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

Abstract

Abstract 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 enhancement index that discerns the areas with the high occurrence frequencies of the PM2.5 enhancement under the flow regime 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 northward shift in the PM2.5-enhanced areas under the subtle transition of the background wind directions. The areas with indices 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 vector 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. Significance Statement Our study investigates the challenging question of local circulation patterns affected by mountain orography and the associated pollutant transport. We analyzed long-term balloon sounding and ground station observations to select the weather regime favoring the formation of lee vortices on Taiwan island. We then quantified the areas with a frequent enhancement of particulate pollutants. The long-term trend of the lee-vortex days exhibited a significant increase in the past decades. The pollution enhancement areas are highly consistent with the regions dominated by leeside local circulation. Together with the idealized high-resolution simulations, we identified that the detailed evolution of the lee vortices is highly sensitive to the subtle changes in background wind direction and hence the redistribution of local pollution.