Simulation of the Effect of Small-Scale Mountains on Weather Conditions During the May 2021 Ultramarathon in Gansu Province, China

Hypothermia killed 21 participants of the 100 km cross-country Ultramarathon Mountain race in Baiyin, Gansu Province in northwestern China on 22 May 2021, when a large-scale cold front passed by the race site. The hypothermia was caused by low temperatures, high winds, and hail-like precipitation, as reported by survivors. However, there is no meteorological station near the race site, leading to a lack of data on the exact weather conditions during the race. Considering the complex topography of the race site, the weather conditions must be re-built using high-resolution model simulations, so that the tragedy can be investigated. In this study, the weather conditions in this mountainous area were simulated using the Weather Research and Forecasting model with a 333 m horizontal grid size. The results show that interaction between the mountain wind and orographic precipitation led to a 6.7 degrees C decrease in temperature and a 12 m s(-1) increase in wind speed. Precipitation increased owing to orographic accent and convergence, and fell as graupel. High-speed downslope winds were the result of large-amplitude trapped lee waves. The combination of low temperatures, high winds, and spillover graupel on the lee resulted in the blizzard-like weather during the race, thus the hypothermia amongst the runners. The apparent temperature was estimated to be as low as -10 degrees C. Our study shows that mesoscale numerical models with a grid size of hundreds of meters should be used to improve weather forecasts in mountainous regions and prevent future tragedies for special events.