Influence des effets d'échelle de la prise en compte du relief sur les écoulements de l'air et les précipitations. Exemples dans les modèles météorologiques GFS, MM5 et RAMS

National audienceSelon l'échelle et la précision à laquelle est pris en compte le relief d'une région, et sont opérés les calculs dans un modèle météorologique déterministe, des biais, sinon des erreurs sont générés. Il en résulte des différences importantes, en particulier pour les précipitations et surtout les vents en sortie de modèles aussi divers que GFS, BOLAM, MM ou RAMS, par exemple

Implementation and assessment of a model including mixotrophs and the carbonate cycle (Eco3M_MIX-CarbOx v1.0) in a highly dynamic Mediterranean coastal environment (Bay of Marseille, France) – Part 1: Evolution of ecosystem composition under limited light and nutrient conditions

Many current biogeochemical models rely on an autotrophic versus heterotrophic food web representation. However, in recent years, an increasing number of studies have begun to challenge this approach. Several authors have highlighted the importance of protists capable of combining photoautotrophic and heterotrophic nutrition in a single cell. These mixotrophic protists are known to play an important role in the carbon cycle. Here, we present a new biogeochemical model that represents the food web using variable stoichiometry. It contains the classic compartments such as zooplankton, phytoplankton, and heterotrophic bacteria and a newly added compartment to represent two types of mixotrophic protists: non-constitutive mixotrophs (NCMs) and constitutive mixotrophs (CMs). We demonstrate that the model correctly reproduces the characteristics of NCMs and CMs and proceed to study the impact of light and nutrient limitation on planktonic ecosystem structure in a highly dynamic Mediterranean coastal area, namely the Bay of Marseille (BoM, France), paying special attention to the dynamics of mixotrophic protists in these limiting conditions. In addition, we investigate the carbon, nitrogen, and phosphorus fluxes associated with mixotrophic protists and showed the following: (i) the portion of the ecosystem in terms of the percentage of carbon biomass occupied by NCMs decreases when resources (nutrient and prey concentrations) decrease, although their mixotrophy allows them to maintain a carbon biomass almost as significant as the copepod one (129.8 and 148.7 mmol C m−3, respectively), as photosynthesis increases as a food source, and (ii) the portion of the ecosystem in terms of the percentage of carbon biomass occupied by CM increases when nutrient concentrations decrease due to their capability to ingest prey to supplement their N and P needs. In addition to providing new insights regarding the conditions that lead to the emergence of mixotrophs in the BoM, this work provides a new tool to perform long-term studies and predictions of mixotroph dynamics in coastal environments under different environmental forcings.</p

WRF Sensitivity Analysis in Wind and Temperature Fields Simulation for the Northern Sahara and the Mediterranean Basin

Different configurations for the Weather Research and Forecasting (WRF) model were evaluated to improve wind and temperature fields predictions in the Northern Sahara and the Mediterranean basin. Eight setups, associated with different combinations of the surface layer physical parameters, the land surface model, and the grid nudging parameters, were considered. Numerical simulations covered the entire month of November 2017. Model results were compared with surface data from meteorological stations. The introduction of the grid nudging parameters leads to a general improvement of the modeled 10 m wind speed and 2 m temperature. In particular, nudging of wind speed parameter inside the planetary boundary layer (PBL) provides the most remarkable differences. In contrast, the nudging of temperature and relative humidity parameters inside the PBL may be switched off to reduce computational time and data storage. Furthermore, it was shown that the prediction of the 10 m wind speed and 2 m temperature is quite sensitive to the choice of the surface layer scheme and the land surface model. This paper provides useful suggestions to improve the setup of the WRF model in the Northern Sahara and the Mediterranean basin. These results are also relevant for topics related with the emission of mineral dust and sea spray within the Mediterranean region

Genèse du mistral par interaction barocline et advection du tourbillon potentiel

Cette étude introduit la notion de cyclonisme dynamique de Lee dans la genèse de la situation météorologique de Mistral. Ensuite, le processus d’interaction barocline renforçant les conditions de violence, de sécheresse et de fraîcheur de ce “maître des vents” de Provence est décrit. Enfin, le cas des régimes de vents de nord-ouest, ayant une dynamique atmosphérique bien différente de celle du Mistral bien qu’ils soient de même secteur, sera détaillé afin de discriminer les deux cas et de spécifier les critères permettant d’affiner la définition du Mistral. La nouvelle définition dite dynamique montre que le nombre de jours de Mistral serait en moyenne de 30 jours contre 80 jours par an

