The transition region between the troposphere and the stratosphere in the tropics, the so-called tropical tropopause layer (TTL), has a key role in the climate system. Located above the mean detrainment level of deep convection, the TTL is the seat of a slow wave-driven upward motion where air progressively acquires stratospheric characteristics. In particular, air parcels are strongly dehydrated as they encounter the cold temperatures that prevail in the TTL, and eventually reach their ‘Lagrangian’ cold point temperature that sets the initial water vapor mixing ratio in the stratosphere. Coupling between the tropical troposphere and the stratosphere also occurs through dynamical processes. Namely, waves with a wide range of scales (from short-scale gravity waves to planetary Kelvin and Rossby-gravity waves) are generated by convective clouds in the tropics and propagate upward in the atmosphere. At altitudes between 20 and 30 km, a large fraction of these waves break and drive the quasi-biennial oscillation (QBO) in equatorial zonal winds, which has profound influence on the whole stratosphere. Currently, climate models have still difficulties in reproducing the observed inter-annual variations of stratospheric water vapour, and simulating a realistic QBO is still challenging. This clearly points to deficiencies in our understanding of important processes acting the tropical upper troposphere and lower stratosphere, and consequently to likely defects in parameterizations used in these models. Such shortcomings are notably problematic as the TTL structure is expected to undergo significant changes in relation to the increase of greenhouse gases in the atmosphere.
In this context, French and American groups have proposed the Strateole 2 project that use long-duration stratospheric balloons to provide novel high-resolution in situ measurements in the TTL and lower stratosphere. Such balloons, which have been successfully used at (mostly) polar latitudes before, can carry a set of instruments that address the relevant TTL dynamical, microphysical and chemical processes, as well as their interactions. The balloons, which are advected by the winds, can fly over the whole equatorial belt for multiple months, and will consequently provide observations over both continents/oceans or convection/clear sky.
The presentation will focus on the scientific objectives of Strateole 2 and give details on the instruments that the balloons will host during the campaign. It will also illustrate the studies that such quasi-Lagrangian observations enable. For this, we will use measurements gathered during previous balloon flights, and we will in particular show indications of deficiencies in equatorial winds in state-of-the-art analyses that are commonly used to diagnose transport in the TTL.