Periodic Reporting for period 1 - ELISIR (Elucidating the role of ice crystal number and ice crystal size for high cloud feedbacks)
Reporting period: 2021-07-01 to 2023-06-30
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.
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.
Work Package 1: Global simulations
In WP1, the Fellow studied how changes in sea surface temperatures affect high clouds using coarse-resolution global climate model simulations. The cirrus freezing code was improved to better represent how ice forms in cirrus cloud conditions. The simulations were run for current and warmer climate scenarios, and the results were compared with satellite data. However, due to the complexity of attributing changes to specific causes, the publication of this work has been delayed.
Work package 2: High-resolution modeling
The WP2 was divided into three tasks. The first concentrated on analyzing data from the high cloud and radiation data from a multimodel dataset of simulations in radiative-convective equilibrium (RCEMIP). This analysis revealed a robust increase in cloud radiative heating, closely associated with the upward shift of high clouds in warmer climates. The model results were subsequently combined with satellite observations of cloud radiative heating and idealized simulations of upward cloud shift with a cloud-resolving model.
The second direction involved improvements to the microphysics in the cloud-resolving model SAM, due to delays with the setup of ICON model simulations. A careful reconsideration and relaxation of microphysical code limiters and the use of ice nucleation parameterization in their predefined temperature range only can lead to rapid improvements in simulations of tropical cirrus cloud properties. In addition, a more physically accurate ice nucleation mechanism further improved the simulated cirrus cloud properties compared to in-situ and satellite observations. Furthermore, the improved SAM model will be used in simulations under radiative-convective equilibrium conditions as part of Phase 2 of the RCEMIP project in spring 2024. This represents an exciting collaboration with the tropical climate dynamics community and opens up new avenues for research in the field of climate dynamics.
The Fellow also coordinated an opinion piece on tropical cirrus clouds that reviews recent progress in the field, highlights important questions that remain unanswered, and discusses promising paths forward. Moreover, the Fellow contributed to a book chapter on radiatively-driven circulations in clouds.
Work package 3: Multimodel datasets
The Fellow supervised a BSc thesis examining changes in vertically integrated ice across CMIP6 models using available ice water path and radiative flux data. Initial results showed a wide range of responses in simulated ice water paths to warmer surface temperatures, warranting further data analysis. The topic of CMIP6 ice cloud analysis will be offered as a thesis topic for MSc students. Additionally, the Fellow initiated a working group within the Global Atmospheric System Studies (GASS) panel of WRCP to facilitate comparisons of ice microphysical properties in global storm-resolving models with in-situ aircraft data. The working group is co-chaired by several researchers, including the Fellow.
Secondment:
During the 6-week stay at LMD Paris, the Fellow had the opportunity to meet experts in various fields, from climate dynamics to satellite observations. Collaborations with Claudia Stubenrauch, Sandrine Bony's group, and others opened up new avenues of research, including access to unique datasets and the possibility of unraveling the secrets of high cloud opacity feedback. The visit to the University of Lille strengthened ties with a satellite expert, Odran Sourdeval, and his valuable ice crystal number dataset. Overall, the secondment provided valuable insights and connections, contributing to the project's progress. The publication on the changes in cloud radiative heating in a warmer climate is a direct outcome of the secondment.
In WP1, the Fellow studied how changes in sea surface temperatures affect high clouds using coarse-resolution global climate model simulations. The cirrus freezing code was improved to better represent how ice forms in cirrus cloud conditions. The simulations were run for current and warmer climate scenarios, and the results were compared with satellite data. However, due to the complexity of attributing changes to specific causes, the publication of this work has been delayed.
Work package 2: High-resolution modeling
The WP2 was divided into three tasks. The first concentrated on analyzing data from the high cloud and radiation data from a multimodel dataset of simulations in radiative-convective equilibrium (RCEMIP). This analysis revealed a robust increase in cloud radiative heating, closely associated with the upward shift of high clouds in warmer climates. The model results were subsequently combined with satellite observations of cloud radiative heating and idealized simulations of upward cloud shift with a cloud-resolving model.
The second direction involved improvements to the microphysics in the cloud-resolving model SAM, due to delays with the setup of ICON model simulations. A careful reconsideration and relaxation of microphysical code limiters and the use of ice nucleation parameterization in their predefined temperature range only can lead to rapid improvements in simulations of tropical cirrus cloud properties. In addition, a more physically accurate ice nucleation mechanism further improved the simulated cirrus cloud properties compared to in-situ and satellite observations. Furthermore, the improved SAM model will be used in simulations under radiative-convective equilibrium conditions as part of Phase 2 of the RCEMIP project in spring 2024. This represents an exciting collaboration with the tropical climate dynamics community and opens up new avenues for research in the field of climate dynamics.
The Fellow also coordinated an opinion piece on tropical cirrus clouds that reviews recent progress in the field, highlights important questions that remain unanswered, and discusses promising paths forward. Moreover, the Fellow contributed to a book chapter on radiatively-driven circulations in clouds.
Work package 3: Multimodel datasets
The Fellow supervised a BSc thesis examining changes in vertically integrated ice across CMIP6 models using available ice water path and radiative flux data. Initial results showed a wide range of responses in simulated ice water paths to warmer surface temperatures, warranting further data analysis. The topic of CMIP6 ice cloud analysis will be offered as a thesis topic for MSc students. Additionally, the Fellow initiated a working group within the Global Atmospheric System Studies (GASS) panel of WRCP to facilitate comparisons of ice microphysical properties in global storm-resolving models with in-situ aircraft data. The working group is co-chaired by several researchers, including the Fellow.
Secondment:
During the 6-week stay at LMD Paris, the Fellow had the opportunity to meet experts in various fields, from climate dynamics to satellite observations. Collaborations with Claudia Stubenrauch, Sandrine Bony's group, and others opened up new avenues of research, including access to unique datasets and the possibility of unraveling the secrets of high cloud opacity feedback. The visit to the University of Lille strengthened ties with a satellite expert, Odran Sourdeval, and his valuable ice crystal number dataset. Overall, the secondment provided valuable insights and connections, contributing to the project's progress. The publication on the changes in cloud radiative heating in a warmer climate is a direct outcome of the secondment.
The concrete outcomes of the project all go beyond the state of the art. The work on changes in radiative heating of high clouds in a warmer climate sticks out as a highlight and identified an additional important factor in the response of high clouds to warming. However, only follow-up studies will be able to fully assess the broad implications of this finding.
Moreover, the project widely exceeded expectations on the communication activities to the broader public. The Fellow has been very active both in communicating science to a broader public both through public talks and his media presence.
Moreover, the project widely exceeded expectations on the communication activities to the broader public. The Fellow has been very active both in communicating science to a broader public both through public talks and his media presence.