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

The project was based on a number of complementary approaches, but with a strong instrumental component or focus on the physics of measurement. Innovative instrumental developments were carried out thanks to the project at the GSMA (Groupe de Spectromètrie Moléculaire et Atmosphérique, Université de Reims Champagne Ardennes) to design and produce a methane measurement instrument adapted to the flight constraints of stratospheric balloons (work coordinated by Mélanie Ghysels). A detailed understanding of the physics of balloon flight has enabled measurements of meteorological variables to be used to deduce information about the turbulence present in the lower stratosphere (work coordinated by Richard Wilson at LATMOS, the Atmosphere, Media and Space Observations Laboratory). A detailed understanding of the physics of measurement was also essential to validate and interpret the measurements of new instruments, in particular the BeCOOL lidar (work co-ordinated by François Ravetta at LATMOS, and involving in particular Thomas Lesigne).

One of the strengths of the project has been the close collaboration between observational experts and modellers: several modelling approaches have been involved, from idealised models to explore the physics of processes (work at the Dynamic Meteorology Laboratory, as part of Miléna Corcos' thesis, supervised by Albert Hertzog and Riwal Plougonven), to global models. The global models have included weather forecasting models (work carried out at Météo-France, coordinated by Alexis Doerenbecher) and climate models (work on stratospheric aerosols, coordinated by Gwenael Berthet at the LPC2E, Laboratoire de Physique et Chimie de l'Environnement et de l'Espace).

BOOST3R has enabled progress to be made in the observation of active processes in the lower tropical stratosphere, with the analysis of original and previously unpublished observations. The BeCOOL Lidar (developed at LATMOS, Paris and Guyancourt) measured the clouds below the balloon on several flights. The proximity of the balloons to the fine ice clouds that form just above the tropopause means that the instrument can detect very fine clouds, finer than those detected by lidar from space. These clouds play a radiative role and affect the composition of the air entering the stratosphere: sedimenting ice crystals dry out the air and reduce its water vapour content.

BOOST3R has successfully supported instrumental developments, enabling the rapid deployment of a measurement instrument for water vapour and carbon dioxide (Piso-Strat Bi-Gaz, developed at the GSMA in Reims). The measurement of methane, with the constraints imposed by long-duration flights in stratospheric balloons, required innovative developments. A one-piece cell was designed and manufactured to extend the optical path of the laser beam without the risk of misalignment due to thermal variations. This development paves the way for unprecedented measurements of methane variations in the lower stratosphere in future campaigns.

Dynamic measurements from the first two Strateole 2 campaigns (in 2019-2020 and 2021-2022) have been analysed as part of BOOST3R to quantify equatorial waves, particularly gravity waves, and to detect turbulence. The theses of Miléna Corcos at the LMD and Clara Pitois at LATMOS have led to major advances. The characteristics measured for gravity waves and the relationship highlighted with the underlying deep convection serve as a reference for work on the parametrisation of gravity waves (activities around François Lott at the LMD in particular). The quasi-Lagrangian nature of the observations also makes them a valuable dataset for analysing the impact of temperature fluctuations on microphysics - an idealised model has been developed to explore these non-linear interactions theoretically as part of Miléna Corcos' thesis.

Stratospheric balloon campaign operations and weather forecasting depend on the quality of models for winds in the lower stratosphere. BOOST3R has made it possible to assess the quality of winds in meteorological models in the tropical lower stratosphere, to quantify errors in balloon trajectory forecasts and to explore the impact of wind observations in terms of data assimilation and forecast improvement. The wind measurements also contributed to the validation of wind measurements made from the innovative ADM-Aeolus instrument, which provided lidar wind measurements from space from 2018 to 2023.

BOOST3R has advanced the development of instruments and the analysis of observations from balloon campaigns in the lower tropical stratosphere, which were carried out as part of the Strateole 2 project. The results obtained under the project have contributed to the emergence of new projects based on what was initiated during BOOST3R:

- the ANR JCJC TURTLES project (Turbulent and Radiatively-driven Transport in the tropical tropopause Layer and lower Equatorial Stratosphere, ANR-21-CE01-0016) coordinated by Aurélien Podglajen (LMD) explores the occurrences of turbulence in the lower equatorial stratosphere, relying in particular on the detection of turbulence from balloon measurements developed in BOOST3R (Wilson et al, 2023).

- The results obtained as part of BOOST3R on equatorial waves, and in particular on internal gravity waves, formed the starting point for analyses combining observations and ‘machine learning’ in order to identify the relationship between large-scale flow and submesh processes in models. Funding from the Institut des Mathématiques pour la Planète Terre (IMPT) has been obtained for a post-doctorate (Sothea Has, co-supervised by Aurélie Fischer, from the Laboratoire Probabilités Statistiques et Modélisation, and Riwal Plougonven, LMD, 2022-2024; Sothea Has has obtained a position at the University of Phnom Penh, Cambodia, at the end of 2024). The SEEDLING project (SEEking Data-driven Levers to Improve and re-New Gravity wave parameterizations, ANR as part of the PEPER Maths-Vives programme), which is rooted in this collaboration but clearly broadens the scope of the approaches involved and the scientific ambitions, was prepared in autumn 2024 and obtained funding for 5 years (coordinated by Aurélie Fischer and Riwal Plougonven).

- The success of the stratospheric balloon campaigns, the relevance of the measurements made and the information obtained on trajectory forecasts have all been arguments for pursuing the development of these platforms. In addition to the third Strateole 2 campaign, scheduled for 2026, the CNES (Centre National d'Etudes Spatiales) selected long-duration balloon campaigns in the lower stratosphere as one of its priority projects during its Scientific Prospective Seminar (SPS 2024). The Stratofleet project, with the development of manoeuvrable balloons, will enable long-term monitoring of the lower stratosphere.

- The instrumental developments for measuring water vapour in the lower stratosphere and the results obtained on its variability were all motivations and arguments for preparing the DEEP-CONVECT project, coordinated by Mélanie Ghysels (Groupe de Spectroscopie Moléculaire et Atmosphérique, Reims).

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.