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Luc Musson-Genon

Abstract

A comparison of simple turbulence closures with a one-dimensional boundary layer model is presented in order to select one for a three-dimensional nonhydrostatic model applied to the transport-diffusion problem in complex terrain. This study is based on a comparison between measurements and numerical simulations during three experiments: the Wangara experiment; a case of diurnal evolution of a boundary layer in clear sky conditions; and the Cabauw and JASIN (Joint Air–Sea Interaction) experiments concerning the interaction between turbulent and radiative processes in cloud layers (fog and stratocumulus). The results obtained with the e-ε equations and with the turbulent kinetic equation using dissipative and mixing lengths are rather similar. The formulation of Louis using a diagnostic diffusion coefficient, slightly modified to take cloud layers into account, gives satisfactory results in view of its simplicity.

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Luc Musson-Genon

Abstract

A one-dimensional (I-D) planetary boundary-layer model, including a complete set of simple physical parameterizations, has been used since July 1986 to predict daily soundings at Trappes in the suburbs of Paris. This model is coupled to the French operational spectral hemispheric model, representing the large-scale atmospheric environment by means of horizontal gradients and vertical velocity in the advective terms, and geostrophic wind. The study of the statistical scores (mean absolute error and mean error) for 220 cases during the year 1986–87 provides good accuracy for the 12-h forecast (as compared to fine mesh modeling and statistical interpretation methods) but exhibits a loss of accuracy for periods longer than 12 h. Improvement in the results through adjustment of some of the model parameters with the help of a file containing 22 test situations appreciably reduces the mean absolute error, this method would thus be useful for local forecasts of sensitive weather elements. Further improvement could be obtained by coupling this I-D model with a three-dimensional, fine mesh model and by refining cloudy turbulence and soil evaporation schemes.

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Luc Musson-Genon

Abstract

A one-dimensional boundary layer model is used to simulate a fog event. This model describes the condensation process at subgrid-scale, the gravitational settling of fog droplets and their interactions with solar and thermal radiation, as well as the turbulent transport associated with turbulent kinetic energy. The different parameterizations used are rather simple, aimed at operational forecasting. Computed results are compared to the measurements of a fog event at Cabauw in the Netherlands on 3 August 1977. The model seems to be able to describe the mechanisms occurring in fog evolution from its appearance to its disappearance. The dataset is the most complete ever published, but as yet it is difficult to validate the different parameterizations. Nevertheless, the importance of turbulent transport is pointed out again. The sensitivity of the model to thermal cooling, the gravitational setting velocity and the initial data is described together with the usefulness of subgrid-scale parameterization. In this work emphasis has been paced on the quantitative comparison between computed and observed evolutions.

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Christine Marais and Luc Musson-Genon

Abstract

A simple method of dynamical adaptation of a mesoscale model has been tested to produce meteorological parameters locally adapted at meteorological stations. This method is based on the use of a soil model in association with a surface boundary-layer model (MUSCLES, modélisation uni-dimensionnelle du sol et de la couche limite en surface), coupled with the outputs of the French operational mesoscale model (PERIDOT). The meteorological station-dependent characteristic constants used for describing the soil properties are identified by comparisons with observations. This adjustment is achieved by a variational method consisting in minimizing a cost function that measures the distance between the output parameters computed by MUSCLES and the observed ones. The minimization algorithm developed for that purpose requires the computation of the gradient of this cost function, which is done in practice by using the adjoint of the MUSCLES code.

For forecasting purposes, it was found that the best way is to adjust the local constants by computing the cost function on the ten previous days. The results are encouraging; for five of the six stations considered, the quality of the gains is significant, even if they are lower than what is achieved by the operationally used statistical adaptation.

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Yongfeng Qu, Maya Milliez, Luc Musson-Genon, and Bertrand Carissimo

Abstract

In many micrometeorological studies with computational fluid dynamics, building-resolving models usually assume a neutral atmosphere. Nevertheless, urban radiative transfers play an important role because of their influence on the energy budget. To take into account atmospheric radiation and the thermal effects of the buildings in simulations of atmospheric flow and pollutant dispersion in urban areas, a three-dimensional (3D) atmospheric radiative scheme has been developed in the atmospheric module of the Code_Saturne 3D computational fluid dynamic model. On the basis of the discrete ordinate method, the radiative model solves the radiative transfer equation in a semitransparent medium for complex geometries. The spatial mesh discretization is the same as the one used for the dynamics. This paper describes ongoing work with the development of this model. The radiative scheme was previously validated with idealized cases. Here, results of the full coupling of the radiative and thermal schemes with the 3D dynamical model are presented and are compared with measurements from the Mock Urban Setting Test (MUST) and with simpler modeling approaches found in the literature. The model is able to globally reproduce the differences in diurnal evolution of the surface temperatures of the different walls and roof. The inhomogeneous wall temperature is only seen when using the 3D dynamical model for the convective scheme.

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Laurent Makké, Luc Musson-Genon, Bertrand Carissimo, Pierre Plion, Maya Milliez, and Alexandre Douce

Abstract

The atmospheric radiation field has seen the development of more accurate and faster methods to take into account absorption. Modeling fog formation, where infrared radiation is involved, requires accurate methods to compute cooling rates. Radiative fog appears under clear-sky conditions owing to a significant cooling during the night where absorption and emission are the dominant processes. Thanks to high-performance computing, high-resolution multispectral approaches to solving the radiative transfer equation are often used. Nevertheless, the coupling of three-dimensional radiative transfer with fluid dynamics is very computationally expensive. Radiation increases the computation time by around 50% over the pure computational fluid dynamics simulation. To reduce the time spent in radiation calculations, a new method using analytical absorption functions fitted by Sasamori on Yamamoto’s radiation chart has been developed to compute an equivalent absorption coefficient (spectrally integrated). Only one solution of the radiative transfer equation is needed against N band × N gauss for an N band model with N gauss quadrature points on each band. A comparison with simulation data has been done and the new parameterization of radiative properties proposed in this article shows the ability to handle variations of gas concentrations and liquid water.

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