The Observation-Based Index to Investigate the Role of the Lee Vortex in Enhancing Air Pollution over Northwestern Taiwan

This study quantifies the potential effect of the lee vortex on the fine particulate matter (PM2.5) pollution deterioration under the complex topography in Taiwan using observational data. We select the lee-vortex days that favor the development of the lee vortices in northwestern Taiwan under the southeasterly synoptic winds. We then define the en-hancement index that discerns the areas with the high occurrence frequencies of the PM2.5 enhancement under the flow re-gime relative to the seasonal background concentrations. Under the lee-vortex days, the center of western Taiwan exhibits enhancement indices higher than 0.65. In addition, during the consecutive lee-vortex days, the index characterizes a north-ward shift in the PM2.5-enhanced areas under the subtle transition of the background wind directions. The areas with indi-ces higher than 0.65 expand on the second day in northwestern Taiwan; the number of stations exhibiting indices higher than 0.8 increases by threefold from the first to the second day. The idealized numerical simulations using the Taiwan vec-tor vorticity equation cloud-resolving model (TaiwanVVM) explicitly resolve the structures of leeside circulations and the associated pollutant transport, and their evolutions are highly sensitive to the background winds.