Periodic Reporting for period 4 - GLAD (Global Lagrangian Cloud Dynamics)
Berichtszeitraum: 2024-07-01 bis 2024-12-31
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.
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.
In the final phase of the project, the team focused on the use of the novel Lagrangian PV gradient method in high- and low-resolution climate datasets aligned with the second WP and the publication of the results. The aim is to quantify the role of cloud-related diabatic processes in the large-scale circulation. A second focus is on the interpretation of the results and their placement in the broader context of geophysical fluid dynamics. Specifically, the PV gradient framework will be used to quantify the role of diabatic processes compared to adiabatic processes in shaping the variability of the jet stream, and thus the Rossby waveguide, on seasonal time scales. The work was published in Bukenberget et al. (2023) and highlighted as Feature Article. The PI has published an article on the role of cloud processes as the underlying cause of decade old biases in the representation of the midlatitude circulation in climate models (Schemm 2023). The article was highlighted as Editor Highlight.
After the novel Lagrangian PV gradient method was successfully published in the Quartlerly Journal of the Royal Meteorological Society, the method was adapted to work with low resolution data. The team analysed the entire ERA5 reanalysis period (1979-present) for the occurrence of extreme jet streaks and used statistical clustering to sort the events into categories with similar characteristics. A perhaps surprising finding is that extreme jets only occur in conjunction with zonal or anticyclonic flow, but not in cyclonic flow situations. A possible interpretation is given, and the team was able to highlight the role of adiabatic and diabatic processes at play. Furthermore, extreme jet streams tend to increase in number during winter over the North Atlantic, but the annual and decadal variability still dominates the overall signal. The work was published in Bukenberger et al. (2025).
Finally, the team has begun to revisit the four-quadrant jet stream model and based on their findings, is attempting to enrich this model, which is based on dry atmospheric dynamics, with the action of diabatic processes. A paper is in preparation that will highlight the importance of clouds at the right entry of the jet, radiation along the jet axis, and turbulence in the jet exit region. The work will be published after the end of the project.
After the novel Lagrangian PV gradient method was successfully published in the Quartlerly Journal of the Royal Meteorological Society, the method was adapted to work with low resolution data. The team analysed the entire ERA5 reanalysis period (1979-present) for the occurrence of extreme jet streaks and used statistical clustering to sort the events into categories with similar characteristics. A perhaps surprising finding is that extreme jets only occur in conjunction with zonal or anticyclonic flow, but not in cyclonic flow situations. A possible interpretation is given, and the team was able to highlight the role of adiabatic and diabatic processes at play. Furthermore, extreme jet streams tend to increase in number during winter over the North Atlantic, but the annual and decadal variability still dominates the overall signal. The work was published in Bukenberger et al. (2025).
Finally, the team has begun to revisit the four-quadrant jet stream model and based on their findings, is attempting to enrich this model, which is based on dry atmospheric dynamics, with the action of diabatic processes. A paper is in preparation that will highlight the importance of clouds at the right entry of the jet, radiation along the jet axis, and turbulence in the jet exit region. The work will be published after the end of the project.
Two milestones go beyond the state of the art. The successful technical development underlying the porting of a trajectory calculation software component of a numerical weather prediction model to a hybrid GPU-based supercomputing architecture goes beyond the state of the art in the field. It is likely that this effort will serve as a blueprint for future model development on application-specific integrated circuits (ASICs) rather than central processing units (CPUs), which will form the core of the next generation of supercomputing centres.
The novel Lagrangian PV gradient method provides the research community with a completely new tool for quantifying the role of clouds in atmospheric dynamics in models and observations. In the final phase, the team prepared the method to be flexible and easy to use in many applications. It opens up new opportunities to study the role of diabatic processes in atmospheric dynamics, how they might change in a warmer climate, and can potentially serve as a benchmark for model development. It also advances the theory of PV gradients with the influence of moisture and other diabatic processes.
The novel Lagrangian PV gradient method provides the research community with a completely new tool for quantifying the role of clouds in atmospheric dynamics in models and observations. In the final phase, the team prepared the method to be flexible and easy to use in many applications. It opens up new opportunities to study the role of diabatic processes in atmospheric dynamics, how they might change in a warmer climate, and can potentially serve as a benchmark for model development. It also advances the theory of PV gradients with the influence of moisture and other diabatic processes.