Search

This dissertation aims at laying a foundation on upscaling work of the impact of stratospheric overshootingconvection (SOC) on the water vapor budget in the tropical tropopause layer (TTL) and lowerstratosphere at a continental scale.To do so, we take advantage of the TRO-Pico field campaign measurements held at Bauru, Brazil, duringtwo wet/convective seasons in 2012 and 2013, and perform accordingly several numerical simulations ofthe TTL which encompass through a large part of South America using the BRAMS mesoscale model.Firstly, we adopt a strategy of simulating a full wet season without considering SOC. This simulationis then evaluated for other typical key features (e.g., TTL temperature, convective clouds, gravity wave)of the TTL. In the absence of SOC and before upscaling its impact, we demonstrate that the model has afair enough ability to reproduce a typical TTL. The importance of large-scale upwelling in comparison tothe finite-scale deep convective processes is then discussed.Secondly, from fine-scale BRAMS simulations of an observational case of SOC during TRO-Pico, wededuce physical parameters (mass flux, ice mass budget, SOC size) that will be used to set a nudging ofthe SOC impact in large-scale simulations. A typical maximum impact of about 2 kt of water vapor, and6 kt of ice per SOC cell is computed. This estimation is 30% lower for another microphysical setup of themodel. We also show that the stratospheric hydration by SOC is mainly due to three types of hydrometeorsin the model.; Cette thèse a pour but de préparer un travail d’extrapolation de l’impact des overshoots stratosphériques(SOC) sur le bilan de vapeur d’eau (VE) dans la couche de la tropopause tropicale (TTL) et dansla basse stratosphère à l’échelle continentale.Pour ce faire, nous profitons des mesures de la campagne de terrain TRO-Pico tenue à Bauru, auBrésil, pendant deux saisons convectives/humides en 2012 et 2013, et de plusieurs simulations numériquesde la TTL sur un domaine englobant une grande partie de l’Amérique du Sud avec le modèle méso-échelleBRAMS.Premièrement, nous avons effectén une simulation d’une saison humide complète sans tenir compte desSOC. Cette simulation est ensuite évaluée pour d’autres caractéristiques clés typiques (température de laTTL, VE, sommets de nuages et ondes de gravité) dans la TTL. En l’absence de SOC, et avant d’extrapolerleur impact, nous démontrons que le modèle reproduit correctement les caractéristiques principales de laTTL. L’importance de l’ascension lente à grande échelle par rapport aux processus convectifs profonds àéchelle finie est ensuite discutée.Deuxièmement, à partir de simulations BRAMS à fine à échelle de cas de SOC observés pendantTRO-Pico, nous déduisons des quantités physiques (flux de glace, bilan de masse de glace, tailles desSOCs), qui serviront à définir un forçage de l’impact des overshoots dans des simulations à grande échelle.Nous montrons un impact maximum d’environ 2 kt en VE, et 6 kt de glace par SOC. Ces chiffres sont30% inférieurs pour un autre réglage microphysique du modèle. Nous montrons que seuls trois typesd’hydrométéores du modèle contribuent à cette hydratation.

[1] In this paper, we report a detailed description of a thin cirrus at the tropopause above a cumulonimbus (Cb) convective cluster observed during the Airborne Platform for Earth Observation-Third European Stratospheric Experiment for Ozone (APE-THESEO) campaign in February-March 1999 in the western Indian Ocean. The thin cirrus (Ci) has an optical depth at 532 nm below 0.1, with extended subvisible stretches, and is located directly below the tropopause, which was supersaturated with respect to ice. A direct comparison between the optical depth retrieved by Meteosat and that obtained by means of the hygrometers installed on the M55-Geophysica aircraft is discussed showing discrepancies ranging from 10 to 20%. Combining satellite and aircraft data, we show that the observed Ci is not due to cirrus outflow from Cb anvils. In the absence of any deeply convective clouds reaching altitudes above 15 km, we propose a possible mechanism of Ci formation based on a net mesoscale transport of water vapor from altitudes above 16 km to the tropopause region around 18 km. This transport could be driven by the critical layer and turbulence induced by gravity waves that could have been generated by lower level Cb cluster activity. The proposed mechanism for high-altitude Ci formation corroborates the new paradigm of a tropical tropopause layer (TTL) or "substratosphere,'' several kilometers thick, which is decoupled from the convection-dominated lower troposphere.