Bertrand Carissimo

On the Influence of a Simple Microphysics Parametrization on Radiation Fog Modelling: A Case Study During ParisFog

A detailed numerical simulation of a radiation fog event with a single column model is presented, which takes into account recent developments in microphysical parametrizations. One-dimensional simulations are performed using the computational fluid dynamics model Code_Saturne and the results are compared to a very detailed in situ dataset collected during the ParisFog campaign, which took place near Paris, France, during the winter 2006–2007. Special attention is given to the detailed and complete diurnal simulations and to the role of microphysics in the fog life cycle. The comparison between the simulated and the observed visibility, in the single-column model case study, shows that the evolution of radiation fog is correctly simulated. Sensitivity simulations show that fog development and dissipation are sensitive to the droplet-size distribution through sedimentation/deposition processes but the aerosol number concentration in the coarse mode has a low impact on the time of fog formation.

Micrometeorological Modeling of Radiative and Convective Effects with a Building-Resolving Code

AbstractIn 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...

On the Definition of the Cloud Water Content Fluctuations and Its Effects on the Computation of a Second-Order Liquid Water Correlation

In subgrid-scale condensation schemes of cloud models, the majority of previous authors have relied on results presented in a paper by Bougeault. In the present paper, second-order liquid water correlations are restated that differ from the former paper but are coherent with the corrigendum of Mellor. These differences are explained here through two different underlying definitions of cloud water content fluctuations; they can be summarized by whether or not unsaturated air within a grid box contributes to the eddy flux of the cloud water content. Taking into account the issue mentioned above, the “buoyancy flux” is also derived. Although the full impact of these changes has not been evaluated here, it may become important for future global cloud-resolving climate models.

Effect of Atmospheric Stability on the Atmospheric Dispersion Conditions Over a Industrial Site Surrounded by Forests

This work investigates the impact of terrain heterogeneity and stability conditions (stable and unstable) on the local turbulence kinetic energy (TKE) around an industrial site using 36 months of data collected at the experimental site SIRTA (Instrumental Site of Atmospheric Research by Remote Sensin). These measurements show that the normalized TKE (TKE/U2: where U is the wind speed) is highly depended on the upstream complexity of the terrain (presence of forested areas or buildings) whatever the stability conditions (stable, unstable). The highest values of normalized TKE are obtained for unstable thermal stratification for all wind direction sector in the zones located behind the forested areas.

Development of a Building Resolving Atmospheric CFD Code Taking into Account Atmospheric Radiation in Complex Geometries

In order to take into account atmospheric radiation and the thermal effects of the buildings in simulations of atmospheric flow and pollution dispersion in urban areas, we have developed a three-dimensional atmospheric radiative scheme in the atmospheric module of the open-source CFD model Code_Saturne. This paper describes our ongoing work on the development of this model. The radiative scheme has been previously validated with idealized cases and the results of a real case. Here we present results of the full coupling of the radiative and thermal schemes with the 3D dynamical flow model and compare the results with simpler approaches found in the literature.

Near-Field Pollutants Dispersion in a Stratified Surface Layer: Comparison of Numerical Study and Field Measurements of SIRTA

In order to study pollutants dispersion in a stable atmospheric boundary layer, an experimental program consisting in measuring turbulence fluctuations and pollutants dispersion in a stratified surface layer and in near field is being conducted on the site ‘SIRTA’ in the southern suburb of Paris. Turbulence is characterized to be strongly anisotropic in a stable surface layer. Forest to the north of experiment field induces a strong wind directional shear between 3 m and 30 m levels and a wind speed decrease. Numerical simulation in RANS mode with k-e turbulence closure and a canopy model enables to reproduce the observed wind channeling effect of forest on the mean flow and turbulence kinetic energy. A preliminary simulation taking into account the real pollutants flow rate has been performed and compared with measurements.

A New Method for Fast Computation of Three-Dimensional Atmospheric Infrared Radiative Transfer in a Nonscattering Medium, with an Application to Dynamical Simulation of Radiation Fog in a Built Environment

AbstractThe 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). Onl...

3D Radiative and Convective Modeling of Urban Environment: An Example for the City Center of Toulouse

A three dimensional tool coupling the thermal energy balance of buildings and the modeling of the atmospheric flow in urban areas has been developed. In this work the effects of complexity in real urban geometry on the interaction between airflows and radiation exchanges with different surfaces is tested. The model results are encouraging and give insight into local surface-atmosphere processes, but further and more rigorous testing has to be performed with other datasets.

A preliminary analysis of measurements from a near-field pollutants dispersion campaign in a stratified surface layer

In order to study pollutants dispersion in a stably stratified surface layer at small scales, a pollutants dispersion field experiment programme is being conducted on the site SIRTA in the southern suburb of Paris. In a preliminary campaign held during the winter of 2012 with continuous gas releases, we had 12 ultrasonic anemometers measuring wind velocity and temperature at 10 Hz, and six photo-ionisation detectors measuring gas concentration at 50 Hz. Turbulence is verified to be anisotropic by calculating variances, integral length scales and power spectra of velocity. Some evidence of a –1 power law sub-range has been found in velocity power spectra. Also, we have deduced eddy advection velocity from spatial velocity cross-correlation and found it greater than mean wind speed measured directly by sensors. Concentration measurements are corrected by using a baseline method. Significant relationships between concentration and turbulence measurements at high frequency are still under further study.

Poster 8 Radiative transfers in CFD modelling of the urban canopy

Abstract The processes that govern air pollution in the urban canopy highly depend on the shape and spacing of the buildings. The dynamical effects have been extensively studied using CFD techniques, usually assuming a neutral atmosphere and neglecting the thermal effects. Nevertheless, radiative transfers play an important role because of their influence on the urban canopy budget. In order to take into account the radiation budget in simulations of pollution dispersion in urban areas, we have developed a radiative scheme in the atmospheric CFD model Mercure_Saturne .