Quantifying Dust Ice Nucleation Effects on Mixed-phase Clouds and Contributions from Arctic Emissions and Agricultural Sources

Abstract

Mineral dust plays an important role in the primary formation of ice crystals in mixed-phase clouds by acting as ice nucleating particles (INPs). It can influence the cloud phase transition and radiative forcing of mixed-phase clouds, both of which are crucial to global energy budget and climate. In this dissertation, I investigate the dust indirect effects on mixed-phase clouds through heterogeneous ice nucleation with the U.S. Department of Energy (DOE) Energy Exascale Earth System Model (E3SM). First, constrained by observations, E3SM shows that dust INPs induce a global mean net cloud radiative effect of 0.05 to 0.26 W m-2 with the predominant warming appearing in the Northern Hemisphere midlatitudes. However, a cooling effect is found in the Arctic due to reduced longwave cloud forcing. Next, I examine the contribution of an overlooked source, dust emitted from local Arctic sources (i.e., high-latitude dust (HLD)), to the Arctic INP population, and its effects on Arctic mixed-phase clouds. It is found that modeled INP concentrations in the Arctic are in better agreement with observations after including HLD INPs. The HLD INPs are found to induce a net cooling effect (-0.24 W m-2 above 60 °N) on the Arctic surface downwelling radiative flux by changing the cloud phase of the Arctic mixed-phase clouds. The magnitude of this cooling is larger than that induced by North African and East Asian dust, which highlights the importance of HLD emissions to the Arctic regional climate. Furthermore, we conduct a model-observation comparison study to examine the model fidelity in simulating INP concentrations in the Arctic. Model performance in simulating meteorological conditions, aerosol properties, and INP concentrations are evaluated against multi-year observation datasets collected from Ny-Ålesund. Finally, a new and physically based method is proposed to parameterize anthropogenic dust (AD) emissions in E3SM. According to this method, the AD contributes to 13.5 % of total dust emission in present day. It is also found that AD emission increases by more than 300 Tg yr-1 from preindustrial to present day, which results in a direct AD radiative forcing of -0.028 W m-2.