Periodic Reporting for period 1 - STEP-CHANGE (State-dependent cloud phase feedbacks: enhancing understanding and assessing global effects)
Okres sprawozdawczy: 2023-03-01 do 2025-08-31
The ways in which clouds change with global warming remain elusive, as are the associated cloud-climate feedbacks that govern most of the spread in climate sensitivity simulated by current Earth System Models. This uncertainty in turn limits society’s ability to take necessary action to avoid dangerous climate change. Despite considerable research progress in recent decades, additional complexities have been uncovered that further add to the uncertainty. For example, the understanding that many cloud-climate feedbacks change with time, due to their dependence on warming levels or patterns, is relatively recent. Cloud thermodynamic phase changes are the root cause of some of this state-dependence, and new research has revealed that these feedbacks could shift Earth’s climate into a state that is more sensitive to greenhouse gas forcing than at present. Understanding and quantifying this state-dependence is therefore critically important, but such progress will require deep understanding of processes on a range of scales, from the microphysics that control cloud phase to large-scale impacts on climate. Furthermore, it has become evident that different cloud-climate feedback regimes are governed by different processes with their own unique state-dependence that must be investigated separately. Therefore, the overall objective of STEP-CHANGE is to understand and quantify feedbacks associated with cloud phase changes, including their state-dependence, for three distinct cloud regimes in the following regions: the Arctic, the Tropical deep convective region, and the Southern Hemisphere storm tracks. A bold research strategy which includes aircraft measurements, lab experiments, space-borne remote sensing, and a hierarchy of numerical model simulations will allow STEP-CHANGE to answer the following key research questions (RQs) associated with process understanding (PU) and large-scale (LS) impacts for each of the three cloud regimes referred to above:
1) Arctic cloud phase feedback
RQ1_PU: What processes govern Arctic mixed-phase cloud longevity, and what levels of warming and sea ice retreat are required for substantial shifts in Arctic mixed-phase cloud abundance?
RQ1_LS: What role have Arctic mixed-phase clouds played in Arctic Amplification to date, and will this role change in the future?
2) Mid-latitude (Southern Ocean) cloud phase feedback
RQ2-PU:How do cloud phase and optical properties vary with sea surface temperature and other cloud-controlling factors in the Southern Ocean region?
RQ2_LS: At what level of warming will practically all Southern Ocean cloud ice be lost, and the cloud phase feedback effectively vanish?
3) Tropical cloud phase feedback
RQ3_PU: What processes determine precipitation efficiency in tropical deep convective clouds?
RQ3_LS: Does precipitation efficiency of deep convective clouds in the Tropics change with warming, and if yes, what is the associated net cloud feedback?
1) Arctic cloud phase feedback
RQ1_PU: What processes govern Arctic mixed-phase cloud longevity, and what levels of warming and sea ice retreat are required for substantial shifts in Arctic mixed-phase cloud abundance?
RQ1_LS: What role have Arctic mixed-phase clouds played in Arctic Amplification to date, and will this role change in the future?
2) Mid-latitude (Southern Ocean) cloud phase feedback
RQ2-PU:How do cloud phase and optical properties vary with sea surface temperature and other cloud-controlling factors in the Southern Ocean region?
RQ2_LS: At what level of warming will practically all Southern Ocean cloud ice be lost, and the cloud phase feedback effectively vanish?
3) Tropical cloud phase feedback
RQ3_PU: What processes determine precipitation efficiency in tropical deep convective clouds?
RQ3_LS: Does precipitation efficiency of deep convective clouds in the Tropics change with warming, and if yes, what is the associated net cloud feedback?
The following overview of main achievements is divided into the three research themes:
Theme 1: Arctic mixed-phase clouds
Work within this theme has taken advantage of recent field campaigns targeting Arctic mixed-phase clouds. This has resulted in studies which combine in situ observations and cloud-resolving model simulations to shed light on the processes that govern the phase of Arctic clouds (Schafer et al., Atm. Chem. Phys. 2025, Dammann et al., J. Geophys. Res. - Atmospheres, 2025), and how these clouds in turn would respond to warming (Schafer et al., Geophys. Res. Lett, 2025). The next step is now to evaluate how this advance in process understanding can help us assess climate feedbacks in Arctic mixed-phase clouds by extending our cloud-resolving model domain to cover a larger area of the Arctic for a longer period of time, so we can make robust conclusion regarding the implications for Arctic mixed-phase cloud feedbacks.
In our global climate modelling tool, NorESM2, we implemented a new parameterization of ice nucleating particles (INPs) based on our own field data and found that this made a very big difference for the radiative properties of Arctic mixed-phase clouds (Bragstad Gjelsvik et al., Atm. Chem. Phys, 2025). The next step with respect to global modeling is to run simulations in which we investigate how the Arctic cloud feedback has changed with the new parameterization in place.
