38 results on '"Bartzokas A"'
Search Results
2. An objective definition of seasons for the Mediterranean region
- Author
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Nikolaos Hatzianastassiou, George Kotsias, Piero Lionello, Aristides Bartzokas, and Christos J. Lolis
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Mediterranean climate ,Atmospheric Science ,Geography ,Climatology ,Principal component analysis - Published
- 2020
3. On the connection between large-scale atmospheric circulation and winter GPCP precipitation over the Mediterranean region for the period 1980-2017
- Author
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Nikolaos Hatzianastassiou, Aristides Bartzokas, Christos J. Lolis, George Kotsias, and Vincenzo Levizzani
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Mediterranean climate ,Atmospheric Science ,Geopotential ,geography ,geography.geographical_feature_category ,Winter precipitation ,010504 meteorology & atmospheric sciences ,Atmospheric circulation ,Anomaly (natural sciences) ,010501 environmental sciences ,GPCP ,01 natural sciences ,Troposphere ,Peninsula ,North Atlantic oscillation ,Climatology ,Mediterranean Sea ,Precipitation ,Geology ,0105 earth and related environmental sciences - Abstract
The influence of the large-scale atmospheric circulation on winter precipitation over the Mediterranean region is studied with the use of monthly 2.5° × 2.5° ERA-Interim 500 and 1000 hPa geopotential heights and GPCP-v2.3 precipitation, for the period 1980–2017. At first, Principal Component Analysis (PCA) is applied both to geopotential heights and precipitation datasets and the main modes of inter-annual variations are found. Then, these modes are investigated through Canonical Correlation Analysis (CCA) to identify the centers of action of the lower and the middle troposphere that control precipitation variability over the Mediterranean region. Three statistically significant canonical pairs are found. The first one, in the North Atlantic, controls precipitation over western Asia Minor, northern Libya and the western Iberian Peninsula. Τhe second mode corresponds to the see-saw between Greenland and central Europe and controls precipitation over the northern Mediterranean region. The third one is located north of Britain and controls precipitation over NW Africa and northern France. For each canonical pair, the mean geopotential heights and precipitation anomaly patterns for the four (10%) years with the highest and the lowest canonical scores are constructed, confirming the results of CCA. Moreover, the correlation between the canonical scores and the indices of the large-scale atmospheric oscillations, namely the North Atlantic Oscillation (NAO), the Arctic Oscillation (AO), the North Sea – Caspian Pattern (NCP) and the East Atlantic/West Russia Pattern (EA/WR), was studied.
- Published
- 2020
- Full Text
- View/download PDF
4. On the intra-annual variation of cloudiness over the Mediterranean region
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Christos J. Lolis, C. D. Papadimas, Eleftherios Ioannidis, Aristides Bartzokas, Nikolaos Hatzianastassiou, and Earth and Climate
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Mediterranean climate ,Atmospheric Science ,010504 meteorology & atmospheric sciences ,Cloud cover ,0208 environmental biotechnology ,02 engineering and technology ,Subtropics ,Seasonality ,medicine.disease ,Atmospheric sciences ,01 natural sciences ,020801 environmental engineering ,NCEP/NCAR Reanalysis ,Anticyclone ,Climatology ,medicine ,Environmental science ,Moderate-resolution imaging spectroradiometer ,0105 earth and related environmental sciences ,Teleconnection - Abstract
The seasonal variability of cloudiness in the Mediterranean region is studied, through the examination of the mean day-to-day intra-annual variation of gridded NCEP/NCAR total cloud cover for the period 1948–2014. A dimensionality reduction process is followed by using a multivariate statistical methodology and three main modes of seasonal variation are found. The first mode is characterized by a winter maximum and a summer minimum and prevails mainly over the Mediterranean and Black Sea areas; a weak see-saw teleconnection over the Alps represents the opposite intra-annual variation. The second mode presents maxima in early autumn and late spring and minima in late summer and winter and prevails over the SW Europe and NW Africa inland regions. The third mode shows a maximum in June and a minimum in October and prevails over the eastern part of central Europe. The main characteristics of these modes are attributed to the seasonal variation of: i) the temperature and humidity of the lower and the middle troposphere affecting static stability and ii) the intensity and the position of the main circulation systems affecting the region, as for example the Azores subtropical anticyclone and the Mediterranean depressions. The examination of inter-decadal changes in the seasonal variation of cloud cover in the above core regions reveals a long-term decrease of cloudiness during the 66-year period 1948–2014. A comparison with contemporary satellite MODerate resolution Imaging Spectroradiometer (MODIS) and surface based European Climate Assessment & Dataset (ECA&D) station cloud cover data is carried out for the above core regions, evidencing a similarity among the data sets as to their seasonal variability, which supports the validity of the NCEP/NCAR reanalysis cloud cover results.
- Published
- 2018
5. On the seasonal variability and the spatial distribution of lightning activity over the broader Greek area and their connection to atmospheric circulation
- Author
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Aristides Bartzokas, Christos J. Lolis, Vassiliki Kotroni, C. Gatidis, and Kostas Lagouvardos
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Mediterranean climate ,021110 strategic, defence & security studies ,Atmospheric Science ,010504 meteorology & atmospheric sciences ,Atmospheric circulation ,0211 other engineering and technologies ,Cold air ,02 engineering and technology ,Spatial distribution ,01 natural sciences ,Lightning ,Low-pressure area ,Climatology ,Atmospheric instability ,Period (geology) ,Geology ,0105 earth and related environmental sciences - Abstract
The spatio-temporal regime of lightning strokes over the broader Greek area is studied using 15-day ZEUS cloud-to-ground lightning data for a 10-year period (2005–14). The multivariate statistical method Factor Analysis (S-mode and T-mode) is applied on the mean intra-annual variation of lightning in order to reveal: i) the main modes of intra-annual variation of lighting activity that correspond to specific sub-regions of the Greek area and ii) the main characteristic patterns of strokes prevailing during specific periods of the year (“seasons”). According to the results there are three main modes of intra-annual variation: a) the continental one with maximum lightning activity in early summer, b) the Ionian Sea one with maximum in early autumn and c) the Aegean Sea one with maxima in middle autumn and late May. Also, according to the patterns of strokes, three main “seasons” are found: a) the summer one (early May–middle August) with highest lightning activity over the continental regions of the country due to the atmospheric instability associated with the intense land heating and the persistence of cool upper air masses, b) the winter one (middle December–middle February) with the maximum of lightning activity over the sea where instability is high due to the frequent prevalence of cold air masses over the warm sea-surface and the associated passages of Mediterranean depressions and c) the autumn one (early September–middle October) with maximum over northwestern Greece associated with the early cold season low pressure systems, the warm sea body of Ionian Sea and the essential role of topography.