Impact des conditions météorologiques locales sur la variabilité spatiale de l'ozone à échelle fine à Nice

International audienceThe city of Nice is strongly affected each summer by ozone pollution. Thanks to many measurement campaigns from April to September 2007 in Nice, the ozone spatial variability was apprehended at fine spatial scale. The secondary pollutant spatial average offers a coastal and suburban areas characterized by higher concentrations than the rest of the surveyed areas. Apart from the impact of primary pollutants fugitive emissions by the motor traffic in urban and suburban, local weather conditions also govern the spatial ozone distribution. To what extent the spatial variability of certain meteorological variables influence the ozone spatial average ? The use of deterministic model RAMS (Regional Atmospheric Modelling System) provide numerical simulations to fine scale to answer this question

Rétroactions des incendies de forêt sur le vent : exemples mesurés sur parcelles végétales expérimentales dans le sud-est de la France.

International audienceIt is well known that wind plays a major role in forest fire propagation. However, the fire itself strongly influences wind characteristics by accelerating wind speeds near the flames. This phenomenon has been observed by fire fighters but rarely quantified. The objective of the study was to measure changes in wind characteristics during field scale experiments. It was observed that not only does wind speed pick up in proximity to the fire front but strong convective currents generated by the fire reinforce vertical wind speeds as well.Le vent propage les incendies de forêt, mais ces derniers rétroagissent également sur le vent, en accélérant les vitesses dans les parages des flammes. Ce fait connu des pompiers mais non quantifié, est ici précisément mesuré et analysé au cours d'expériences de terrain en grandeur réelle, qui ont montré également que les vitesses verticales se renforcent lorsque les flammes approchent, ascendances liées à la convection provoquée par le feu

Influence of Meteorological Conditions and Aerosol Properties on the COVID-19 Contamination of the Population in Coastal and Continental Areas in France: Study of Offshore and Onshore Winds

International audienceHuman behaviors probably represent the most important causes of the SARS-Cov-2 virus propagation. However, the role of virus transport by aerosols—and therefore the influence of atmospheric conditions (temperature, humidity, type and concentration of aerosols)—on the spread of the epidemic remains an open and still debated question. This work aims to study whether or not the meteorological conditions related to the different aerosol properties in continental and coastal urbanized areas might influence the atmospheric transport of the SARS-Cov-2 virus. Our analysis focuses on the lockdown period to reduce the differences in the social behavior and highlight those of the weather conditions. As an example, we investigated the contamination cases during March 2020 in two specific French areas located in both continental and coastal areas with regard to the meteorological conditions and the corresponding aerosol properties, the optical depth (AOD) and the Angstrom exponent provided by the AERONET network. The results show that the analysis of aerosol ground-based data can be of interest to assess a virus survey. We found that moderate to strong onshore winds occurring in coastal regions and inducing humid environment and large sea-spray production episodes coincides with smaller COVID-19 contamination rates. We assume that the coagulation of SARS-Cov-2 viral particles with hygroscopic salty sea-spray aerosols might tend to inhibit its viral infectivity via possible reaction with NaCl, especially in high relative humidity environments typical of maritime sites

Occurrence of perfluoroalkyl substances in the Bay of Marseille (NW Mediterranean Sea) and the Rhône River

International audienceFour perfluoroalkyl substances (PFAS) were analyzed in 62 duplicate surface water samples from the Rhône River and Marseille Bay (France; NW Mediterranean Sea). Perfluorooctane sulfonate (PFOS) was detected in all samples and exceeded the European Environmental Quality Standard (EQS) values in over 80% of the cases. The most contaminated samples were from the Rhône River (up to 200 ng L-1 ∑ 4 PFAS), as well as those collected near a wastewater treatment plant outlet in Marseille Bay (up to 9 ng L −1 ∑ 4 PFAS). While PFOS was the predominant PFAS in Marseille Bay, remarkably high concentrations of perfluorohexanoic acid (PFHxA) were measured in the Rhône River (8-193 ng L −1). The relative abundances of individual compounds differed thus significantly between the Rhône River and Marseille Bay, indicating different sources. A simulation made with the MARS3D model showed that PFOS inputs from the Rhône River can enter Marseille Bay at levels > EQS

Wind and Wave Dependent Sea Spray Generation Functions - From the Laboratory to the Field

International audienc