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

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) and 1.17°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.