Météorologie | EPFL Graph Search

La météorologie est une science qui a pour objet l'étude des phénomènes atmosphériques tels que les nuages, les précipitations ou le vent dans le but de comprendre comment ils se forment et évoluent en fonction des paramètres mesurés tels que la pression, la température et l'humidité. Le mot vient du grec ancien (« qui est au-dessus de la terre »), qui désigne les particules en suspension dans l'atmosphère et , « discours » ou « connaissance ». C'est une discipline qui traite principalement de la mécanique des fluides et de la thermodynamique mais qui fait usage de différentes autres branches de la physique, de la chimie et des mathématiques. Purement descriptive à l'origine, la météorologie est devenue un lieu d'application de ces disciplines. Pour ce faire elle doit s'appuyer sur un réseau cohérent d'observations : le premier du genre — qui concerne un territoire multinational étendu — apparaît en 1854, sous la direction du français Le Verrier qui établit un réseau européen de do

Hydrological Importance and Water Quality Treatment Potential of a Small Freshwater Wetland in the Humid Tropics of Costa Rica

Manon Bachelin

Rapid increases in population and growing food demand are causing widespread deterioration of tropical wetlands globally, and an increased focus on the role and function of these imperiled ecosystems is required. Objectives of this study were to investigate the hydrological dynamics and water quality treatment potential of a small freshwater wetland in the humid tropics of Costa Rica. High-resolution, spatially distributed surface water and meteorological data were combined with a detailed topographical survey to quantify the wetland water balance, hydroperiod, and seasonal variability of wetland area, volume, and residence time. The water balance was dominated by precipitation and outflow, with little contribution from runoff, except during the largest storms. Over 80% of the wetland was flooded continuously; hydroperiods in remaining areas were bi-modal. Small seasonal variations in wetland area, volume, and residence times yielded high and sustained water quality treatment potential. Potential pollutant removal efficiencies were 63.6-99.8% for biological oxygen demand; 60.0-99.8% for total suspended solids; 51.1-98.5% for total nitrogen; and 34.2-99.7% for total phosphorous. The study provides insights into the hydrological functions of this and similar small Central American wetlands and provides a template for extending in-depth hydrological monitoring to other tropical wetland sites.

2011

Improvement of an urban turbulence parametrization for meteorological operational forecast and air quality modeling

Clive Muller

During the last century, urban pollution has increased with the growth of cities. Urban air quality has become a high priority as it is directly linked to concerns such as human exposure and health. The present work is dedicated to urban air quality modeling with focus on urban meteorology. The main goal is to improve meteorological and air quality simulations in urban areas. Based on measurements and numerical air quality simulations, Chapter 3 describes the meteorological situation and tests an emission inventory for an ozone pollution episode in Mexico City (2 March 1997), in order to determine the principal factors to be accounted in an air quality study. The Mexico City case shows the great influence of meteorological conditions and pollutant emissions on air quality. A thorough understanding of the phenomena governing the meteorological conditions, like small scale convergence, and an accurate emission inventory validated by VOC measurements in the city, are sensitive elements in an air quality study. In Chapter 4, the presence of an Urban Heat Island (UHI) is underlined over the city of Basel (Switzerland) with the BUBBLE measurements. Further on, the ability of aLMo (the operational weather prediction model of MeteoSwiss) to reproduce the effect of a city on the boundary layer atmospheric flow fields is investigated. Results show that aLMo is not able to reproduce the UHI and hence that its surface scheme based on the Monin-Obukhov Similarity Theory (MOST) is not adapted for the urban areas. Therefore, the Buildings Effects Parametrization (BEP) which has been developed by Martilli et al.(2002) especially for urban areas, is implemented in aLMo. Results show that alMo is now able to reproduce the main behavior of the urban boundary layer as the UHI and BEP has hence a real enhancement potential for aLMo. Chapter 5 shows the sensitivity of aLMo with respect to its vertical resolution. In order to limit the sensitivity of aLMo to the grid resolution, BEP is modified. The mesoscale model furnishes the upper boundary conditions for the inner calculation of BEP. That is, BEP recalculates independently the vertical profiles of wind, temperature and energy based on the surface fluxes of momentum, heat and turbulent kinetic energy. The results obtained show a decrease in the sensitivity to the resolution and a better agreement with the measurements. Furthermore, the modified version of BEP gives additional meteorological fields (temperature, wind and TKE) in the urban canopy. The computed temperature in the urban canopy shows a good agreement with measurements in a Basel street canyon. This work shows that it is not necessary to have a high resolution for taking into account modification of the atmospheric flow fields induced by a city.

EPFL2007

BIOPHYSICAL CONTROLS ON EVAPOTRANSPIRATION: NUMERICAL MODELS AND EXPERIMENTAL OBSERVATIONS

Federico Croci

Water and food are basics of human life, unfortunately climate changes and water pollution have reduced the amount of water available for human uses. In this context the study of the hydrologic cycle is becoming of growing importance in order to optimize the anthropogenic exploitation of water resource. Evapotranspiration is the term used to describe the part of the water cycle which removes liquid water from an area with vegetation into the atmosphere by the processes of both transpiration and evaporation. Evapotranspiration is an important part of the hydrologic cycle because it represents a considerable percentage of water losses in watersheds. This process is also one of the main consumers of solar energy at the Earth's surface. The estimation of evapotranspiration it is of great importance for agriculture and water resources management, for example, by knowing the rate of water loss from a region, farmers will be better placed to efficiently manage the irrigation strategies. The rate of evapotranspiration at any location on the Earth's surface is controlled by several factors such as energy availability, humidity gradient, wind speed immediately above the surface, water availability, type of vegetation, stomatal resistance and soil characteristics. The problem of estimating evapotranspiration has been widely studied. During the past several decades, different approaches have been proposed and many formulas are available in the literature to fulfill this task. The complexity of the phenomena and its great variability all over the world make difficult to find a universal and robust method for the evapotranspiration prediction and estimation. Evapotranspiration can be obtained by many estimation methods. Some of these methods need many weather parameters as inputs while others need fewer. Factors such as data availability, the intended use, and the time scale required by the problem must be considered when choosing the evapotranspiration calculation technique. The Penman-Monteith equation was derived from the classic Penman equation adding to the terms already considered in the Penman formulation (aerodynamic resistance and energy balance) the canopy resistance. This approach requires many meteorological data, such as maximum and minimum air temperatures, relative humidity, solar radiation, and wind speed. If some of these data are not available, alternative techniques have been developed to overcome this drawback. However, this estimations of meteorological data affect the quality of the result, alternatively, in such cases other simpler methods may be used. The Penman-Monteitu equation is assumed to be the most reliable because is based on physical principles and because it considers all the climatic factors, which affect evapotranspiration.

2013