Strateole-2 addresses climate processes acting at the Tropical Tropopause Layer (TTL), the interface between the troposphere and the stratosphere in the tropics, as well as in the equatorial stratosphere. Long-duration balloons are unique vectors to enable gathering in-situ measurements in those remote places of the atmosphere that even high-altitude research aircrafts hardly reach.

Transport through the TTL

The TTL is the gateway to the global stratosphere for tropospheric species, whether of natural or anthropogenic origin. The stratosphere chemical composition is thus, to a large extent, set by complex physical, dynamical and chemical processes acting in the TTL.

The purple box schematically represents the Tropical Tropopause Layer. Air masses penetrates the stratosphere through the tropical tropopause, near 17 km. Strateole-2 balloon will fly near the top of the TTL. After Randel & Jensen (2013).

The TTL is primarily ventilated from below by deep tropical convective systems (cumulonimbus clouds) that detrain near 14-15 km in average, and rapidly transport atmospheric compounds from the surface to upper altitudes. The TTL is also more marginally ventilated sideways through wave-induced exchanges with the mid-latitude stratosphere.

In the TTL, air masses continue to rise up to the stratosphere, but more slowly: it typically takes one month for an air parcel to go through the whole TTL (Tissier and Legras, 2016). During that time, air masses are affected by photochemical reactions, as well as by physical (e.g., ice crystal formation) or dynamical processes (e.g., wave-induced temperature fluctuations).

Strateole-2 will notably document how air masses are dehydrated during their transit in the TTL. The stratospheric water vapor amount indeed exhibits inter-annual variations as large as 30%, which are essentially linked to variations in the tropical tropopause temperature (see figure below).

Water vapor variations
Timeseries of water vapor mixing ratio at different altitudes in the mid-latitude stratosphere (Boulder, Colorado). After Hurst et al., 2011.

Strateole-2 instruments will measure water vapor, as well as other atmospheric tracers, and perform high-resolution observations of temperature perturbations associated with convectively-generated gravity waves. Those temperature perturbations strongly influence the nucleation of ice crystals, which may sediment downward and thus remove water vapor from air masses. Such nucleation events manifest themselves as thin (often subvisble) cirrus clouds in the TTL, which will also be observed by Strateole-2 instruments.

Dynamics of the equatorial stratosphere

The dynamics of the equatorial lower stratosphere is dominated by the Quasi-Biennial Oscillation (QBO), an oscillation of the zonal-mean zonal wind with a mean period of 27 months.

The Quasi-Biennial Oscillation as seen in ECMWF analyzed winds since 2006 (blue: westward winds, green/red: eastward winds). Strateole-2 balloons fly between 50 and 70 hPa. Notice the QBO disruption in 2016. Credits: A. Hertzog (LMD).

This oscillation results from the forcing of the zonal-mean stratospheric circulation by a wide spectrum of waves generated by tropical convection. The QBO has a strong influence on the dynamics of the global stratosphere, and e.g. modulates the severity of the springtime ozone hole each year. The evolution of the QBO in a changing climate is a real challenge for current climate models. Yet, the recent and unusual disruption of the eastward phase in 2016 suggests that the QBO will inevitably be affected by climate change.

Strateole-2 will provide high-resolution observations of the atmospheric wave spectrum over the whole tropical belt, and relate wave activity to underlying convection. These observations will be used to precisely document the contribution of various types of waves to the driving of the QBO, and ultimately to improve some of the parameterizations used in climate models.

Improving meteorological forecasts

Strateole-2 meteorological measurements will be disseminated in near-real time through a collaboration with Meteo-France, so as to be assimilated by numerical weather prediction systems worldwide. This is achieved thanks to the hourly Iridium communications between the ground control center and the scientific gondolas, wherever their location. Strateole-2 balloon-borne observations will notably fill in a gap of wind measurements over the tropical oceans. Impact assessment experiments will be carried out to determine how the weather forecasts have evolved due to the inclusion of Strateole-2 observations.

Validating Aeolus winds

The ESA Aeolus mission, which has been launched in August 2018, has also been designed to increase the number of wind observations in the Earth atmosphere. Aeolus is the first ever wind lidar in space. It provides continuous vertically-resolved wind measurements from the surface to the lower stratosphere.

Since Strateole-2 balloons are advected by stratospheric winds at their float altitude, monitoring their positions (with a GPS receiver) provides highly accurate, in-situ wind measurements that will be used to validate Aeolus observations. This validation is particularly important in the tropics, since it is the place where Aeolus’ impact on weather forecasts is expected to be the largest.