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 addressed 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, to identify the small-scale structure of mixed-phase clouds.
(ii) High-resolution model simulations were employed to identify the underlying physics responsible for the observed structures, and the field measurements provided case studies for regional cloud-resolving modelling that allowed us to test and revise state-of-the-art cloud microphysics parameterizations.
(iv) GCMs, with revised microphysics parameterizations, were confronted with cloud phase constraints available from space.
(v) The validated GCMs were used to re-evaluate the climate impact of mixed-phase clouds in terms of their contribution to climate forcings and feedbacks.
Through a synergistic combination of the above tools for the study of mixed-phase clouds at a range of scales, the MC2 project has contributed to moving 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.
