Marine boundary layer cloud condensation Nuclei bias over the Southern Ocean: Comparisons between the community atmosphere model 6 and field observations

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.