Spatial and temporal characteristics and meteorological indexes of late spring coldness in Qingdao

Meteorological services play a role in agriculture and improve defense capability. This study used data from seven national meteorological stations in Qingdao, China, including the daily mean and minimum temperatures during the spring (3-5 months) of 1961-2015 and the crop frost or low-temperature damage from disasters, as categorized by the national standard of GB/T 34816-2017 "meteorological indicators of late spring coldness."The meteorological index and temporal and spatial characteristics of the late spring coldness were statistically analyzed. The results showed that there were two grades of mild and moderate cold periods (no severe) in late spring (67.3% and 32.7%, respectively). The average annual occurrence of late spring coldness was 1.1 stations, mainly in April. Since the 1990s, the interannual or intergenerational changes had decreased, and the spatial distribution gradually decreased from the northwest to the southeast coast. Moderate Jimo had the greatest probability of occurrence, and the probability of causing late spring coldness was 26.3%. Based on the meteorological conditions of the disaster, we can derive the meteorological index of disaster-induced late spring coldness. The mild minimum temperature dropped to 0-5℃, the average temperature anomaly was -4 to -2℃, and the duration was 3-5 days. The moderate minimum temperature dropped below 0℃, and the average temperature anomaly was < -4℃ for more than 6 days. From the influence scope, occurrence time, and disaster-causing analysis, the moderate spring coldness impact range was regional and occurred from late March to mid-April, which may cause crop freezing damage. The mild influence range was small, generally < 2 stations, the time was late, and the probability was greater in the middle and late April, which may cause frost or chilling damage. The cold spring weather caused late spring coldness; 26.3% of the coldness resulted in agricultural production-associated economic losses. Interestingly, the frequency of mild late spring coldness was higher than that of moderate late spring coldness, and the occurrence time overlapped with the critical growth period of the main crops (i.e., the wheat jointing stage to booting stage, the flowering stage of most fruit trees). This needs to be prevented. Frost damage, late frost injury, or low-temperature damage in the Qingdao area were all caused by late spring coldness. The interannual variability of the late spring coldness in Qingdao was not obvious, even interdecadal variability had even increased, which was not consistent with the overall distribution in Qingdao. This was related to the regulatory effects of the ocean. There are temperature differences from the south-eastern coast to the north-western inland region of Qingdao, and the marine climate effects also gradually weaken.