The goSouth-2 campaign in Invercargill, New Zealand:First insights into heterogeneous ice formation and the effects of Australian aerosols on clouds at the edge of the Southern Ocean

The region of the Southern Ocean (SO) is a current hot-spot of research related to aerosol-cloud interactions. On the one hand, detailed observations of aerosols and clouds and their interactions are required to explain the shortwave radiation bias in model simulations. On the other hand, the clean environment of the SO atmosphere allows the study of the impact of even slight variabilities in the aerosol load on cloud and precipitation processes and on the radiation budget of the atmosphere.

The goSouth-2 project led by Leibniz Institute for Tropospheric Research (TROPOS) contributes with a unique long-term dataset of remote-sensing and in-situ measurements of cloud-relevant aerosol properties and associated cloud properties for the northern edge of the SO. The core facility of goSouth-2 is the ACTRIS station LACROS (Leipzig Aerosol and Cloud Remote Observations System) which has been deployed on the premises of the New Zealand MetService in Invercargill (46.4173 °S, 168.3307 °E, 3 m a.s.l., https://cloudnet.fmi.fi/site/invercargill) at the southern tip of the New Zealand’s South Island. The characterization of aerosol optical properties is observed by a newly-built  PollyXT lidar. This system yields backscatter coefficient and depolarization ratios at 355, 532, and 1064 nm. Extinction coefficients are derived from Raman scattering at 387 and 607 nm. Raman scattering by water vapour at 407 nm yields observations of the water vapour mixing ratio. In addition, it is the first PollyXT lidar which includes a 460-nm fluorescence channel enabling a more refined discrimination between smoke, biological aerosols, and other types of pollution. Cloud properties are covered by 94- and 35-GHz cloud radar observations, of which the latter provides RHI and PPI scans for characterization of hydrometeor shapes and the horizontal wind field.  Surface in-situ observations of the aerosol size distribution, cloud condensation nuclei concentrations, and off-line characterization of ice nucleating particle (INP) concentrations are performed. goSouth-2 is involved in the project ACADIA jointly run by Leipzig University and TROPOS, the HALO-South aircraft campaign, ongoing EarthCARE Cal/Val activities, and is conducted in close collaboration with partners from University of Canterbury, Earth Sciences New Zealand, and MetService, NZ. The latter contributes 2 radiosonde launches per day and weather radar observations.

First conclusions drawn from the dataset to date are that aerosol in the cloud-free troposphere is rare. If present, it can mostly be assigned to wildfires or dust from Africa or Australia. In SO air masses ice formation in clouds warmer than -4°C is frequently absent, confirming the lack of efficient INPs. The majority of stratiform precipitation systems is found to be embedded in Australian air masses. A remarkable feature is that enhanced loads of Australian aerosols (dust, smoke) are frequently associated with enhanced turbulence in the affected cloud systems, similar to the dusty-cirrus phenomenon. Observations of the lowest 3 km of marine atmosphere, show a complex aerosol structure containing multiple embedded sub-layers of differing aerosol properties, whose effects on cloud formation remain to be identified. We use co-located observations from goSouth-2, the HALO-South aircraft campaign, and EarthCARE to characterize the observed scenarios.