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  • Author or Editor: C. G. Helmis x
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J. A. Kalogiros
and
C. G. Helmis

Abstract

A simple method for correcting the time series of a wet temperature sensor characterized by slow response is presented in this paper. A first-order linear approximation of the heat transfer equation is used for correcting the wet temperature measurements. The sensor cutoff frequency is found by an easy experimental method using the phase spectrum between the wet temperature sensor and a much faster platinum thermometer under low wind speeds (up to 3 m s−1) and relatively dry atmosphere (relative humidity 20%–40%). If such a correction is applied, the wet sensor output along with the computed vapor pressure time series using the psychrometric equation are considerably recovered.

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J. A. Kalogiros
and
C. G. Helmis

Abstract

A method to detect and remove fixed echoes in sodar facsimile records is described. This method is based on the capability of the wavelet transform to objectively detect singularities in signals, and it is adapted to two-tone facsimile records. At the present time, no other relevant method for facsimile record processing, except human observation, has been described in the literature. Automatic removal of fixed echoes is useful during computer processing of facsimile records with modern algorithms in order to prevent false layer detection when it is not possible to avoid or remove fixed echoes before recording. A test of the method on complex examples shows its success and limitations.

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K. H. Papadopoulos
,
C. G. Helmis
, and
G. T. Amanatidis

Abstract

Based on an extensive wind dataset over complex terrain, the commonly used small-angle approximation σ v ≈ σθ V is studied and found to overestimate over all wind speeds and σθ values observed. This should be anticipated due to the assumptions necessary to derive the approximation. Overestimation (of 10%–30%) is also observed in the small σθ range. The three parameters involved are further discussed to gain better understanding of the behavior of the approximation under different conditions. The standard deviation of wind direction σθ is shown to vary inversely with wind speed not only under stable, but also under convective conditions, reaching a site-dependent constant value at high wind speeds. The dependence of the ratio of the mean longitudinal wind component to the scalar mean wind speed on wind speed and σθ is examined, as well as that of the relevant standard deviations (σ u V a ). While the former obtains small values in the high-σθ, or low-wind range, or both, estimated values of the latter justify equivalence of σ u , σ V a under most conditions. Finally, the effects of wind speed and σθ on σ v are examined.

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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. 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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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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