Statistical Analysis of Spatial and Temporal Characteristics and Risk Probability of Late Spring Coldness in Beijing-Tianjin-Hebei Region

Based on the daily temperature data of 99 national meteorological stations in Beijing-Tianjin-Hebei region from March to May 1961 to 2020， the statistical methods such as Mann-Kendall trend test were used to analyze spatial and temporal distribution characteristics of late spring coldness from frequency， duration days and ratio of occurring stations to all stations， and the risk probability of different levels of late spring coldness was evaluated based on the information diffusion theory.The results indicated that： In the past 30 years， 2570 cases of late spring coldness occurred at 99 national meteorological stations， with the mildest and the least severe.In terms of frequency， the mild and moderate late spring coldness decreased from north to south， while severe late spring coldness decreased from northeast to southwest， the mild， moderate-severe and the total late spring coldness were most in April， the inter-annual occurrence frequency showed a downward trend， with a significant decrease from March to May and May.In terms of total duration days， mild and severe late spring coldness decreased from northeast to southwest， with moderate late spring coldness showing a pattern of high in north and south and low in middle.The average duration days of mild late spring coldness at each station was not significantly different， while the difference was significant in moderate and severe late spring coldness， the average duration days increased sharply with the increase of level.The annual mean duration days of late spring coldness was 6.9 d·a-1， showing a downward trend， with a decrease rate of 1.5 d·（10a）-1.The ratio of the stations showed an inter-annual downward trend in duration days was 88.9%， with 35 stations showing a significant decrease and the southern region experiencing a greater decrease than the northern region.The inter-annual downward trend in ratio of late spring coldness stations in March to May and May was significant， with the highest ratios of mild and moderate-severe late spring coldness in April， and the ratio of mild late spring coldness was much higher than that of moderate-severe late spring coldness in each month.The risk of late spring coldness decreased in a stepwise manner as the level increased， The stations with risk was mild and a return period of about 2 years or less was about 34.3% of all stations， mainly in Beijing， Shijiazhuang， Xingtai and Handan； The stations with risk was moderate and a return period of 5~10 years and 10 years or less was about 90% of all stations， and the former was concentrated at the north and south ends of Beijing-Tianjin-Hebei region， while the latter was mostly located in the central part of Beijing-Tianjin-Hebei region.The risk of severe late spring coldness in the research area was relatively low， with up to 97% of severe stations with a return period of about 25 years or less.