Date of Award

1-1-2021

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

College/School/Department

Department of Atmospheric and Environmental Sciences

Content Description

1 online resource (xii, 185 pages) : color illustrations, color maps.

Dissertation/Thesis Chair

Fangqun Yu

Committee Members

Ryan Torn, Qilong Min, Kara Sulia

Keywords

aerosol-cloud interactions, cloud microphysics, dust indirect effects, Mineral dust, The GEOS-Chem model, WRF model, Mineral dusts, Cloud physics, Precipitation (Meteorology), Microphysics

Subject Categories

Atmospheric Sciences

Abstract

For its enormous emission rate and long-range transport ability, mineral dust is one of the most abundant aerosol species, contributing to about half of the global atmospheric aerosol dry mass burden. A series of studies demonstrated the significant influences of mineral dust on air quality, public health, biogeochemical cycles, and climate systems. In the atmosphere, mineral dust is known to have important impacts on the Earth system through direct effects (scattering and absorbing shortwave and longwave radiations), semi-direct effects (changing atmospheric temperature structure and cloud lifetime), and indirect effects (influencing cloud microphysics processes as cloud condensation nuclei (CCN) and ice-nucleating particles (INPs)).Although dust impacts on clouds and precipitations are recognized as important, the dust effects remain to have significant uncertainties and are highly dependent on the specific cloud types and meteorological conditions. This study, based on numerical simulations and analysis of data from multiple sources, focuses on the characteristics of dust aerosol and dust effects on climate/weather systems over two representative areas, the Taiwan and northeastern United States (NEUS) regions. There are three components of this research. (1) By analyzing long-term simulations from a global chemical transport model with size-resolved particle microphysics (GEOS-Chem-APM), we examine the variations of the dust number concentration in the two regions in the last two decades. The results demonstrate that the seasonal variations and vertical distributions of the mineral dust over these two regions are controlled by the long-range transport of dust. During dust events, the dust number concentration over these remote regions can be high enough to influence the formation and development of the ice/mixed-phase cloud. The results also show similar decreasing trends of the atmospheric dust loading and event frequencies in both Taiwan and the NEUS regions, which can be related to the decline in East Asian dust emission in the last decades. (2) Based on the understanding of dust properties over the selected regions, the impacts of mineral dust on cloud and precipitation are investigated through analysis of long-term multiple observational and modeling data sets. After eliminating the potential influences from some co-varying meteorological conditions (water vapor, wind speed, low-level moisture flux, and sea surface temperature), the result indicates an evident positive correlation between the long-range transported mineral dust and the summertime clouds/precipitations over the mountainous region in Taiwan. The precipitation and ice/liquid water paths in the non-typhoon precipitation days show positive responses to the increasing atmospheric dust loading. (3) To investigate the detailed physical processes and mechanisms of the dust-cloud interaction, numerical experiments are carried out to study the dust effects on clouds and precipitations. Over the Taiwan region, we run the Weather Research and Forecasting model (WRF) coupled with the modified spectral-bin microphysics and Morrison two-moment schemes, with the dust number concentrations from the GEOS-Chem-APM simulation. The result indicates that the long-range transported dust particles, with relatively low number concentrations (~0.6 cm-3 under 3 km), can notably affect the convective cloud (ice/liquid water content, updraft intensity, cloud top height, cloud coverage, etc.) and precipitation (intensity, spatial pattern, and evolution) through microphysical and invigoration effects. In this numerical study, the saturation adjustment approach utilized in bulk microphysics schemes is suggested to influence the simulation of the dust-cloud interactions.

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