The Impact of Taiwan’s Rugged Orography on Air Pollutant Transport and the Numerical Modeling of 20 March 2018 Case

Taiwan is a subtropical island with an area of only about 36,000 km² and yet packed with high density of mountains. There are 268 peaks that are taller than 3000 m in elevation and, as a result, the mountains are extremely rugged. Such rugged orography will certainly have great influence on the local circulation and consequently impact on the transport of air pollutants. It is thus necessary to understand the impact of the orography on air flow before we can interpret the measured data during the EMeRGe-Asia campaign in March-April 2018 correctly.

For the above purpose, we performed high resolution numerical simulations of the flow around Taiwan region for two cases using the Weather Research and Forecast (WRF) model. The first one is a highly stagnant case where Taiwan was under the influence of a high pressure system occurring on 10 November 2018. Two horizontal resolutions are used: 1 km and 2 km, both show very similar flow and cloud patterns as revealed by satellite images of the day. Detailed analysis of the simulated results including the flow pattern and isentropic analysis will be shown to illustrate that low level pollutants can be transported upward to at least 1 km altitude even under such calm weather.

The second one is the 20 March 2018 case which occurred during the EMeRGe-Asia campaign. Unlike the above stagnant case, this was a more turbulent situation when a typhoon was approaching from the east and a southerly flow carried air pollutants from SE Asia. The 1 km resolution simulation shows good match with satellite observation. The simulation results show a substantial concentration of VOC at ~ 3000 m altitude near Taiwan whereas the VOC was very low near the surface. The model reproduces this feature well and hence it appears that the model’s predictions are credible. More detailed analyses are being performed and comparison of the results with combined ground and aircraft observations to illustrate the impact of the orography on the transport of pollutants.