Letteratura scientifica selezionata sul tema "Météorologie – Observations – Laboratoires"

The MEthane Remote sensing Lidar missioN (MERLIN), currently in phase C, is a joint cooperation between France and Germany on the development of a spatial Integrated Path Differential Absorption (IPDA) LIDAR (LIght Detecting And Ranging) to conduct global observations of atmospheric methane. This presentation will focus on the status of a LIDAR mission data simulator and processor developed at LMD (Laboratoire de Météorologie Dynamique), Ecole Polytechnique, France, for MERLIN to assess the performances in realistic observational situations.

Atmospheric general circulation models are believed to be appropriate tools for studying airborne impurity cycles in order to supplement observations and to improve our knowledge of gaseous and particulate pollutant cycles in the atmosphere. The main aspects of the modelling of tracer cycles are reviewed and illustrated by two particular examples: desert dust particles in the 1 μm range and water isotope species HDO and H218O. Some results from a first simulation including desert dust and water isotope cycles using the model developed at the Laboratoire de Météorologie Dynamique (LMD) are presented and compared to observations, with particular emphasis on ice-sheet data. The relatively good agreement with observations obtained so far is encouraging and should stimulate further applications to other types of tracers.

Atmospheric general circulation models are believed to be appropriate tools for studying airborne impurity cycles in order to supplement observations and to improve our knowledge of gaseous and particulate pollutant cycles in the atmosphere. The main aspects of the modelling of tracer cycles are reviewed and illustrated by two particular examples: desert dust particles in the 1 μm range and water isotope species HDO and H2 18O. Some results from a first simulation including desert dust and water isotope cycles using the model developed at the Laboratoire de Météorologie Dynamique (LMD) are presented and compared to observations, with particular emphasis on ice-sheet data. The relatively good agreement with observations obtained so far is encouraging and should stimulate further applications to other types of tracers.

Abstract Vertical rainfall profiles obtained with TRMM-PR 2A25 standard products are compared with rain profiles deduced from the Laboratoire de Météorologie Dynamique second generation global climate model (LMDZ, the Z stands for zoom capability) with two parameterization schemes: Emanuel’s and Tiedke’s. This paper focuses on the low layers of the atmosphere over West Africa during the monsoon [June–September (JJAS)]. The precipitation decrease above 4 km is systematically not represented in rainfall profiles generated by Emanuel’s parameterization scheme. However, Emanuel’s scheme shows a decrease similar to the observation from 4 km down to the surface, especially in the Sahel (proper depth of the layer dominated by reevaporation). As for Tiedtke’s scheme, it best describes the downward increase in the upper levels of the atmosphere, whereas the downward decrease in the lower levels begins too low when compared to the observations. Tiedtke’s parameterization shows an overestimation of liquid water production over the ocean and over the Guinean region and a slightly too strong reevaporation in the Sahara and Sahel. The zonal distribution of vertical rain profiles is then biased with this model scheme compared to the 2A25-PR product. On the other hand, although Emanuel’s scheme detects too much reevaporation over the Sahara and underestimates liquid water production over the ocean compared to PR observation, it shows a good meridional distribution of these parameters. This is especially true in the Sahel where Emanuel’s scheme gives the best representation of reevaporation.

Many questions remain concerning whether or not an ice cap existed during the mid-Cretaceous period (120–90 Ma). Other than data and observations from ice-rafted materials, atmospheric general circulation models (AGCMs) may be appropriate tools to investigate whether changes in atmospheric composition, land-sea distribution, or oceanic circulation (used as boundary conditions to constrain the model), provide the climatic conditions that enable ice caps to be formed.This study uses an AGCM developed by the Laboratoire de Météorologie Dynamique (LMD) to perform a set of numerical sensitivity experiments to investigate plate tectonics (land–sea distribution and orography), CO2 partial pressure in the atmosphere and changes in prescribed sea-surface temperatures. The main goals are to quantify the effect of each forcing factor on the increase of Northern Hemisphere high-latitude temperatures, and to investigate whether combining these factors produce temperatures that would allow the ice-sheet formation.

Many questions remain concerning whether or not an ice cap existed during the mid-Cretaceous period (120–90 Ma). Other than data and observations from ice-rafted materials, atmospheric general circulation models (AGCMs) may be appropriate tools to investigate whether changes in atmospheric composition, land-sea distribution, or oceanic circulation (used as boundary conditions to constrain the model), provide the climatic conditions that enable ice caps to be formed. This study uses an AGCM developed by the Laboratoire Météorologie Dynamique (LMD) to perform a set of numerical sensitivity experiments to investigate plate tectonics (land-sea distribution and orography), CO2 partial pressure in the atmosphere and changes in prescribed sea-surface temperatures. The main goals are to quantify the effect of each forcing factor on the increase of Northern Hemisphere high-latitude temperatures, and to investigate whether combining these factors produce temperatures that would allow the ice-sheet formation.

