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?
