Conductor Icing in Mountainous Beijing: Synoptic Drivers, Accretion Pathways, and Elevation Dependence

Abstract

Conductor icing threatens overhead transmission lines in mountainous North China, yet the relative contributions of different accretion pathways, including freezing rain, rime, and wet snow, remain poorly quantified. Five conductor icing events occurring in mountainous Beijing between February 2015 and March 2026 were analysed. In situ fibre-optic conductor icing observations were combined with meteorological fields from the fifth-generation European Centre for Medium-Range Weather Forecasts reanalysis (ERA5) and ERA5-Land, together with 30 m digital elevation model (DEM) data. A Makkonen-based accretion-flux framework was applied to distinguish between the freezing-rain/freezing-drizzle, in-cloud-rime, and wet-snow pathways. For the two events with complete hourly observations, the simulated numbers of observation sites with icing were positively correlated with observations (Spearman ρ=0.716 and 0.928 for Cases 1 and 2, respectively), while the corresponding correlations for mean ice thickness were 0.394 and 0.749. Sensitivity experiments showed that Cases 2 and 4 were robustly rime-dominated, Case 3 was robustly wet-snow-dominated, and Case 5 was predominantly rime-dominated; Case 1 represented a near 0 °C mixed event in which the quantitative rime–wet-snow partition was parameter-sensitive. Freezing rain/freezing drizzle did not become the dominant pathway in any baseline or sensitivity configuration. The events shared a common sequence involving a 500 hPa short-wave trough, 850 hPa moisture transport, low-level convergence, orographic ascent, and the formation of supercooled low clouds at conductor elevation. Icing occurrence and larger ice thicknesses were more frequent at higher elevations, and the diagnosed rime accretion potential generally increased with elevation. These event-based results indicate that conductor icing assessments for mountainous North China should consider in-cloud rime, wet-snow adhesion, and terrain-modulated cloud exposure in addition to classical freezing-rain diagnostics, while broader regional generalisation requires a larger event sample.