P. Zanis

An estimate of the impact of Stratosphere to Troposphere Transport (STT) on the lower free tropospheric ozone over the Alps using 10Be and 7Be measurements

Jungfraujoch (JUN), Switzerland and Zugspitze (ZUG), Germany. Inspection of the variability of the ratio 10 Be/ 7 Be in relation to 10 Be, 7 Be, and relative humidity (RH) reveals that the ratio is independent from processes that have a clear effect on both radionuclides, such as wet scavenging. High ratio values are generally met under cyclonic or northerly advective conditions, which are the synoptic situations mostly related to stratosphere-to-troposphere transport (STT) events over central Europe, while the 10-day back trajectories indicate a stratospheric source for the majority of the cases within the upper 10% quantile of 10 Be/ 7 Be ratios. The monthly 10 Be/ 7 Be ratios show a clear May and June peak at JUN and a much weaker seasonality at ZUG. A simple mixing model is used for an independent estimate of the strength of STT throughout the year based on the 7 Be and 10 Be measurements. In spite of the various uncertainties, the results indicate a seasonal cycle of stratospheric ozone percentage contribution with an early spring maximum (3– 11%) and autumn minimum (1–2%) at ZUG, while at JUN, a primary maximum in May and June (6–18%), a secondary maximum in March (4–13%), and a minimum again in autumn (1–4%) are revealed. Although the simple method applied here provides an independent estimate for the impact of STT to the lower troposphere, it nevertheless shows relatively good agreement with Lagrangian model calculations, especially for ZUG. INDEX TERMS: 0341 Atmospheric Composition and Structure: Middle atmosphere—constituent transport and chemistry (3334); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 3329 Meteorology and Atmospheric Dynamics: Mesoscale meteorology; 3362 Meteorology and Atmospheric Dynamics: Stratosphere/troposphere interactions; KEYWORDS: stratospheric intrusions, cosmogenic radionuclides, tropospheric ozone, 7 Be, 10 Be, Alps Citation: Zanis, P., et al., An estimate of the impact of stratosphere-to-troposphere transport (STT) on the lower free tropospheric ozone over the Alps using 10 Be and 7 Be measurements, J. Geophys. Res., 108(D12), 8520, doi:10.1029/2002JD002604, 2003.

Low-frequency variability of beryllium-7 surface concentrations over the Eastern Mediterranean

7Be measurements, performed in Northern Greece (40°N) since 1988, were analyzed, in order to investigate the variability of the surface concentrations that can be attributed to processes with frequencies below the synoptic variability. Spectral analysis on the 7Be time series revealed three characteristic spectral regions, a 1-year periodicity corresponding to the well-established annual cycle, a periodicity between 20 and 30 months and a peak corresponding to a period of 11 years. The relationship with the 11-year cycle of solar activity was investigated through the correlation between the sunspot number and 7Be (−0.86) and between the heliocentric potential and 7Be (−0.80). 7Be was in coincidence with the heliocentric potential whereas it lagged the sunspot number by 5 months, expressing the time needed for the solar wind variations to be reflected on the production of 7Be during the specific solar cycle. The amplitude of the annual cycle was not constant throughout the whole period. An anti-correlation of −0.83 was revealed between the amplitude and the sunspot number with higher amplitudes during solar minimum and smaller amplitudes during solar maximum. With the use of a simplistic model, the amplitude modulation was attributed to the changing with the 11-year cycle contribution of the upper tropospheric and lower stratospheric reservoirs to the surface concentrations. More specifically, during solar maximum the contribution of upper troposphere down to the surface via winter-mixing is as much as 55% compared to the summer-mixing contribution, whereas during solar minimum winter-mixing is no more than 35%, thus inducing a higher amplitude. The superposed epoch method revealed that the 20–30 months periodicity is related to the quasi-biennial oscillation (QBO), with the 7Be minimum following the QBO maximum with ≈8 months time delay, whereas for the same time lag total ozone showed a reverse behavior. Cross spectrum analysis between 7Be and total ozone with the QBO shows a significant (90% confidence level) squared coherence, indicating that 68% and 86% of 7Be and total ozone variability of the 20–30 months periodicity, respectively, can be explained by the QBO.

