Compound extreme events of rapid warming and air pollution driven by foehn over the northern slope of the middle tianshan mountains

This study examines a severe foehn event that occurred in winter on 19–20 February 2021 over the northern slope of the Middle Tianshan Mountains in Xinjiang, China, which produced extreme warming, gale-force winds, and heavy air pollution. By combining high-density surface observations, radiosonde data, microwave radiometer measurements, environmental monitoring records, reanalysis products, and WRF model simulations, we investigate the touchdown dynamics and the associated multi-phase environmental impacts. The main findings are as follows: (1) The foehn touchdown was not progressive but abrupt, triggered by intense mechanical turbulence from extreme vertical wind shear (150.1(m/s)/km between 850 and 890 hPa, with a Richardson number of 0.036). This shear broke through the low-level temperature inversion and drove a peak hourly warming of 20.9 °C. (2) WRF simulations reveal a distinct surface thermal structure during the mature stage, characterized by a warm core inside the canyon and cooler temperatures in the surrounding plains. Within the canyon, the convergence of southeasterly and northwesterly flows produced a micro-front, which was responsible for the pronounced local temperature fluctuations. (3) The foehn exhibited a phased dual effect on air quality. Before touchdown, the stable inversion layer trapped pollutants, allowing PM 2.5 to build up to 224 µg/m³ at Urumqi Station. After touchdown, while strong winds enhanced particulate dispersion, the extreme warmth and intensified solar radiation concurrently accelerated photochemical ozone formation, with the maximum O₃-8 h reaching 158 µg/m³ at Miquan Station. This case study reveals the coupling among thermal, dynamic, and environmental effects during foehn events, providing a scientific basis for forecasting and combined pollution management over complex terrain.