Marine Boundary Layer Cloud Condensation Nuclei Bias Over the Southern Ocean: Comparisons Between the Community Atmosphere Model 6 and Field Observations

[Geoengineering News]

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2024JD042734

Authors

Qing Niu, Christina McCluskey, Greg M. McFarquhar

First published: 19 April 2025

https://doi.org/10.1029/2024JD042734

Abstract

Marine boundary layer (MBL) clouds play a crucial role in regulating radiative balance in the atmosphere. Previous studies identified that MBL cloud droplet number concentration (Nd) is underestimated by a factor of 2 over the summertime Southern Ocean (SO) close to the Antarctic coast in many models. Here, comparisons between cloud condensation nuclei (CCN) observations from field campaigns during the 2017–2018 Austral summer over the Australasian sector of the SO and simulated CCN from the Community Atmospheric Model 6 (CAM6) are presented. Modeled MBL CCN number concentration (NCCN) is underestimated, by close to 100% at latitudes south of 55°S with the NCCN bias (a) largest close to the Antarctic Plateau during summer, implying the biased CCN type has seasonal and latitudinal variation and, (b) three times larger over sea ice than over open water, implying sea spray CCN are better simulated compared to secondary CCN. Assessments of aerosol size distributions indicate an underestimation of accumulation-mode-aerosols (Ac) with diameters 70 nm < D < 100 nm. CCN supersaturation spectra indicate that the observed CCN had lower hygroscopicity compared to simulated CCN, implying differences in CCN chemical composition. With secondary aerosols including sulfate being less hygroscopic than sea salt CCN, the CCN activation ratio derived using bulk hygroscopicity kappa in the Abdul-Razzak function leads to an underestimation of critical supersaturation south of 62°S. The biases reported here highlight important shortfalls in simulated CCN that can be important to the well-documented underestimated Nd by Earth System Models, a key feature and uncertainty of pre-industrial conditions.

Key Points

There is low bias of cloud condensation nuclei number concentration (NCCN) south of 55°S over the Southern Ocean

The NCCN-supersaturation spectra show that the Community Atmosphere Model 6 overestimates CCN hygroscopicity

CCN type over the open water is better simulated over the open water compared to that over sea ice

Plain Language Summary

Over the Australasian sector of the Southern Ocean (SO), the particles suspended in the air right over the water are collected from a field campaign during the 2017–2018 Austral summer and are simulated using the Community Atmospheric Model 6 (CAM6). Comparisons show that the number concentration of particles that can serve as cloud embryos are underestimated close to the Antarctic coast during February 2018. The analysis demonstrates that the particle types over the open water are better simulated than those over the sea ice, implying the missing particles can be related to the chemical reaction outputs and their transportation instead of direct emissions from the water with blowing wind. This is consistent with the finding that the simulated particles do not match the observed seasonal or latitudinal dependence. This conclusion is important for SO clouds considering cloud droplets form upon these particles under favorable conditions and provides insights for Earth System Model evaluation.

Source: AGU