| Summary: | This work addresses the shortwave radiation bias over the Southern Ocean (SO). Climate models often simulate insufficient upwelling shortwave radiation at the top of the atmosphere. This bias has been linked to a deficit in supercooled liquid over the SO in models, which could be caused by using parameterizations of ice nucleation tuned to northern hemisphere observations, where mineral dust is much more prevalent. In the pristine atmosphere over the SO, however, primary marine organic aerosol (PMOA) is in fact the dominant source of INPs. PMOA has recently been implemented in the UK Earth System Model (UKESM1), from which the NZESM is derived. The purpose of this work is to allow PMOA to function as a source of ice nucleating particles (INPs) within the NZESM, extending earlier work which enabled dust particles to function as INPs in the NZESM. Although the NZESM has no explicit INP functionality, the globally uniform heterogeneous freezing temperature can optionally be replaced by a three-dimensional distribution that is a function of the dust number density. This increases the freezing temperature for high dust densities, mimicking the function of dust INPs. A similar approach will be used in this work, but with PMOA. Different parameterisations of INP number density for PMOA exist in the literature, representing PMOA INPs as a function of temperature and total organic carbon mass concentration or temperature and sea spray aerosol mixing ratio. These will be implemented within the NZESM and assessed for their ability to reduce the SO SW radiation bias.
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