- Published
- 2018
6. A comparison of different approaches for the definition of seasons in the Mediterranean region.
- Author
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Kotsias, George, Lolis, Christos J., Hatzianastassiou, Nikolaos, Lionello, Piero, and Bartzokas, Aristides
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SEASONS ,PRINCIPAL components analysis ,K-means clustering ,CLUSTER analysis (Statistics) - Abstract
Α new method, based on the definition of weather types (WTs), for the determination of seasons in the Mediterranean region is adopted. This method combines principal component analysis and k‐means Cluster Analysis on daily grid point meteorological data for the 70‐year period 1949–2018 obtained from the NCEP/NCAR database. At first, the method is applied to the classification of WTs and subsequently to the definition of seasons based on the mean intra‐annual variations of the WTs' frequency. The resulting seasons are compared to a previous method of the seasons' definition, based on the long‐term mean intra‐annual variations of climatological parameters. Furthermore, a synthesis of the two methods is proposed. The analysis is applied not only to the 70‐year period 1949–2018, but also to the five overlapping 30‐year sub‐periods 1949–1978, 1959–1988, 1969–1998, 1979–2008, 1989–2018 in order to investigate possible long‐term changes of the seasons' characteristics. Four seasons are identified for all periods and for both methodological approaches. These seasons broadly correspond to the four conventional ones, but they differ in their onset and cessation dates as well as their duration. For the composite method, it is found that winter lasts about 4 months, summer is a little longer than 3 months and spring and autumn last around 2.5 months. The most remarkable findings regarding the seasons' characteristics are: (a) the start/end dates and duration of winter are almost identical in all three approaches, while summers' characteristics differ; (b) shortening of winter and spring in recent periods, associated with the delayed start and expedited end dates, respectively; and (c) the prolongation of autumn and summer, due to the delay of its end date, and the earlier onset and delayed end dates, respectively. [ABSTRACT FROM AUTHOR]
- Published
- 2022
- Full Text
- View/download PDF
7. Objective Definition of Climatologically Homogeneous Areas in the Southern Balkans Based on the ERA5 Data Set
- Author
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Georgios Kotsias, Christos J. Lolis, and Aristides Bartzokas
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objective classification ,Atmospheric Science ,Humid continental climate ,Convective inhibition ,010504 meteorology & atmospheric sciences ,Anomaly (natural sciences) ,Cloud cover ,0208 environmental biotechnology ,02 engineering and technology ,01 natural sciences ,Convective available potential energy ,020801 environmental engineering ,Latitude ,Altitude ,Dew point ,Climatology ,Environmental science ,ERA5 ,lcsh:Q ,Mediterranean climate ,lcsh:Science ,0105 earth and related environmental sciences ,cluster analysis - Abstract
An objective definition of climatologically homogeneous areas in the southern Balkans is attempted with the use of daily 0.25° × 0.25° ERA5 meteorological data of air temperature, dew point, zonal and meridional wind components, Convective Available Potential Energy, Convective Inhibition, and total cloud cover. The classification of the various grid points into climatologically homogeneous areas is carried out by applying Principal Component Analysis and K-means Cluster Analysis on the mean spatial anomaly patterns of the above parameters for the 10-year period of 2008 to 2017. According to the results, 12 climatologically homogenous areas are found. From these areas, eight are mainly over the sea and four are mainly over the land. The mean intra-annual variations of the spatial anomalies of the above parameters reveal the main climatic characteristics of these areas for the above period. These characteristics refer, for example, to how much warmer or cloudy the climate of a specific area is in a specific season relatively to the rest of the geographical domain. The continentality, the latitude, the altitude, the orientation, and the seasonal variability of the thermal and dynamic factors affecting the Mediterranean region are responsible for the climate characteristics of the 12 areas and the differences among them.
- Published
- 2018
8. The atmospheric circulation characteristics favouring snowfall in an area with complex relief in Northwestern Greece
- Author
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Aristides Bartzokas, Stavros Dafis, E. E. Houssos, and Christos J. Lolis
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Mediterranean climate ,Atmospheric Science ,010504 meteorology & atmospheric sciences ,Atmospheric circulation ,Geopotential height ,010501 environmental sciences ,Snow ,Atmospheric sciences ,01 natural sciences ,Low-pressure area ,Troposphere ,Anticyclone ,Climatology ,Trough (meteorology) ,Geology ,0105 earth and related environmental sciences - Abstract
The connection between atmospheric circulation over Europe and snowfall in Ioannina, Northwestern (NW) Greece, is examined. The study deals with 160 snow events, referring to the 56-year period 1956–2011. For each of the events, the patterns of 500 and 1000 hPa geopotential height, 500 and 850 hPa air temperature, 1000–500, 1000–700 and 700–500 hPa thickness are constructed for the European area, whereas static stability K-index, relative humidity at 500, 700 and 850 hPa levels and finally relative vorticity at 500 and 1000 hPa are also used for the Southeastern (SE) Mediterranean region, for the previous day (D-1), the starting day (D) and the day following the cessation of snowfall (END). Factor analysis and cluster analysis are applied to the above datasets and the evolutions of the above circulation parameters are classified into eight clusters. Snowfall in NW Greece is generally associated with a low pressure system over the Ionian Sea combined with an anticyclone over Western or NW Europe, causing cold northwesterly flow in the lower troposphere over NW Greece. In the middle troposphere, a deep 500 hPa trough usually extends from Western Russia to Italy and the Ionian Sea. The eight atmospheric circulation structures differ mainly in the exact location and/or the intensity of the depression and/or the anticyclone, in the orientation of the 500 hPa trough axis and in the trajectories of the above synoptic systems between D-1 and END days.
- Published
- 2015
9. The atmospheric conditions over Europe and the Mediterranean, favoring snow events in Athens, Greece
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E. E. Houssos, Aristides Bartzokas, Christos J. Lolis, Laboratory of Meteorology, Okayama University, and EGU, Publication
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Mediterranean climate ,010504 meteorology & atmospheric sciences ,lcsh:Dynamic and structural geology ,Atmospheric circulation ,[SDU.STU]Sciences of the Universe [physics]/Earth Sciences ,Athens greece ,Geopotential height ,Atmospheric sciences ,01 natural sciences ,Troposphere ,010104 statistics & probability ,lcsh:QE500-639.5 ,0101 mathematics ,lcsh:Science ,0105 earth and related environmental sciences ,lcsh:QE1-996.5 ,General Medicine ,Snow ,Low-pressure area ,lcsh:Geology ,13. Climate action ,Anticyclone ,Climatology ,[SDU.STU] Sciences of the Universe [physics]/Earth Sciences ,Environmental science ,lcsh:Q - Abstract
The 3-dimensional structure and the evolution of atmospheric circulation favoring snowfall in Athens are examined. The study refers to 61 snow events, which occurred during the period 1958–2001. For each one of the events, the patterns of MSL pressure, 850 hPa and 500 hPa air temperatures, 500 hPa geopotential height and 1000–500 hPa thickness are constructed for the European region, for the day before (D-1), the first day (D) and the day after the end of the event (END). A statistical methodology involving Factor Analysis and Cluster Analysis is applied to the above data sets and the 61 cases are finally classified into five clusters. These clusters are generally characterized by a north-easterly flow in the lower troposphere over the Athens area. This flow is associated with the presence of a low pressure system around Cyprus and an anticyclone over Europe. The position, the intensity and the trajectories of the surface and the upper air systems during D-1, D and END days are generally different among the five clusters.
- Published
- 2018
10. Relationship between the Indian summer monsoon and the large-scale circulation variability over the Mediterranean
- Author
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Helena A. Flocas, Aristides Bartzokas, Despoina Rizou, and Panos Athanasiadis
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Mediterranean climate ,Troposphere ,Atmospheric Science ,Atmospheric circulation ,Climatology ,Rossby wave ,East Asian Monsoon ,Environmental science ,Subsidence (atmosphere) ,Jet stream ,Atmospheric sciences ,Monsoon - Abstract
In this study the impact of the Indian summer monsoon on the large scale variability of the atmospheric circulation over the Mediterranean is investigated on an inter-annual time scale. Composite and correlation analysis results are presented, outlining different circulation patterns in the upper and lower troposphere for strong and weak monsoon years respectively. For this purpose ERA-40 Reanalysis monthly mean data at various isobaric levels together with the standardized All India Rainfall Index for boreal summer (June–July–August–September) of a 44-year period were employed. During strong monsoon years many atmospheric circulation systems appear strengthened over Eurasia, resembling a well-organized Rossby wave train over the area. In the upper troposphere a meridional shift of the jet streams over the examined area was also identified during extreme monsoon years. On the other hand, in the lower troposphere enhanced northerlies (Etesians) appear to dominate over Eastern Mediterranean along with intensified subsidence during strong monsoon years.
