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.

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 -38^oC!

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. 

In the media:

• Super-cooled liquid clouds: Why researchers want to know more, Radio NZ
• New Zealand’s Next Top Model, New Zealand Geographic
• New Zealanders out to uncover icy secrets, Scoop.co.nz
• Breaking the ice, NIWA