributions however are the most difficult to measure accurately so far. This latest analysis of the 1993-present SLR data set from LAGEOS and LAGEOS 2 data for the International Earth Rotation Service (IERS) TRF (ITRF) development includes the weekly monitoring of such compound changes in the low degree and order harmonics. Along with the static parameters of the TRF we have determined a time series of variations of its origin with respect to the center of mass of the Earth system (geocenter). These estimates provide a measure of the total motion due to all sources of mass transport within the Earth system and can be used to either complement the estimates from the future missions or to validate them through comparisons with their estimates for the same quantities. The data were reduced using NASA Goddard's GEODYN/SOLVE II software, resulting in a final RMS error of~8 mm - close to the data noise level. We will discuss our solution for the GM, the geo- center (degree one terms), and the C 2,1, S 2,1, C 2,2, S 2,2 time series, compare them to variations inferred from geophysical processes, and examine their spectrum.
[Show abstract][Hide abstract] ABSTRACT: Intensive measurements of UV solar irradiance, total ozone and surface ozone were carried out during the solar eclipse of 11 August 1999 at Thessaloniki, Greece and Stara Zagora, Bulgaria, located very close to the footprint of the moon's shadow during the solar eclipse with the maximum coverage of the solar disk reaching about 90% and 96% respectively. It is shown that during the eclipse the diffuse component is reduced less compared to the decline of the direct solar irradiance at the shorter wavelengths. A 20-minute oscillation of erythemal UV-B solar irradiance was observed before and after the time of the eclipse maximum under clear skies, indicating a possible 20-minute fluctuation in total ozone presumably caused by the eclipse induced gravity waves. The surface ozone measurements at Thessaloniki display a decrease of around 10–15 ppbv during the solar eclipse. Similarly, ozone profile measurements with a lidar system indicate a decrease of ozone up to 2 km during the solar eclipse. The eclipse offered the opportunity to test our understanding of tropospheric ozone chemistry. The use of a chemical box model suggested that photochemistry can account for a significant portion of the observed surface ozone decrease.
Full-text · Article · Dec 2001 · Advances in Space Research
[Show abstract][Hide abstract] ABSTRACT: Spectral measurements of global solar irradiance, obtained under cloud-free conditions during the SUSPEN campaign (July 1997) in Thessaloniki, Greece, are compared with radiative transfer model calculations, showing an agreement to within +/-5% for wavelengths higher that 305 nm. The uncertainties in the modeled spectra were analyzed with respect to the aerosol-related model input parameters (single-scattering albedo and asymmetry factor), which were not derivable from measurements. A range of single-scattering albedo values was used to investigate its impact on surface UV irradiance through comparison of measurements with model calculations. It was found that a difference in the single-scattering albedo of 0.1 changes the model-measurement ratio by 7%-14%, depending on solar zenith angle. Finally, an attempt was made to relate the estimated values of single-scattering albedo to wind direction and relative humidity, which control the origin and type of the aerosols in the area.
Full-text · Article · Jun 2001 · Journal of the Atmospheric Sciences
[Show abstract][Hide abstract] ABSTRACT: Spectral measurements of the aerosol particle scattering coefficient sigmas and the aerosol optical depth taua were conducted at Ouranouspolis (Greece, 40°23'N, 23°57'E) and at Sde Boker (Israel, 30°51'N, 34°47'E) between June and September 1998. Measurements were related to 5-day three-dimensional back trajectories at 950, 850, and 550 hPa to assess the influence of long-range transport from particular source regions to the aerosol load at the two sites. Our measurements show that the eastern Mediterranean basin is moderately to highly polluted during summer. Daily average sigmas values at 550 nm were typically in the range of 30-200 Mm-1 at both sites. The range obtained for the summer regional aerosol optical depth taua was 0.03-0.52 at 500 nm. Enhanced aerosol extinction was related to transport of polluted air masses from western and eastern Europe. High-altitude transport of mineral dust from northern Africa was observed at both sites, particularly in Israel.
