As a consequence of increased greenhouse gas concentrations in the atmosphere, the global mean temperature has increased in comparison to pre-industrial times. Now that the increase is a recognized fact, scientific attention has shifted towards the manifestation of global warming on regional scales, where climate variability is largely controlled by atmospheric dynamics. On these scales, low model agreement on future circulation changes is reflected by the fact that climate models agree on a global mean temperature increase but not on precipitation trends. While the former is controlled by the thermodynamics properties of the atmosphere, the latter is much more controlled by weather dynamics. Indeed, Midlatitude circulation changes in response to warming are associated with considerable uncertainties, and models even partly disagree on the sign of regional climate change. Cloud-circulation interaction is believed to be the main cause behind this disagreement. The World Climate Research Programme (WCRP) consequently identifies a better process understanding of the couplings between cloud-related processes and the large-scale circulation as one of its Grand Challenges.
The question of how clouds interact with and influence midlatitude circulation is at the heart of some of the most challenging research questions in climate and weather science. While this question has long been explored in weather science, climate science has given it increased attention in recent years motivated by the projected increase in water vapor in a warmer atmosphere. Clouds interact with the atmospheric circulation in numerous ways. To name only a few processes, scattering and reflection or emission of radiation, turbulent mixing, evaporation of rain droplets, and latent heat release during condensation are some of the processes that underly the highly complex interaction. The representation of cloud-related diabatic processes poses a significant challenge for both weather and climate models, largely because clouds are unresolved, and processes that occur below the models’ resolution are parameterized. Given the potentially impacts of projected global warming and the significant benefits of improved weather predictions, it is imperative to improve the representation of cloud-circulation interactions in models. This requires in the first place an improvement of our mechanistic understanding of how diabatic processes affect the mid-latitude circulation.
The bold goal of this project is to develop a new process-oriented framework for the study of cloud-circulation interactions based on the history of air parcels (Lagrangian perspective) within a revised potential vorticity (PV) framework. It will enable us to develop a fundamentally better mechanistic understanding of the diabatic to adiabatic interactions in the atmosphere. Further, the revised Lagrangian PV-gradient diagnostic is a particularly powerful framework because the combination of a Lagrangian perspective on a conserved variables allows not only to disentangle adiabatic from diabatic processes but even to establishes a direct link between cloud-related diabatic processes and associated circulation changes via change in the gradient of PV. A systematic Lagrangian-based investigation of cloud-circulation couplings based on PV gradients is a novelty and can be key for closing existing knowledge gaps. In the first part, theoretical considerations are made that form the basis for a Lagrangian PV gradient framework. Next, kilometre-scale simulations are performed, and associated biases arising from model resolution are examined from a jet-stream centred perspective. These biases are linked to observed errors in the representation of the mid-latitude circulation in climate models. Finally, the findings of this project will be embedded into a holistic concept of how clouds and related small-scale processes affect the midlatitude jet stream.
