Variability in the Raindrop Size Distribution During an Extreme Large-Scale Freezing Rain Event in Northeast China

Freezing rain (FZR) presents significant risks to energy, transportation, and agriculture, leading to substantial economic losses and casualties, particularly in Southwest China and central, eastern China, with only occasional occurrences in northern China. This study investigates an extreme, large-scale FZR event that occurred during 8–9 November 2021 in Heilongjiang Province of Northeast China, marking the region’s most intense FZR since 1958. Surface station observations revealed distinct characteristics of the FZR, and the stations were classified into three types by using the k-means clustering: stations with continuous FZR (FZR_Con), stations with FZR of mixed hydrometeor types (FZR_Mix), and stations with FZR transitioning to rain (FZR_Rain). Vertical atmospheric temperature and humidity profiles significantly influenced the raindrop size distribution (DSD) for the three station types. All three station types exhibited an inversion layer in the upper atmosphere, though they formed through two distinct mechanisms: (1) the supercooled warm rain mechanism and (2) the melting mechanism. This study found that the mass-weighted mean diameters (D_m) were larger than those observed in FZR events in central China and in stratiform rain in northern and northwestern China. FZR_Mix, which formed through the supercooled warm rain mechanism, exhibited the largest D_m among the three types. In contrast, FZR_Con and FZR_Rain formed through the melting mechanism, involving the melting of ice crystals and snow particles. The drier refreezing layer in FZR_Rain, compared to FZR_Con, resulted in a lower normalized number concentration (N_w) and a larger D_m. Positive exponential relationships between D_m and R, as well as N_w and R, across all FZR types, highlighting dominant role of microphysical processes such as collision and coalescence. Variations in the μ–λ and Z–R relationships among the FZR types further underscore differences in the microphysical processes and regional precipitation characteristics. This study enhances our understanding of the macro- and microphysical properties of FZR formed through different mechanisms, providing valuable reference for improved radar-based precipitation estimation in mid- and high-latitude regions.