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Cheng-Ku Yu
,
Wei-Fan Liu
,
Lin-Wen Cheng
, and
Chuan-Yao Lin

Abstract

This study explores the mechanisms responsible for valley precipitation enhancement over Da-Tun Mountain under the prevailing northeasterly monsoonal flow. Da-Tun Mountain, located adjacent to the northern coast of Taiwan, is a small-scale (15 km), concave-like terrain feature with two ridge arms and a funnel-shaped valley. A typical valley precipitation enhancement event that occurred on 13 December 2018 was chosen for detailed analyses. Upstream conditions were characterized by the absence of convective available potential energy with a large-Froude-number (>1) flow regime. Observational and modeling results indicate a consistent, important signature of flow splitting due to partial blocking as the low-level northeasterly flow encountered the ridge arms. Fine-scale structures of airflow and precipitation evident from the simulations further reveal that the deflected flows over the two ridge arms interacted with each other to produce lateral convergence and enhanced precipitation inside the valley. The smaller-scale splitting flows tended to occur over the ridge arms as upstream moist Froude number decreased from relatively higher (5–11) to lower (3–5) values due to the temporal change in moist static stability. Quantitative diagnoses of vertical velocities performed over the region of primary precipitation support that upward motions associated with lateral convergence greatly overwhelmed the upslope-forced lifting over the valley region during the valley precipitation enhancement periods. However, vertical motions over the ridge arms with steeper slopes were dominantly contributed by the upslope forcing, but their intensities were also modulated by the flow-splitting-induced divergence.

Significance Statement

Many mountain ranges around the world exhibit a concave-like terrain feature with various spatial scales and orientations. Orographic modulations of rainfall by concave mountains are of great importance to local weather, as torrential rainfall has been frequently reported over these ridges in different geographical locations. This study aims to advance our knowledge of precipitation mechanisms over Da-Tun Mountain, a small-scale concave topography located in northern Taiwan. Observational and modeling analyses reveal evidence of splitting flows over different ridge arms of this mountain barrier. These smaller-scale splitting flows and their interactions play important roles in modulating the intensity of upslope-forced lifting and contributing to valley precipitation enhancement. These identified processes are anticipated to be commonly active over highly three-dimensional, concave-like topography.

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