Spatio–Temporal Characteristics of Freezing Rain in China and Its Impact on Ultra-High Voltage Transmission Lines

The spatio–temporal distribution characteristics of freezing rain are of high guiding significance for many departments, including electric power, transportation, communications, agriculture, and forestry. Previous studies on freezing rain were mostly based on observation data. Due to the short length, uneven distribution, and partial lack of data, the current understanding of spatio–temporall distribution characteristics may still be insufficient. The ERA5 reanalysis data contains freezing rain data that were not contained in other reanalysis data, which provides the possibility of further understanding the distribution characteristics. This paper uses the ERA5 freezing rain data to analyze the annual and interdecadal characteristics of China’s freezing rain days and freezing rainfall from 1979 to 2020. The results show that China’s annual freezing rain days and rainfall are mostly concentrated in Guizhou, Hunan, etc., directly affecting seven “West-to-East” ultra-high voltage direct current transmission lines, covering a total length of about 4,900 kilometers. The annual freezing rain days and rainfall in concentrated areas show a downward trend. As for the first EOF (Empirical orthogonal function) mode (variance contribution 36.96%) of the number of freezing rain days, it is mainly distributed along the east of the Heihe−Tengchong line, showing an overall decreasing trend, and there is a north−south reverse distribution with the Qinling–Huaihe line as the boundary. The second EOF mode (variance contribution 11.56%) of the number of freezing rain days reflects two anti-phase regions in the southern freezing rain concentrated area, and the phase alternate period spans from one to five years. The EOF mode distribution of the annual freezing rainfall is similar to the annual number of freezing rain days.