- Published
- 2015
11. Atmospheric circulation evolution related to desert‐dust episodes over the Mediterranean
- Author
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E. E. Houssos, Aristides Bartzokas, Nikolaos Hatzianastassiou, Nikos Mihalopoulos, Antonis Gkikas, and Christos J. Lolis
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Troposphere ,Mediterranean climate ,Atmospheric Science ,Atmospheric circulation ,Anticyclone ,Climatology ,Environmental science ,Geopotential height ,Storm ,Atmospheric sciences ,Desert dust ,Aerosol - Abstract
Cases of atmospheric circulation evolution favouring the occurrence of desert aerosol episodes (DAEs) over the broader Mediterranean region were investigated using an objective and dynamic algorithm, with daily satellite data for the period 2000–2013. After identifying strong and extreme DAEs, at a 1°×1° geographical-cell level, 255 dust aerosol episode days (DAEDs) and 148 cases of consecutive DAEDs, namely desert aerosol episode cases (DAECs), are defined. For each DAEC, the evolution of the lower tropospheric circulation 1 and 2 days before, during the initiation and after the cessation of the DAEC, is considered. S-mode factor analysis and k-means cluster analysis are applied on mean sea-level pressure and 700 hPa geopotential height fields obtained from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis Project, classifying the 148 cases of atmospheric-circulation evolution into six homogeneous and discrete clusters. The mean intra-annual variation of the DAECs reveals a primary maximum in May (18.9%), and their mean annual number is equal to 11.4 DAECs. On a seasonal basis, the highest percentage of the DAECs is found in spring (51.4%). Maximum duration of the DAECs is 7 days, with 58.8% lasting 1 day. Annually, the mean monthly number of the DAEs varies from 35.8 (September) to 58.0 (April). The western parts of the Mediterranean are affected by the DAEs when cyclonic conditions prevail in the western Mediterranean and northwestern Africa. In contrast, the central and eastern parts of the study region are affected by dust storms when a low-pressure system in the central Mediterranean or central Europe and an anticyclone in the eastern Mediterranean prevail. As to the mean regional intensity (aerosol optical depth at 550 nm) the strong DAEs vary from 0.67 to 0.77, while the extremes vary from 1.14 to 2.06. Generally, strong DAEs are more frequent than extremes (in five out of six clusters).
- Published
- 2014
12. An objective definition of seasons for the Mediterranean region.
- Author
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Kotsias, George, Lolis, Christos J., Hatzianastassiou, Nikolaos, Lionello, Piero, and Bartzokas, Aristides
- Subjects
CLOUDINESS ,PRECIPITABLE water ,PRINCIPAL components analysis ,MERIDIONAL winds ,GEOPOTENTIAL height - Abstract
An objective definition of seasons for the Mediterranean region is performed using the mean intra‐annual variations of 12 climatological parameters. The aim is to achieve a climatologically appropriate determination of the onset and cessation dates, as well as the duration, of each season, better than the conventional seasons (winter, spring, summer and autumn). Moreover, possible changes of the defined seasons during the 70‐year study period 1949–2018 are investigated over the climatologically sensitive Mediterranean region. The data used are daily NCEP/NCAR Reanalysis grid point values of precipitation rate, convective precipitation rate, 2 m temperature, total cloud cover, 2 m zonal and meridional wind, 500 and 1,000 hPa geopotential height, 500 and 850 hPa temperature, 850 hPa specific humidity and precipitable water over the Mediterranean region, for the period 1949–2018. Firstly, Principal Component Analysis is applied to the mean intra‐annual variations of the above parameters in order to reduce the dimensionality and then k‐means Cluster Analysis is applied to the resultant Principal Components in order to group dates with similar regimes of the above parameters. This methodology is applied for the 70‐year period 1949–2018 as well as for the five overlapping 30‐year subperiods 1949–1978, 1959–1988, 1969–1998, 1979–2008, and 1989–2018. According to the results, four seasons are defined for all periods. Although these seasons correspond to the four conventional seasons, there are differences regarding the onset and cessation dates and the duration. In general, it is found that winter lasts about 4 months, spring and summer are a little shorter than 3 months and autumn lasts about 2.5 months. The most remarkable long‐term changes are: (a) the warming during the last 30‐year period, which agrees with the ongoing climate change, (b) the shortening of winter and spring due to the delay of their onset date, and (c) the extension of autumn due to the delay of its cessation date. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
- View/download PDF
13. Seasonal Variability of Total Cloud Cover in the Mediterranean Region for the Period 1948–2014
- Author
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Eleftherios Ioannidis, Christos J. Lolis, and Aristides Bartzokas
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Mediterranean climate ,010504 meteorology & atmospheric sciences ,Cloud cover ,0207 environmental engineering ,02 engineering and technology ,Seasonality ,medicine.disease ,01 natural sciences ,Late summer ,Point data ,Climatology ,Period (geology) ,medicine ,Environmental science ,Day to day ,020701 environmental engineering ,0105 earth and related environmental sciences - Abstract
The seasonal variability of total cloud cover in the Mediterranean region is studied, using NCEP/NCAR daily grid point data. The mean day to day intra-annual variations of total cloud cover during the hydrological year (1 October–30 September) are extracted for the 66-year period 1948–2014. A dimensionality reduction method is applied on the 324 time series and three main modes of seasonal variation of cloud cover over the Mediterranean region are found. The first one is simple, characterized by a maximum in winter and a minimum in summer, prevailing mainly over the sea. The second one is double showing maxima in early autumn and late spring and minima in late summer and winter and prevails over SW Europe and NW Africa inland regions. Finally, the third one shows a maximum in June and a minimum in October and prevails over the eastern part of central Europe. The main characteristics of these modes are attributed to the seasonal variation of the various thermal and dynamical factors affecting the region. An examination of the seasonal variability of cloud cover in the above core regions during sub-periods of the total period 1948–2014 reveals some long-term changes during the last 66 years.
- Published
- 2016
14. Factors Regulating the Air–Sea Heat Fluxes Regime over the Aegean Sea
- Author
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D. Georgopoulos, George Ferentinos, Themistoklis Chronis, Vassilis P. Papadopoulos, and Aristides Bartzokas
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Mediterranean climate ,Atmospheric Science ,Mediterranean sea ,Heat flux ,North Atlantic oscillation ,Climatology ,Latent heat ,Environmental science ,Radiative forcing ,Shortwave ,Sea level - Abstract
The authors examine the impact of low-frequency atmospheric forcings on the air–sea heat fluxes over the Aegean Sea. The correlation between the air–sea heat flux components and three established [North Atlantic Oscillation (NAO), east Atlantic–western Russian pattern (EAWR), and North Sea–Caspian pattern (NCP)] and two testing climatic indices of potential effect over the eastern Mediterranean Sea region underlines significant discrepancies between the radiative (shortwave and longwave radiation) and the turbulent (sensible and latent heat) components. The NAO index affects the air–sea heat fluxes over the Aegean Sea region much less than the two innovative indices, the “Mediterranean index” and the “Eastern Europe index,” which play more effective roles. Moreover, the influence of the sea level atmospheric pressure (SLP) variability over an extended area (Europe and North Africa) on surface fluxes regime is investigated. The SLP anomalies are corroborated as a prominent regulating factor of the air–sea heat fluxes over the Aegean Sea region, especially during the cold season of the year. The analysis of the extreme values in the heat exchange anomalies for the period 1958–2001 highlights the role of SLP field on determining the air–sea heat fluxes regime, mainly during winter, when, occasionally, large amounts of heat loss from the sea surface trigger the mechanism of intermediate- and deep-water formation. It is suggested that wind regime and turbulent components are the modulators of the net air–sea heat flux anomalies throughout the year.
