1981—2020年西藏初霜日、终霜日及无霜期的时空变化特征

Spatial and Temporal Distribution of the First Frost Date,Last Frost Date and the Frost-Free Period in Tibet from 1981 to 2020

选取1981—2020年西藏38个气象站点逐日最低气温资料,以日最低气温≤0℃为霜冻指标,采用线性倾向估计、Pearson系数、Mann-Kendall和优势分析等方法,分析近40 a西藏高原初霜日、终霜日和无霜期的时空变化特征。结果表明:(1)西藏总体上表现为自东南向西北初霜日提早、终霜日推迟和无霜期减少的分布特征,并具有明显的海拔垂直地域性差异。1991—2020年与1981—2010年平均值比较,绝大部分站点初霜日推迟,终霜日提早,无霜期增加。(2)近40 a西藏38个站中,36个站初霜日推迟、终霜日提前,35个站无霜期延长。西藏平均初霜日显著推迟5.1 d/10 a,终霜日显著提早3.3 d/10 a,无霜期显著增加8.4 d/10 a。(3)初霜日在海拔3000~3500 m地区趋于提早,海拔3500 m以上地区随海拔升高推迟率增大。终霜日在不同海拔高度上提早特征趋同存异,低海拔地区(2000~3000 m)最明显。无霜期在各海拔高度上都呈增加趋势,海拔4500 m以上地区增幅最大。(4)20世纪80年代初霜日偏早,终霜日偏晚,无霜期偏短;90年代初霜日和终霜日均略早,无霜期变化不大;进入21世纪后,初霜日偏迟,终霜日偏早,无霜期明显增加,以21世纪10年代最为突出。(5)西藏平均初霜日、终霜日和无霜期分别在2005年、1994年和2003年发生了突变。

Based on the daily minimum temperature data of 38 meteorological stations in Tibet from 1981 to 2020,the daily minimum temperature≤0℃was defined as the frost index,the spatial and temporal variation characteristics of the first frost date(FFD),the last frost date(LFD)and the frost-free period(FFP)in the Tibetan Plateau in the past 40 years were analyzed by using linear tendency estimation,Pearson coefficient,Mann-Kendall and dominance analysis.The results showed that:(1)In general,Tibet showed the distribution characteristics of early FFD,delayed LFD and reduced FFP from southeast to northwest,with significant vertical zonal characteristics of altitude.Compared with the average value from 1981 to 2010,at most stations,the FFD was postponed,the LFD was earlier,and the FFP increased.(2)Among the 38 stations in Tibet,there were 36 stations where the FFD was delayed and the LFD became earlier,and there were 35 stations where the FFP prolonged from 1981 to 2020.The FFD significantly delayed at the rate of 5.1 d·10 a^(−1),the LFD significantly advanced by the rate of 3.4 d·10 a^(−1),and the FFP significantly prolonged with a rate of 8.4 d·10 a^(−1) during the past 40 years.(3)The FFD advanced in the altitude of 3000~3500 m,and the delay rate of the FFD increased with the increase of altitude in the area above 3500 m.The early characteristics of the LFD were similar in different altitude areas,especially in low altitude areas(2000~3000 m).The FFP increased at all altitudes,with the largest increase in areas above 4500 m.(4)In the 1980s,the FFD was earlier,the LFD was later,and the FFP was shorter.In the early 1990s,the FFD and LFD were slightly earlier,and the FFP did not change much.After entering the 21st century,the FFD was late,the LFD was early,and the FFP increased,especially in the 2010s.(5)The average of FFD,the LFD and FFP in Tibet had abrupt change in 2005,1994 and 2003,respectively.