Modelling clouds, aerosols and atmospheric chemistry

Budget: $670,000

In the final two years of the Challenge, investigated marine organic aerosol and its coupling to clouds –the “missing” piece of the puzzle, and brought together our previous model development efforts to best represent clouds and aerosols over the Southern Ocean. Our model formulations can be used by future generations of climate models to produce quality simulations of climate change. We had two work strands:

1. Parts of the Southern Ocean are biologically very productive and rich with marine life. Some of these life forms, or their detritus, float to the surface and get airborne. In some cases, these may become freezing nuclei. We assessed available measurements and formulations of Primary marine organic aerosol (PMOA) acting as freezing nuclei and develop a formulation of the impact of PMOA on freezing temperature simple enough to work with the UKESM/NZESM model.
2. A wealth of research has been conducted through the Deep South National Science Challenge on sea salt aerosol fluxes. We integrated these strands of research to arrive at a recommended set of parameters optimized for the Southern Ocean, but without compromising the quality of simulation for other parts of the globe. Biologically produced volatile organic compounds, such as Dimethyl Sulfide (DMS), are excellent candidates for contributing to the formation of cloud condensation nuclei. However, whether or not they contribute to cloud formation depends on the presence of inorganic halogens in the atmosphere originating from sea spray. We implemented the source of inorganic halogens in the NZESM and examined its effect on DMS and cloud formation.

Budget: $1,000,000

Using observations from the Processes and Observations programme, we addressed aerosol activation to cloud droplets, cloud droplet freezing, and the interactions between clouds and aerosols over the Southern Ocean in the NZESM. We undertook three work streams:

1. Based on the research in the Clouds and Aerosols Observations project, we implemented and tested new model code to simulate ice-nucleating particles (predominantly dust) in the NZESM, which are important precursors to cloud development and rainfall.
2. In the NZESM, sea salt aerosol fluxes are highly dependent on wind speed. Previous research developed two new parameterisations of sea salt aerosol flux based on observations made as part of the Clouds and Aerosols Observation project. We tested these parameterisations in the NZESM and found an improvement in the bias in wintertime aerosol optical depth (Revell et al. 2019, 2021).
3. Finally, we undertook a sensitivity analysis to explore interacting uncertainties on key processes and parameterisations controlling simulated aerosol-cloud interactions over the Southern Ocean in the NZESM.

Budget: $255,710

A particularly important type of aerosol, sulfate aerosol, forms when sea ice melts. Algae growing on the underside of sea ice produce dimethyl sulphide, which – when the ice melts – is released into the atmosphere. Dimethyl sulfide then undergoes a series of chemical reactions to form sulfate aerosol. Sea salt and sulfate aerosols can act as cloud condensation nuclei, and thus initiate cloud formation. In this project we analysed and fine-tuned the way the NZESM simulates sulfate aerosols, with the aim of improving climate simulations of the southern hemisphere.

Budget: $300,000

Ozone depletion is a major (and, seasonally, the dominant) driver of southern hemisphere climate change, and counteracts global warming by the cooling effect due to heat loss from the atmosphere. The substances that deplete ozone (like CFCs, or chlorofluorocarbons), however, are powerful greenhouse gases, and contribute to warming of the atmosphere. Challenges with simulating ozone mean it is difficult to have confidence that its considerable influence is properly represented in climate models. 

This research used satellite-based observational datasets to validate NZESM model runs and its simulation of key chemical compounds in the stratosphere. This validation was used to improve our understanding of stratospheric ozone depletion, and in particular Antarctic ozone depletion, to ultimately contribute to more realistic simulations of southern hemisphere climate and its response to human activity.

These projects build on the research undertaken in the early research project Clouds and Aerosols Over the Southern Ocean. Read more about that project here.