Dust particles are increasingly considered as having important effects on weather through their influence on atmospheric dynamics, radiation budget, clouds and precipitation. As a major dust contributor in the world, East Asian dust is considered to play a key role in determining global and regional dust variations, and impose significant effects on regional or even global weather system. This thesis presents the work on improving the numerical simulation of dust activity and its effects over East Asia in a regional modeling system.

Large uncertainties exist in numerical dust emission simulations over East Asia due to errors in physical parameterizations or model configurations. The sensitivity of the Weather Research and Forecast model coupled with Chemistry module (WRF-Chem) to diffe...[ Read more ]

Dust particles are increasingly considered as having important effects on weather through their influence on atmospheric dynamics, radiation budget, clouds and precipitation. As a major dust contributor in the world, East Asian dust is considered to play a key role in determining global and regional dust variations, and impose significant effects on regional or even global weather system. This thesis presents the work on improving the numerical simulation of dust activity and its effects over East Asia in a regional modeling system.

Large uncertainties exist in numerical dust emission simulations over East Asia due to errors in physical parameterizations or model configurations. The sensitivity of the Weather Research and Forecast model coupled with Chemistry module (WRF-Chem) to different dust emission schemes and soil data is investigated in this study, in order to quantify the uncertainties and look for a better configuration for stable long-term dust simulation over East Asia.

Dust particles are considered as effective ice nuclei, which play an important part in ice nucleation process in the atmosphere, affecting the formation and evolution of ice and mixed-phase clouds. However, there is no effective and efficient way to evaluate this microphysical effect of dust in regional models. To fill the gap, a coupled dust-microphysics scheme is implemented into WRF-Chem, enabling the model to estimate the effect of dust in atmospheric microphysical processes, especially ice nucleation process within ice clouds and mixed-phase clouds. Further sensitivity experiments are carried out to optimize the parameters in the ice nucleation scheme. This work substantially improves the capability of WRF-Chem in simulating atmospheric ice water content, cloud cover, and cloud radiative effect enhanced by dust.

Based on the work above, the full effects of dust aerosol, including direct radiative effect, cloud radiative effect, and indirect microphysics effect, on the weather system over East Asia during a typical dust-intensive period are evaluated. This is the first time that the microphysical and full effects of dust can be numerically estimated in East Asia.

The comparison with comprehensive stationary and satellite-observations reveals that the model is capable of well reproducing dust emission, surface PM₁₀ concentration during dust events. Furthermore, the model has a reasonable performance in simulating the dust-induced modification on optical properties, such as AOD and extinction coefficient over East Asia during dust events. The model yields the best performance in dust simulation with Shao’s emission scheme and USGS soil data.

By comparing with satellite-observations, the GOCART-Thompson microphysics scheme is demonstrated to remarkably improve the performance of the model in simulating the atmospheric ice water content, with effect of dust taken into account in the atmospheric microphysical processes. The comparison between observational and simulated precipitation at over 50 meteorological stations over China also suggests that, the performance of WRF-Chem in simulating precipitation over East Asia is improved by taking the effect of dust in microphysical processes into account in the simulation.

For the modification of the regional weather system induced by the effects of dust over East Asia during the investigated period, the direct radiative effect of dust combining with the cloud radiative effect enhanced by dust causes a loss of net radiation at the Earth’s surface, whereas a gain of net radiation within the atmosphere, leading to a cooling effect at the surface and lower troposphere, and a warming effect in mid to upper troposphere. The radiative forcing caused by the cloud radiative effect enhanced by dust is much more significant than that caused by direct radiative effect of dust, especially for LW radiative forcing, which is highly affected by cloud cover.

The atmospheric ice water path and ice crystal number density is significantly increased over East Asia, with abundant ice nuclei served by dust particles. By contrast, the atmospheric cloud water path and cloud droplet number density is substantially reduced over East Asia.

The convective precipitation is inhibited at most areas in East Asia, which is resulted from the enhancement of atmospheric stability over East Asia, and the reduction of cloud droplets that can grow into rain droplets in proper conditions. Much less cloud droplets in the atmosphere, combining with the warming effect within the atmosphere caused by the radiative forcing and latent heat released by the freezing of super-cooled water droplets, results in higher saturation pressure for water vapor and faster evaporation rate for cloud droplets, and further leads to an inhibition of non-convective precipitation. The decrease in convective and non-convective precipitation results in a reduction of total precipitation over East Asia.