Two mechanisms appear to be primarily responsible for the formation of cirrus clouds in Tropical Tropopause Layer (TTL): detrainment from deep convective anvils and in situ initiation. Here we propose to identify TTL cirrus clouds by analyzing water content measurements from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and Aura Microwave Limb Sounder (MLS). Using ice water content (IWC) and water vapor (H2O) abundances we identify TTL cirrus clouds that contain too much ice to have been formed in situ — and therefore must be of convective origin. We use two methods to infer amounts of water vapor available for in situ formation. Clouds with IWC greater than this threshold are categorized as being of convective origin; clouds with IWC below the threshold are ambiguous — they could either form from in situ or still be of convective origin. Applying the thresholds from December 2008 to November 2009, we found that at least 19.2% of tropical cirrus were definitively of convective origin at the tropopause (375 K) during boreal winter. At each level, we found three maxima in the occurrence of convective cirrus: western Pacific, equatorial Africa, and South America. Averaged over the entire tropics (30oS to 30oN), we found convective cirrus occurs more frequently in boreal winter and less frequently in boreal summer, basically following the a decreasing trend from DJF, MAM, SON, to JJA. During boreal summer, we found that only 4.6% of tropical cirrus come from convection. Sensitivity tests show that the thresholds derived at 390 K have the largest uncertainty. At lower levels, especially 375 K, our thresholds are robust.

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 dans la basse stratosphère à l’échelle continentale.Pour ce faire, nous profitons des mesures de la campagne de terrain TRO-Pico tenue à Bauru, au Brésil, pendant deux saisons convectives/humides en 2012 et 2013, et de plusieurs simulations numériques de la TTL sur un domaine englobant une grande partie de l’Amérique du Sud avec le modèle méso-échelle BRAMS.Premièrement, nous effectuer une simulation d’une une saison humide complète sans tenir compte des SOC. Cette simulation est ensuite évaluée pour d’autres caractéristiques clés typiques (température de la TTL, VE, sommets de nuages et ondes de gravité) dans la TTL. En l’absence de SOC et avant d’extrapoler son leur impact, nous démontrons que le modèle reproduit correctement les caractéristiques principales de la TTL. L’importance de l’ascension lente à grande échelle par rapport aux processus convectifs profonds à échelle finie est ensuite discutée.Deuxièmement, à partir de simulations BRAMS à fine à échelle de cas de SOC observés pendant TRO-Pico, nous déduisons des quantités physiques (flux de glace, bilan de masse de glace, tailles des SOCs), 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 sont 30% nférieurs pour un autre réglage microphysique du modèle. Nous montrons que seul trois types d’hydrométéores du modèle contribuent à cette hydratation. / This dissertation aims at laying a foundation on upscaling work of the impact of stratospheric overshooting convection (SOC) on the water vapor budget in the tropical tropopause layer (TTL) and lower stratosphere at a continental scale.To do so, we take advantage of the TRO-Pico field campaign measurements held at Bauru, Brazil, during two wet/convective seasons in 2012 and 2013, and perform accordingly several numerical simulations of the 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 simulation is 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 a fair enough ability to reproduce a typical TTL. The importance of large-scale upwelling in comparison to the finite-scale deep convective processes is then discussed.Secondly, from fine scale BRAMS simulations of an observational case of SOC during TRO-Pico, we deduce physical parameters (mass flux, ice mass budget, SOC size) that will be used to set a nudging of the SOC impact in large-scale simulations. A typical maximum impact of about 2kt of water vapor, and 6kt of ice per SOC cell is computed. This estimation is 30% lower for another microphysical setup of the model. We also show that the stratospheric hydration by SOC is mainly due to two types of hydrometeors in the model.

