Cloud microphysical properties over the Southern Ocean: First results from the HALO-South airborne campaign

Extensive cloud cover over the Southern Ocean is a key contributor to global cloud radiative forcing. The region hosts some of the most pristine clouds on Earth, as its air masses originate largely over the open ocean and Antarctica with minimal influence from continental emissions. This provides an opportunity to investigate aerosol–cloud interactions under near-preindustrial aerosol conditions. However, the scarcity of in-situ measurements in this region leads to a misrepresentation of the Southern Ocean cloud properties in climate models, resulting in biases of simulated shortwave radiation and near-surface temperatures. To address these points, the HALO-South airborne campaign was conducted in September and October 2025, based out of Christchurch, New Zealand. Using DLR’s High Altitude and Long Range Research Aircraft (HALO), 20 research flights were carried out over the Southern Ocean in the vicinity of New Zealand, extending into the Antarctic marginal sea ice zone. The campaign targeted a broad suite of cloud regimes, from boundary-layer clouds to multilayer mixed-phase systems and high-level cirrus clouds. In addition, the flights sampled clouds embedded in a variety of synoptic weather systems, including cold-air outbreaks and convective systems. Flights were planned in synergy with satellite overpasses and with support from weather prediction models to ensure coverage of representative conditions. Here, we present a statistical overview of over 20 hours of cloud dataset collected by underwing probes during the HALO-South campaign, including cloud microphysical properties such as particle number concentration, liquid and ice water content, and particle size distributions. This dataset enables a deeper understanding of aerosol–cloud interactions, mixed-phase processes, and cloud radiative effects in the Southern Ocean. It provides critical observational constraints for evaluating satellite retrievals, assessing weather and climate model performance, and informing model development aimed at reducing long-standing regional radiation biases. Comparisons with Northern Hemisphere field campaigns further highlight hemispheric contrasts in cloud–aerosol coupling, offering new opportunities to investigate how differing aerosol environments shape cloud properties and climate feedbacks.