Statistical Characteristics and Formation Mechanisms of Night Warming Events at Yunding Winter Olympic Stadium in Chongli

Based on the observed data from automated weather stations during November 2018–March 2019 and November 2019–March 2020 at the Yunding Winter Olympic Stadium in Chongli, Hebei Province, nocturnal warming events at the stadium are statistically analyzed in this paper. Several possible formation mechanisms for these events are then discussed based on microwave radiometer, three-dimensional laser radar and the wind profiler data, and reanalysis data from NCEP/NACR. The results show that the occurrence probability of nocturnal warming events in Yunding Stadium reaches 76.9% from November to the following March, indicating that such warming events are a common phenomenon there. The frequency and amplitude of the warming decrease with increasing altitude. The triggering mechanisms of nocturnal warming events at Yunding Stadium can be classified into four categories: (1) mixed warming in the inversion layer caused by vertical wind shears, (2) Foehn-type warming, (3) whole-layer subsidence warming, and (4) warm advection in the mid and low levels. During the warming process of the second through 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 an initial temperature inversion in the valley. In this scenario, warming amplitude decreases with increasing altitude. The warming at the top of the mountain is only affected by the third and fourth types of formation mechanisms, while all four types can trigger warming events in the valley. This explains why the occurrence frequency of warming events decreases with increasing altitude.