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  • Author or Editor: D. N. Asimakopoulos x
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C. Varotsos
,
N. Dris
, and
D. Asimakopoulos

Abstract

Two recent reference atmosphere models for ozone and temperature, which are deduced from satellite data, are employed to detect the existence and the behavior of a terannual wave both in ozone and temperature.

Through the photochemical and the radiative processes, physical considerations are given in an attempt to explain the cause of the formation of the terannual wave.

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D. N. Asimakopoulos
,
D. G. Deligiorgi
, and
D. P. Lalas

Abstract

Preliminary results from a series of experiments utilizing an acoustic sounder located at the top of a 1026 m mountain are presented which show some interesting features of the evolution of the atmospheric boundary layer.

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C. P. Jacovides
,
Michael D. Steven
, and
D. N. Asimakopoulos

Abstract

This paper investigates the influence of gaseous pollutants and aerosol on the spectral composition of various segments of the solar spectrum in cloudless conditions. This investigation is done by using data of the spectral energy distribution of global and diffuse solar irradiances collected during a field experiment in Athens. The authors found that the Ångström turbidity coefficient β always shows a temporal pattern with high values in the morning and the afternoon and low values at midday, and the wavelength exponent α widely varies over 1.02–1.4.

Atmospheric turbidity produced a measurable but variable effect on spectral solar irradiances. The authors found that the relative attenuations caused by high urban aerosol can exceed 36% ± 7.5%, 30% ± 5.8%, and 26% ± 4.1% in the ultraviolet, visible, and near-infrared portions of the solar spectrum, respectively, as compared with “background” values. On the other hand, the relative increase in scattered irradiance was greater in the near-infrared band (40% ± 4.8%), and in visible and ultraviolet bands the relative increase reached 31% ± 5.5% and 18% ± 6.5%, respectively.

Spectrally reduced (Rayleigh corrected) and aerosol (Ångström) optical depths were retrieved, representing different aerosol loadings over the Athens atmosphere. The effects of altitude and the temporal and spatial variability of spectral optical depth values were analyzed. The overall results suggest that the shortest wavelengths are very sensitive to aerosol loading.

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D. N. Asimakopoulos
,
C. G. Helmis
, and
G. J. Stephanou

Abstract

An improved version of a monostatic acoustic minisounder for the investigation of the structure and dynamics of the lower boundary layer, operating at high frequencies and equipped with a specially designed micro-antenna and advanced electronic circuits, is presented. This minisounder is shown to be capable of high resolution of less than 0.7 m and a minimum discernable height of less than a meter above ground. Examples of facsimile records under both convective and stable conditions obtained by the improved minisounder are given to demonstrate its capabilities.

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C. G. Helmis
,
D. N. Asimakopoulos
,
D. P. Lalas
, and
T. J. Moulsley

Abstract

Experimental results of simultaneous measurements of temperature differences in the streamwise, lateral and vertical directions during convective, new-neutral and stable conditions in the atmospheric boundary layer of an urban site are presented and discussed. The lateral and vertical temperature difference variances are found to be almost equal regardless of the conditions but only agree with the streamwise difference variance in convective or rear-neutral conditions. This points out the need for further investigation of the effects of stratification on the scalar fields. When local isotropy is satisfied, the difference spectra are shown to follow the theoretical behavior derived by Van Atta (1977).for temperature gradients in the inertial subrange.

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C. G. Helmis
,
D. N. Asimakopoulos
,
D. G. Deligiorgi
, and
M. C. Petrakis

Abstract

Observations of the vertical thermal and wind structure made in a large and broad valley are reported here. The growth of the convective boundary layer is found to be responsible for the destruction of the nocturnal temperature inversion. No sinking of the inversion layer was observed in response to the morning upslope flow following the profiles of both potential temperature and wind data. These observations alongside previous published work do not permit a generalization of the behavior of the flow and thermal ameture for all valleys. Characteristics like the length, the depth and the ridgetop width of the valley play an important role to the mechanism of the destruction of the temperature inversion.

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C. G. Helmis
,
C. Jacovides
,
D. N. Asimakopoulos
, and
H. A. Flocas

Abstract

An experimental campaign was carried out on a small Greek island that is characterized by complex terrain; its aim was to study the local characteristics of the vertical structure of the atmospheric boundary layer (ABL). The instrumentation was installed close to the shoreline and consisted of a 13-m-high meteorological mast instrumented at three levels, and a high-range vertical monostatic sodar. Tethered balloon flights were carried out for 3 days under different atmospheric conditions. The analysis of the available data revealed interesting features of the vertical structure of the atmosphere over the island, with the development of a convective internal boundary layer (IBL) within the first 150 m above the ground, while the marine boundary layer (MBL) formed at higher altitudes, up to 450 m. Buoyant oscillations appear within the MBL in the form of gravity waves with frequencies of 7 min. Theoretical calculations of the IBL height verified the experimental results. During the night, a complex wind flow forms in the lower 250–300 m, resulting from the development of katabatic flows and topographic channeling.

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M. Santamouris
,
G. Mihalakakou
,
B. Psiloglou
,
G. Eftaxias
, and
D. N. Asimakopoulos

Abstract

Three methods for analyzing and modeling the global shortwave radiation reaching the earth’s surface are presented in this study. Solar radiation is a very important input for many aspects of climatology, hydrology, atmospheric sciences, and energy applications. The estimation methods consist of an atmospheric deterministic model and two data-driven intelligent methods.

The deterministic method is a broadband atmospheric model, developed for predicting the global and diffuse solar radiation incident on the earth’s surface. The intelligent data-driven methods are a new neural network approach in which the hourly values of global radiation for several years are calculated and a new fuzzy logic method based on fuzzy sets theory. The two data-driven models, calculating the global solar radiation on a horizontal surface, are based on measured data of several meteorological parameters such as the air temperature, the relative humidity, and the sunshine duration.

The three methods are tested and compared using various sets of solar radiation measurements. The comparison of the three methods showed that the proposed intelligent techniques can be successfully used for the estimation of global solar radiation during the warm period of the year, while during the cold period the atmospheric deterministic model gives better estimations.

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