北极地区春季降水呈现固态向液态转变的态势

Arctic has been going through a transition from solid precipitation to liquid precipitation in spring

降水形态的变化可以影响地表的温度和反照率,对下垫面物质和能量平衡、陆地水文及生态系统均产生极大影响.基于美国阿拉斯加8站和加拿大11站日平均气温和固态、液态降水资料拟合的固-液态降水临界气温,辨析了1961~2010年环北极地区253个站点的降水形态时空变化特征.结果表明:60°N以北地区,降雨量占总降水量的比值(rainfall to total precipitation ratio,RPR)随纬度升高而减小.RPR气候平均态在夏季最高,秋季、春季次之,冬季最小.在不同季节,RPR变化趋势存在明显的区域差异.在春季,RPR变化趋势较为一致,在北极大部分地区(82.46%站点)呈增加趋势,且有22.37%站点通过显著性检验,表明北极大部分地区春季降水在过去50多年间呈现由固态向液态转变的趋势.使用95%置信区间上限和下限临界温度对降水形态进行划分和趋势分析,其结果与使用最优解的计算结果一致.在北极冰雪开始消融的春夏季节转换期(3~7月),阿拉斯加、中西伯利亚和北欧部分地区存在明显的固态降水向液态降水转变的趋势,这一趋势可能正在对北极地-气相互作用施加着影响.

The changes of precipitation form have important impacts on mass balance and energy flux of the underlying Cryosphere that closely related to hydrological cycles and the health of the ecosystem over the Arctic.So far,most of land stations around Arctic record the total precipitation amount only,except for dozens of stations in Alaska and northern Canada,where the solid and liquid precipitation are recorded respectively.Based on these different precipitation form records and daily mean air temperature observations during 1961–2015,we developed a special standard of optimum temperature threshold to separate rainfall from the total precipitation.That is,if the air temperature is greater than the temperature threshold,the probability of precipitation occurring as rain is larger than 0.5,then we assume the precipitation falls in liquid form.We derived the seasonal optimum temperature threshold of 2.01°C（spring）,0.54°C（summer）,1.14°C（fall）and1.17°C（mean annual）,respectively.We did not discuss the situation in winter because of rare liquid precipitation observations.By using these temperature thresholds,we distinguished the solid and liquid phase from total precipitation archived at 253 stations over the northern Eurasia region and investigated their variation trend during 1961–2010.Results showed that the proportion of rainfall in total precipitation decreased with latitudes in the region north of 60°N.In general,we found the maximum and minimum of the rainfall to total precipitation ratio（RPR）in summer and winter.Further analysis demonstrated that the precipitation forms showed obvious seasonal and interannual variability.Specially,the spring RPR increased over most of the terrestrial Arctic（82.46%stations）,22.37%of which passed 95%statistical significance test,indicating that there was a general transition trend from solid precipitation to liquid precipitation in the spring Arctic continents over the past decades.In summer and autumn,there was obvious regional variability in the south of the research area,but in the arctic coastal stations the increasing trend appeared uniform.The RPR trends,respectively derived from the lower and upper limits of 95%confidence interval of temperature threshold,were highly consistent with the trend in the spatial distribution that derived from the optimum temperature threshold.Additionally,more precipitation had fallen as rainfall instead of snowfall in Alaska,Siberia and Northern Europe during the ablation season（March-July）,which would make profound impacts on the Arctic land-atmosphere interaction.