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Philippe Bougeault

Abstract

A simple upper boundary condition for hydrostatic, Boussinesq models is derived from a linear internal wave theory, assuming a uniform stratification and no Coriolis effects. This condition is applied in a two-dimentional nonlinear model of the planetary boundary layer. The numerical implementation and some stability problems are discussed. A comparison of the results of numerical experiments using different vertical extensions with analytical solutions is used to show that the condition provides a satisfactory solution to the problems of radiation of upward propagating energy.

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Philippe Bougeault

Abstract

A third-order turbulence model of the marine stratocumulus layer, including cloud ensemble relations and detailed radiative computation, is used to study a case observed over the North Sea during the JASIN experiment. It is shown that the model is able to reconstruct the observed structure of the boundary layer, starting from only the large-scale information. A strong diurnal cycle, induced by the absorption of solar radiation of the cloud layer, is studied in some detail. The role of intermittent cumuli observed below the stratocumulus, and the significance of the cloud-top instability criterion are also investigated.

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Philippe Bougeault

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A parameterization of the effects of deep cumulus convection on the large-scale heat and water budget is proposed. It is based on a simple prediction of the convective mass flux, and a global treatment of the detrainment by relaxation of the large-scale values of the temperature and the moisture towards a single cloud profile. The performances of the scheme and its sensitivity to some details of the formulation are tested, both in one-dimensional semi-prognostic computations and in three-dimensional forecasts.

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Philippe Bougeault

Abstract

This paper aims to develop and test a parameterization scheme for partial cloudiness, to be used in the framework of higher-order models of the turbulent planetary boundary layer. The proposed scheme is designed to be general enough and fairly accurate, although slightly at the expense of simplicity. It is based upon the assumption that the total moisture and temperature fluctuations follow gamma probability density functions, which allow for a variable skewness factor, and therefore for different cloud layer regimes. It is nevertheless believed that simpler parameterizations can be used in a number of ways, depending upon specific uses.

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Haraldur Ólafsson and Philippe Bougeault

Abstract

A numerical, hydrostatic model is used to investigate the form and magnitude of the pressure drag created by 3D elliptical mountains of various heights (h) and aspect ratios (R) in flows characterized by uniform upstream velocity (U) and stability (N). Three series of simulations, corresponding to increasing degrees of realism, are performed: (i) without rotation and surface friction; (ii) with rotation, but no surface friction; (iii) with rotation and surface friction. For the simulations with rotation, the Coriolis parameter has a typical midlatitude value and the upstream flow is geostrophically balanced. The surface friction is introduced by the use of a typical roughness length.

For low values of the nondimensional height (Nh/U), the pressure drag is reduced by the effect of rotation, in agreement with well-known results of linear theory. This seems to be valid until Nh/U ∼ 1.4, that is, in the high drag regime. On the other hand, for large values of Nh/U, that is, in the blocked flow regime, rotation has the opposite effect and increases the drag. The authors propose a simple interpretation of these results: that geostrophic adjustment acts to first order as a relaxation toward the upstream velocity. For low Nh/U, the acceleration above the mountain is a dominating feature of the flow and here the flow is slowed by the presence of rotation. For high Nh/U, when upstream blocking is dominant, the flow is slowed by the mountain and therefore accelerated by rotation. For values of Nh/U ∼ 1.4, the rotation is sufficient to force a transition from the blocked state to the unblocked state. The influence of rotation may therefore extend the range of usefulness of linear theory.

Surface friction dramatically suppresses wave breaking at all values of Nh/U. The induced effect on the drag is negligible for Nh/U > 3, but there is a strong reduction at smaller values of Nh/U. In fact, the high-drag regime is nearly suppressed.

The overall combined effect of rotation and surface friction is to constrain the drag (and to some extent, the flow patterns) to values remarkably close to the linear prediction. This sheds some light on recent, but as yet unexplained, results from the PYREX field experiment. The authors conclude this paper by running a real case drawn from this experiment, which reveals a behavior consistent with the idealized scenarios.

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Valéry Masson and Philippe Bougeault

Abstract

Three-dimensional simulations of the IOP 10 of the Pyrénées Experiment are presented. In this case, the northerly synoptic flow forces two regional winds around the Pyrénées Mountain range: the cierzo in the Ebro Valley and the tramontana over the Mediterranean Sea. First, experimental data are used to validate the simulation. The local winds are well reproduced, and the computed flow splitting upwind of the Pyrénées compares favorably with the real flow. The computed turbulence kinetic energy and the turbulent fluxes are in fair agreement with the observations in the cierzo.

