Modelling clouds, aerosols and atmospheric chemistry

Have you ever looked at the ocean and noticed that the sky above it appeared hazy? This is caused by the presence of tiny particles or droplets in the air. In remote, unpolluted regions such as the Southern Ocean, such “aerosols” form from natural sources, such as ocean waves breaking and releasing sea salt into the air.

Aerosols over the Southern Ocean are important because they influence cloud formation and play a role in the energy budget. However, the way aerosols behave in the atmosphere is complex, and it’s currently difficult to model them accurately. This may be one reason why contemporary climate models, including the NZ Earth System Model (NZESM), has difficulty simulating the energy budget over the Southern Ocean, which has flow-on implications for simulating New Zealand’s climate.

Clouds and aerosols are a leading source of errors in climate models, particularly over the Southern Ocean which plays a key role in determining New Zealand’s weather and climate. Throughout the Deep South National Science Challenge we have developed capacity in Earth System modelling, with considerable expertise in modelling clouds and aerosols.

Key outcomes:

The introduction of a new functionality into the Global Coupled 5 (GC5) configuration of the Unified Model (the physical base model of NZESM and UKESM) to make dust particles freezing nuclei, meaning they can initiate the freezing of cloud droplets. This improvement has significantly improved the realism of the model simulations and will impact future climate models that are based on the Unified Model.
We have worked with the Deep South observations team to improve marine aerosol fluxes, developed new aerosol chemistry schemes, and enhanced the ability of aerosols to interact with clouds, thus improving performance of the New Zealand Earth System Model in the Southern Ocean region.
Updates to heterogeneous chemistry and inclusion of solar variability as a climate forcer resulted in more realistic simulations of the Antarctic ozone hole.

How this research is being used:

We are involving MetOffice colleagues in this research who will be instrumental for making sure that our advances are lodged into the mainstream models in use at the UM Consortium.
This research is working in close collaboration with the Observing Clouds and Aerosols projects to better understand the processes in relation to aerosols and cloud formation that are being modelled in the NZESM.

This project in the media:

New Zealand’s Next Top Model, New Zealand Geographic
Breaking the ice, NIWA
Just the ticket! Not one, not two, but up to four “tickets” to improve the world’s centralised global climate model, Deep South Challenge news story
Simulating Earth’s changing climate: why some models exaggerate future warming, The Conversation
Climate Lessons: How clouds complicate climate models, Stuff

Organic aerosol-cloud coupling (2021-24)

Budget: $670,000

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

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 will assess 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.
A wealth of research has been conducted through the Deep South National Science Challenge on sea salt aerosol fluxes. We will integrate 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 will implement the source of inorganic halogens in the NZESM and examine its effect on DMS and cloud formation.

Simulating clouds and aerosols in the NZESM (2019-22)

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:

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.
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).
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.

Sulfate aerosols over the Southern Ocean (2017-19)

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.

Stratospheric chemistry in the NZESM (2016-18)

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.

Clouds and Aerosols Over the Southern Ocean (2015-19)

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.

Acknowledgements

This significant endeavour is supported by an international partnership with other weather and climate modelling centres led by the UK Meteorological Office. Our global climate model is based on the UK’s version, and our international partnership is essential. We simply couldn’t do this on our own.

We would like to acknowledge the passing of Mike Harvey in 2022, who was involved in Phase 1 of the Clouds & Aerosols project. Read our farewell to Mike here.