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Dotsika E.,Greek National Center For Scientific Research | Lykoudis S.,Institute of Environmental Research and Sustainable Development | Lykoudis S.,University of Patras | Poutoukis D.,General Secretariat for Research and Technology
Global and Planetary Change | Year: 2010

This paper reviews all available stable isotopic data concerning precipitation and spring water in Greece, from the 1960s until today. Spatial variability is investigated in order to provide basic information and identify the locally significant parameters that affect isotopic distributions. The area of interest was partitioned into eight sections according to geographical location and climatic characteristics. The distance of the station from the sea and the altitude are the main factors imprinted in the isotopic signature of precipitation. Local meteoric water meteoric line (LMWL) for precipitation and spring water were calculated for each section and for Greece as a whole. Precipitation LMWLs differ from the Global Meteoric Water Line (GMWL) in various ways across Greece. Elevated deuterium excess values are observed, probably due to water vapour originating from the Aegean or the Mid-eastern Mediterranean. Spring LMWLs are more or less consistent throughout the country. Furthermore, a high resolution map of precipitation and freshwater spring (Cl- < 200 ppm and T < 25 °C) δ18O reveals several interesting features such as an orographic shadow effect induced by the Pindos Mountains, a strong climatic signal in southern Greece and a local city-microclimate effect around Athens. © 2009 Elsevier B.V. All rights reserved. Source

Pantavou K.,National and Kapodistrian University of Athens | Lykoudis S.,Institute of Environmental Research and Sustainable Development
International Journal of Biometeorology | Year: 2014

A simple thermo-physiological model of outdoor thermal sensation adjusted with psychological factors is developed aiming to predict thermal sensation in Mediterranean climates. Microclimatic measurements simultaneously with interviews on personal and psychological conditions were carried out in a square, a street canyon and a coastal location of the greater urban area of Athens, Greece. Multiple linear and ordinal regression were applied in order to estimate thermal sensation making allowance for all the recorded parameters or specific, empirically selected, subsets producing so-called extensive and empirical models, respectively. Meteorological, thermo-physiological and overall models - considering psychological factors as well - were developed. Predictions were improved when personal and psychological factors were taken into account as compared to meteorological models. The model based on ordinal regression reproduced extreme values of thermal sensation vote more adequately than the linear regression one, while the empirical model produced satisfactory results in relation to the extensive model. The effects of adaptation and expectation on thermal sensation vote were introduced in the models by means of the exposure time, season and preference related to air temperature and irradiation. The assessment of thermal sensation could be a useful criterion in decision making regarding public health, outdoor spaces planning and tourism. © 2013 ISB. Source

Kalogiros J.,Institute of Environmental Research and Sustainable Development
Journal of Atmospheric and Oceanic Technology | Year: 2012

A least squares method for the reconstruction of Doppler spectra of weather radars with irregular pulse repetition time used to increase the range of unambiguous velocity is presented and evaluated. This method is a robust spectral method that is based on the least squares minimum norm principle and reconstructs both the magnitude and the phase of the discrete Fourier transform of the signal. The phase spectrum is useful in the estimation of the differential phase in dual-polarization radars with staggered sampling schemes, which is a case of irregular sampling. A computationally efficient iterative algorithm for estimating the mean frequency of the signal, which is required for the reconstruction of the spectrum, is described for possible realtime applications. Aclutter filter method based on spectral interpolation, which can be applied to echoes with generally nonzero mean velocity, is also described and combined with the spectrum reconstruction method. Using simulated data it is shown that the least squares reconstruction method with or without the presenceof clutter gives results with small bias and standard error and can be applied to wide spectra. The application of the method to real X-band radar data with a low signal-to-noise ratio and a high stagger ratio value of 5/6 showed that the least squares method has low sensitivity to the stagger ratio and satisfactorily gives spectral reconstruction for signal-to-noise ratio values as low as 10 dB. © 2012 American Meteorological Society. Source

Kitsara G.,National and Kapodistrian University of Athens | Papaioannou G.,National and Kapodistrian University of Athens | Papathanasiou A.,National and Kapodistrian University of Athens | Retalis A.,Institute of Environmental Research and Sustainable Development
Water Resources Management | Year: 2013

Evidence of global dimming/brightening is mainly based upon few measurements of solar radiation. A need for more research supported and extended with the use of other climatic variables, such as sunshine duration, recorded for a longer time period and successfully used as a proxy for solar radiation over the past 80 years, has already been urged. Thus, in this study, residual sunshine duration series computed after removal of the cloudiness-related variability, from daily sunshine duration and cloudiness data measured at the National Observatory of Athens are used for highlighting global dimming or brightening periods in Athens, during the period 1951-2001. Furthermore, the consistency of trends in radiation records and their implications for the hydrological cycle and especially the trends in reference evapotranspiration are examined, during the period 1951-2001. The analysis focuses on the seasonal decadal variations, determines and explains the causes of the seasonal trends from the inter-annual and decadal variability of the sunshine duration during the last half past century, with a special emphasis in detecting possible sub-periods in Athens. The signs of trends in the modeled annual and seasonal reference evapotranspiration are estimated, according to the indicated global dimming/brightening periods. © 2012 Springer Science+Business Media Dordrecht. Source

Kotroni V.,Institute of Environmental Research and Sustainable Development | Lagouvardos K.,Institute of Environmental Research and Sustainable Development
Environmental Research Letters | Year: 2016

Here we present the analysis of lightning activity over the Mediterranean, based on a 10 year long dataset (2005-2014) provided by the ZEUS long-range lightning detection system. The major hot-spots of lightning activity are identified, with a clear predominance during the warm period of the year over land in the vicinity of the major topographic features of the area. Special emphasis is also given on the discussion of the seasonal distribution of lightning. In addition, we investigate the relationship of lightning with sea-surface temperature, obtained by high-resolution satellite measurements and we conclude that the number of lightning strokes is positively correlated with the sea-surface temperature during autumn when also the maximum lightning activity over the sea is depicted. We suggest that higher sea surface temperature further destabilises the lower tropospheric layers, enhancing thus convection and therefore lightning. © 2016 IOP Publishing Ltd. Source

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