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Pierre Durand, Gabriel Abadie, and Aimé Druilhet

Abstract

The authors show how a capacitive device measuring moisture can be used aboard instrumented atmospheric aircraft as an alternate sensor for turbulence measurement. Using a calibrated Lyman-α sensor as a reference, the time response of the capacitive device can be improved in such a way that turbulent latent heat flux and moisture variance can be calculated with a good level of accuracy. This improvement is done by correction of the amplitude as well of the phase of the signal. These corrections are determined from in-flight measurements therefore, they take into account the time response of the sensor itself, as well as its airborne installation.

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Pierre Durand, Aime Druilhet, and Serge Briere

Abstract

A sea wind situation was analyzed during the COAST (Cooperative Operations with Acoustic Sounding Techniques) experiment. The thermal internal boundary layer (TIBL) which develops inland from the coast was investigated by an instrumented aircraft and fixed measurements, and by a two dimensional version of a third order turbulence closure model. The two-dimensional structure of the TIBL was demonstrated in the vertical, perpendicular to the shore. The mean quantities (temperature, humidity, and wind), as well as their turbulent moments, were computed and comparison made between experiment and model. The experimental mean fields were well reproduced by the, model. The turbulence fields were reproduced in their general features as well as in their magnitude, but not in local singularities.

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Claude Estournel, Raoul Vehil, Daniel Guedalia, Jacques Fontan, and Aimé Druilhet

Abstract

Pollutants (gaseous and aerosol) contained in urban atmospheres alter radiative fluxes at the surface.Numerous radiative models have been developed, and while few experimental data are available, results areoften contradictory. We have taken measurements, over several weeks, of downward radiation (solar andinfrared) over the city of Toulouse and a rural reference site. The downward IR flux was larger at the urbansite by day and night (increase between 15 and 25 W m2). Attenuation of the incident solar radiation atthe urban site was observed (30 W m2 in the middle of the day). A radiative model enabled us to show thatthe IR flux increase is mainly due to higher air temperature associated with the urban "heat island." Theenhissivity increase due to the addition of absorbing constituents in the urban area was very weak. Theattenuation of solar radiation was due to absorption by urban aerosol. Total incoming radiation (solar+ infrared) was similar at the two sites by day and was slightly higher at the urban site during the night.

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Daniel Guedalia, André Ntsila, Aimé Druilhet, and Jacques Fontan

Abstract

This study presents a method of obtaining the quantitative intensity of vertical diffusion during periods of atmospheric stability. This method associates the continuous measurement of radon concentration at ground level and the use of a monostatic sodar. The value of “equivalent mixing height” he is calculated using the radon variation which can be related to the global exchange coefficient of the inversion layer. The sodar detects the thickness of the nocturnal inversion layer.

Both systems operated simultaneously over several months at two sites (urban and suburban). In most cases the nocturnal layer was indicated both by sodar echoes and by a large decrease of the he value. In other cases the use of sodar alone can lead to a lack of detection of stable periods which, nevertheless, are shown clearly by radon. The comparison of measurement at two sites shows a modification of nocturnal stability above the urban site. The equivalent exchange coefficients are ∼0.3 m2 s−1 (at the urban site) and 0.08 m2 s−1 (at the suburban site).

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Marc Georgelin, Evelyne Richard, Monique Petitdidier, and Aime Druilhet

Abstract

A subgrid-scale orography parameterization based upon the use of an effective roughness length has been implemented in a mesobeta-scale model. The impact of such a parameterization is investigated in the framework of orographic flow simulations. Three mountain flow situations observed during the Pyrenees Experiment (PYREX) are studied. When subgrid-scale orography is accounted for, the mountain wave amplitude is reduced, the blocking is increased, the leeside low-level turbulence is intensified, and the regional wind characteristics are modified. Detailed comparisons made with the PYREX data indicate that the inclusion of subgrid orography yields a significant improvement in the model results.

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Pierre Durand, Leonardo De Sa, Aimé Druilhet, and Frédérique Said

Abstract

Airborne measurements are currently used for computing turbulence fluxes of heat and momentum. The method generally used is the eddy correlation technique, which requires sophisticated equipments to calculate the absolute velocities of the air. We used the well-known inertial dissipation method to calculate the turbulent fluxes of heat and momentum from low-level airborne measurements This only requires knowledge of inertial subrange characteristics of velocity and scalars. The method was validated by comparing dissipation fluxes with those computed by the eddy correlation method. The agreement between the two is very good, particularly for heat fluxes. Last, it is shown how the turbulent kinetic energy dissipation rate can be easily calculated, using a single measurement (the attack angle by example), and therefore how turbulent fluxes can be simply calculated from low level airborne measurements.

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