Ice nucleation in mixed-phase clouds has recently been identified as a critical factor in projections of future climate. Here we explore how this process influences climate sensitivity using the Community Earth System Model 2 (CESM2). We find that ice nucleation affects simulated cloud feedbacks over most regions and levels of the troposphere, not just extratropical low clouds. Ice nucleation’s impact on climate sensitivity is found to primarily operate through this process’s role setting global-scale cloud phase. Conversely, whether ice nucleation is treated as aerosol-sensitive is of limited importance. In satellite-constrained model experiments, dissimilar ice nucleation realizations all result in a strongly positive total cloud feedback, as in the default CESM2. A microphysics update from CESM1 to CESM2 had substantially weakened ice nucleation, due partly to a model issue. Our findings suggest that this contributed to increased climate sensitivity by reducing global cloud phase bias, resulting in more realistic mixed-phase clouds.

We conduct a global assessment of the spatial heterogeneity of cloud phase within the temperature range where liquid and ice can coexist. Single-shot CALIOP lidar retrievals are used to examine cloud phase at scales as fine as 333 m, and horizontal heterogeneity is quantified according to the frequency of switches between liquid and ice along the satellite’s path. In the global mean, heterogeneity is greatest between $-$15 and $-$4 \textdegree C with a peak at $-$5 \textdegree C, when small patches of ice are prevalent within liquid-dominated clouds. Heterogeneity “hot spots” are typically found over the extratropical continents, whereas phase is relatively homogeneous over the Southern Ocean and the eastern subtropical ocean basins, where supercooled liquid clouds dominate. Even at a fixed temperature, heterogeneity undergoes a pronounced annual cycle that, in most places, consists of a minimum during autumn or winter and a maximum during spring or summer. Based on this spatial and temporal variability, it is hypothesized that heterogeneity is affected by the availability of ice nucleating particles. These results can be used to improve the representation of subgrid-scale heterogeneity in general circulation models, which has the potential to reduce longstanding model biases in cloud phase partitioning and radiative fluxes.

Mixed-phase clouds play an important role in determining Arctic warming, but are parametrized in models and difficult to constrain with observations. We use two satellite-derived cloud phase metrics to investigate the vertical structure of Arctic clouds in two global climate models that use the Community Atmosphere Model version 6 (CAM6) atmospheric component. We report a model error limiting ice nucleation, produce a set of Arctic-constrained model runs by adjusting model microphysical variables to match the cloud phase metrics, and evaluate cloud feedbacks for all simulations. Models in this small ensemble uniformly overestimate total cloud fraction in the summer, but have variable representation of cloud fraction and phase in the winter and spring. By relating modelled cloud phase metrics and changes in low-level liquid cloud amount under warming to longwave cloud feedback, we show that mixed-phase processes mediate the Arctic climate by modifying how wintertime and springtime clouds respond to warming.

An observationally-constrained time series of historical aerosol effective radiative forcing (ERF) from 1750 to 2019 is developed in this paper. We find that the time history of aerosol ERFs diagnosed in CMIP6 models exhibits considerable variation and explore how the time history of aerosol forcing influences the probability distributions of present-day aerosol forcing and emergent metrics such as climate sensitivity. Using a simple energy balance model, trained on CMIP6 climate models and constrained by observed near-surface warming and ocean heat uptake, we derive estimates for the historical aerosol forcing. We find 2005-2014 mean aerosol ERF to be -1.1 (-1.8 to -0.5) W m-2 relative to 1750. Assuming recently published historical emissions from fossil fuel and industrial sectors and biomass burning emissions from SSP2-4.5, aerosol ERF in 2019 is -0.9 (-1.5 to -0.4) W m-2. There is a modest recovery in aerosol forcing (+0.025 W m-2 decade-1) between 1980 and 2014. This analysis also gives a 5-95% range of equilibrium climate sensitivity (ECS) of 1.8-5.1C (best estimate 3.1C) with a transient climate response (TCR) of 1.2-2.6C (best estimate 1.8C).

