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1.
Summary Hourly measurements of solar irradiance in the wave band excluding photosynthetically active radiation (PAR) and solar irradiance (SI) were made over a 12-month period at the National Observatory of Athens, for obtaining the ratios of PAR to SI. These irradiance ratios exhibit dependence on sky conditions, with slightly larger values being observed under cloudy skies and seasonal variations, attributable to changes in local air mass climatology. The highest values have been obtained during the growing season (April–September). The mean annual value of 0.473 observed for the irradiance ratio in the PAR band compares favorably with values reported in the literature for different locations over a wide geographical area.With 2 Figures 相似文献
2.
Dimitris A. Retalis 《Pure and Applied Geophysics》1977,115(3):575-581
In the present study the influence of smoke and sulfur dioxide concentrations and of wind velocity on the concentration of small positive and negative ions above Athens area (=37°58.3N, =23°43E,h=107 m) is examined.Between the above mentioned elements simple linear correlation and multiple linear correlation is attempted. This correlation is based on mean daily values of the elements, for the two-year period 1971–1972, for each month separately.From the above examination it has been ascertained that small ions concentration increases as smoke and/or sulfur dioxide concentration decreases. Also, small ions concentration increases as wind velocity increases.In addition, it was ascertained that sulfur dioxide contributes the least to the determination of the variance of small ions concentration, whilst the greatest contribution is made by smoke for the positive small ions or by wind velocity for the negative ones. 相似文献
3.
D.G. Hadjimitsis C.R.I. Clayton A. Retalis 《International Journal of Applied Earth Observation and Geoinformation》2009
Because atmospheric effects can have a significant impact on the data obtained from multi-spectral satellite remote sensing, it is frequently necessary to make corrections before any other image processing can be started. This paper describes a robust and relatively simple atmospheric correction method that uses pseudo-invariant targets (PITs) in conjunction with the empirical line method. The method is based on the selection of a number of suitable generic PITs, on the basis that they are large, distinctive in shape, and occur in many geographical areas. Whereas the multi-temporal normalization method corrects all images to a selected reference image, in this method images are simultaneously corrected using targets with a range of estimated surface reflectance values. The paper describes some applications of the method for a range of environmental studies involving water quality and air pollution monitoring, and mapping land-cover changes. 相似文献
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5.
In the frame of this paper a heat wave that resulted in record high temperatures in Athens Greece is analysed. Namely the analysis focuses in the ability of three widely used planetary boundary layer parameterisations to reproduce the heat wave temperatures. The simulations were performed with the Pennsylvania State University – National Center for Atmospheric Research MM5 model that is also used for operational weather forecasting at the National Observatory of Athens since 2002. The 2-m temperature at a grid increment of 1-km over the highly complex terrain of the Athens Area is statistically verified against the available surface station observations. The results of the analysis showed that the two nonlocal schemes, namely Blackadar and MRF succeeded much better than the local scheme ETA to reproduce the heat wave 2-m temperature although they considerably underestimated the maximum observed temperatures. In addition, it was found that the model grid points characterised by “urban” land-use provided better statistical verification results, with reduced cold bias. Further, in order to study the role of the initial skin temperature conditions on the 2-m temperature forecasts, satellite observed skin temperatures have been used to initialise the high resolution simulations. This sensitivity test showed that when using the satellite observed skin temperature in the model initial conditions, the simulation of the 2-m temperature is positively affected during the early stages of the simulation while later on the model physical parameterisations are decisive for the time evolution of temperature. 相似文献
6.
Optimizing statistical classification accuracy of satellite remotely sensed imagery for supporting fast flood hydrological analysis 总被引：1，自引：0，他引：1
Dimitrios D. Alexakis Athos Agapiou Diofantos G. Hadjimitsis Adrianos Retalis 《Acta Geophysica》2012,60(3):959-984
The aim of this study is to improve classification results of multispectral satellite imagery for supporting flood risk assessment
analysis in a catchment area in Cyprus. For this purpose, precipitation and ground spectroradiometric data have been collected
and analyzed with innovative statistical analysis methods. Samples of regolith and construction material were in situ collected and examined in the spectroscopy laboratory for their spectral response under consecutive different conditions
of humidity. Moreover, reflectance values were extracted from the same targets using Landsat TM/ETM+ images, for drought and
humid time periods, using archived meteorological data. The comparison of the results showed that spectral responses for all
the specimens were less correlated in cases of substantial humidity, both in laboratory and satellite images. These results
were validated with the application of different classification algorithms (ISODATA, maximum likelihood, object based, maximum
entropy) to satellite images acquired during time period when precipitation phenomena had been recorded. 相似文献
7.
Summary The annual and diurnal variation of the conductivity near the ground at Athens, based on long-term data, are presented. The diurnal double oscillation is a characteristic of the atmospheric electric climate of Athens. In addition, the influence of air pollution due to smoke and sulfur dioxide and of wind speed (as an element affecting the dispersion of pollulants) on large ion concentration and on conductivity has been examined. The air pollution, which is often severe in Athens area, results in decreasing conductivity and increasing large ion concentration, while the influence of wind speed is also pronounced. The results justify the use of conductivity as a pollution index. Finally the plausible influence of air temperature and relative humidity and of their rate of change on air electrical conductivity, has been examined. The results are ambiguous and depending on the season, especially for relative humidity.With 3 Figures 相似文献
8.
Dimitris A. Retalis 《Pure and Applied Geophysics》1991,136(2-3):217-233
In the present study, we examine the daily and annual variation of the air-earth current density in Athens for all weather as well as for fair weather for a 16-year period 1965–1980.The daily variation of the air-earth current density has a definite bimodal form while the annual variation has no definitive form but simply reaches its larger values during the summer months. There has also been a Fourier analysis of the daily variation.Furthermore, and for a 3-year period 1968–70, we present the mean daily and annual variation of the electric field, air-earth current density and total conductivity of the air. Based on these parameters we have estimated the conduction and the convection current and we have correlated them to the total (measured) current. In addition, we have estimated the values of the parameter, in order to test the fulfillment of Ohm's law in atmosphere. 相似文献
9.
Summary In the present study, the daily, annual and seasonal variations of the electric field in Athens-Greece, for all weather as well as for fair weather conditions, for the 16-years period 1965–1980 were examined. The daily variation of the electric field follows a double oscillation. A Fourier analysis was also used for the study of its diurnal variation. The results of the Man-Kendall test showed that the electric field at National Observatory of Athens (NOA), during the 16-years period, is essentially constant. In addition, the influence of the air pollution due to smoke and of wind speed on electric field was examined. It was deduced that an increase of air pollution results in an increase of electric field, while the electric field decreases as the wind speed increases.With 4 figures 相似文献
10.
Measurements of positive and negative small atmospheric ion concentrations have been made regularly since 1968 at the National Observatory of Athens (NOA). In this paper the 17-year period 1968–1984 is summarized. The diurnal and annual variations are examined, and Fourier analysis is also used for the study of the diurnal variation. The concentrations of small ions follow a double diurnal course. The maxima occur near 3–5 h and 13–16 h local time (LT = GMT + 2 h). The minima are observed at 6–8 h and 21–23 h. The annual course of small ions presents maximum concentration values around the summer season. The mean of the small ion concentration (SIC) for the 17-year period (1968–1984) is n+ = 188.8 ions/cm3 for positive ions and n− = 151.1 ions/cm3 for negative ions. Their ratio is equal to 1.25. The year-to-year variation of SIC for the examined period shows a negative trend. The results from multiple regression analysis show that wind speed and SIC are positively correlated, while relative humidity, smoke and sulphur dioxide are negatively correlated. 相似文献