A Simulation Study On Dust Aerosol Transport From Troposphere To Stratosphere Over The Tibetan Plateau

Using NCEP/FNL data and satellite observations, together with a mesoscale chemical transport model (WRF-Chem) and the mesoscale model WRF coupled with a dust module (WRF-Dust), a dust event occurred in the Taklimakan desert, north of Tibetan Plateau(TP) on April17,2007is analyzed to understand the characteristics and mechanisms of dust aerosol transport from the surface to the upper troposphere and the lower stratosphere over Tibetan Plateau and its vicinity. In addition, differences in the transport of particles with different sizes and the factors affecting the dust aerosols transport over the TP are also discussed. Finally, the effects of the coagulation, the below-cloud scavenging and the dry deposition on dust transport are analyzed using the WRF-Dust. Main conclusions are summarized as the following:(1) Using a mesoscale chemical transport model (WRF-Chem), a dust event occurred in the Taklimakan desert is simulated. The results indicate that vertical transport of dust aerosols is closely related to the vertical distribution of background horizontal winds. In the absence of the cloud microphysical processes, when a deep convergence zone of northerly and southerly winds forms over Tibetan Plateau, the vertical motions resulted from the convergence could transport dust aerosols, originated from the Taklimakan desert, to the lower stratosphere with an evident inclined transport pathway over Tibetan Plateau. The result implies that the position of the dust reached the lower stratosphere may not colacate with the surface dust source. In addition, dust particles with different sizes present different transfer characteristics. Small dust particles can be transported to the lower stratosphere relative easily by upward motions, while particles with size larger than8.0μm could not be transported to the lower stratosphere due to strong deposition process.(2) The effects of the surface emission of aerosols and orography height on the dust transport over the TP are investigated through a series of WRF-Chem sensitivity simulations. The result demonstrates that:The dust transport changes significantly when the orography height decreased by30%and50%. The results from sensitivity experiments show that the stronger zonal wind over the North TP depresses upward and southward transport of dust aerosols when orography height is decreased. The result of the emission-controlled experiments shows that, the dust aerosol can be transported to the lower stratosphere over the Tianshan Mountains, Himalayas and the South TP more easily due to the higher orography height of these areas.(3) Finally, dust aerosol transport over Tibetan Plateau is investigated by using the WRF-Dust model. The result demonstrate that WRF-Dust model can also simulate the basic characteristics of dust transport, although the radiative and microphysical processes associated with dust aerosols are not included in the model. Due the effect of a cold trough located over the Tibetan Plateau, a deep convergence zone of northerly and southerly winds formed which gives rise to upward transport of aerosols from the troposphere to the lower stratosphere. Due to the effect of strong vertical diffusion in the WRF-Dust model, the distributions of the dust aerosols in the upper troposphere are similar to those in lower stratosphere. The coagulation, dry deposition and bellow-cloud scavenging processes have no significant effects on the distributions of the dust aerosols, but can cause significant decrease of the dust aerosol concentration in the upper troposphere. The numerical experiments with and without coagulation, dry deposition and bellow-cloud scavenging processes show that the coagulation process have no significant effect on the concentrations of the dust aerosols. The dry deposition plays an important role in clearing the dust aerosols, the below-cloud scavenging has little effect on aerosol concentrations near the ground, but has a strong effect on the dust aerosol concentration in the upper troposphere and lower stratosphere, and its impact on aerosol concentration which is comparable to that of the dry deposition process in magnitude.