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A. Ben Mohamed, J-P. Frangi, J. Fontan, and A. Druilhet

Abstract

Spectral aerosol optical depths were measured in seven localities of Niger (West African Sahel) from February 1986 to June 1987. Results of these observations, together with related climatic parameters, are presented and discussed in this paper.

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J. Noilhan, B. Bénech, G. Letrenne, A. Druilhet, and A. Saab

Abstract

Some aspects of the mean and turbulent structures of artificial thermal plumes in the boundary layer (BL) are presented. This analysis is based mainly on measurements with an instrumented aircraft. As initial conditions for plume rise, the characteristics of the BL during the 10 experiments are summarized. Under neutral conditions, plume rise in the BL follows approximately the classical prediction. Plume structure aspects are analyzed inside the upwind active zone corresponding to the region of strongest gradients within the convective column. In an attempt to compare the various experiments, scaling parameters are derived from energetic considerations for both dissipative and first-order parameters. Individual and normalized vertical profiles are given. The statistics presented discriminate between dry and condensed plumes. For both populations, vertical profile tendencies are rather similar in the BL. However, condensed plumes show higher values of the perturbed variables inside the whole column. Mean and turbulent structure within the plume is sensitive to the stratification of the environment. In the BL, the main structural aspects consist of quasi-constant profiles of vertical velocity and specific humidity excess, and of rapid decreases of temperature excess, dissipation rate and temperature structure parameter, such decreases are due to mixing with the surrounding atmosphere. Penetration into the drier stable layers results in a reversal of the sign of θp, an increase in the specific humidity excess and a small decrease in the dissipative parameters. The intensity of the plume turbulence is found to be more dependent on wind speed and condensation processes than on atmospheric turbulence.

The oscillations inside the horizontal part of the plume in the upper stable layer are modulated by mean advection and by possible interaction between the buoyant cells and natural waves.

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A. Druilhet, J. P. Frangi, D. Guedalia, and J. Fontan

Abstract

We present a collection of experimental data concerning turbulent characteristics of the convective boundary layer. These data were obtained by means of instrumented aircraft during numerous experiments carried out above five flight areas from 1973 to 1978. Only dry convective conditions with weak dynamic instability are contained in this report. The computed quantities are vertical turbulent fluxes of sensible and latent heat, second- and third-order moments of vertical wind component, potential temperature and humidity, dissipative and spectral characteristics and length scales. All these quantities are normalized using the convective similarity hypothesis of Deardorff. A particular case is considered for humidity where it is necessary to choose between the two boundary conditions: evaporation and entrainment flux. The latter is chosen.

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B. Bénech, E. Koffi, A. Druilhet, P. Durand, P. Bessemoulin, J. Campins, A. Jansa, and B. Terliuc

Abstract

The Pyrénées experiment (PYREX), launched by the French and Spanish meteorological services, had provided an extensive database that was used in the present work to describe the airflow around the Pyrénées and to evaluate the applicability of the linear theory to stable flows that are blocked by a relatively large mountain range.

The direction of incidence of the incoming synoptic wind was used to classify the flow in northerly and southerly categories. In both categories, the directions of incidence bear westward from normal incidence to the main axis of the mountain range. Froude (Fr) and Rossby (Ro) numbers were used to represent the thermodynamic characteristics of the incoming air masses and the scale of interaction in terms of the mountain shape. Here, Fr and Ro were found linearly correlated, and their combination corresponding to blocked flow in all the cases.

Average cross correlations between the pressure drag across the mountain and the vertical profiles of the wind components at selected locations demonstrate that the horizontal flow is clearly separated in an upper undisturbed regime and a lower blocked regime in which regional winds are induced. The separating layer corresponding to the mean Froude number encountered during PYREX was found at an altitude of about two-thirds of the mountaintop. Due to the perturbation induced by the deflected wind, the total wind during northerly flows is stronger on the east side than on the west side, while during southerly incoming flows, the total wind distribution is almost symmetric.

Good correlation was found among the pressure field perturbation, the pressure drag, and Froude number of the incoming flow. Clear correlation rules can be deduced from the analysis of the experimental data at different altitudes for both categories of incoming flows. Therefore, since the pressure drag can be easily determined using field measurements, it becomes a powerful tool for the prediction of the regional winds around the Pyrénées.

The experimental data are in agreement with the linear theory predictions of the linear model in regarding (a) the perturbation of the surface pressure field, which resembles the predicted bipolar distribution; (b) the dependence of the drag on Fr−1, which enables the assessment of the linear theory and the definition of the conditions of applicability of two models [(i) a two-dimensional model, for which it was possible to define quantitatively the effective blocked area, and (ii) a three-dimensional model, for which a scaling function that combines the direction of incidence, the mountain shape, and the Coriolis effect was found almost constant, with an average value of 0.2 for all the cases under study]; (c) the extension of the area affected by the blocking effect, estimated to be 4.5–5 times the width of the barrier and the drift of the strong deceleration point due to the Coriolis effect; (d) the dependence of the wind velocities on Fr−1 at the edges of the barrier; and (e) the asymmetric flow deviation induced by the Coriolis effect and biased by the departure of the flow from normal incidence.

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M. Lothon,B. Campistron, S. Jacoby-Koaly, B. Bénech, F. Lohou, F. Girard-Ardhuin, and A. Druilhet
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B. Bénech, J. Noilhan, A. Druilhet, J. M. Brustet, and C. Charpentier

Abstract

The work reported here describes the environmental impact of emitting about 1000 MW of dry heat from a concentrated source into the atmosphere. It is based on a large field program conducted jointly by the Centre de Recherches Atmosphériques and Electricité de France. This program provided an opportunity to evaluate the actual environmental impacts of large-scale heat release and to obtain data required to develop parameterization schemes for use in modeling heat releases by intense sources such as dry cooling towers.

The heat source is an array of 105 fuel-oil burners distributed over 15 000 m2. An aerial assemblage suspended at two levels (25 and 50 m) over the burner array has been used to collect data (temperature and velocity fields) for analyzing aspects of both the mean and the turbulent components of the flow near the heat source.

The flow field near the heat source comprises a cold downdraft upwind zone which supplies the burner area with ambient air, a convective zone containing a hot vertical air stream with rotation effects, and a cold updraft downwind zone where numerous vortices are initiated.

In the upwind zone, the horizontal flow is accelerated, steady state and divergent. In the convective zone, temperature and vertical velocity are closely correlates, as are temperature and horizontal velocity. The downstream flow shows strong convergence (∼0.3 s−1) and contains two counter-rotating vortices. Cross-correlation and spectral analysis of temperature and vertical velocity in the convective zone show that the major spectral energy contribution is located at wavelengths between 30 and 70 m. The slope of the temperature spectra tends to increase with the standard deviation of the temperature fluctuations. The turbulence in the core of the convective zone is characterized by large values of the dissipation rate ε (∼1 m2 s−3) and of the temperature structure parameter CT 2 (∼10 m−⅔ K). The comparison between the turbulent and advective heat fluxes suggests that the turbulence is not yet fully developed at the vicinity of the heat source. Finally, an estimation of the mean initial conditions as a function of the wind is given.

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