5.5      Impact of topography on aerosol transport from the southern Tibetan Plateau and its implication for aerosol climatic impact.

Zhao, Chun, Meixin Zhang, University of Science and Technology of China (USTC), China, Zhiyuan Cong, Institute of Tibetan Plateau research, China, Qiuyan Du, Mingyue Xu, Yu Chen, Rui Li, Yufei Fu, USTC, Ming Chen, and Jimy Dudhia, NCAR

The Tibetan Plateau (TP) has significant impacts on climate of Asia. Previous studies have shown that the glaciers on the TP are accelerating melting in the last decades, which will affect the Asian climate and also threaten the water resources in neighboring countries. Part of the reason may be that the light absorbing aerosols (LAA) can be transported from the southern slope of TP into the surface snow and atmosphere over the TP, and thereby affect the radiation budget of TP. Most of previous modeling studies about aerosol transport and impact over the TP used models with resolutions coarser than 10 km that may not be able to resolve well the complex topography of the southern slope of TP. In this study, the experiments with WRF-Chem at multiple resolutions are conducted for pre-monsoon season (April) to investigate the impacts of topography on modeling the aerosol transport and distribution over the TP. The WRF-Chem simulation is first time conducted at very high resolution (1 km) over the southern slope of TP as the reference to fully investigate the impact of complex topography. The station observations near the Mt. Everest and the surface and satellite retrievals near the southern slope of TP are used to evaluate the model simulations. The simulations at all resolutions show that the prevailing up-flow over the southern slope during the daytime is the dominant transport mechanism of aerosols over the TP, compared to the valley and glacier wind circulations. However, the simulations at relatively coarser resolution (22.5 km) and relatively finer resolution (4.5 km) show large differences in representing the distributions of topography of the southern slope of TP, which results in significantly different transport pathways of aerosols over the TP and also leads to large difference in simulated aerosol radiative forcing in the atmosphere and surface snow of TP. This implies that global climate models generally with even coarser resolutions than 22.5 km may introduce significant biases in estimating aerosol radiative forcing over the TP. It is noteworthy that the simulation at 1 km resolution produces similar results with the one at 4.5 km, which may suggest that the resolution around 4 km is fine enough to resolve the complex topography and hence the transport pathways of aerosols over the southern slope of TP.