Abstract. An interactive sulphate aerosol chemistry module has been incorporated in the Laboratoire de Météorologie Dynamique General Circulation Model (LMD-GCM) to simulate the sulphur chemistry during the Indian Ocean Experiment (INDOEX) Intensive Field Phase-1999 (INDOEX-IFP). The originality of this module is its ability to predict particle mass and number concentration for the Aitken and accumulation modes. The model qualitatively reproduces the spatial patterns of observations on sulphate aerosol during INDOEX. On the basis of size distribution retrieved from the observations made along the cruise route during 1998 and 1999, the model successfully simulates the order of magnitude and the general north-south gradient in aerosol number concentration. The result shows the southward migration of minimum concentrations, which follows ITCZ (Inter Tropical Convergence Zone) migration. Sulphate surface concentration during INDOEX-IFP at Kaashidhoo (73.46° E, 4.96° N) gives an agreement within a factor of 2 to 3. Predicted sulphate aerosol optical depth (AOD) matches reasonably with measured values, indicating the capability of this model to predict the vertically integrated column sulphate burden. The Indian contribution to estimated sulphate burden over India is more than 60% with values upto 40% over the Arabian Sea.

Abstract The Laboratoire de Météorologie Dynamique atmospheric general circulation model with zooming capability (LMDZ) has been used in a nudged mode to enable comparison of model outputs with routine observations and evaluate the model physical parameterizations. Simulations have been conducted with a stretched grid refined over the vicinity of Paris, France, where observations, collected at the Trappes station (Météo-France) and at the Site Instrumental de Recherche par Télédétection Atmosphérique observatory, are available. For the purpose of evaluation of physical parameterizations, the large-scale component of the modeled circulation is adjusted toward ECMWF analyses outside the zoomed area only, whereas the inside region can evolve freely. A series of sensitivity experiments have been performed with different parameterizations of land surface and boundary layer processes. Compared with previous versions of the LMDZ model, a “thermal plume model,” in association with a constant resistance to evaporation improves agreement with observations. The new parameterization significantly improves the representation of seasonal and diurnal cycles of near-surface meteorology, the day-to-day variability of planetary boundary layer height, and the cloud radiative forcing. This study emphasizes the potential of using a climate model with a nudging and zooming capability to assess model physical parameterizations.

Abstract. The quality of the representation of greenhouse gas (GHG) transport in atmospheric general circulation models (GCMs) drives the potential of inverse systems to retrieve GHG surface fluxes to a large extent. In this work, the transport of CO2 is evaluated in the latest version of the Laboratoire de Météorologie Dynamique (LMDz) GCM, developed for the Climate Model Intercomparison Project 6 (CMIP6) relative to the LMDz version developed for CMIP5. Several key changes have been implemented between the two versions, which include a more elaborate radiative scheme, new subgrid-scale parameterizations of convective and boundary layer processes and a refined vertical resolution. We performed a set of simulations of LMDz with different physical parameterizations, two different horizontal resolutions and different land surface schemes, in order to test the impact of those different configurations on the overall transport simulation. By modulating the intensity of vertical mixing, the physical parameterizations control the interhemispheric gradient and the amplitude of the seasonal cycle in the Northern Hemisphere, as emphasized by the comparison with observations at surface sites. However, the effect of the new parameterizations depends on the region considered, with a strong impact over South America (Brazil, Amazonian forest) but a smaller impact over Europe, East Asia and North America. A finer horizontal resolution reduces the representation errors at observation sites near emission hotspots or along the coastlines. In comparison, the sensitivities to the land surface model and to the increased vertical resolution are marginal.

The Laboratoire de Météorologie Dynamique (LMD) variable-grid atmospheric general circulation model (AGCM) was used in this study for a five-year high-resolution simulation of the Antarctic climate. The horizontal resolution is about 100 km over a large part of the ice sheet. This study focuses on the simulated surface mass balance (precipitation-evaporation sublimation-melt) and on the spatial and temporal variability of snowfall in Antarctica. The simulated annual mean surface mass balance for the whole continent is close to the observed value, and the model simulates well the spatial distribution of the surface mass balance. The annual cycle of snowfall exhibits a clear minimum in summer over the high interior plateau as well as for Antarctica as a whole, in agreement with the observations. In the interior of the continent, the model produces a permanent light background snowfall that accounts for about 5% of the total annual precipitation. The bulk of the snowfall is produced irregularly during periods that generally last only two or three days that are caused by cyclones off the coast.

