| dc.description.abstract | During autumn and winter, the southern Lanyang Plain in Taiwan frequently experiences heavy rainfall, significantly impacting local residents and agricultural activities. As early as 1950, Kabasawa(1950) identified unique rainfall characteristics in Yilan and proposed a conceptual model of possible rainfall mechanisms related to Yilan′s topography. Despite over seventy years of research utilizing model simulations and data analysis, a comprehensive understanding still lacks detailed three-dimensional observational data. Since 2020, the Yilan Experiments of Severe Rainfall (YESR) have provided intensive vertical observations using UAVs, soundings, radars, and wind profilers, aiming to explore and understand the rainfall mechanisms in Yilan. This study primarily uses vertical intensive observations from UAVs (82 flights) during YESR and integrates long-term data from the Central Weather Bureau′s meteorological stations from autumn and winter seasons of 2018 to 2024 (total 721 days). It investigates rainfall and wind field characteristics in Yilan under winter monsoon condition, constructing a rainfall mechanism model for the southern Lanyang Plain in the absence of synoptic systems.
Using the average data from Taoyuangu and Yingziling stations as the background field for Yilan, the study segments observation periods based on background wind directions, focusing on periods with wind directions from 350 to 110 degrees (winter monsoon). Statistical results indicate the influence range of topographic return flow spans south of Dafu station and north of Su’ao station, with a maximum thickness of 800m or more, up to 1100m. The most favorable conditions for topographic return flow formation occur with background wind directions between 50 and 70 degrees, with westerly winds dominating 42% to 63% of the time in central and southern Lanyang Plain.
Further analysis using intensive vertical observations from the YESR reveals distinct rainfall mechanisms in the southern Lanyang Plain under different background wind directions. With background wind directions from 350 to 30 degrees (north wind) and wind speeds of 9 to 11 m s-1, rainfall is primarily influenced by orographic lifting, with high background wind speeds suppressing upstream convective movement, concentrating rainfall over the Central Mountain Range (rainfall occurrence rate > 50%). When the background wind is from 70 to 110 degrees (east wind) with speeds of 6 to 9 m s-1, background wind is blocked by the Xu Mountain Range, creating near-surface terrain-blocking flows. It converges with background wind offshore and create the absolute instability layer inland (with a thickness of about 1200m), provides favorable conditions for convective rainfall. During background wind directions of 30 to 70 degrees (northeast wind) with speeds of 6 to 9 m s-1, the interaction of orographic lifting and terrain-blocking flows leads to the highest frequency of intense, sustained rainfall (rainfall occurrence rate of 30% to 60%) in Yilan. Initial convective activity generated by orographic lifting over the Central Mountain Range is followed by new convection formed by the interaction of gust fronts and background winds, progressively moving upstream. Concurrently, terrain-blocking flows aids in the continuous development of offshore convection, which is enhanced by orographic lifting when advected over the Central Mountain Range, resulting in sustained heavy rainfall in central and southern Lanyang Plain. Therefore, under northeast wind conditions, the combined effects of orographic lifting and return flow mechanisms lead to the most intense and frequent rainfall in Yilan.
In summary, rainfall mechanisms in Yilan are complex. This study finds that topography aids convective development through orographic lifting and terrain-blocking flows, with background wind direction determining the rainfall mechanism in the southern Lanyang Plain, and background wind speed influencing the type of orographic rainfall. Under north wind conditions, orographic lifting dominates, concentrating rainfall over the Central Mountain Range. Under northeast wind conditions, the interaction between orographic lifting and terrain-blocking flows cause persistent and intense rainfall in central and southern Lanyang Plain. Under east wind conditions, offshore convection uniformly affects the entire Yilan, with terrain-blocking flows dynamically and thermally supporting convective development. | en_US |