Recent past and future patterns of the Etesian winds based on regional scale climate model simulations

The aim of this work is to investigate the recent past and future patterns of the Etesian winds, one of the most persistent localized wind systems in the world, which dominates the wind regime during warm period over the Aegean Sea and eastern Mediterranean. An objective classification method, the Two Step Cluster Analysis (TSCA), is applied on daily data from regional climate model simulations carried out with RegCM3 for the recent past (1961–1990) and future periods (2021–2050 and 2071–2100) constrained at lateral boundaries either by ERA-40 reanalysis fields or the global circulation model (GCM) ECHAM5. Three distinct Etesian patterns are identified by TSCA with the location and strength of the anticyclonic action center dominating the differences among the patterns. In case of the first Etesian pattern there is a ridge located over western and central Europe while for the other two Etesian patterns the location of the ridge moves eastward indicating a strong anticyclonic center over the Balkans. The horizontal and vertical spatial structure of geopotential height and the vertical velocity indicates that in all three Etesian patterns the anticyclonic action center over central Europe or Balkan Peninsula cannot be considered as an extension of the Azores high. The future projections for the late 21st century under SRES A1B scenario indicate a strengthening of the Etesian winds associated with the strengthening of the anticyclonic action center, and the deepening of Asian thermal Low over eastern Mediterranean. Furthermore the future projections indicate a weakening of the subsidence over eastern Mediterranean which is rather controlled by the deepening of the south Asian thermal Low in line with the projected in future weakening of South Asian monsoon and Hadley cell circulations.

Spatiotemporal variability and contribution of different aerosol types to the aerosol optical depth over the Eastern Mediterranean

This study characterizes the spatiotemporal variability and relative contribution of different types of aerosols to the Aerosol Optical Depth (AOD) over the Eastern Mediterranean as derived from MODIS Terra (3/2000-12/2012) and Aqua (7/2002-12/2012) satellite instruments. For this purpose, a 0.1° × 0.1° gridded MODIS dataset was compiled and validated against sunphotometric observations from the AErosol RObotic NETwork (AERONET). The high spatial resolution and long temporal coverage of the dataset allows for the determination of local hot spots like megacities, medium sized cities, industrial zones, and power plant complexes, seasonal variabilities, and decadal averages. The average AOD at 550 nm (AOD550) for the entire region is ~ 0.22 ± 0.19 with maximum values in summer and seasonal variabilities that can be attributed to precipitation, photochemical production of secondary organic aerosols, transport of pollution and smoke from biomass burning in Central and Eastern Europe, and transport of dust from the Sahara Desert and the Middle East. The MODIS data were analyzed together with data from other satellite sensors, reanalysis projects and a chemistry-aerosol-transport model using an optimized algorithm tailored for the region and capable of estimating the contribution of different aerosol types to the total AOD550. The spatial and temporal variability of anthropogenic, dust and fine mode natural aerosols over land and anthropogenic, dust and marine aerosols over the sea is examined. The relative contribution of the different aerosol types to the total AOD550 exhibits a low/high seasonal variability over land/sea areas, respectively. Overall, anthropogenic aerosols, dust and fine mode natural aerosols account for ~ 51 %, ~ 34 % and ~ 15 % of the total AOD550 over land, while, anthropogenic aerosols, dust and marine aerosols account ~ 40 %, ~ 34 % and ~ 26 % of the total AOD550 over the sea, based on MODIS Terra and Aqua observations.

Changes in surface UV solar irradiance and ozone over the balkans during the eclipse of August 11, 1999

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 moons 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.

Modeling and mapping temperature and precipitation climate data in Greece using topographical and geographical parameters

This study presents a methodology for modeling and mapping the seasonal and annual air temperature and precipitation climate normals over Greece using several topographical and geographical parameters. Data series of air temperature and precipitation from 84 weather stations distributed evenly over Greece are used along with a set of topographical and geographical parameters extracted with Geographic Information System methods from a digital elevation model (DEM). Normalized difference vegetation index (NDVI) obtained from MODIS Aqua satellite data is also used as a geographical parameter. First, the relation of the two climate elements to the topographical and geographical parameters was investigated based on the Pearson’s correlation coefficient to identify the parameters that mostly affect the spatial variability of air temperature and precipitation over Greece. Then a backward stepwise multiple regression was applied to add topographical and geographical parameters as independent variables into a regression equation and develop linear estimation models for both climate parameters. These models are subjected to residual correction using different local interpolation methods, in an attempt to refine the estimated values. The validity of these models is checked through cross-validation error statistics against an independent test subset of station data. The topographical and geographical parameters used as independent variables in the multiple regression models are mostly those found to be strongly correlated with both climatic variables. Models perform best for annual and spring temperatures and effectively for winter and autumn temperatures. Summer temperature spatial variability is rather poorly simulated by the multiple regression model. On the contrary, best performance is obtained for summer and autumn precipitation while the multiple regression model is not able to simulate effectively the spatial distribution of spring precipitation. Results revealed also a relatively weaker model performance for precipitation than that for air temperature probably due to the highly variable nature of precipitation compared to the relatively low spatial variability of air temperature field. The correction of the developed regression models using residuals improved though not significantly the interpolation accuracy.

