Research Project

Observing cloud and aerosol interactions

Clouds have a massive effect on climate. Cloud cover reflects radiation from the sun that would otherwise be absorbed by oceans, raising their temperatures. Cloud cover can also act as a blanket, keeping warmth near the surface.

Dark clouds

The frequency of clouds over the Southern Ocean is often underestimated, causing models to predict warmer sea surface temperatures than observed. This underestimation, and the challenges of climate models in accurately simulating clouds has been long-documented. It is an area of focus for the Deep South’s Observations programme due to the significant impact it has on modelling Southern Hemisphere climate.

Models often misrepresent the composition of clouds because of how they model the small particles, called aerosols, that are required for cloud formation. These tiny particles help the cloud water droplets grow and eventually become ice crystals, however, the extremely low aerosol concentrations in the Southern Hemisphere mean that we often observe liquid water clouds in sub-zero environments, with records of water droplets in clouds at temperatures down to -38oC!

These biases, or errors in the models, also mean that climate features that are critical to the climate over New Zealand, such as the Southern Hemisphere storm track and jet, are incorrectly modelled, either in position or intensity. These storm tracks impact New Zealand directly via their influence on rainfall and extreme weather events.  

There are much fewer observational data on clouds and aerosols over the Southern Ocean than for the Northern Hemisphere, and we are using various observation techniques to better understand, and ultimately better model, Southern Hemisphere climate into the future.

Key outcomes:

  • Targeted observation campaigns have provided valuable data and knowledge on low-level clouds and aerosol composition that cannot be derived from satellite observations. 
  • Alongside satellite datasets, these in-situ observations were used to evaluate the performance and sensitivity of the NZESM over NZ and the Southern Ocean, comparing them with the presence and composition of aerosols and resulting clouds that are simulated in the model.
  • This work led to improvements in the representation of ice formation processes for generating clouds in the NZESM which has reduced biases in the model, as well as the development of innovative machine learning techniques to identify cloud types and their properties.
  • More broadly, analysis of CMIP6 models simulations against observational and satellite datasets exposed compensating errors in the modelled composition of cloud cover over the Southern Ocean and New Zealand, which are known to have large impacts on the accuracy of projections. 

How this research is being used:

  • A ground-based LIDAR simulator developed in this project to enable “apples-to-apples” comparisons between models and observations is available as a creative commons-licenced community product. The team worked with instrument technology collaborators to enhance and promote uptake of this tool in the scientific community.
  • This research has worked in close collaboration with the Modelling Clouds and Aerosols projects to implement model improvements to the NZESM.

In the media:

Measuring cloud and aerosol interactions: 2021 – 2024

Budget: $733,000

Phase 1: Developed the understanding required to improve cloud simulations by analysing the treasure trove of previously collected airborne and ground-based datasets on aerosols and cloud processes across the Southern Ocean and Aotearoa. Satellite data was utilised to gain a better understanding of how aerosols impact cloud formation. These data were subset based on meteorological conditions, to better understand the critical factors responsible for cloud formation in different circumstances. The ground-based lidar tool was improved to include the aerosol environment, allowing us to better determine how clouds simulated in models differ from reality. 

Phase 2: Targeted observations of clouds and our environment in Aotearoa, onboard ships in the Southern Ocean and in Antarctica, and collaborations with national and international measurement campaigns, gaining the knowledge needed to guide specific improvements in the NZESM, and eventually, reducing model biases around Aotearoa. This process knowledge contributed to making climate models both more accurate and robust.

Cloud & aerosol observations to improve the NZESM: 2019 – 2020

Budget: $230,000

Alongside the research in the Earth Systems Modelling Programme, and using data from observational campaigns in the Southern Ocean, this project investigated the prevalence and profiles of the types of clouds seen, whether they be ice clouds, or mixed phase clouds (which include super-cooled liquid cloud mentioned above). Comparing observational data to NZESM model output using the simulator technology developed in the previous project, the team improved the way that the NZESM models ice cloud formation.

Using these detailed measurements, the team also investigated the composition and abundance of aerosols, like sulfate and sea salt, that are observed the Southern Ocean, likewise comparing to NZESM output and improving the way these are represented in the model.

New methods to simulate Southern Ocean clouds: 2016- 2018

Budget: $150,000

Comparing a model with observations is a fundamental way to test the skill of a climate model; however, the variables representing clouds in models and the physical observations from satellites or ground-based instruments that you would compare them against are not directly comparable. This project utilised a novel methodology to couple a satellite simulator (which mimics what a satellite would “see” of a given cloud distribution produced by a model) to the NZESM to be able to compare model output to satellite observations, to assess how well the NZESM is modelling clouds. It also developed a ground-based simulator, which allowed for vertical profiling of the cloud composition from the bottom-up to correspond to ship-based observations. This work improved our understanding of the types, properties and extent of clouds that form over the Southern Ocean, and identified areas of the NZESM that could be improved to reduce the current biases in the NZESM, and many other climate models.

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Clouds & aerosols over the Southern Ocean: 2015 – 2019

These projects built on the research undertaken in the early research project Clouds & aerosols over the Southern Ocean. Read more about that project here.


This was an international collaboration effort and we would like to thank our colleagues for providing their data and being part of this effort.

We would also like to acknowledge the Atmospheric Radiation Measurement (ARM) user facility, a U.S. Department of Energy (DOE) Office of Science user facility managed by the Biological and Environmental Research Program.