- Published
- 2012
15. The main characteristics of atmospheric circulation associated with fog in Greece
- Author
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Aristides Bartzokas, E. E. Houssos, and Christos J. Lolis
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Mediterranean climate ,genetic structures ,Atmospheric circulation ,forecast ,Geopotential height ,Atmospheric sciences ,lcsh:TD1-1066 ,lcsh:Environmental technology. Sanitary engineering ,lcsh:Environmental sciences ,lcsh:GE1-350 ,model ,thessaloniki ,lcsh:QE1-996.5 ,lcsh:Geography. Anthropology. Recreation ,climatology ,area ,simulation ,new-york ,Point data ,lcsh:Geology ,Warm front ,lcsh:G ,Anticyclone ,Climatology ,General Earth and Planetary Sciences ,Environmental science ,radiation fog - Abstract
The characteristics of the atmospheric circulation over Europe and the Mediterranean associated with the formation and the dissipation of fog in Greece are examined. The data used consists of: i) 3-hourly meteorological observations recorded at 16 meteorological stations in Greece and ii) daily (00:00 UTC) 2.5×2.5 grid point values of mean sea-level pressure, 500 hPa geopotential height, 850 hPa and 500 hPa air temperatures and 1000–500 hPa thickness over Europe for the period 1957–2002. 1055 fog events are extracted from the 3-hourly meteorological observations. A specific methodology scheme including S-mode Factor Analysis and k-means Cluster Analysis is applied to the grid point data sets for the first day of a fog event (D day), the day prior to D day (D-1 day) and the day that follows the last day of a fog event (END day) and the 1055 evolutions of the atmospheric circulation associated with fog events in Greece are classified into 10 clusters. The mean patterns of MSL Pressure, 850 hPa and 500 hPa air temperatures, 1000–500 hPa thickness and 500 hPa geopotential height show that in most of the clusters, the presence of anticyclonic conditions over the Balkans, a warm front passage, or a weak, humid southerly flow induced by the presence of a shallow depression over the western Mediterranean favor fog formation in Greece, while the dissipation of fog occurs when drier air masses are transferred over the Balkans. The main differences among the 10 clusters refer to the exact position, the intensity and the specific evolution of the surface and the upper air systems, the season of their predominance and the area of the Greek territory that mainly refer to.
- Published
- 2009
16. On the intra‐annual variability of atmospheric circulation in the Mediterranean region
- Author
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D. A. Metaxas, Christos J. Lolis, and Aristides Bartzokas
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Mediterranean climate ,Atmospheric Science ,Circulation (fluid dynamics) ,Atmospheric circulation ,Climatology ,Humidity ,Environmental science ,Vorticity ,Mediterranean Basin ,Sea level ,Geostrophic wind - Abstract
Some aspects on the intra-annual variability and the seasonal peculiarities of the atmospheric circulation in the Mediterranean region are examined. T-mode factor analysis is applied on the long-term (1948–2003) 5-day mean values of relative geostrophic vorticity for the 73 5-day periods of the year. The analysis is applied for sea-surface and 500-hPa levels. For sea level, two circulation types are found to be dominant: the type of the warm period and the type of the cold period, characterizing the so-called Mediterranean climate; they are strongly dependent on the thermal and humidity characteristics of the earth's surface and the position and strength of the sub-tropical (Azores) anti-cyclone. For 500-hPa level, two extra but weaker circulation types are added, prevailing mainly during the transitional seasons and modifying the smooth intra-annual variation of the atmospheric circulation. In this case, the thermal influence of the earth's surface appears to be weaker, while other circulation features, such as blocking activity and/or cut-off lows, seem to play an important role too. Copyright © 2007 Royal Meteorological Society
- Published
- 2008
17. Temperature fluctuations in the mediterranean area during the last 120 years
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D. A. Metaxas, Aristides Bartzokas, and A. Vitsas
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Mediterranean climate ,Atmospheric Science ,Trend analysis ,Eastern mediterranean ,Mediterranean sea ,Climatology ,Air temperature ,Northern Hemisphere ,Climate change ,Mediterranean area ,Environmental science - Abstract
Based upon sea-surface temperature (SST) data, recently corrected for bucket-intake observational change, the SST fluctuations since 1873 in the Mediterranean are studied and compared with air temperature (AT) fluctuations at some land stations of this area and for the Northern Hemisphere (ATNH). Instead of the actual temperature, we used the frequency difference (per cent) of the number of the warm minus cold months, (fw—fc) annually and seasonally, smoothed using 10-year moving averages. This frequency difference was found to be a linear function of the actual temperature. For the whole Mediterranean, the trend of the SST is simple enough, with a minimum in about 1910 and a double maximum in about 1940 and 1965. A secondary minimum follows in 1975–1980, very strong in the eastern Mediterranean, with a rise after that to the present. This trend when compared with ATNH presents some differences: in the ATNH a minimum appears earlier, in 1890, but this is shown only slightly in the Mediterranean SST. The second SST maximum, in 1965, is not shown in ATNH, and the general rise of recent years starts in the ATNH about 10 years earlier. When considering the AT of the Mediterranean stations, it can be seen that the rise, after the minimum of 1910, stops 15 years earlier than the SST and ATNH. Then, an anomalous and slow decrease follows until about 1975–1980, followed by a rise to the present, which is delayed in the eastern Mediterranean.
- Published
- 2007
18. The 850 hPa relative vorticity centres of action for winter precipitation in the Greek area
- Author
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Aristides Bartzokas, D. A. Metaxas, and Christos J. Lolis
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Mediterranean climate ,Atmospheric Science ,Atmospheric circulation ,mediterranean ,sea-surface temperature ,pollution ,patterns ,Precipitation ,canonical correlation analysis ,winter precipitation ,principal components ,variability ,Anomaly (natural sciences) ,greece ,climatology ,Vorticity ,west african depressions ,Anticyclone ,North Atlantic oscillation ,Climatology ,850 hpa relative vorticity ,Period (geology) ,circulation ,anomalies ,cyclogenesis ,Geology ,cluster analysis - Abstract
In this work, the relationship between atmospheric circulation over Europe and precipitation in Greece is studied during high winter (January-February). Specifically, the 850 hPa relative vorticity centres, affecting or related to precipitation in the various areas of Greece, are defined and the corresponding temporal variability of both parameters is examined. Factor analysis is first applied on both fields, in order to. reduce the dimensionality of the original data sets. Canonical correlation analysis is then applied on the factor analysis results, in order to interrelate the two parameters. It is found that: (i) precipitation over western and northern Greece, the eastern Aegean islands and the western Turkish coasts is mainly controlled by vorticity over Italy and the Gulf of Genoa; (ii) southern Aegean Sea and Crete precipitation is controlled by relative vorticity west of Crete; and (iii) eastern mainland areas precipitation is significantly affected by a vorticity seesaw between Tunisia and the northern Aegean. The temporal variations of precipitation and relative vorticity parameters over the above areas show a precipitation reduction over most Greek stations, combined with enhanced anticyclonic activity over almost the whole Mediterranean during the late 1980s and the early 1990s, a period characterized by high North Atlantic oscillation index values. Finally, cluster analysis is applied on the factor scores time series of precipitation, in order to classify the winter precipitation anomaly patterns into objectively defined clusters. Six main precipitation patterns were revealed and the corresponding relative vorticity mean anomaly regimes over southern Europe indicate the atmospheric circulation characteristics causing these specific patterns. Copyright (C) 2003 Royal Meteorological Society. International Journal of Climatology
- Published
- 2003
19. The RISKMED project: Philosophy, methods and results
- Author
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Joel Azzopardi, Federico Porcù, D. Conte, A. Buzzi, Vassiliki Kotroni, S. Music, K. Savvidou, Silas Michaelides, Luciana Bertotti, Mario Marcello Miglietta, Stefano Dietrich, Antonis Gkikas, Luigi Cavaleri, Oxana Drofa, Christos J. Lolis, K. Nikolaides, Silvio Davolio, M. I. Tsirogianni, Kostas Lagouvardos, Aldo Drago, A. Mugnai, Aristides Bartzokas, Bartzokas A., Azzopardi J., Bertotti L., Buzzi A., Cavaleri L., Conte D., Davolio S., Dietrich S., Drago A., Drofa O., Gkikas A., Kotroni V., Lagouvardos K., Lolis C.J., Michaelides S., Miglietta M., Mugnai A., Music S., Nikolaides K., Porcu F., Savvidou K., and Tsirogianni M.
