Global Modeling Study of Aerosol Indirect Effects in Mixed-Phase Clouds.

Abstract

Aerosol indirect effects (AIE) remains one of the largest uncertainties in understanding and projecting anthropogenic climate change. This study aims to improve the understanding of the aerosol indirect effect in mixed-phase clouds by studying the heterogeneous ice nucleation parameterization and aerosol-cloud interactions.

The first part of the dissertation enabled the assessment of aerosol indirect effects in mixed-phase clouds by implementing aerosol-dependent heterogeneous ice nucleation parameterizations into coupled CAM3+ and aerosol transport model IMPACT (CAM-IMPACT). The effect of different parameterizations on cloud liquid/ice water amount, cloud radiative forcing and anthropogenic aerosol forcing were compared. The aerosol-dependent freezing parameterization predicts less ice water path (IWP) than the original formulation, especially in the Southern Hemisphere. The net solar flux at top of atmosphere (FSNT), and net long-wave flux at the top of the atmosphere (FLNT) changes by up to 8.73 W/m2 and 3.52 W/m2, respectively, due to the use of different parameterizations in mixed-phase clouds.

In the second part of the dissertation, a 3-(hydrophobic, hydrophilic, and hygroscopic) soot scheme is developed, which provides a link between sulfuric acid coating, hygroscopicity, and ice nucleation efficiencies, according to laboratory observation. An offline radiation model was used to compare mixed-phase cloud AIE using with 3-soot scheme and 1-soot scheme. The new scheme results in significant changes to anthropogenic forcing in mixed-phase clouds. The net forcing in off-line studies varies from 0.111 to 1.059 W/m2 depending on the ice nucleation capability of hygroscopic soot particles.

The third part of this study is an assessment of the effect of marine organic aerosol (MOA) as ice nuclei on a global scale. The emission, transport, deposition, and ice nucleation for MOA were implemented into the coupled CAM-IMPACT model. Multiple sensitivity experiments were designed to test the effect of sea-spray emission functions, organic enrichment functions, and MOA ice nucleation efficiencies. The modeled ice water path and cloud radiative forcing were evaluated against various satellite observations. MOA is found to be the dominant ice nuclei species compared to dust and soot. The comparison of ice water path to ISCCP observation improves when MOA is included as ice nuclei.