Photo: Cumulonimbus over Africa
This combination of instruments will make it possible to gather information about the vertical transport of components near the tropical tropopause (Tropical Tropopause Layer), at around 18 km of altitude.
The TTL is a key region of the Earth's climate system because it is the gateway to the stratosphere for chemical species in the lower layers of the atmosphere. Among these species, the STRATEOLE-2 mission will pay particular attention to water vapour, the most important greenhouse gas in the Earth's atmosphere.
In the tropical region, powerful convective systems help to rapidly transport water vapour from the Earth's surface to the base of the TTL, located at 14 km altitude. The cooling that occurs during this ascent results in significant condensation of water vapour, associated with the formation of clouds with strong vertical extension (cumulonimbus) and intense precipitation.
But the mechanisms allowing water vapour to travel the few kilometres left to reach the stratosphere are the subject of scientific controversy.
There are roughly two opposing theories:
- The first states that a small fraction of the convective systems is able to penetrate directly into the stratosphere and is the main source of stratospheric water vapour.
- The second, on the other hand, proposes that the last kilometres of ascent are associated with slower movements and the formation of sub-visible thin clouds (cirrus), indicating a gradual drying up of the air masses.
Even the most recent observations of cloud systems, such as those made by the Franco-American space mission CALIPSO, have not definitively resolved this controversy. Indeed, these observations only partially describe the tropical convective systems, which are characterised by a very marked diurnal cycle as well as by fluctuations at spatial scales of the order of the kilometre.
In this context, the STRATEOLE-2 campaign will bring new observations, as they are collected in-situ during the flight of pressurised balloons.
The TTL1 configuration instruments will typically perform measurements every 15 min during the three-month flight, sampling the spatio-temporal scales of the convective systems.
The combination of tropospheric and stratospheric ozone and vapour measurements will be used to characterise the nature of the air masses sampled. The LOAC particle counter will record the frequency of occurrence of clouds entering the lower stratosphere.
Information: Albert Hertzog
Philippe Cocquerez, Project manager STRATEOLE-2, philippe.cocquerez at cnes.fr
Albert Hertzog, Principal Investigator STRATEOLE-2 (LMD) et PI SAWfPHY - hertzog at lmd.polytechnique.fr