- Subjects
Meteorology ,cyclone ,forecast ,Population ,Severe storms -- Forecasting ,precipitation ,system ,lcsh:TD1-1066 ,Weather forecasting ,Meteorology -- Case studies ,Cyclone forecasting -- Mediterranean Sea ,Extreme weather ,scheme ,MEDITERRANEAN CLIMATE ,FLOOD ,lcsh:Environmental technology. Sanitary engineering ,education ,lcsh:Environmental sciences ,PRECIPITATION ,METEOROLOGICAL FORECAST ,ALERT ,lcsh:GE1-350 ,validation ,education.field_of_study ,model ,Severe weather ,lcsh:QE1-996.5 ,lcsh:Geography. Anthropology. Recreation ,flood ,sensitivity ,Surface weather observation ,lcsh:Geology ,Model output statistics ,NATURAL HAZARDS ,Geography ,lcsh:G ,Climatology ,Thunderstorm ,General Earth and Planetary Sciences ,Distribution of lightning ,Early warning system ,Climatology -- Environmental aspects ,NUMERICAL WEATHER FORECAST ,simulations - Abstract
This paper presents RISKMED, a project targeted to create an Early Warning System (EWS) in case of severe or extreme weather events in the central and eastern Mediterranean and specifically in southern Italy, northwestern Greece, Malta and Cyprus. As severe or extreme weather events are considered, cases when the values of some meteorological parameters (temperature, wind, precipitation) exceed certain thresholds, and/or a severe weather phenomenon (thunderstorm, snowfall) occurs. For an accurate weather forecast, selected meteorological models have been operated daily, based on a nesting strategy using two or three domains, providing detailed forecasts over the above mentioned areas. The forecast results are further exploited for the evaluation and prediction of human discomfort and fire weather indices. Finally, sea wave models have also been operating daily over the central and eastern Mediterranean Sea. In case a severe or extreme weather event is forecasted within the next 48 or 72 h for selected target areas (sub-regions defined by their morphological and population characteristics), the local authorities and the public are informed via a user-friendly graphic system, the so-called RISK MAP. On the web page of the Project (http://www.riskmed.net), additional information is provided about the real-time values of some meteorological parameters, the latest satellite picture and the time and space distribution of lightning during the last 24 h. The RISKMED project was financed by the EU and the Ministries of National Economy of Greece, Italy, Malta and Cyprus, in the frame of INTERREG IIIB/ARCHIMED programme., peer-reviewed
- Published
- 2010
20. A statistical study of precipitation in northwest Greece
- Author
-
Aristides Bartzokas, D. A. Metaxas, and A.K. Fotiadi
- Subjects
Mediterranean climate ,northwest greece ,Atmospheric Science ,cyclone ,Stepwise regression analysis ,factor analysis ,Regression analysis ,precipitation ,Spatial distribution ,stepwise regression analysis ,Analisis factorial ,topography ,Climatology ,harmonic analysis ,Environmental science ,Statistical analysis ,Precipitation ,Maxima - Abstract
In this paper, using 20 year mean monthly precipitation totals from 54 stations, the general statistics for the time and space distribution of precipitation in northwest Greece is at first described. Precipitation varies from about 1000 mm at the coast to 2500 mm on the mountains, with a large variability. Then, the intra-annual variation is studied, using Fourier analysis. This variation is described well by two harmonics, explaining about 90% of the total variance. The first harmonic, over 80% of the variance, shows a maximum from late December (coastal areas) to early January (continental areas) and is caused by the northwest Mediterranean depression activity. The second harmonic exhibits maxima in May and November, also delayed in the continental areas, and is mainly caused by upper air troughs and instability. The spatial distribution of precipitation is then studied by using factor analysis. The 12 months of a year are classified in two parts corresponding to two statistically significant factors, explaining 88.5% of the total variance. Finally, the combination of positive and negative standardized scores of the two factors showed that northwest Greece is characterized by four precipitation regimes, depending on the combination of depression activity, the effects of the sea and of topography. Copyright (C) 1999 Royal Meteorological Society. International Journal of Climatology
- Published
- 1999
21. A Study of Divergence and Convergence of the Wind Field over Europe and the Mediterranean
- Author
-
O. Pezoula and A. Bartzokas
- Subjects
Atmosphere ,Mediterranean climate ,Atmospheric pressure ,Climatology ,Cyclogenesis ,Period (geology) ,Convergence (economics) ,Geology ,Latitude ,Divergence - Abstract
In this work, the divergence and convergence of the wind field over Europe, the Mediterranean and North Africa is studied for the 60-year period 1950–2009. Mean monthly values (s−1) are utilized at 273 grid points, spaced by 2.5° × 2.5°, at 10 atmospheric pressure levels from 1,000 hPa up to 100 hPa. It is found that on a seasonal basis considerable differences appear among the various European regions both near the surface of the Earth and in the upper atmosphere. During winter, near the surface, high values of convergence appear in the cyclogenesis areas and along the tracks of depressions. Specifically: in the Mediterranean, with maximum values in the Gulf of Genoa, the Black Sea and from the North Sea up to the Baltic Sea. On the contrary, over the rest of the areas divergence prevails. This situation is reversed above approximately 600 hPa with positive values (divergence) over the Mediterranean and the Seas of northern Europe and negative (convergence) over central Europe. During summer, over the seas, where air is cooler than over land, positive values are observed (divergence). This difference is more intense over lower latitudes. Similarly to winter, in the upper atmosphere, the convergence-divergence field becomes smoother changing signs at about 600 hPa.
- Published
- 2012
22. Desert Dust Properties, Modelling, and Monitoring
- Author
-
Achuthan Jayaraman, Aristides Bartzokas, Pawan Gupta, Ralph A. Kahn, and Dimitris G. Kaskaoutis
- Subjects
Mediterranean climate ,Atmospheric Science ,Article Subject ,Meteorology ,Central asia ,lcsh:QC851-999 ,Pollution ,Geophysics ,Optical radar ,Geography ,Lidar ,Climatology ,Satellite data ,Climate model ,lcsh:Meteorology. Climatology ,Desert dust - Abstract
This paper is just the three-page introduction to a Special Issue of Advances in Meteorology focusing on desert dust. It provides a paragraph each on 13 accepted papers, most relating to the used of satellite data to assess attributes or distribution of airborne desert dust. As guest Associate Editors of this issue, we organized the papers into a systematic whole, beginning with large-scale transport and seasonal behavior, then to regional dust transport, transport history, and climate impacts, first in the Mediterranean region, then India and central Asia, and finally focusing on transport model assessment and the use of lidar as a technique to constrain dust spatial-temporal distribution.
