Source: Dissertation Abstracts International, Volume: 78-07(E), Section: B.

Adviser: Trude Storelvmo.

Motivated by the fact that clouds and aerosols currently continue to contribute the largest uncertainty to estimates of Earth's changing energy budget, this thesis addresses the climatic impact of mixed-phase clouds and the dust aerosols that act as ice nuclei (IN) to form the ice crystals in these clouds. Mixed-phase clouds are composed of mixtures of supercooled liquid droplets and ice crystals that form at temperatures between 0 and ~ --35°C and are ubiquitous in Earth's mid- and high-latitudes. Since liquid droplets tend to be smaller in size and larger in quantity compared to ice crystals, mixed-phase clouds with higher liquid fractions tend to be more reflective to sunlight than those with lower liquid fractions. Earth's energy budget thus delicately depends on the thermodynamic phase partitioning of these clouds.

This thesis begins by first examining the ice nucleating potential of dust aerosols as well as polluted dust and smoke aerosols at mixed-phase cloud temperatures using global satellite observations from NASA's dual-wavelength polarization lidar, CALIOP. A global-scale analysis suggests that dust aerosols are more efficient at ice nucleation than polluted dust aerosols and that smoke aerosols may also be able to act as IN.

Next, the global satellite observations are applied to constrain the thermodynamic phase partitioning of mixed-phased clouds in an atmospheric global climate model (GCM) in which the supercooled liquid content of the mixed-phase clouds, like many other GCMs, is underestimated. The GCM is constrained using Quasi-Monte Carlo sampling of six cloud microphysical parameters. A global-scale variance-based sensitivity analysis reveals that the Wegener-Bergeron-Findeisen timescale for the growth of ice crystals at the expense of liquid droplets and the ice nucleation by large dust particles are the two parameters that exert the largest influence on phase partitioning of mixed-phase clouds.

Finally, two of the observationally-constrained simulations deemed to best match the observations, along with a control simulation as well as an upper and lower bound simulation in which the mixed-phase clouds have maximally high and low percentages of liquid are run to equilibrium in a fully-coupled climate model configuration. The results reveal a strong positive correlation between the percentage of supercooled liquid in the mixed-phase clouds and the equilibrium climate sensitivity (ECS). Moreover, the control simulation has an ECS that is 1.0 and 1.3°C lower than that of the observationally-constrained simulations. Simulations with higher supercooled liquid fractions in their mixed-phase clouds prior to CO2 doubling exhibit a weaker cloud phase feedback. The cloud phase feedback is a negative feedback that counteracts global warming by increasing the amount of shortwave radiation reflected back out to space. In a global warming situation, the troposphere deepens, causing isotherms to be located higher in altitude compared to where they were prior to global warming. This results in an additional layer of liquid in the global warming situation, which increases the optical thickness of the mixed-phase clouds. Thus, mixed-phase clouds with lower fractions of liquid will always exhibit a stronger cloud phase feedback than those with higher fraction of liquid. The feedback is particularly prominent in the extratropics. Its weakening suggests that GCMs with low biases in their mixed-phase cloud supercooled liquid contents are also underestimating the ECS by overestimating the cloud phase feedback.