Seminar

Abstract

The tropical tropopause is the gateway to the stratosphere for all tropospheric species. The stratospheric water vapor content, of prominent importance for the global climate and the stratospheric ozone chemistry, is for instance set as air masses ascent and dehydrate in the Tropical Tropopause Layer (TTL). Observed decadal variations of stratospheric water have actually been shown to modulate the long-term increase of global temperatures. The effective transport of water vapor into the stratosphere depends on the interplay of complex microphysical and dynamical processes. Wave-induced temperature fluctuations are notably important to create favorable conditions for the nucleation of ice crystals.

In the tropics, waves are primarily generated by deep convective systems, with spatial scales ranging from individual convective cores to the planetary scale. They drive the lower-stratosphere quasi-biennial oscillation (QBO) in zonal-mean zonal winds, which has impacts on the global stratospheric circulation. Despite its importance, the QBO evolution with climate change is still uncertain, mainly because climate models rely on parameterizations to represent unresolved convection and short-scale gravity waves.

In this context, Strateole-2 scientific objectives are aimed at providing in-situ, high resolution observations of the TTL dynamics and composition in order to better constrained those parameterizations. The project uniqueness comes from the use of long-duration stratospheric balloons. These balloons can drift for several months in the lower stratosphere in a quasi-Lagrangian manner, and thus provide observations over the whole tropical band. The Strateole-2 balloons are launched from Seychelles Islands (5°g;S) in three different campaigns in 2019-2020, 2021-2022, 2024-2025. More than ten instruments have been developed to perform measurements on these balloons. Observations of dynamics, greenhouse gases, aerosols and clouds are thus gathered along the balloon flights, both at the balloon flight level and remotely. Balloon-borne meteorological observations are furthermore transmitted to weather prediction centers in real-time to improve forecasts during the campaign.

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Albert Hertzog is assistant professor at Sorbonne Université, Paris, France. He has been doing his research at Laboratoire de Météorologie Dynamique since 1999, when he got involved in the Vorcore project that used long-duration balloon flights to better understand the dynamics of the stratospheric polar vortex above Antarctica. He has continued his research on scientific ballooning, and is now PI of the French-American Strateole-2 project dedicated to the tropical lower stratosphere. He is particularly interested in better characterizing the impacts of small-scale gravity waves on the general circulation of the stratosphere and on cirrus microphysics.