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B. Benech

Abstract

An experimental project on convective plumes initiated from the ground by an exceptionally powerful artificial heat source has been carried out. The heat source consisted of 97 oil burners releasing a total power of 600 MW. The measuring equipment consisted of a ground network of temperature and velocity sensors, and an instrumented aircraft for one experiment.

The values of the plume radius, the temperature difference inside and outside of the plume, and the vertical velocity were measured at different heights. To give the scale of the phenomenon, the following values were obtained at a height h of 10 m above the ground:R 0 = 36 m, &DeltaT 0 = 50°C, w 0 = 3 m s−1; these values have been taken as the initial conditions for the theoretical approach to the problem.

Two types of diffusion have been found along the plume: a relative diffusion in the lower part [ΔR(t) 2ΔR(0) 2t 2] for h < 300 m, and an absolute diffusion above [ΔR(t) 2ΔR(0) 2t] for h > 300 m.

Plume rise is discussed in term of atmospheric instability. When a temperature inversion is present, the plume penetrates the inversion if it occurs below 1000 m and has a thickness of no more than 150 m; in all other inversion cases, the plume only reaches the inversion layer, or occasionally raises it.

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J. Noilhan and B. Bénech

Abstract

An experimental study of the dynamics within artificial thermal plumes rising in the boundary layer is presented.

In this third part, measurements just above the heat source and aircraft investigations in the plume aloft are used to reveal the internal structure of the airflow within the buoyant column. Analysis of the pressure perturbation obtained both by direct measurements and as a residual in the mean vertical motion equation for a plume, shows that the vertical pressure gradient accelerates the airflow near the heat source and then reduces the buoyancy in the upper levels. The pressure deficits, attaining maximum values of 1 mb in the core of the lower portion of the plume, are well correlated with large vertical velocities. During light ambient wind conditions, the reduced pressure near the heat source produces a large converging inflow sufficient to cause the lower portion of the plume to go into rotation as a whole. An analysis of the components of the velocity field and momentum fluxes within the column underscores the convergent and divergent characters of the flow, respectively, at the lower and upper portions of the plume. Strong vorticity concentration (∼4 10−2 s−1) is associated with a reduction of entrainment into the column.

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J-L. Caccia, B. Benech, and V. Klaus

Abstract

The third intensive observation period (IOP3) of PYREX was a case of strong lee waves generated by a southerly wind crossing the Pyrenees chain. Upstream radiosounds and measurements obtained by aircraft along the chain transect and by constant volume balloons launched near the crest provided spatial characteristics of the lee wave at different times and heights. Values ranging from 7 to 14 km for the horizontal wavelength, and from 3 to 5 m s−1 for the maximum amplitude of the air vertical velocity, were observed. The lee wave horizontal extent, measured from the crest line, reached 30 to 55 km. In addition, two very high frequency stratosphere-troposphere (VHF ST) radars, one on the mountain mean axis and another downstream in the lee wave field, observed the temporal variations of the vertical profiles of the vertical velocity. The analysis of those observed variations and their vertical distribution allowed the stationarity of the wave to be studied. The lee wave was found to be far from stationary during its lifetime, but there were some periods, never longer than 1.5 h, during which the wave was quasi-stationary. Data obtained by the airborne instruments revealed that the wavelength, amplitude, and downstream wave extent underwent temporal variations. The time evolution of the vertical velocity profile observed by the radar below the wave field revealed that, most of the time, the lee wave was trapped, which was found to be consistent with radiosounding data. In addition, favorable comparison between airborne and radar data added further evidence that direct VHF ST radar measurement of the vertical air motion induced by lee waves is not seriously affected by beam tilting.

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J-M. Brustet, B. Benech, and P. Waldteufel

Abstract

The possibility of applying infrared imagery to the study of a large, hot plume materialized by carbon particles resulting from the incomplete combustion of fuel oil is investigated.

In a specific case (the PROSERPINE experiment), due to the high carbon particle content, the lower part of the plume acts as a semi-opaque target. Using an infrared camera equipped with a detector sensitive in the 2–5.8 μm band, the thermal images are found to yield a plume geometry in good agreement with visible contours retrieved from visible photographs.

Thermal images provide access to the internal structure of a plume, down to scales which depend on the plume opacity. It appears that IR imagery is able to yield improved information concerning the turbulent fields of motion and temperature.

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E. Koffi, B. Bénech, J. Stein, and B. Terliuc

Abstract

This paper considers a linear hydrostatic model of a stable, uniform, constant rotational airflow over three- dimensional, elliptic, cross-sectional families of mountains in a z system. The surface pressure and the winds that are induced around the mountain chain are deduced using Fourier representation in both horizontal directions. The surface pressure perturbations and the induced wind intensities are linked to 1) the incoming airmass thermodynamic properties through Froude and Rossby numbers, 2) the geometrical aspect ratio of the mountain, 3) the direction of incidence of the incoming flow relative to the mountain orientation, and 4) the Coriolis effect through the Rossby number. The balance between the different factors that contribute to the morphology of the pressure and wind fields was established for northerly and southerly incoming flows that were blocked by an elliptical barrier resembling the Pyrénées mountain chain. Fair agreement was found between the results of the model and the experimental data collected during PYREX (Pyrénées experiment) intensive operational periods, with special regard to the asymmetry of the lateral flow for northerly incoming air masses.

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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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P. Bougeault, A. Jansa Clar, B. Benech, B. Carissimo, J. Pelon, and E. Richard

Although the qualitative influence of mountains over the atmosphere has been known for a long time, numerous deficiencies, linked to orography, are still noted, either in forecasts by regional models, or in the long-term behavior of climate models. This is why the French and Spanish weather services are undertaking an important field campaign to document the dynamic modifications to the atmospheric flow generated by the Pyrenean range during a 2-month period (October and November 1990) with six intensive observation periods (IOPs) of 2 to 3 days. The experimental strategy is based largely on mesoscale numerical-model results and will help to validate these models. The main focus is on the documentation of clear-air turbulence generated either by breaking mountain waves, by surface roughness, or by the wind shear induced by the lateral-flow deviation around the mountain. Experimental means include several networks of surface stations, radio soundings, constant-level balloons, four wind profilers, and several research aircraft.

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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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M. Lothon, B. Campistron, S. Jacoby-Koaly, B. Bénech, F. Lohou, and F. Girard-Ardhuin

Abstract

A C-band meteorological Doppler radar has been used to investigate the dynamic processes and the coherent organizations within a clear air atmospheric boundary layer (ABL). Depending on the atmospheric conditions, this moderate sensitive radar was able to provide a coherent and continuous velocity field from 0.1 km up to 3 km above ground, and over a horizontal range reaching at least 30 km. Here the focus is on the profiling capability of this Doppler radar in a clear air boundary layer. The velocity volume processing method was used to deduce vertical profiles of the wind field from the panoramic conical scannings. A comparison between the observations of this C-band radar and two UHF wind profilers is presented. Good agreement was obtained in the measurements of the wind velocity and of the vertical and temporal evolution of the reflectivity. In particular, as for UHF wind profilers, the ABL top was found coincident for the C-band radar with a bright band of reflectivity maximum, and both types of radar detected the same thin layered echoes above the ABL. The advantage of the C-band radar over the UHF wind profiler is its steering capability, which was used in particular to obtain the two-dimensional topographical map of the ABL top. A discussion on the echo sources and vertical velocity measurement is also presented.

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