Characteristics of First/Last Frost Date Events and Frost-free Period in Dalian Area During Recent 46 Years

Based on the frost index defined by the daily minimum air temperature ≤ 0℃, and the method of climate diagnosis, the climatic features of the first/last frost dates and frost-free period changes are analyzed with 3 national stations data of daily minimum temperatures in Dalian area from 1961 to 2006. Results show that the first frost dates are postponed, while the last frost dates occur earlier, leading to the significant lengthening of frost-free periods at both Dalian and Wafangdian stations during recent 46 years, with the change being more significant at Wafangdian station. The tendency of the first/last frost dates is statistically significant at Dalian and Wafangdian stations, but it is not for Zhuanghe station, though the increase in frost-free periods at the station can still pass the confidence test. On the other hand, the characteristics in 1960s and 1970s are that the first frost dates comes earlier, the last frost dates ends later so that the frost-free period is short at each station. There is a trend that the first frost dates come later and the last frost dates appear earlier from 1960s to 1980s. The first frost dates are the latest, the last frost dates end the earliest and the frost-free period is the longest in early 1990s at Dalian station. It is notable that, in the past 6 years, the first frost dates appear much later and the last frost dates end much earlier at Wafang-dian and Zhuanghe stations, but the dates tend to be stable near the normal values at Dalian station. The change of extreme frost dates for the study area is also analyzed. It is found that the frequency of extreme early first-frost events ranges from 1 to 3 times, with Dalian station witnessing the highest value, and Wafangdian station the lowest. The frequency of extreme late last-frost events is from 1 to 4 times, but it is also the highest at Dalian station and the lowest at Wafangdian station. In addition, the variability of first frost dates indicates 2-year and 11-year cycle and that of last frost dates bears 22-year remarkable quasi-periods at Dalian station, while the first frost dates and frost-free periods at Zhuanghe station have undergone 11-year quasi-periods during recent 46 years. The frequency of anomalous frost events is the highest in 1960s at Dalian and Zhuanghe stations, and the lowest in early 21 century, but it is the highest in early 21 century and the lowest in 1980s at Wafangdian station.