- Published
- 2012
23. Intra-annual variation of atmospheric static stability in the Mediterranean region: a 60-year climatology
- Author
-
Aristides Bartzokas, Christos J. Lolis, D. A. Metaxas, and Kostas Lagouvardos
- Subjects
Mediterranean climate ,Atmospheric Science ,greek area ,Atmospheric circulation ,covariability ,Longitudinal static stability ,reanalysis ,temperature ,Atmospheric sciences ,Spatial distribution ,thunderstorms ,northern-hemisphere blocking ,indexes ,Climatology ,Thermal ,Atmospheric instability ,Period (geology) ,Environmental science ,Annual variation ,europe ,winter precipitation ,principal components - Abstract
The seasonal characteristics of atmospheric static stability in the Mediterranean region are examined, for the 60-year period 1948-2007 and for the four 15-year sub-periods 1948-1962, 1963-1977, 1978-1992 and 1993-2007. S-Mode and T-Mode Factor Analysis are applied to the mean 5-day values of K static stability index over the Mediterranean region. Three dominant modes are revealed for both, the intra-annual variation and the spatial distribution of K-index. It is found that these modes are connected to the seasonal characteristics of the main atmospheric circulation systems affecting the region and the thermal properties of the Earth's surface (land or sea). The differences among the results of the four sub-periods partially reflect the inter-decadal variations of the strength of the above factors. Theoretical and Applied Climatology
- Published
- 2012
24. Synoptic conditions favouring the occurrence of aerosol episodes over the broader Mediterranean basin
- Author
-
Antonis Gkikas, Nikolaos Hatzianastassiou, C. D. Papadimas, E. E. Houssos, and Aristides Bartzokas
- Subjects
Mediterranean climate ,Atmospheric Science ,Atmospheric circulation ,aerosol particles ,Geopotential height ,aerosol transport ,synoptic conditions ,Subtropics ,Mediterranean ,Thermal low ,Mediterranean Basin ,Aerosol ,Anticyclone ,Climatology ,Environmental science ,dust - Abstract
In the present study, the synoptic conditions that favour the occurrence of aerosol episodes over the broader Mediterranean basin are investigated. For this purpose daily satellite measurements of aerosol optical depth (AOD) at 550 nm from MODIS-Terra (Collection 5) for the period from 1 March 2000 to 28 February 2007 were used together with mean-sea-level pressure and geopotential height at 700 hPa obtained from the NCEP/NCAR Reanalysis Project. An objective and dynamic algorithm was developed in order to determine strong and extreme aerosol episodes in the study area. According to the applied methodology and the relevant criteria, 322 days with aerosol episodes have been identified. The atmospheric circulation of each day was objectively classified by the implementation of Factor Analysis and Cluster Analysis, resulting in eight representative synoptic conditions (clusters). The aerosol episode days (AEDs) are most frequently observed during the dry period of the year, and especially in July–August (108 days or 33.5%) and April–May (106 days or 33%) and give rise to desert dust, anthropogenic pollution and biomass-burning episodes. The most frequent synoptic conditions are those of Clusters 4 (30.1%) and 5 (21%), inducing episodes that occur primarily in the western and secondarily in the central Mediterranean. In the former case, which is observed in summer, Europe is under the influence of the extended subtropical anticyclone of the Atlantic (Azores), while the eastern Mediterranean is under the influence of the extended thermal low of south-west Asia. In the latter case, in spring and summer, anticyclonic conditions prevail over central Europe and over the east Atlantic Ocean, and low pressures at the northwestern parts of the broader Mediterranean basin and across the Sahara. During the aerosol episode days of the various clusters, the mean regional AOD can reach values up to 0.8 ± 0.2 and 2.3 ± 1.3 for strong and extreme episodes, respectively. Copyright © 2011 Royal Meteorological Society
- Published
- 2012
25. Large-Scale Atmospheric Circulation Favoring Deep- and Intermediate-Water Formation in the Mediterranean Sea
- Author
-
Samuel Somot, Aristides Bartzokas, Vassilis P. Papadopoulos, Paraskevi Drakopoulou, Simon A. Josey, and Simón Ruiz
- Subjects
Mediterranean climate ,Atmospheric Science ,010504 meteorology & atmospheric sciences ,Atmospheric pressure ,Atmospheric circulation ,Air-sea interactions ,Humidity ,Sensible heat ,010502 geochemistry & geophysics ,01 natural sciences ,Mediterranean sea ,13. Climate action ,Anticyclone ,North Atlantic oscillation ,Climatology ,Mediterranean Sea ,Environmental science ,14. Life underwater ,Deep convection ,0105 earth and related environmental sciences - Abstract
Atmospheric circulation patterns that are conducive to extreme ocean heat loss are investigated at four sites of special interest in the Mediterranean Sea. The Gulf of Lions, the South Adriatic Sea, the Cretan Sea, and the Levantine Sea are areas where anomalously high winter heat loss may lead to deep- or intermediate-water formation. At each of the above sites, the atmospheric circulation during such events is derived by averaging the sea level pressure (SLP) fields during the lower decile of the wintertime series of the net heat exchange.A relatively simple SLP pattern dominated by an anticyclone over northwestern Europe with a weaker cyclone to the southeast is found to be associated with strong heat loss in the selected sites with minor variations in pattern structure depending on the site. The SLP composite pattern reflects the combined effect of different atmospheric modes of variability and the authors consider the impacts on heat loss of a number of these modes (North Atlantic Oscillation, east Atlantic pattern, east Atlantic–west Russia pattern, and Scandinavian pattern), together with the North Sea–Caspian pattern and the Mediterranean index. The extremes in heat loss are strongly connected with the intensity and the positions of the poles of these patterns that modulate, through the necessary SLP gradient and associated northerlies, the transfer of cold and dry air over the areas of dense-water formation. Analysis of air–sea temperature difference, specific humidity, and evaporation anomalies indicates that the extremes of the net heat fluxes are primarily due to the latent and sensible heat flux components, This study was funded by the European Community’s Seventh Framework Programme (FP7/2007-2013) under Grant 287600 with the Policyoriented marine Environmental Research for the Southern European Seas (PERSEUS) project.MichelDe´que´ is acknowledged for running the ARPEGE-Climate model in its ARPERA configuration. The HIPOCAS dataset has been provided by Puertos de Estado (Spain).
- Published
- 2012
26. Spatial and temporal sea-surface temperature, covariances in the mediterranean
- Author
-
D. A. Metaxas, I. S. Ganas, and Aristides Bartzokas
- Subjects
Mediterranean climate ,Atmospheric Science ,Eastern mediterranean ,Sea surface temperature ,Geography ,Mediterranean sea ,Climatology ,Principal component analysis ,Period (geology) - Abstract
Using sea-surface temperature data for the post-war period, the Mediterranean Sea is divided objectively (principal components factor analysis) into subareas with similar covariances. This division is not the same in every season of the year. In summer, two subareas are found, the west and the east. In spring and autumn three, and in winter four significant subareas denote physical oceanographic and meteorological processes. The scores of the analyses show clearly the minimum of the late 1970s and then the warming of recent years somewhat delayed in the eastern Mediterranean.
- Published
- 1994
27. Extreme precipitation events in NW Greece
- Author
-
Aristides Bartzokas, E. E. Houssos, and EGU, Publication
- Subjects
Mediterranean climate ,geography ,geography.geographical_feature_category ,lcsh:Dynamic and structural geology ,lcsh:QE1-996.5 ,General Medicine ,Thermal low ,Low-pressure area ,lcsh:Geology ,lcsh:QE500-639.5 ,Climatology ,Spring (hydrology) ,Period (geology) ,[SDU.STU] Sciences of the Universe [physics]/Earth Sciences ,Environmental science ,lcsh:Q ,Precipitation ,lcsh:Science ,Trough (meteorology) ,Sea level - Abstract
In this work, the extreme precipitation events in NW Greece are studied. The data used are daily precipitation totals recorded at the meteorological station of Ioannina University for the period 1970–2002. 156 days with precipitation totals above 35 mm (5% upper limit) are only considered. It is seen that, a minimum frequency of extreme precipitation events appears in the period 1986–1991, which is characterized by a high positive NAO index. For each of the 156 extreme precipitation days, at first, the mean sea level pressure pattern over Europe is constructed by using 273 grid point values. Using Factor Analysis, the dimensionality of the 156×273 data matrix is reduced to 156×5 (84% of the total variance) and then, Cluster Analysis is applied on the results of Factor Analysis. Thus, the 156 cases are grouped objectively to 11 clusters, revealing the main pressure patterns, which favour extreme precipitation in NW Greece. Seven of the patterns are encountered in winter and autumn, while three of them cover a period from autumn to spring and one appears mainly in summer. In all of them the cause of the extreme precipitation event is a low pressure system centred west of Greece or a low pressure trough extended eastwards or southwards up to Greece. In some cases the depression is so strong and extended that it covers the whole Europe and the Mediterranean. In the single summer pattern, rainfall is caused by an extension of the SW Asia thermal low up to the central Mediterranean.
