Hebei Meteorological observatory
Based on the observation data from the automatic weather stations during November 2018–March 2019 and November 2019–March 2020 at Yunding Winter Olympic Stadium in Chongli, Hebei Province, the night warming events in the stadium are statistically analyzed in this paper, and then several types of possible formation mechanisms for these events are further discussed through the microwave radiometer, the three-dimension laser radar and the wind profiler data, and the reanalysis data from NCEP/NACR. The results show that the occurrence probability of night warming events in Yunding Stadium reaches 76.9% from November to the March next year, indicating that warming event is a common phenomenon there. The frequency and amplitude of warming decrease with the increase of the station altitude. The triggering mechanisms of night warming events in Yunding Stadium can be classified into four categories such as the mixed warming in the inversion layer caused by vertical wind shears, the foehn warming, the whole-layer sinking warming and the warm advection warming in mid and low level. During the warming process of the second to the fourth types, the warming amplitude at the bottom of the valley may be significantly larger than that at the top of the mountain when there is a temperature inversion in the valley before the start of warming, thereby the warming amplitude decreases with the increase of altitude. The warming at the top of the mountain is only affected by the third and fourth types of formation mechanisms, while all the four types can form warming events at the valley. That is why the occurrence frequency of warming events decreases with the increase of altitude.