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P. Bougeault
,
J. Noilhan
,
P. Lacarrère
, and
P. Mascart

Abstract

As a follow-up to the HAPEX-MOBILHY experiment in southwestern France, an advanced parameterization of land-surface processes has been designed. This paper and two companions report on the implementation of this new parameterization in the limited-area model of the French Weather Service and on results obtained in the numerical simulation of one of the most widely studied periods of the experiment, 16 June 1986. In Part I, we discuss the production of maps of parameters to be used in the three-dimensional model. The useful parameters are produced by tabulation of their most probable values, as function of some primary parameters, which are for the present time the soil texture, the soil depth, the dominant vegetation type, and the albedo. The methods giving the maps of these primary parameters are also briefly discussed. They rely on existing information, such as several soil maps and satellite products from AVHRR and Meteostat. Parts II and III discuss the results of the simulation of 16 June 1986.

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P. Bechtold
,
J. W. M. Cuijpers
,
P. Mascart
, and
P. Trouilhet

Abstract

A simple method is proposed to extend a low-order turbulence scheme including a subgrid-scale cloudiness scheme to represent not only nonconvective (stratiform) cloudiness and turbulence but also shallow, nonprecipitating cumulus convection by utilizing an appropriate subgrid-scale distribution function. A simple approach is chosen that avoids the knowledge of the skewness parameter. All cloud water-related variables (cloud water content, partial cloudiness, liquid water flux) are computed by interpolating linearly as a function of the saturation deficit between two limit cases: the stratocumulus case, which can be well represented by a Gaussian distribution function, and the trade wind cumulus case, characterized by a positively skewed distribution function with a skewness of 2.

Comparisons of the scheme with a third-order turbulence scheme and large-eddy simulations (LES) for the Puerto Rico Field Experiment show satisfying results. It is shown that the liquid water flux term, which is strongly dependent on the distribution chosen, is the crucial parameter of the scheme.

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J. F. Mahfouf
,
E. Richard
,
P. Mascart
,
E. C. Nickerson
, and
R. Rosset

Abstract

Various parameterizations of the planetary boundary layer (PBL) currently used in three-dimensional (3D) mesoscale models are compared with a more complex scheme including a turbulent kinetic energy (TKE) equation. In the first set of simulations made with a ID model against the classical Wangara data, the mean wind, temperature and moisture calculated in the PBL are nearly insensitive to the choice of the parameterization. In the second set of simulations, the TKE parameterization is used in a 3D mesoscale model to simulate sea breeze flows over south Florida. A comparison is presented with previous simulations of Pielke, and Pielke and Mahrer, for the mean flow, and with the third-order turbulence closure model of Brière for the turbulent variables, including a discussion of the turbulent energy budget, The analysis of the results obtained with the TKE scheme shows that the predicted turbulent fields are qualitatively realistic and interact significantly with the sea breeze circulation. Finally, a comparison is made between the TKE scheme and the simpler parameterization of Pielke and Mahrer. It shows only slight differences as far as the mesoscale structure of the mean variables is concerned.

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Florian P. Pantillon
,
Jean-Pierre Chaboureau
,
Patrick J. Mascart
, and
Christine Lac

Abstract

The extratropical transition (ET) of a tropical cyclone is known as a source of forecast uncertainty that can propagate far downstream. The present study focuses on the predictability of a Mediterranean tropical-like storm (Medicane) on 26 September 2006 downstream of the ET of Hurricane Helene from 22 to 25 September. While the development of the Medicane was missed in the deterministic forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) initialized before and during ET, it was contained in the ECMWF ensemble forecasts in more than 10% of the 50 members up to 108-h lead time. The 200 ensemble members initialized at 0000 UTC from 20 to 23 September were clustered into two nearly equiprobable scenarios after the synoptic situation over the Mediterranean. In the first and verifying scenario, Helene was steered northeastward by an upstream trough during ET and contributed to the building of a downstream ridge. A trough elongated farther downstream toward Italy and enabled the development of the Medicane in 9 of 102 members. In the second and nonverifying scenario, Helene turned southeastward during ET and the downstream ridge building was reduced. A large-scale low over the British Isles dominated the circulation in Europe and only 1 of 98 members forecasted the Medicane. The two scenarios resulted from a different phasing between Helene and the upstream trough. Sensitivity experiments performed with the Méso-NH model further revealed that initial perturbations targeted on Helene and the upstream trough were sufficient in forecasting the warm-core Medicane at 84- and 108-h lead time.

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