ALBERT

Description: The Southern Ocean (SO) exhibits some of the most pristine air on Earth, particularly during the winter season, when the lowest concentrations of cloud condensation nuclei (CCN) are observed. Historically, research has focused on the biogenic production of dimethyl sulfide as the primary explanation for the observed seasonal cycle in CCN and in support of the ‘CLAW’ hypothesis. More recent research, however, suggests that this hypothesis is incomplete and there is a need to better understand alternate sources (e.g., sea spray) and sinks (e.g., coalescence scavenging) to fully constrain the CCN budget. We examine the potential impact of the structure of marine boundary layer clouds on the CCN concentration through precipitation and wet deposition. Marine boundary layer clouds dominate the lower latitudes of the Southern Ocean, specifically the various states (open, closed, disorganised) of mesoscale cellular convection (MCC). Using a cloud climatology based on Himawari-8 observations, the relationship between CCN concentrations and precipitation from Kennaook/Cape Grim Baseline Air Pollution Station was examined. A lower median CCN concentration (68.9 cm^-3) was observed when open MCC was dominant upwind of the site under ‘baseline’ conditions, as compared to when closed MCC (88.6 cm^-3) was dominant. This difference is statistically significant. It was observed that open MCC precipitated more heavily (1.72 mm/day) than closed MCC (0.29 mm/day), establishing a negative relationship between CCN concentration and precipitation. The most pristine air is observed when open MCC is directly upwind of the Kennaook/Cape Grim station. This negative relationship was observed at both diurnal and seasonal time scale.

Description: Looking at the past sharpens our understanding of possible future climate changes. We focus on Earth system modeling, paleoclimate data analysis, and theoretical aspects. Predicting the future spread of possible climates, the risk of climate extremes and the risk of rapid transitions is of high relevance. The past provides evidence of abrupt climate change and the frequency of extremes. Earth system models applied both to past and future scenarios enhance our ability to detect regime shifts in climates and extremes. We consider the response of the system to long-term forcing and then focus on shorter time scales down to weather. Particular aspects are: Model resolution matters for climate change and extremes; recorder systems such as O, H, C-isotopes enable a suitable interpretation of the past; data assimilation can yield a dynamically consistent picture. New high-resolution models can quantify the feedbacks in the atmosphere-ocean-ice system and inform us about the full range of climate variability and extremes.With our holistic approach, we seek to overcome known biases of deep-time polar amplification, the stochastic nature of centennial-to-millennial climate variability, as well as extremes. Here, we put emphasis on the concept of a hierarchy of models as this provides a linkage between theoretical understanding and the complexity of the system. Lohmann, Butzin, Eissner, Shi, Stepanek, 2020: Abrupt climate and weather changes across timescales. Paleoc. Paleoclim., doi:10.1029/2019PA003782 Lohmann, 2020: Temperatures from energy balance models: the effective heat capacity matters. ESD, doi:10.5194/esd-11-1195-2020 Contzen, Dickhaus, Lohmann, 2023: Long-term temporal evolution of temperature extreme in a warming Earth. PLOS, doi:10.1371/journal.pone.0280503

Description: Antarctic precipitation remains poorly characterised and understood, especially within the boundary layer. This is due in part to a still-limited amount of surface-based remote sensing observations. A suite of cloud and precipitation remote-sensing instruments including a W-band cloud radar and a K-band Micro Rain Radar (MRR) were used to characterise snowfall over Davis (69S, 78E). Surface snowfall events occurred when boundary layer wind speeds were weaker, temperatures were warmer, and relative humidity over ice higher, than when virga were present. The presence of virga is associated with Fohn conditions due to the location of Davis in the lee of an ice ridgeline. Dual wavelength ratio values from the summer indicate particle aggregation at temperatures of -14C to -10C, consistent with observations made elsewhere, including in the Arctic. In-cloud updrafts were stronger in summer than in winter at these temperatures. Larger downward velocities and the presence of super-cooled liquid layers suggest some rimed particles at warm temperatures above -10C during summer. Sublimation of snowfall mass aloft was 50\% between the accumulation peak at 1.2~km and 205~m altitude, which occurs within CloudSat's `blind zone'. An estimated lower bound of blowing snow fraction is 30%. Given the common prevailing winds and numerous ice ridgelines along much of the East Antarctic coastline, these Davis results can be used as a basis to further understand snowfall across the region.