- Published
- 2006
28. Spatial and temporal variability of precipitation over the Mediterranean Basin based on 32-year satellite Global Precipitation Climatology Project data, part I: evaluation and climatological patterns.
- Author
-
Hatzianastassiou, Nikolaos, Papadimas, Christos D., Lolis, Christos J., Bartzokas, Aristides, Levizzani, Vincenzo, Pnevmatikos, John D., and Katsoulis, Basil D.
- Subjects
METEOROLOGICAL precipitation ,RAINFALL ,MEDITERRANEAN climate ,SPATIOTEMPORAL processes - Abstract
ABSTRACT The precipitation regime over the Mediterranean basin is investigated for the period 1979-2010 using monthly mean satellite data from the Global Precipitation Climatology Project ( GPCPv2). The results show that a clear contrast exists between the more rainy northern part of the study region (Southern Europe) and the drier southern area (North Africa, Iberian Peninsula) and between the western sides (rainsides) of the Iberian, Italian and Balkan peninsulas and their eastern sides (rainshadows). The mean annual precipitation averaged over the study area is P = 593 ± 203 mm year
−1 , but it has a strong spatial variability ranging from 20 mm year−1 (North Africa) to 1500 mm year−1 (Alps). A significant seasonal variability exists, with the early winter and late autumn months (November and December) being the wettest with precipitation amounts larger than 60 mm month−1 . The GPCPv2 satellite precipitation data are satisfactorily correlated with rain gauge measurements from 433 stations within the study area (correlation coefficient R = 0.78 for all stations on a yearly basis, with values ranging between 0.72 and 0.82, depending on the season) with a slight overestimation. They also compare well with the higher spatial and temporal resolution Tropical Rainfall Measuring Mission ( TRMM) data, which supports the validity of the present study. [ABSTRACT FROM AUTHOR]- Published
- 2016
- Full Text
- View/download PDF
29. Spatial and temporal variability of precipitation over the Mediterranean Basin based on 32-year satellite Global Precipitation Climatology Project data. Part- II: inter-annual variability and trends.
- Author
-
Hatzianastassiou, Nikolaos, Papadimas, Christos D., Lolis, Christos J., Bartzokas, Aristides, Levizzani, Vincenzo, Pnevmatikos, John D., and Katsoulis, Basil D.
- Subjects
METEOROLOGICAL precipitation ,MEDITERRANEAN climate ,TELECONNECTIONS (Climatology) ,METEOROLOGICAL satellites - Abstract
ABSTRACT Monthly mean satellite data from the Global Precipitation Climatology Project ( GPCPv2) are used to examine the year-by-year variability of precipitation over the Mediterranean Basin and its changes over the period 1979-2010. The results show that the mean annual precipitation averaged over the study area has slightly increased from 1979 to 2010 by 1.28 mm or by 0.2% (trend not statistically significant at the 95% confidence level). Nevertheless, examining the trends at a local scale, spatial and temporal patterns are revealed, with opposite trends in adjacent areas and increasing precipitation in summer and autumn against almost unchanged or decreasing precipitation in winter and spring, respectively. Inter-decadal changes of precipitation are detected, with precipitation decreasing in the 1980s, then increasing through the late 1990s and finally declining in the 2000s before levelling off since 2007. These changes are significantly anti-correlated ( R = −0.57, up to −0.66 in winter) with the North Atlantic Oscillation ( NAO) index, thus confirming the critical role of this large-scale teleconnection for the regional precipitation over the basin. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
- View/download PDF
30. Relation between sensible and latent heat fluxes in the Mediterranean and precipitation in the Greek area during winter
- Author
-
B. D. Katsoulis, Aristides Bartzokas, and Christos J. Lolis
- Subjects
Mediterranean climate ,Atmospheric Science ,air-temperature ,atmospheric circulation ,mediterranean ,factor analysis ,Sensible heat ,precipitation ,Mediterranean sea ,Latent heat ,sea-surface temperature ,Precipitation ,northern oceans ,canonical correlation analysis ,principal components ,Lead (sea ice) ,greece ,connection ,climatology ,last century ,sensible heat flux ,Heat flux ,latent heat flux ,Climatology ,Heat transfer ,Environmental science ,anomalies ,cyclogenesis - Abstract
The variability of sensible and latent heat fluxes in the Mediterranean Sea and their effect on precipitation in the Greek area during the winter months are investigated for the 39 year period 1959-97 by using multivariate statistical methods. First, factor analysis is applied, mainly as a data reduction tool, and then canonical correlation analysis is applied on the factor scores time series of the two pairs, i.e. sensible heat flux-precipitation and latent heat flux-precipitation, to reveal their interrelations. The results show there is a statistically significant relationship between sensible and latent heat fluxes over the western Mediterranean Sea and precipitation in the western windward areas of Greece. The sensible and latent heat fluxes in the western Mediterranean Sea contribute significantly to depression development. The depressions formed advect cold and dry air masses over the western Mediterranean, reinforcing these sensible and latent heat fluxes. Furthermore, these depressions lead to a southerly or southwesterly airflow over Greece. This flow, along with the existence of potential instability, significantly contributes to precipitation formation in the windward areas of western Greece and also in the islands of the eastern Aegean Sea. Copyright (C) 2004 Royal Meteorological Society. International Journal of Climatology
- Published
- 2004
31. Sky luminance distribution in Central Europe and the Mediterranean area during the winter period
- Author
-
R. Kittler, H.D. Kambezidis, S. Darula, and Aristides Bartzokas
- Subjects
Mediterranean climate ,Atmospheric Science ,media_common.quotation_subject ,Illuminance ,mediterranean ,sky luminance distribution ,Atmospheric sciences ,Luminance ,diffuse illuminance ,central europe ,Geophysics ,Overcast ,Space and Planetary Science ,Sky ,daylight climate ,Period (geology) ,Environmental science ,zenith luminance ,Daylight ,Zenith ,media_common - Abstract
Five-minute data of global and diffuse irradiance and illuminance along with zenith luminance from Bratislava and Athens for a period of 5 years are used in order to define the prevailing sky luminance distributions at the two sites in the winter period. The classification into 15 sky types is mainly based on the value of the ratio of zenith luminance to diffuse illuminance with solar altitude as parameter (SSLD method). This is applied for the first time in a research work. It is found that the most frequent sky type in Bratislava is the overcast (with steep luminance gradation and slight brightening towards the sun); while in Athens it is the cloudless polluted sky (with a broad solar corona). That is, the winter daylight climate is quite different in Central Europe in comparison to the Mediterranean. An analysis for various solar altitudes reveals the frequency distribution of the 15 sky types suitable for the application in daylight climate studies. (C) 2002 Published by Elsevier Science Ltd. Journal of Atmospheric and Solar-Terrestrial Physics
- Published
- 2003
32. Spatial and temporal 850 hPA air temperature and sea-surface temperature covariances in the Mediterranean region and their connection to atmospheric circulation
- Author
-
B. D. Katsoulis, Christos J. Lolis, and Aristides Bartzokas
- Subjects
Mediterranean climate ,Atmospheric Science ,Atmospheric circulation ,covariability ,factor analysis ,precipitation ,Mediterranean Basin ,Mediterranean sea ,mediterranean sea ,mediterranean oscillation ,canonical correlation analysis ,principal components ,climatology ,last century ,oscillation ,Atmospheric temperature ,winter ,Sea surface temperature ,850 hpa air temperature ,Climatology ,Environmental science ,anomalies ,Spatial variability ,sea-surface temperature (sst) ,europe ,atlantic ,Teleconnection - Abstract