The mechanisms which amplify small orbitally driven changes in insolation and drive the glacial-interglacial cycles of the past 2.7 million years are poorly understood. Previous studies indicate that cloud feedbacks oppose ice sheet initiation at times when orbital configuration supports ice sheet growth. A recent study in which cloud phase was observationally constrained by satellite measurements provides evidence for a weaker opposing cloud feedback than previously found in response to carbon dioxide doubling (Tan et al., 2016). We observationally constrain cloud phase in the Community Earth System Model. We find a weaker cloud phase feedback, which unmasks water vapor and cloud feedbacks that extend cooling to lower latitudes. Snowfall accumulation and ablation metrics also support ice sheet expansion as seen in proxy records. Our results indicate that well understood cloud and water vapor feedbacks are the amplifying mechanism driving orbital climates.

One of the largest uncertainties in numerical weather prediction and climate models is the representation of mixed-phase clouds, in which supercooled liquid water and ice can coexist. The aim of our study is to understand how the supercooled liquid fraction (SLF) in clouds with temperature from -40°C to 0°C is related to temperature, geographical location, and cloud type. Our analysis contains a comparison of four satellite-based datasets, one derived from active and three from passive satellite sensors, and focuses on SLF distribution near-globally, but also stratified by hemisphere and continental/maritime regions. Despite the SLF differences found among the datasets, they commonly indicate an increase of SLF with COT, and generally larger SLF in the Southern Hemisphere than in the Northern Hemisphere (up to about 20% difference), with the exception of continental low-level clouds, for which the opposite is true.

We conduct a global assessment of the spatial heterogeneity of cloud phase within the temperature range where liquid and ice can coexist. Single-shot CALIOP lidar retrievals are used to examine cloud phase at the 333-m scale, and heterogeneity is quantified according to the frequency of switches between liquid and ice along the satellite's path. In the global mean, heterogeneity is greatest from -15 to -2C with a peak at -4C, when small patches of ice are prevalent within liquid-dominated clouds. Above -20C, heterogeneity is greatest in the northern midlatitudes and lower over the Southern Ocean, where supercooled liquid clouds dominate. Zonal mean heterogeneity undergoes an annual cycle with a peak that follows seasonal shifts in the extratropical storm track. These results can be used to improve the representation of subgrid-scale heterogeneity in general circulation models, which has the potential to reduce model biases in phase partitioning and radiation balance.

Shortwave radiative feedbacks from Southern Ocean clouds are a major source of uncertainty in climate projections. Much of this uncertainty arises from changes in cloud scattering properties and lifetimes that are caused by changes in cloud thermodynamic phase. Here we use satellite observations to infer the scattering component of the cloud-phase feedback mechanism and determine its relative importance by comparing it with an estimate of the overall temperature-driven cloud feedback. The overall feedback is dominated by an optical thinning of low-level clouds. In contrast, the scattering component of cloud-phase feedback is an order of magnitude smaller and is primarily confined to free-tropospheric clouds. The small magnitude of this feedback component is a consequence of counteracting changes in albedo from cloud optical thickening and shifts in the scattering direction of cloud particles. These results indicate that shortwave cloud feedback is likely positive over the Southern Ocean and that changes in cloud scattering properties arising from phase changes make a small contribution to the overall feedback. The feedback constraints shift the projected 66% confidence range for the global equilibrium temperature response to doubling atmospheric CO2 by about +0.1 K relative to a recent consensus estimate of cloud feedback.

Aerosols interact with radiation and clouds. Substantial progress made over the past 40 years in observing, understanding, and modeling these processes helped quantify the imbalance in the Earth’s radiation budget caused by anthropogenic aerosols, called aerosol radiative forcing, but uncertainties remain large. This poster presents the outcome of an international workshop and subsequent review paper, which quantify the likely range of aerosol radiative forcing over the industrial era based on multiple lines of evidence, including modelling approaches, theoretical considerations, and observations. Improved understanding of aerosol absorption and the causes of trends in surface radiative fluxes narrow the range of the forcing from aerosol-radiation interactions compared to the latest assessment by the Intergovernmental Panel on Climate Change (IPCC). A robust theoretical foundation and convincing evidence constrain the forcing caused by aerosol-driven increases in liquid cloud droplet number concentration. However, the influence of anthropogenic aerosols on cloud liquid water content and cloud fraction and on mixed-phase and ice clouds remains poorly constrained. Observed changes in surface temperature and radiative fluxes provide additional constraints. These multiple lines of evidence lead to total aerosol radiative forcing ranges that are of similar width to the last IPCC assessment but more clearly based on physical arguments.