Kinematics and Microphysics of Orographic Precipitation Associated with Different Background Precipitation of Typhoon Chanthu (2021)Tsubaki Hosokawa, Cheng-Ku YuNational Taiwan University |
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The understanding of detailed processes of tropical cyclone (TC)-related orographic precipitation is limited due to a lack of detailed observations over mountainous regions. This study uses unique observations from polarimetric radars, a dense rain gauge network, and disdrometers to document the kinematic and microphysical characteristics of orographic precipitation over Da-Tun Mountain in northern Taiwan during Typhoon Chanthu (2021). Heavy precipitation occurred over Da-Tun influenced by two distinct TC background features: the Outer Rainband (OR) and the Weak Stratiform (WS).
Radar observations showed that the OR was a stratiform-like rainband, characterized by strong reflectivity and modest upward motion. This inherent convective updraft favored active ice crystal production, evidenced by a distinct increase in Zdr and Kdp between ?15C and ?10C, signifying the dendritic growth layer (DGL). Subsequent aggregation and riming drove further growth underneath DGL. Conversely, the WS exhibited weak vertical air motions, leading to inactive ice-phase growth and smaller, fewer raindrops. Precipitation enhancement through the collision and coalescence was evident in both stages, consistent with the seeder-feeder concept, where the TC background (seeder cloud) collects orographically generated clouds (feeder cloud). The OR's larger drops and higher concentrations suggest a higher washout efficiency than the WS, resulting in more prominent orographic enhancement over the windward slope. The smaller and fewer drops in the WS resulted in less efficient collection and a greater downstream shift of the enhanced precipitation. These microphysical differences in seeder clouds critically determine the degree of low-level orographic enhancement. Our results reveal distinct microphysical pathways, offering important insights for understanding and predicting severe rainfall as TCs approach mountainous regions." |