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Mixed-phase clouds and climate (MC2) – from process-level understanding to large-scale impacts

Periodic Reporting for period 2 - MC2 (Mixed-phase clouds and climate (MC2) – from process-level understanding to large-scale impacts)

Reporting period: 2019-09-01 to 2021-02-28

The importance of mixed-phase clouds (i.e. clouds in which liquid and ice may co-exist) for weather and climate has become increasingly evident in recent years. We now know that a majority of the precipitation that reaches Earth’s surface originates from mixed-phase clouds, and the way cloud phase changes under global warming has emerged as a critically important climate feedback. Atmospheric aerosols may also have affected climate via mixed-phase clouds, but the magnitude and even sign of this effect is currently unknown. Satellite observations have revealed that cloud phase is misrepresented in global climate models (GCMs), suggesting systematic GCM biases in both precipitation formation and cloud-climate feedbacks. Such biases give us reason to question GCM projections of the climate response to increasing CO2 concentrations or changing atmospheric aerosol loadings.
In the MC2 project we seek to address the above issues, through a multi-angle and multi-tool approach:
(i) By conducting extensive field measurements of cloud phase at mid- and high latitudes, we seek to identify the small-scale structure of mixed-phase clouds.

(ii) Large Eddy Simulations are employed to identify the underlying physics responsible for the observed structures, and the field measurements will provide case studies for regional cloud-resolving modelling in order to test and revise state-of-the-art cloud microphysics parameterizations.

(iv) GCMs, with revised microphysics parameterizations, are confronted with cloud phase constraints available from space.

(v) The validated GCMs will be used to re-evaluate the climate impact of mixed-phase clouds in terms of their contribution to climate forcings and feedbacks.

The overall objective can be summarised as follows: Through a synergistic combination of tools for the study of mixed-phase clouds at a range of scales, the MC2 project will move the field of climate science forward, from improved process-level understanding at small scales, to better climate change predictions on the global scale.

MC2 sub-objectives include:
1) To determine the small-scale structure of mixed-phase clouds, and the extent to which it matters. Specifically, determine to what extent cloud phase is spatially homogeneous, as opposed to non-uniform and ‘patchy’, and whether environmental factors like turbulent mixing play a role in cloud homogeneity.
2) To re-evaluate the aerosol effect on mixed-phase clouds. By first assessing the ability of GCMs with the most sophisticated cloud microphysics representations available to reproduce observed mixed-phase clouds, we will re-evaluate the aerosol effect on mixed-phase clouds.
3) To determine the large-scale variability of mixed-phase clouds, and specifically whether spatial and temporal variations in cloud phase for a given isotherm can be explained predominantly by variations in INP.
4) To provide new and improved estimates of the strength of the cloud-phase feedback and a re-evaluation of its importance for mid- and high-latitude climate change.
Work performed within the project so far has shed new light on how aerosols can influence clouds by acting as ice-nucleating particles in a range of cloud regimes (ice clouds, stratiform mixed-phase clouds, and convective mixed-phase clouds). It has further advanced understanding of the thermodynamic phase composition of clouds, and how it varies spatially and temporally. It has revealed that cloud phase can be heterogeneous both vertically and horizontally, affecting the radiative properties of mixed-phase clouds and hos these change with a changing climate. A new instrument for investigations of these small-scale structures of mixed-phase clouds has been developed and will shortly be ready for field measurements.
As evidenced by the list of publication in leading disciplinary and interdisciplinary journals and associated media attention, findings from the project so far are considered of high interest to the climate research community and beyond. The recent discovery of the state-dependence of climate feedbacks operating through mixed-phase clouds is perhaps most noteworthy, and has wide-reaching implications. Ongoing and upcoming research that we expect important outcomes from before the end of the project relate especially to field measurements and high-resolution modeling, which can help answer the fundamental question of what controls the composition and structure of mixed-phase clouds. This will in turn aid understanding of the role that these clouds have played in global warming so far, and of the role they will play in future climate change.