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

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 (N_CCN) is underestimated, by close to 100% at latitudes south of 55°S with the N_CCN 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 N_d by Earth System Models, a key feature and uncertainty of pre-industrial conditions.