Above the clouds: HALO measurement campaign successfully completed
Will the Cape Verde Islands become wetter or drier in the future? Will it rain more or less in the Sahel? The answers to questions like these depend heavily on how the tropical rain belt might change in the future. Known to experts as the Intertropical Convergence Zone (ITCZ), this band of clouds stretches around the Earth near the equator – but it has a surprisingly complex and variable inner life. The PERCUSION measurement campaign led by the Max Planck Institute for Meteorology (MPI-M) and the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt, DLR) investigated the diverse processes in the Atlantic ITCZ with a focus on the mesoscale – in other words, it considered phenomena bigger than individual clouds, but smaller than large-scale weather patterns. The campaign was part of the more extensive ORCESTRA expedition and has now concluded with a final workshop in Hamburg.
PERCUSION relied on coordinated measurements from two platforms: the German research aircraft HALO and the European Space Agency's (ESA) EarthCARE satellite, which was launched into space in May 2024. HALO's measurement flights during August and September 2024 served several purposes. Firstly, the researchers determined vertical profiles of wind speed, humidity, and temperature in the center and at the edges of the Atlantic ITCZ using meteorological drop sondes that were released from the aircraft. Additionally, the instruments installed on the aircraft continuously recorded relevant variables such as vertical profiles of clouds and radiation. The combination of these data provides a picture of air mass movement and cloud formation in the ITCZ. This approach had already proven successful in 2020 during the EUREC4A measurement campaign led by the MPI-M. Compared to EUREC4A, which focused on shallow trade wind clouds, the measurements this time were much more extensive and aimed mostly at rain clouds in the deep tropics, which are not yet well represented in climate models.
The aircraft provides detail, the satellite long-term observations
The second aim of PERCUSION was to validate the EarthCARE satellite. To this end, HALO was equipped with instruments similar to those on the satellite – including a Lidar and a Radar – and flew underneath it on 33 occasions. "The coordination went very smoothly," summarizes MPI-M group leader Julia Windmiller, who was responsible for the campaign together with Bjorn Stevens, head of the "Climate Physics" department at MPI-M, and Silke Groß from DLR. "The comparison with our measurements makes it easier to interpret the satellite data and use it for statistical observations of the ITCZ." This means that the researchers not only have a detailed snapshot of the ITCZ, but can continue to observe its variability thanks to EarthCARE.
The flights took place at different locations to allow the aircraft and satellite to jointly look at different situations: First starting from Cape Verde in the eastern Atlantic, where the air layers are partly interspersed with Saharan dust, then in the clean-air regime in the western Atlantic off Barbados. In addition, HALO covered tracks over the mid-latitudes in November from its home airport in Oberpfaffenhofen. By this third part of the PERCUSION tour, the other seven sub-campaigns of ORCESTRA had already been completed.
In total, almost 300 terabytes of raw measurement data were collected in around 300 flight hours. This data now needs to be analyzed and interpreted. On top of that, the campaign was a success also beyond data acquisition, as Bjorn Stevens points out: "Many young scientists from different countries were involved in the campaign. They fulfilled their responsible tasks with great commitment," says the MPI-M Director. "I think for many of them, these experiences didn’t just benefit them scientifically, but also personally."
Further information
Contact
Dr. Julia Windmiller
Max Planck Institute for Meteorology
julia.windmiller@mpimet.mpg.de
Prof. Bjorn Stevens
Max Planck Institute for Meteorology
bjorn.stevens@mpimet.mpg.de