Could high resolution simulators be the future of climate change prediction?

Wadham's David Richards Tutorial Fellow in Physics, Hannah Christensen, believes they could provide an essential guide for climate change policy.

Working with Wadham MPhys student Oliver Driver, Hannah has demonstrated that state-of-the-art high-resolution atmospheric simulations are able to mimic the fractal dimensions of clouds, copying their behaviour. 

Hannah explains: “The physics which describes how clouds merge into large organised systems is largely unknown. But we do know that clouds are fractals: they appear similar whether we consider individual storms, or very large organised cloud systems 1000 km across. 

In a forthcoming paper Hannah highlights the ability of simulations to capture cloud behaviour. This means that high-resolution models can be used as digital laboratories to study the physics underpinning this important process.

"By comparing perimeter fractal dimensions we quantify whether the clouds generated in simulations align with reality" explains Oliver.

Hannah adds: “We compute the fractal dimension of clouds in a set of high-resolution simulations. This quantifies the appearance of the clouds - how smooth or crinkly they are – a property that is independent from their size. We find that the fractal dimension of the simulated clouds is very close to that observed - within 10%. The fractal dimension seems to be sensitive to choices made when building the computer model used to produce the simulations. Unexpectedly, we find that it is not the representation of thunderstorms that matters the most, but instead the choices made when deciding how to represent turbulence close to the surface."

Hannah and Oliver believe this is of interest because these high-resolution simulators are arguably the future of climate prediction. “Our current models, such as those used in the IPCC, are very low resolution, with pixels on the order of 100km compared to the 2-5 km pixels in these simulations analysed by Oliver. At 100km many important processes in the atmosphere must be approximated, for example, clouds and thunderstorms, and these approximations lead to errors building up in our predictions. We are moving towards ultra-high-resolution models where we don't need these approximations, such as those that will be used in "digital twin" type experiments.” 

Digital twins are recreations of planet Earth that with simulated atmosphere, ocean, ice, and land, providing forecasts of floods, droughts, and fires from days to years in advance. 

“Before using a models as a digital twins, we need to be sure what they can and can't simulate, one aspect of which is what we set out to assess in the paper,” said Hannah. 

Hannah, although not directly involved in COP 26, was a contributing author of the IPCC AR6 WGI Chapter 4 Future global climate: scenario-based projections and near-term information which guides many COP discussions.

Hannah Christensen

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