EarthCARE's lidar shows particles in the atmosphere in detail

Atmospheric lidar completes the new climate satellite

Equipped with four state-of-the-art instruments -- a cloud profiling radar, an atmospheric lidar, a broadband radiometer and a multispectral imager -- EarthCARE (Earth Cloud Aerosol and Radiation Explorer) will carry out a number of different measurements simultaneously. Together, these measurements will help to better understand how clouds and aerosols reflect incoming solar energy back into space and how they trap thermal radiation emitted by the Earth. This information is important for understanding how climate change affects the Earth's energy balance and for predicting how quickly clouds and aerosols might lose their current cooling effect in the future.

EarthCARE was launched into orbit around the Earth on 29 May 2024. Just one month later, the satellite delivered the first images from its cloud profiling radar, followed shortly afterwards by the first images from its broadband radiometer, the multispectral imager and, finally, the atmospheric lidar in August. This state-of-the-art instrument records detailed vertical profiles of aerosols and clouds in the atmosphere over different regions of the Earth. Aerosols are tiny particles and droplets from natural sources such as dust and sea salt, and from human activities such as industrial emissions or wood burning. The laser emits short pulses of ultraviolet light, which are reflected by atmospheric targets and analysed in a highly sensitive receiver. The distance can be determined from the transit time, the concentration from the signal strength and the type of aerosol from the polarisation. This makes it possible to measure the distribution and properties of aerosols and clouds, including their height, thickness, optical and physical properties. Synergy with the other three instruments on the satellite is crucial to understanding the role of aerosols and clouds in the Earth's energy balance. A new aerosol classification model ("Hybrid End-to-End Aerosol Classification," or HETEAC for short) has been specially developed as the basis for aerosol typing to ensure that these calculations work across the different instruments. The ATLID atmospheric lidar, in particular, will also make an important contribution to improving air quality forecasts. Ulla Wandinger, who has been involved in the development of ATLID for many years, is delighted with the first measurements: "The wealth of data and the detailed insight into the structures of the atmosphere are absolutely impressive." EarthCARE has the potential to significantly advance the understanding of aerosols, clouds and their interactions, as well as climate research.

The first images from August show the diversity of aerosols and clouds in the Earth's atmosphere: for example, a profile of Polar Stratospheric Clouds (PSC) over Antarctica, which play an important role in the formation of ozone holes, or Tropical Storm Debby over the Gulf of Mexico, and plumes of smoke from forest fires in Canada. Simonetta Cheli, ESA's Director of Earth Observation Programmes, said: "Following on from the first images from EarthCARE's other three instruments, we can now also see how well the atmospheric lidar is working. These profiles from the atmospheric lidar come exactly as we had anticipated, after the instrument has been through its routine decontamination and calibration processes. The atmospheric lidar brings us completely new insight into the vertical distribution of clouds and aerosols and, together with the other instruments, puts us on course to gaining new scientific understanding into Earth's energy balance."

Extensive measurement campaigns in the Atlantic and in Europe

To ensure that the data from the new instruments can be used and interpreted in the best possible way, it is important to compare them with ground-based and airborne measurements in a wide range of situations. For this reason, a series of complex international measurement campaigns are currently underway.

Until November, the German research aircraft HALO will fly under the EarthCARE track several times from Cabo Verde in the Atlantic, from Barbados in the Caribbean and from Oberpfaffenhofen in Germany until November. The HALO-PERCUSION validation mission is led by the German Aerospace Center (DLR) together with the Max Planck Institute for Meteorology (MPI-M). A number of partners, including the University of Leipzig, are also involved. PERCUSION is one of several subprojects of the ORCESTRA (Organised Convection and EarthCare Studies over the Tropical Atlantic) research project coordinated by the MPI-M. Another sub-project is CLARINET (CLoud and Aerosol Remote sensing for EarThcare), in which TROPOS researchers use the new ACTRIS remote sensing station of the Cabo Verde Atmospheric Observatory (CVAO) at the Ocean Science Centre in Mindelo (OSCM) to validate EarthCARE data in the tropical Atlantic and compare them with long-term measurements.

The ground stations of the European research infrastructure ACTRIS play an important role in calibrating the data from the EarthCARE satellite: they have been established and expanded in recent years to analyse aerosol particles and clouds using remote sensing instruments such as lidar and radar. Around 50 stations in Europe and overseas are involved in the atmo4ACTRIS measurement campaign. This dense network offers the great advantage that EarthCARE flies over at least one of the stations practically every day, as the low-Earth orbit ensures that the satellite "flies" over our planet in strips, only returning to the same part of the Earth every 25 days. A single ground station is therefore not sufficient for calibration.

"As part of the ATMO-ACCESS infrastructure project, we rehearsed the measurement campaign for two months at the end of last year with simulated overflights to prepare ourselves for the complex task. This was very helpful, because although the ACTRIS stations all work according to the same standards, some of them have very different backgrounds when it comes to validating satellite data. We are all looking forward to comparing the first EarthCARE data with the ground-based measurements," says Dr Holger Baars of TROPOS, who is coordinating the campaign from Leipzig. In addition to the TROPOS stations in Leipzig and Melpitz, stations of the German Weather Service (DWD) in Hohenpeißenberg and Lindenberg, the University of Cologne (UoC) in cooperation with the Jülich Research Centre (FZJ), the Karlsruhe Institute of Technology (KIT) and the University of Leipzig will participate. German partners are also providing important data from overseas: the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) is contributing observations from Ny-Ålesund in the Arctic in cooperation with the UoC, and TROPOS is providing data from three stations in the Earth's dust belt: Cabo Verde in the Atlantic, Limassol in Cyprus and Dushanbe in Tajikistan.