Full-text · Article · May 2001 · Journal of Geophysical Research Atmospheres
[Show abstract][Hide abstract] ABSTRACT: We present measurements of the vertical structure of the aerosol backscattering coefficient in the lower troposphere, which have been performed at the city of Thessaloniki in N. Greece, during the years 1996 and 1997. A ground-based backscatter lidar system operated throughout the year, mostly around local noon hours. The lidar measurements were accompanied by measurements of the aerosol optical depth in the visible spectral region, using a CIMEL sun-tracking photometer. The seasonal variation of the aerosol loading and its vertical distribution in the lower troposphere over the city of Thessaloniki is discussed. The maximum values of the aerosol optical depth are found during the spring season. Indication about the origin of these maxima is given by inspection of the various aerosol layers observed in the lidar profiles. Most of the aerosol loading is present in the first 3 km height, and only in rare cases there are important aerosol layers detectable above 3 km, as in a case of Saharan dust transported over the city of Thessaloniki, in May 1997. Both instruments used in this study show similar seasonal variation of the aerosol load. It was found that almost 85% of the aerosol load is located in the layer below 3 km. There is a bias between the CIMEL and lidar derived optical depth at 532 nm, mainly attributed to the aerosols present between ground level and 600 m height, which represent up to 50% of the total aerosol optical depth.
Full-text · Article · Jan 2000 · Atmospheric Environment
[Show abstract][Hide abstract] ABSTRACT: An improved ozone Differential Absorption Lidar (DIAL) system has been designed for tropospheric and lower stratospheric ozone monitoring in Northern Greece. The system is based on a frequency quadrupled pulsed Nd:YAG laser and the Raman shifting technique in Dz and H2 gases. The lidar system emits simultaneously 4 wavelengths (266 mn, 289 nm, 299 mn and 316 mn) at 10 Hz repetition rate using a single low pressure Raman cell. The optical receiving system is based on a 50 cm concave telescope which is directly coupled to a specially conceived grating spectrometer. This lidar system uses analog (12 bits- 40 MHz) and photon counting (250 MHz) detection systems, able to measure lidar signals up 16 km height. Ozone vertical profiles are to be measured from 0.8 km up to 15 km height, with a 30-500 m spatial and a l-minute temporal resolution. In this paper the major technical characteristics of the improved lidar system are presented. The system is foreseen to provide the first daytime/ nighttime ozone vertical profiles during early winter 1999.
No preview · Article · Dec 1999 · Physics and Chemistry of the Earth Part C Solar Terrestrial & Planetary Science
[Show abstract][Hide abstract] ABSTRACT: We present the results of our investigations into the vertical structure of several North African dust plumes exported to the Mediterranean in 1997. Two backscatter lidar systems were operated in the western and eastern parts of the Mediterranean basin during dust events identified using Meteosat visible images. Dust transport soundings have shown that dust particles are trapped and transported inside well-defined layers in the free troposphere. In general, the dust transport appeared to be multilayered, with several distinct layers at different altitudes between 1.5 and 5 km. The analysis of Meteosat IR images, the Total Ozonc Mapping Spectrometer aerosol index, and back-trajectories clearly shows that these layers have different origins in Africa. Finally, in addition to the free troposphere transport, the presence of dust particles inside the planetary boundary layer has been assessed and quantified for two particular events with aerosol optical thickness of 0.3-0.4, using simultaneous lidar and Sun photometer measurements. In one case only, significant dust load (dust optical thickness of ~0.1) occurred in the boundary layer.
Full-text · Article · Sep 1999 · Journal of Geophysical Research Atmospheres
[Show abstract][Hide abstract] ABSTRACT: A two-wavelength UV (289-316 nm) ozone Differential Absorption Lidar (DIAL) system is very well adapted to perform ozone measurements in the troposphere with adequate spatial and temporal resolutions. The DIAL system is used for systematic STE monitoring in the Eastern Mediterranean region, since March 2000, in the frame of the European STACCATO Project. The ozone DIAL system is validated by com- parisons of the ozone lidar profiles with those obtained by ECC ozonesondes, in the altitude range between 2 and 11 km, and measurement accuracy of these two instru- ments is discussed. Finally, a case study of a STE event over N. Greece is presented and analyzed, using ozone lidar and ECMWF data, as well as air mass back-trajectory data.