L'assimilation de données à méso-échelle permet d'initialiser les modèles régionaux de prévision numérique du temps, en combinant une ébauche du modèle avec les observations disponibles notamment à cette échelle. L'émergence de stations météorologiques connectées fournit une source d'observations d'opportunité permettant de densifier le réseau d'observation de surface. Ces nouvelles stations permettent d'observer à fine échelle la convection profonde près de la surface (Mandement, 2020). Cette thèse propose ainsi d’explorer la capacité de ces observations d'opportunité de température, d’humidité relative et de pression à améliorer la prévision de phénomènes météorologiques de méso-échelle, en les assimilant dans le modèle régional AROME-France.L'étape de pré-traitement de ces observations est nécessaire pour corriger et filtrer les observations considérées anormales vis-à-vis de l'ébauche du modèle, et d'écrémer les observations spatialement proches. Ce pré-traitement permet d'apporter à l'assimilation près de 129 fois plus d'observations au sol de pression, 3 fois plus d'observations de température et 6 fois plus d'observations d'humidité relative que dans le cas où seules les observations standards de surface sont utilisées. Des expériences avec et sans les observations d'opportunité ont été menées (méthode OSE) sur une période d'un mois. Ces OSE ont d'abord été réalisées en assimilant de manière indépendante les trois types d'observations d'opportunité susmentionnés, puis de manière conjointe. Les résultats de l'assimilation des observations d'opportunité de pression montrent une diminution de l'écart quadratique moyen entre la prévision à 1 h d'échéance et les observations standards de pression de 10.3 % en France, avec le schéma 3DEnVar. Il en est de même pour l'assimilation des observations d'opportunité de température et d'humidité relative, qui permettent une diminution de 0.9 % et de 1 % respectivement par rapport aux observations standards de la même grandeur. Avec le réglage actuel du schéma 3DEnVar, il apparaît que les observations de surface disposent d'un poids moindre à l'analyse par rapport aux autres observations et par rapport à l'ébauche. L'assimilation de nombreuses observations d'opportunité de surface contribue à compenser cet effet.Le type de schéma d'assimilation et son réglage jouent un rôle important dans l'assimilation de ces nouvelles observations. Le schéma d'assimilation 3DVar n'est ainsi pas capable de tirer profit des observations d'opportunité, du fait de la création d'incréments à la fois isotropes et de grandes dimensions horizontales. Des études de cas ont été menées, dans un premier temps sur des situations comportant de forts gradients de pression de surface — comme dans le cas d'un système convectif de méso-échelle, d'un front, d'une dépression de méso-échelle ou d'un mascaret atmosphérique ; puis dans des cas comportant de forts gradients de température ou d'humidité relative près de la surface — dans des cas de brise de mer, de brouillard, d'îlot de chaleur urbain ou de ligne orageuse. Ces études de cas ont permis de mettre en évidence la capacité des observations d'opportunité à améliorer la représentation de ces phénomènes, en contraignant l'analyse près de la surface
Mesoscale data assimilation is used to initialise regional numerical weather prediction models, by combining a first guess with the available observations particularly at this scale.The emergence of personal weather stations (PWS) is a source of observations that can be used to densify the surface observation network. These new observations have demonstrated their ability to observe deep convection near the surface at fine scales. This thesis proposes to explore the capacity of these temperature, relative humidity and pressure observations to improve the forecasting of mesoscale meteorological phenomena, by assimilating them in the AROME-France regional model.These observations need to be pre-processed in order to correct and filter out observations that are considered anomalous in relation to the first model guess, and to be thinned out to remove close observations in space.This pre-processing makes it possible to assimilate almost 129 times more pressure observations, 3 times more temperature observations and 6 times more relative humidity observations than in the case where only standard surface observations are used.Experiments with and without the PWS observations were carried out (OSE method) over a period of one month. These OSEs were first carried out by assimilating the three types of PWS observations mentioned above independently, and then jointly. The results of the assimilation of PWS pressure observations show a reduction in the mean square deviation between the 1 h forecast and the standard pressure observations of 10.3 % in France, using the 3DEnVar scheme. The same applies to the assimilation of PWS temperature and relative humidity observations, which result in a reduction of 0.9 % and 1 % respectively compared to standard observations of the same parameter. With the current setting of the 3DEnVar scheme, it appears that surface observations are given less weight in the analysis than both other observations and the first guess. The assimilation of many PWS surface observations helps to compensate for this effect.The type of assimilation scheme and its adjustment play an important role in the assimilation of these new observations. The use of the 3DVar assimilation scheme is thus unable to take advantage of dense PWS observations, due to the creation of increments that are both isotropic and of large horizontal dimensions.Case studies were carried out, firstly on situations involving strong gradients in surface pressure - as in the case of a mesoscale convective system, a front, a mesoscale low pressure system or an atmospheric bore; then on cases involving strong gradients in surface temperature or relative humidity - in the case of a sea breeze, a fog, an urban heat island or a squall line. These case studies highlighted the ability of PWS observations to improve the representation of these phenomena, by constraining the surface analysis