This PhD thesis investigates the transport and distribution of short-lived halogenated organic substances in the tropical tropopause layer (TTL) in the Pacific Ocean. Short-lived halocarbons are one of the major groups of the ozone depleting substances as they provide a source for the active halogens which decrease ozone in the atmosphere. The TTL serves as the primary gateway of tropospheric air to enter the stratosphere. The air which enters the stratosphere is distributed all over the globe. Thus, the research on which tropospheric air masses go into the TTL, its structure and composition and the transport within is crucial. This thesis uses the UK Meteorological Office Lagrangian particle dispersion model NAME to (i) support the flight planning activities and achieve the multi aircraft coordination in CAST, CONTRAST, ATTREX 2014 campaigns, and (ii) quantify the amount and distribution of short-lived halocarbons in the TTL, and explain differences in these vertical distributions and transport characteristics. The halocarbons of interest are methyl iodide (CH3I), bromoform (CHBr3) and dibromomethane (CH2Br2). A new NAME procedure was developed and operated successfully to provide routine simulations and near real-time products suitable for guiding the CAST, CONTRAST and ATTREX aircraft in order to achieve their mission scientific objectives, and to make coordinated measurements. NAME was used post-campaign to analyse distribution of short-lived halocarbons in the TTL, identify their source regions and transport timescales. A new approach is proposed to investigate the TTL composition in terms of the boundary layer air influence, and subsequently quantify CH3I, CHBr3 and CH2Br2 by estimating their boundary layer and background contribution. The sums of these modelled estimates are in good agreement with the ATTREX 2014 and 2013 CH3I, CHBr3 and CH2Br2 observations. The quantification of the contribution of short-lived bromocarbons to the active bromine in the TTL was achieved, and the results lie within the range of the recent literature studies. The final focus of this thesis is on how well NAME represents the particle displacement via convection. Convection is the major transport pathway for the short-lived halocarbons to reach the TTL. The role of convection in transporting CH3I, CHBr3 and CH2Br2 to the TTL is assessed using the new convection scheme in NAME. A validation of the performance of this scheme is provided, showing that it yields improved and more realistic representation of the particle displacement via convection.

Several studies suggest that deep convection that penetrates the tropical tropopause layer may influence the long-term trends in lower stratospheric water vapor. This thesis investigates the relationship between penetrating deep convection and lower stratospheric water vapor variability using historical infrared (IR) observations. However, since infrared observations do not directly resolve cloud vertical structure and cloud top height, and there has been some debate on their usefulness to characterize penetrating deep convective clouds, CloudSat/Calipso and Aqua MODIS observations are first combined to understand how to best interpret IR observations of penetrating tops. The major findings of the combined CloudSat/Calipso and Aqua MODIS analysis show that penetrating deep convection predominantly occur in the western tropical Pacific Ocean. This finding is consistent with IR studies but is in contrast to previous radar studies where penetrating deep convective clouds predominantly occur over land regions such as equatorial Africa. Estimates on the areal extent of penetrating deep convection show that when using IR observations with a horizontal resolution of 10 km, about two thirds of the events are large enough to be detected. Evaluation of two different IR detection schemes, which includes cold cloud features/pixels and positive brightness temperature differences (+BTD), show that neither schemes completely separate between penetrating deep convection and other types of high clouds. However, the predominant fraction of +BTD distributions and cold cloud features/pixels ≤ 210 K is due to the coldest and highest penetrating tops as inferred from collocated IR and radar/lidar observations. This result is in contrast to previous studies that suggest the majority of cold cloud features/pixels ≤ 210 K are cirrus/anvil cloud fractions that coexist with deep convective clouds. Observations also show that a sufficient fraction of penetrating deep convective cloud tops occur in the extratropics. This provides evidence that penetrating deep convection should be documented as a pathway of stratospheric-tropospheric exchange within the extratropical region. Since the cold cloud feature/pixel ≤ 210 K approach was found to be a sufficient method to detect penetrating deep convection it was used to develop a climatology of the coldest penetrating deep convective clouds from GridSat observations covering years 1998-2008. The highest frequencies of the coldest penetrating deep convective clouds consistently occur in the western-central Pacific and Indian Ocean. Monthly frequency anomalies in penetrating deep convection were evaluated against monthly anomalies in lower stratospheric water vapor at 82 mb and show higher correlations for the western-central Pacific regions in comparison to the tropics. At a lag of 3 months, the combined western-central Pacific had a small but significant anticorrelation, where the largest amount of variance explained by the combined western-central Pacific region was 8.25%. In conjunction with anomalies in the 82 mb water vapor mixing ratios, decreasing trends for the 1998-2008 period were also observed for tropics, the western Pacific and Indian Ocean. Although none of these trends were significant at the 95% confidence level, decreases in the frequency of penetrating deep convection over the 1998-2008 shows evidence that could explain in part some of the 82 mb lower stratospheric water vapor variability.