Strateole-2 is a French-US project supported by ESA that uses CNES superpressure balloons drifting around the globe to study the climate processes in the Tropical Tropopause Layer (TTL) and the lower stratosphere, helping Aeolus validation activities along the way.

It turns out that Jules Verne was not far off the mark when he wrote his iconic novel ‘Around the World in 80 days’: on 23 February 2020, one of Strateole-2’s eight superpressure balloons completed its circumnavigation of the globe just an hour and seventeen minutes shy of 80 days. Five days later, the last three balloons of the first leg of the Strateole-2 mission completed their journeys, successfully concluding the campaign.

Inflation of a CNES superpressure balloon, Copyrights: Ph. Cocquerez, CNES

Strateole-2 will release a total of nearly 50 long-duration balloons in three separate campaigns between 2019 and 2024. Balloons are released from Mahé International Airport, Seychelles, with the local support of the Seychelles Meteorological Authority.

The first technological test campaign, which started in November 2019 and ended in February 2020, was comprised of eight CNES superpressure balloons. These spherical balloons, which have been used in several projects by CNES before, are 11 and 13-metre in diameter and made of plastic. They are closed, filled with helium, carry up to 50 kg of equipment and fly at an altitude between 18 and 20 km, sometimes higher, for up to three months.

“The campaign aimed to test the new technological developments of the past few years and set up the system that will be used during the scientific campaign at the end of 2021,” explains Albert Hertzog, Strateole-2 Principal Investigator.

These developments come in the form of two gondolas each balloon carries. One of them, developed by CNES, is for safety and provides services like telecommunications and localisation information, but can also perform actions such as releasing gas or ballast from the system to lighten the balloon and allow it to fly a little higher if needed. However, the balloons have no piloting system, their trajectory is entirely dependent on the winds.

The second gondola is for scientific operations and contains most of the instruments used for measurements and data collection during the flights. It was primarily designed by a consortium of French laboratories, and carries French and American instruments specifically designed for long-duration balloon flights.

The scientific aims of Strateole-2

Strateole-2 has four scientific aims. The primary objective is the study of climate processes acting at the TTL, which is a gateway between the troposphere and the stratosphere at the equator. Strateole-2’s instruments will measure water vapour, as well as other atmospheric tracers, and perform high-resolution observations of temperature variations associated with convectively-generated gravity waves.

The goal is to better understand how water vapour is injected into the stratosphere and the mechanisms responsible for dehydrating air parcels as they rise into the troposphere.

The life cycle of thin cirrus clouds at the top of the troposphere and the interaction of gravity waves with them will also be observed by the Strateole-2 instruments.

The second aim is to study the dynamics of the equatorial lower stratosphere, which is dominated by the Quasi-Biennial Oscillation (QBO), an oscillation of the east-west wind component present at an altitude of 18 to 30 km, the same height at which the Strateole-2 balloons fly. The biyearly change of the wind from east to west is mainly driven by the convectively-generated gravity and planetary waves.

Strateole-2 will help scientists better understand the dynamics of the tropical atmosphere through its balloons’ high-resolution measurements of meteorological variables, during their flight above both land and ocean.

The third objective is to improve meteorological forecasts by sharing Strateole-2 meteorological measurements in near-real-time with meteorological services so they can be assimilated by numerical weather prediction systems worldwide. Strateole-2 balloon observations will therefore help fill in a gap in wind measurements over tropical oceans.

The final objective is the validation of Aeolus observations using Strateole-2’s highly accurate, in-situ wind measurements. This validation is particularly important in the tropics, since it is the place where Aeolus’s impact on weather forecasts is expected to be the largest.

Strateole-2 and Aeolus Cal/Val activities

What was a technological campaign for CNES and Strateole-2 partners, was a validation campaign for ESA and its wind mission, Earth Explorer Aeolus. Launched on 22 August 2018, Aeolus is the first satellite mission to acquire profiles of Earth’s wind on a global scale. Its near-real-time observations have already improved the accuracy of numerical weather prediction models and are helping advance our understanding of tropical dynamics and processes relevant to climate variability.

“For the Aeolus calibration/validation aspects of the campaign the main instrument we use is the Temperature SENsor (TSEN),” Hertzog states. “It provides basic meteorological measurements like wind pressure, temperature and so on. The instrument was used during previous campaigns so we were confident it would work whatever the new developments we had for the technical campaign.”

“This data is unique in a sense that we don’t normally get these kinds of in-situ wind observations in the lower stratosphere,” Thorsten Fehr, Head of the Atmospheric Section at ESA, explains.  “Our initial estimate was that we would get 200 to 300 collocations between the balloons and Aeolus. Our final number, using quite restrictive collocation criteria between the balloons and Aeolus, yielded 231 collocations, we therefore consider the campaign a complete success.”

Collocations between Aeolus and Strateole-2 balloons, Copyrights: ESA/LMD/CNES

Once the consolidated wind data from the Strateole-2 campaign will be available, the Aeolus team will be working hard, comparing the balloons’ measurements to those of ESA’s wind mission to validate the data it collected.

The challenges of flying a balloon around the world

Strateole-2 is a complex campaign and comes with a set of unique challenges. Among them, gargantuan diplomatic feats, wayward balloons and technical mishaps.

The first of these is related to the balloons’ trajectory. Since the balloons fly in the atmosphere, the Strateole-2 team had to request flight authorisations from all the countries the balloons could potentially fly over during the campaign. This meant a lot of work in advance, before the campaign, on the part of CNES and the French Foreign Affairs Ministry.

“Our target was to fly between 20 degrees south and 15 degrees north, so we had to request flight authorisations from almost 100 countries,” Hertzog reveals.

But even those did not prove enough: when the trajectory forecast indicated that two of the balloons would fly above countries for which the campaign did not have overflight authorisation, the team made the call to terminate the flights before they entered their territories.

The second challenge was related to the tropical Quasi-Biennial Oscillation that Strateole-2 aims to study. The balloons fly 18 to approximately 20 km in altitude, which means they are subject to eastward and westward oscillating winds.

At the start of the campaign in November 2019, the winds were eastward, so the balloons flew from the Seychelles over the Indian Ocean to Indonesia and then crossed the Pacific, but they were also influenced by the seasonal cycle of the wind circulation in the stratosphere. Two of the balloons drifted southward and were then embedded in the summertime stratospheric circulation, which is westward, so they wound up flying in the opposite directions than the balloons that were closer to the equator.

After two months of flight, two balloons also suffered a failure: the flights ended over the Pacific Ocean without an explicit end-of-flight command. To investigate what happened, at the end of the campaign, the team purposefully released mass on the three remaining balloons to assess the maximum superpressure that the balloon envelope is able to sustain. The test revealed that the superpressure at which a balloon burst was higher than the one being nominally used for the flights, meaning that the failure of the two balloons was specific to them and not present in the others.

“We were satisfied with the results of the campaign as most of the new developments performed very well, exceeding expectations,” Hertzog says. “With the balloons for instance, we aimed for 90-day flights and we achieved that in average for the eight balloons. There were only a few minor issues, but that’s what the campaign was designed for, to test the system and instruments before the scientific campaigns and improve them where needed.”

Strateole-2’s first scientific campaign is set to kick off in October 2021.