Enhanced surface ozone during the heat wave of 2013 in Yangtze River Delta region, China

Under the background of global warming, occurrence of heat waves has increased in most part of Europe, Asia and Australia along with enhanced ozone level. In this paper, observational air temperature and surface ozone in the Yangtze River Delta (YRD) region of China during summer of 2013, and the regional chemistry-climate model (RegCM-CHEM4) were applied to explore the relationship between heat wave and elevated ground-level ozone. Observations indicated that YRD experienced severe heat waves with maximum temperature up to 41.1°C, 6.1°C higher than the definition of heat wave in China, and can last for as long as 27days. Maximum ozone reached 160.5ppb, exceeding the national air quality standard (secondary level) as 74.7ppb. Moreover, ozone was found to increase at a rate of 4-5ppbK-1 within the temperature range of 28-38°C, but decrease by a rate of -1.3~-1.7ppbK-1 under extremely high temperature. A typical heat wave case (HW: 24/7-31/7) and non-heat wave case (NHW: 5/6-12/6) were selected to investigate the mechanism between heavy ozone and heat waves. It was found that chemical reactions play the most important role in ozone formation during HW days, which result in 12ppb ozone enhancement compared to NHW days. Chemical formation of ozone can be influenced by several factors. During heat waves, a more stagnant condition, controlled by anti-cyclone with sink airflow, led to less water vapor in YRD from south and contributed to less cloud cover, which favored a strong solar radiation environment and ozone significantly increasing. High temperature also slightly promote the effect of vertical turbulence and horizontal advection, which beneficial to ozone remove, but the magnitude is much smaller than chemical effect. Our study suggests that the chemical reaction will potentially lead to substantial elevated ozone in a warmer climate, which should be taken into account in future ozone related issues.

The Etesians: from observations to reanalysis

AbstractThe Etesians are among the most persistent regional scale wind systems in the lower troposphere that blow over the Aegean Sea during the extended summer season. In this study we evaluate the performance of three different reanalysis products (the twentieth century reanalysis, 20CR; the 40-year European Centre for Medium-Range Weather Forecasts, ECMWF, Re-Analysis, ERA40; and the recently released ECMWF reanalysis ERA-20C) in capturing the Etesian wind system. Three-hourly data from 24 stations over Greece are used and compared with reanalysis outputs for the extended summer season (May–September) from 1971 to 2000. An objective classification of Etesians based on the pressure difference over the Aegean is provided. Classified Etesian days are then investigated as well as the associated large scale atmospheric circulation. Results highlight the ability of the investigated reanalyses to adequately describe the Etesian meteorological regimes. Intense Etesians are associated with stronger geopotential height anomalies over western-central Europe and the Eastern Mediterranean and with pronounced changes in the mean position of the jet streams. Finally, station time series provide evidence for less frequent intense Etesian days at the end of the extended summer season.

Trend Analysis for Climatic Time Series for Greece

A database of climatic time series from the network of Hellenic National Meteorological Service has been developed under the research program GEOCLIMA. Initially a quality test was applied to the raw data and then missing data have been imputed with a regularized expectation–maximization algorithm to complete the climatic record. Next, a quantile-matching algorithm was applied in order to verify the homogeneity of the data. The processed time series were used for trend analysis of the time series of maximum and minimum air temperature and precipitation. It is shown that peak temperature extremes are becoming warmer, especially for the minimum temperatures, while precipitation is decreasing over the area with variable local significance though.

Optimization of a Regional Climate Model for High Resolution Simulations over Greece

A set of six yearly high resolution (10 × 10 km) regional climate simulations were carried out over Greece using RegCM3 in the framework of the project GEOCLIMA based on different setups for the convective scheme. Specifically, the simulations comprised two experiments using the Grell convective scheme with Fritsch-Chappell (FC) closure assumption and four experiments using the Emanuel convective scheme. The aim of the study is the optimization of the model for Greece by comparing simulated values of near surface temperature, precipitation and cloudiness with the respective observed values at 84 Greek stations. The model domain is nested to a coarser RegCM3 European domain (at a resolution of 25 × 25 km) driven by the ERA-40 reanalysis dataset. Simulations using the modified Emanuel convective scheme reduce mean bias (and RMSE) in temperature over 25% (20%), in cloudiness over 20% (10%) and in precipitation over 70% (40%). Results show that the model for the Greek area is more sensitive to changes in autoconversion threshold than changes in relaxation rate.