Theme 2: Southern Ocean mixed-phase clouds
We have so far conducted a global study using a combination of active remote sensing from Space (CALIPSO and CloudSat products) and Earth System Modelling (NorESM2 model) which made particularly interesting findings related to Southern Ocean (SO) clouds. While observationally constraining cloud phase in the Northern high latitudes had strong impacts on local climate, constraining SO mixed-phase clouds using satellite observations had large implications not just for projected warming in the SO, but also globally (Hofer et al., 2024, Comm. Earth Env.). This further highlighted the importance of SO mixed-phase clouds for global climate sensitivity and was an incentive for the team to place
even more emphasis on clouds in this region than originally intended. In doing so, we have recently made a very big discovery related to SO mixed-phase clouds, namely that there may be a global climate tipping point associated with them. We came to this conclusion by combining several different approaches in what ended up being three independent lines of evidence for a very strong non-linear sensitivity of SO cloud feedback to warming in this particular
region. A manuscript describing this discovery and the lines of evidence will emerge once the review process has been finalised. The implications of this discovery are potentially wide-reaching and once published we expect this to be
one of the major achievements of STEP-CHANGE.
Theme 3: Tropical deep convective mixed-phase clouds
While the planned STEP-CHANGE field campaign focusing on Tropical mixed-phase clouds has been postponed, our participation in another large Tropical field campaign (ORCHESTRA, in August of 2024) has offered some data on
Tropical mixed-phase clouds that could be of used for cloud-resolving modeling evaluation, in combination with satellite remote sensing, in the next stage of work within this theme.
Theme 1: Arctic mixed-phase clouds
Work within this theme has taken advantage of recent field campaigns targeting Arctic mixed-phase clouds. This has resulted in studies which combine in situ observations and cloud-resolving model simulations to shed light on the processes that govern the phase of Arctic clouds (Schafer et al., Atm. Chem. Phys. 2025, Dammann et al., J. Geophys. Res. - Atmospheres, 2025), and how these clouds in turn would respond to warming (Schafer et al., Geophys. Res. Lett, 2025). The next step is now to evaluate how this advance in process understanding can help us assess climate feedbacks in Arctic mixed-phase clouds by extending our cloud-resolving model domain to cover a larger area of the Arctic for a longer period of time, so we can make robust conclusion regarding the implications for Arctic mixed-phase cloud feedbacks.
In our global climate modelling tool, NorESM2, we implemented a new parameterization of ice nucleating particles (INPs) based on our own field data and found that this made a very big difference for the radiative properties of Arctic mixed-phase clouds (Bragstad Gjelsvik et al., Atm. Chem. Phys, 2025). The next step with respect to global modeling is to run simulations in which we investigate how the Arctic cloud feedback has changed with the new parameterization in place.
Theme 2: Southern Ocean mixed-phase clouds
We have so far conducted a global study using a combination of active remote sensing from Space (CALIPSO and CloudSat products) and Earth System Modelling (NorESM2 model) which made particularly interesting findings related to Southern Ocean (SO) clouds. While observationally constraining cloud phase in the Northern high latitudes had strong impacts on local climate, constraining SO mixed-phase clouds using satellite observations had large implications not just for projected warming in the SO, but also globally (Hofer et al., 2024, Comm. Earth Env.). This further highlighted the importance of SO mixed-phase clouds for global climate sensitivity and was an incentive for the team to place
even more emphasis on clouds in this region than originally intended. In doing so, we have recently made a very big discovery related to SO mixed-phase clouds, namely that there may be a global climate tipping point associated with them. We came to this conclusion by combining several different approaches in what ended up being three independent lines of evidence for a very strong non-linear sensitivity of SO cloud feedback to warming in this particular
region. A manuscript describing this discovery and the lines of evidence will emerge once the review process has been finalised. The implications of this discovery are potentially wide-reaching and once published we expect this to be
one of the major achievements of STEP-CHANGE.
Theme 3: Tropical deep convective mixed-phase clouds
While the planned STEP-CHANGE field campaign focusing on Tropical mixed-phase clouds has been postponed, our participation in another large Tropical field campaign (ORCHESTRA, in August of 2024) has offered some data on
Tropical mixed-phase clouds that could be of used for cloud-resolving modeling evaluation, in combination with satellite remote sensing, in the next stage of work within this theme.
As described above, a few major scientific achievements have already emerged from the project, with highlights as follows:
1) The development of our own parameterization of INPs in the Arctic, which is a region where conventional parameterizations generally fail, and the implementation of this parameterization in NorESM2, including an assessment of how this changed simulated Arctic clouds and comparison to observations.
2) The major discovery that SO mixed-phase clouds may be linked to a global climate tipping point due to their strong and non-linear dependence on surface air temperatures in the region, as described above.
1) The development of our own parameterization of INPs in the Arctic, which is a region where conventional parameterizations generally fail, and the implementation of this parameterization in NorESM2, including an assessment of how this changed simulated Arctic clouds and comparison to observations.
2) The major discovery that SO mixed-phase clouds may be linked to a global climate tipping point due to their strong and non-linear dependence on surface air temperatures in the region, as described above.