Second, the good fit between the observations and the computation permits one to draw some conclusions from the simulation, using the model as numerical laboratory to amplify the utility of the dataset In particular, the acceleration of the wind along the Ebro Valley is examined, and it is found that it is governed in the upper part of the valley by the pressure gradient created by the Pyrénées. Next, the balance of forces in the planetary boundary layer in the two wind systems is considered. Surprisingly, the cierzo has an Ekman-type balance of forces, but not the tramontana. Finally, the authors analyze the variation of the ground pressure drag from three simulations where the mountain height is varied: the result is consistent with the conclusions of Stein. The results also confirm the beneficial effect of an enhanced orography for such simulations.

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Véronique Ducrocq and Philippe Bougeault

Abstract

This paper reports on simulations of an observed squall line with the meso-beta-scale hydrostatic PERIDOT model. An attempt is made to answer the question of what model changes are necessary to roughly simulate a squall line with a hydrostatic model lacking explicit microphysics. A qualitatively satisfactory simulation is obtained by modifying the physical package of the model and by increasing the resolution compared to that of the operational model. Sensitivity experiments are conducted in order to determine the necessary components for the simulation of the squall line. The authors find that both increased resolution and a downdraft parameterization are required.

The energetics of the inner circulation of the squall line are investigated using the best simulation. The main supply for the rear to front flow is identified as conversion of potential energy into kinetic energy. which is linked to the line-normal horizontal gradient of perturbation pressure. This result agrees with the conclusions of the sensitivity experiments and with previous studies on the important role played by the density current in squall-line propagation.

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Haraldur Ólafsson and Philippe Bougeault

Abstract

The hydrostatic flow over an elliptical mountain of aspect ratio 5 is explored by numerical experiments. The upstream profiles of wind and stability are constant, the Coriolis effect is ignored, and there is free slip at the lower boundary. In these conditions, the, flow characteristics depend mainly on the nondimensional mountain height, Nh/U. The authors have conducted experiments with Nh/U varying from 0.500 to 6.818. For low values of Nh/U, the results confirm the linear theory of Smith, which predicts stagnation aloft, leading to wave breaking and, on the upstream slope, leading to flow splitting. For higher values of Nh/U, the authors find that wave breaking ceases on the axis of symmetry but continues on each side of this axis. Even for the highest value of Nh/U used (6.818), significant areas of wave breaking and wave activity aloft are found. For all values of Nh/U, a substantial part of the flow is diverted vertically above the mountain. The detailed study of the kinematic pattern within the upstream blocking reveals an increasing tendency to small vortex creation when Nh/U increases. This, however, does not affect the main flow features. Finally, the authors observe the generation of potential vorticity in the wake of the mountain, leading to the creation of lee vortices. The potential vorticity pattern is very similar to the vorticity pattern shown by Schär and Smith for shallow-water flow. It is found to be insensitive to the turbulence parameterization in our model, as well as the general flow pattern. On the other hand, comparison with an experiment using a circular mountain reveals large differences in the elliptical case.

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Philippe Bougeault and Jean-Claude André

Abstract

It is shown that higher-order turbulence-closure models include as particular solutions damped oscillations in the statically stable case, and possibly amplifying and propagating oscillations in the statically unstable case. These two kinds of oscillatory motions are shown to be strongly affected by molecular and pressure damping, both from analytical linear analysis and one-dimensional numerical simulation. The results are applied to the case of a stratocumulus capped boundary-layer and compared to the ones of Moeng and Randall. Indications are given on how to achieve stable nunneries simulations by improving the formulation of the mixing (dissipative) length.

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Thierry Bergot, Dominique Carrer, Joël Noilhan, and Philippe Bougeault

Abstract

Accurate short-term forecasts of low ceiling and visibility are vital to air traffic operation, in order to maximize the use of an airport. The research presented here uses specific local observations and a detailed numerical 1D model in an integrated approach. The goal of the proposed methodology is to improve the local prediction of poor visibility and low clouds at Paris’s Charles de Gaulle International Airport. In addition to the development of an integrated observations and model-based forecasting system, this study will try to assess whether or not the increased local observing network yields improvements in short-term forecasts of low ceiling and poor visibility. Tests have been performed in a systematic manner during 5 months (the 2002/03 winter season). Encouraging results show that the inclusion of dedicated observations into the local 1D forecast system provides significant improvement to the forecast. Inspection of events indicates that the improvement in very short-term forecasts is a consequence of the ability of the forecast system to more accurately characterize the boundary layer processes, especially during night. Accurate forecast of low cloud seems more difficult since it strongly depends on the 3D mesoscale flow. This study also demonstrates that the use of a 1D model to forecast fogs and low clouds could only be beneficial if it is associated with local measurements and with a local assimilation scheme. The assimilation procedure used in this study is based on different steps: in the first step the atmospheric profiles are estimated in a one-dimensional variational data assimilation (1DVAR) framework, in the second step these atmospheric profiles are modified when fog and/or low clouds are detected, and in the third step the soil profiles are estimated in order to keep the consistency between the soil state and atmospheric measurements.

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