Balloon Observations and mOdelling for the Stratosphere - Troposphere Transition in the Tropical Region – BOOST3R

The upper-troposphere and lower-stratosphere (UTLS) region in the Tropics is a particular region of the atmosphere in three respects. First, numerical weather prediction models and in climate models still exhibit significant defficiencies in this region. Second, multiple processes involving dynamics (global circulation, waves, turbulence), microphysics and radiative effects interplay, with relevant scales ranging from microns, the typical size of ice crystals in high-altitude cirrus, to the planetary scale associated with the largest equatorial waves. Third, the stratospheric circulation involves a slow ascent from the troposphere to the stratosphere in the Tropics, so that the tropical UTLS represents the gateway to the stratosphere. Hence, processes occurring in this region determine the composition and humidity of the stratosphere, and gives the tropical UTLS a disproportionnate importance relative to its volume.

The inherent difficulty in modelling this region (multiple scales, multiple processes, absence of a simple balance such as geostrophy) implies that precise observations, with an appropriate and sufficient sampling, are necessary to better constrain our understanding of processes in this
region.

The BOOST3R project builds on a formidable opportunity for research on the tropical UTLS: three campaigns of superpressure, long-duration balloons will be carried out to observe the tropical UTLS between late 2018 and late 2023. These campaigns are supported by the French space agency, CNES, which is commited to launch 45 balloons, drifting at altitudes between 18.5 and 20.5 km. Instruments are developed in France and in the USA to measure humidity, aerosols, and to detect cirrus clouds and fine-scale temperature structures in the couple of kilometers below the balloon flight level, in addition to meteorological variables (temperature, pressure, wind). These campaigns will therefore provide unprecedented measurements in the tropical UTLS, which is otherwise poorly sampled by classical observations. Measured variables will include winds, but also, importantly, water vapor, aerosols and cirrus clouds. The purpose of BOOST3R is to magnify the outcome of these campaigns, by supporting two sets of studies. The first one is operational and consists in preliminary works aimed at obtaining high-quality measurements (instrument development and calibration) and at coordinating with other, complementary measurements. These tasks require expenditure in equipment for the instrumental refinement, and manpower for preparing coordinated measurements. The second one bears on modelling and observational approaches to understand and constrain the multiple processes that occur in the tropical UTLS. Trajectory calculations and climate modelling will be used in complement and in confrontation with analyses of the observations gathered during the first balloon campaign, to occur in the boreal fall and winter of 2018-2019. Mainly manpower is required for this second task, to ensure a synergy between the different teams involved in the measurements and in the modelling approaches.

What is at stake is to determine which processes primarily control the overall state and evolution of the tropical UTLS under the secular increase of greenhouse gases on the one hand, and with the dramatic increase in pollution tied to the rapid economic development of Eastern Asia in recent
years on the other hand. Expected outcomes of the project include new instruments, unprecedented observational datasets to be made publicly available, and improved modelling of this key region of the climate system.