The spatial and temporal covariability between the lower troposphere and sea surface temperatures (SSTs) are Studied in the Mediterranean basin for the period 1958-98. Monthly air temperature anomalies for the 850 hPa pressure level (T-850hPa) at 2.5degrees x 2.5degrees grid points and SST anomalies in 5degrees x 5degrees grid boxes are utilized. As a first step, factor analysis is applied oil both sets of data in order to reduce their dimensionality. Then, canonical correlation analysis is applied and this leads to one statistically significant pair of canonical variates for winter and to two pairs for summer. In winter, a teleconnection (see-saw) between western Europe and the eastern Mediterranean at the 850 hPa level is revealed, and a corresponding weaker one between the areas of central-west and eastern Mediterranean for SST. The correlation between T-850hPa and SST appears higher over the eastern Mediterranean. In summer, the first pair of canonical variates reveals a covariability between T-850hPa and SST in the western Mediterranean, and the second one shows a covariability in the eastern Mediterranean, without the existence of any strong spatial teleconnection. The analysis is repeated, using time lags of I month, or longer, in order to detect any possible non-synchronous relation. Statistically significant results are found only when T-850hPa leads SST with a time lag of I month. In particular, the results are statistically significant for winter only, and the findings are similar to those of the first analysis. Therefore, the existence of a I month time scale SST persistence is detected for winter months. Copyright (C) 2002 Royal Meteorological Society. International Journal of Climatology
- Published
- 2002
33. Pressure Covariability over the Atlantic, Europe and N Africa - Application - Centers of Action for Temperature, Winter Precipitation and Summer Winds in Athens, Greece
- Author
-
Aristides Bartzokas and D. A. Metaxas
- Subjects
Mediterranean climate ,Atmospheric Science ,Atmospheric circulation ,variability ,Northern Hemisphere ,Subtropics ,Thermal low ,Geography ,Anticyclone ,Climatology ,frequency ,northern-hemisphere ,Icelandic Low ,sea-level pressure ,height field ,Teleconnection ,principal components - Abstract
This paper deals with the surface pressure covariability over the Altantic/European sector of the Northern Hemisphere. using monthly grid point data for 100 year period 1890 1989. Factor analysis is applied to 90 grid point time series for January, February. July, and August. The initial 90 pressure variables can be reduced to 7-8 factors in winter and 10 in summer. A winter teleconnection was identified. known as the seesaw phenomenon, between the Icelandic low and the Azores subtropical anticyclone. In order to define the centers of action for temperature, winter precipitation and summer northern wind frequency (etesian days) in Athens and in the Aegean sea, the variability of the factor scores and of these weather elements is compared. It is shown that the center of action for temperature in Athens is found to be in north and northwest Europe (centered over southern Scandinavia). For winter precipitation, the center of action is located in the west and southwest Mediterranean and northwest Africa. Finally, for the etesian winds frequency variability, this center of action is found over the northern Adriatic and northern former Yugoslavia, while there is no evidence of influence by the southwest Asia thermal low. Theoretical and Applied Climatology
- Published
- 1994
34. Editorial
- Author
-
Aristides Bartzokas
- Subjects
Mediterranean climate ,Current (stream) ,Atmospheric Science ,Geography ,Climatology ,Volume (compression) - Published
- 2004
35. On the intra-annual variability of atmospheric circulation in the Mediterranean region.
- Author
-
Lolis, C. J., Metaxas, D. A., and Bartzokas, A.
- Subjects
ATMOSPHERIC circulation ,SYNOPTIC climatology ,HOMOGENEITY ,TROPOSPHERE ,CLIMATOLOGY ,METEOROLOGY ,MEDITERRANEAN climate ,RESEARCH - Abstract
The article presents a study that measures the intra-annual variation of the atmospheric circulation and detecting potential seasonal oddments in the Mediterranean regions. It uses the geostrophic vorticity and analysis is enforced for lower troposphere and middle troposphere. The vertical homogeneity of the troposphere is tested as the vertical structure of depressions in the regions depicts an important spatial and seasonal variability. Moreover, the thermal influence of the surface of the Earth appears to be weaker, while the blocking activity and/or cut-off lows have also significant roles.
- Published
- 2008
- Full Text
- View/download PDF
36. Annual variation of pressure over the Mediterranean area
- Author
-
Aristides Bartzokas
- Subjects
Mediterranean climate ,Atmospheric Science ,Meteorology ,Atmospheric pressure ,Seasonality ,medicine.disease ,Low-pressure area ,Amplitude ,Mediterranean sea ,Anticyclone ,Climatology ,medicine ,Environmental science ,Sea level - Abstract
This study deals with the annual variation of pressure over the Mediterranean area using monthly values from 24 stations, for a period of 22 years. The Azores and the Siberian anticyclones as well as the low pressure system of SW-Asia are responsible for the observed distribution pattern of the pressure over the area and for the seasonal changes at each station. Fourier analysis shows that for Central Mediterranean stations, which record two minima and two maxima of pressure during the year, the amplitude of the second harmonic is smaller than the amplitude of the third.
- Published
- 1989
37. Objective Definition of Climatologically Homogeneous Areas in the Southern Balkans Based on the ERA5 Data Set.
- Author
-
Lolis, Christos J., Kotsias, Georgios, and Bartzokas, Aristides
- Subjects
ATMOSPHERIC temperature ,POTENTIAL energy ,CLOUDINESS ,PRINCIPAL components analysis - Abstract
An objective definition of climatologically homogeneous areas in the southern Balkans is attempted with the use of daily 0.25° × 0.25° ERA5 meteorological data of air temperature, dew point, zonal and meridional wind components, Convective Available Potential Energy, Convective Inhibition, and total cloud cover. The classification of the various grid points into climatologically homogeneous areas is carried out by applying Principal Component Analysis and K-means Cluster Analysis on the mean spatial anomaly patterns of the above parameters for the 10-year period of 2008 to 2017. According to the results, 12 climatologically homogenous areas are found. From these areas, eight are mainly over the sea and four are mainly over the land. The mean intra-annual variations of the spatial anomalies of the above parameters reveal the main climatic characteristics of these areas for the above period. These characteristics refer, for example, to how much warmer or cloudy the climate of a specific area is in a specific season relatively to the rest of the geographical domain. The continentality, the latitude, the altitude, the orientation, and the seasonal variability of the thermal and dynamic factors affecting the Mediterranean region are responsible for the climate characteristics of the 12 areas and the differences among them. [ABSTRACT FROM AUTHOR]
- Published
- 2018
- Full Text
- View/download PDF
38. Objective Definition of Climatologically Homogeneous Areas in the Southern Balkans Based on the ERA5 Data Set
- Author
-
Christos J. Lolis, Georgios Kotsias, and Aristides Bartzokas
- Subjects
Mediterranean climate ,cluster analysis ,objective classification ,ERA5 ,Science - Abstract
An objective definition of climatologically homogeneous areas in the southern Balkans is attempted with the use of daily 0.25° × 0.25° ERA5 meteorological data of air temperature, dew point, zonal and meridional wind components, Convective Available Potential Energy, Convective Inhibition, and total cloud cover. The classification of the various grid points into climatologically homogeneous areas is carried out by applying Principal Component Analysis and K-means Cluster Analysis on the mean spatial anomaly patterns of the above parameters for the 10-year period of 2008 to 2017. According to the results, 12 climatologically homogenous areas are found. From these areas, eight are mainly over the sea and four are mainly over the land. The mean intra-annual variations of the spatial anomalies of the above parameters reveal the main climatic characteristics of these areas for the above period. These characteristics refer, for example, to how much warmer or cloudy the climate of a specific area is in a specific season relatively to the rest of the geographical domain. The continentality, the latitude, the altitude, the orientation, and the seasonal variability of the thermal and dynamic factors affecting the Mediterranean region are responsible for the climate characteristics of the 12 areas and the differences among them.
- Published
- 2018
- Full Text
- View/download PDF
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