ELISIR · Elucidating the role of ice crystal number and ice crystal size for high cloud feedbacks

Elucidating the role of ice crystal number and ice crystal size for high cloud feedbacks

Clouds, particularly high clouds, play a crucial role in influencing atmospheric circulation patterns and the regional manifestation of global climate change. High clouds are expected to intensify global warming by remaining at near-constant temperature and shifting to higher altitudes, which leads to a positive climate feedback. However, changes in their extent, cloud optical depth, and cloud properties, specifically ice crystal number and ice crystal size, remain uncertain. These uncertainties, particularly in tropical high cloud feedback, contribute significantly to the overall uncertainty in projections of future climate. This project investigated the responses of high cloud properties to global warming. These properties play a key role in determining cloud optical properties and cloud lifetime, which, in turn, influence the high cloud feedback and changes in atmospheric circulation patterns. By focusing on the interactions of ice crystals and radiative fluxes, the project tried to enhance the accuracy of climate projections and reduce uncertainties related to cloud feedback and its impacts on the Earth's climate. While recent empirical and theoretical work has led to advances in the understanding of radiation-climate feedbacks in the cloud-free atmosphere, the radiation-cloud interaction is still subject to large uncertainties and lacks an underlying fundamental theory. Uncertainties are particularly large for the radiative fluxes within the atmosphere (i.e., radiative heating) that drive planetary-scale circulation patterns and their responses in a warmer climate. The key conclusion of the project uncovers the role of density in interactions between high clouds and radiation. These interactions robustly increase when clouds shift upward, where air density decreases. The identified mechanism was confirmed in multimodel simulations and satellite retrievals of observed interannual variations in cloud radiative heating. In addition, a theory based on simple physics was developed that predicts the simulated increase in cloud radiative heating in a warmer climate. Such changes are likely to influence the regional manifestations of global climate change. The enhanced cloud-radiation interactions will amplify the role of high clouds in modulating planetary-scale circulation patterns in a future warmer climate, including basic phenomena such as the poleward shifts of the Hadley cell and the extratropical jet stream. While the work performed so far focused on the scientific basis of the mechanism, additional studies are planned that will explore the above-mentioned implications of the project’s findings. In addition, part of the work was dedicated to improving the representation of tropical cirrus in high-resolution models. The key conclusion is that already quick, easy changes to the model code with no/little computational cost can substantially improve the realism of simulations of ice clouds.

Data: CORDIS, © European Union