A new method to separate precipitation phases

Abstract. Separating the solid precipitation from liquid precipitation in an existing historical precipitation observation data series is a key problem in the monitoring and study of climate anomaly and long-term change of extreme precipitation events in difference phases. In this study, based on the comprehensive analysis of the historical daily temperature, precipitation data, and weather phenomenon records in the northern areas of Mainland China (north of 30° N), the threshold temperature of rainfall and snowfall in historical precipitation data for a complex and diverse geographical and climatic region were determined. A statistical model was established, and a method of separating solid precipitation from liquid precipitation was proposed. The main conclusions include: (1) in northern China, the actual threshold temperature range of the daily mean temperature of rain and snow determined based on weather phenomenon records was between −1.2–6.3 °C, with a difference of 7.5 °C among areas, and a mean threshold value of 2.81 °C for the whole region. The actual threshold temperature in the northern Tibetan Plateau was the highest (generally higher than 4 °C). The low threshold temperature values appeared in eastern Northeast China, North China, and northern Xinjiang Autonomous Region, which were less than 2 °C. (2) The actual threshold temperature decreased with increase in longitude east of 105° E; meanwhile, it was more dispersed in the areas west of 105° E. The actual threshold temperature was generally higher and more variable in the low latitude areas, while it was lower and more concentrated in the high latitude; the threshold temperature was lower in the low-altitude areas and higher in the high-altitude areas, and it generally increased with altitude. (3) There was a negative correlation between the actual threshold temperature and the annual precipitation; the actual threshold temperature was higher in the areas with less precipitation, and lower in the areas with more precipitation. The actual threshold temperature was negatively correlated with the annual average relative humidity, and was generally low in humid areas with relatively large humidity and vice versa. (4) The multivariate regression fitting model developed in this paper based on latitude, altitude, and annual precipitation was able to simulate the actual threshold temperature of the precipitation phase in northern China well. According to the calculated threshold temperature based on the model, the relative deviation of snow days and snowfall are smaller, and the stations with less than 10 % of relative deviation reached 95.1 % and 90.7 %, respectively. The results of this study can be used for the separation of solid and liquid precipitation events in the areas without sufficient weather phenomenon records or metadata.