Observing severe precipitation near complex topography during the Yilan Experiment of Severe Rainfall in 2020 (YESR2020)

This study introduces a recent field experiment investigating multiscale terrain–circulation–precipitation interactions. When a synoptic-scale northeasterly wind prevails under the active East Asian winter monsoon, stratocumulus cloud decks with severe rainfall exceeding 100 mm·day⁻¹ frequently occur in the northeastern plain area and adjacent mountains in Yilan, Taiwan. The Yilan Experiment of Severe Rainfall (YESR2020) is a field campaign from November 20, 2020, to November 24, 2020, to survey the physical processes leading to severe wintertime rainfall. The three-dimensional structure of the wind field and the atmospheric environment can be identified through high temporal and spatial resolution sounding observations, which is empowered by the novel Storm Tracker mini-radiosonde. During YESR2020, the continuously collected meteorological data of two northeasterly episodes captured the variability of local-scale wind patterns and the features of the severe rainfall induced by stratocumulus. A preliminary analysis indicated that a local-scale convergence line could appear over the plain area of Yilan under the northeasterly environmental condition. The precipitation hotspot was located in the mountain region of southern Yilan, where the local winds signified turbulence features. Moreover, the severe rainfall of the two northeasterly episodes spotlighted shallow cumulus under stratus with pure warm rain processes. The results of YESR2020 inspire the arrangement of future field observations to explore detailed mechanisms of heavily precipitating stratocumulus over complex topography.