Projet ANR-07-JCJC-0016 – TropIce : Vapeur d'eau à la tropopause et nuages de glace
Programme : Jeunes chercheuses & jeunes chercheurs (JCJC) 2007
Année de financement : 2007
Coordinateur du projet : CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE ILE-DE-FRANCE SECTEUR OUEST ET NORD (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE ILE-DE-FRANCE SECTEUR OUEST ET NORD)
Partenaires : Centre national de la recherche scientifique
Montant : 148717.0 euros
Date de début : 2007
Durée : 36 mois
Résumé : "Water vapour is an essential component of the climatic system. On one hand, it has a strong influence on chemical components of the atmosphere, due to it being the major source of OH radicals; on the other hand, its emissions of infrared radiations make it the most important greenhouse gas. -  In the stratosphere, water vapour concentrations are much lower than in the troposphere (less than 2%). The reasons for this extreme dryness are still unclear; however it appeared very early that, in order to reach such low levels of humidity, air masses had to go through the very cold temperatures of the tropical tropopause, where they would get dehydrated (Brewer, 1949). Even if this theory has gained widespread acceptance, several aspects of it are still debated. This has become a pressing problem, as humidity in the stratosphere has been steadily increasing for the past 50 years without any good explanation (Rosenlof et al. 2001), and stratospheric conditions have a profound impact on the global climate (Forster and Shine 2002; Scaife et al. 2005). -  Two mechanisms are mainly considered to explain the dehydration: First, large-scale dehydration during troposphere-to-stratosphere transports, mainly through the very cold tropical tropopause region over the Western Pacific (the opposed ""stratospheric fountain"" and ""cold trap"" hypotheses), after potentially large horizontal transports (Fueglistaler et al. 2004); second, small-scale dehydration in deep tropical convective systems ascending past the tropopause (""overshoots"", Sherwood and Dessler, 2000). Recent studies suggest that both of these phenomena are simultaneously occurring, but their respective temporal and spatial frequencies are still largely unknown, even if anthropogenic conditions seem to increase the overshoots (Sherwood 2002). In an effort to explain the annual, large-scale variations in water vapour circulation, we plan, on one hand, to study the correlation between water vapour concentration and variations in large-scale atmospheric circulation (e.g. the El Nino /Southern Oscillation), using retrievals of Lyapunov exponents to identify large-scale dynamic structures in the atmosphere. On the other hand, we intend to quantify the role of convective events for dehydration, using satellite data combined with Lagrangian methods (Sect. 2.1). -  When studying water vapour transport, it is important and useful to simultaneously consider cirrus and ice clouds. These very cold clouds, composed of ice crystals, are generally close to the tropopause (more than 50%) and therefore are often associated with dehydration of air masses before their entry in the stratosphere. On one hand, the presence of a very thin layer of subvisible cirrus related to large-scale dehydration is currently supposed over the West Pacific (Fueglistaler et al. 2004), and its observation would confirm the cold trap hypothesis; unfortunately subvisible cirrus are extremely hard to detect with today's passive remote sensing satellite imagery (Chiriaco et al 2004). On the other hand, ice clouds are often located on top of convective systems (the ""anvil"") and actively participate to air dehydration through freeze drying; the intensity of the overshooting is generally considered to depend on the properties of the generated ice particles - unfortunately microphysical properties of convective ice clouds are very variable and still poorly understood, as thick convective systems typically mask their icy top from view (Noel et al 2004). We plan to use active remote sensing satellite observations to alleviate these two problems and quantify these two phenomena (Sect. 2.2). Finally, cirrus clouds directly impact the Earth's radiative budget through their albedo and greenhouse effects, which are both poorly quantified (Stephens 1990; Baker 1997); thus they indirectly affect water vapour transport dynamics. -  The long-term questions we are trying to address through these studies are (1) how can we link the diffe