Cheng-Ku Yu

Here we present a 15-year record (2009–2023) of quality-controlled, one-minute rainfall observations from the Da-Tun rain gauge network (DTRGN), deployed to capture fine-scale precipitation variability over Da-Tun Mountain (DT). DT, situated immediately inland of northern Taiwan’s coast, is an isolated three-dimensional volcanic massif ~15 km wide and ~1 km high, and represents a key climatological center of heavy orographic rainfall because tropical cyclones and northeasterly monsoonal flow frequently bring abundant moisture and precipitation to this mountain barrier. The DTRGN comprises 25 tipping-bucket rain gauges measuring rainfall across major mountain peaks, slopes, and valleys that are potential hotspots of orographically enhanced precipitation. We additionally provide temporal aggregations of one-minute rainfall observations at 10-minute, hourly, daily, and monthly intervals to support the research community. As a high-density, station-based rainfall dataset, DTRGN serves as an essential benchmark and resource for a broad spectrum of research and operational applications, including studies of orographic precipitation processes, tropical cyclone rainfall characterization, radar-derived precipitation correction, hydrological modeling, and assessments of long-term precipitation trends in complex terrain.

Outer tropical cyclone rainbands (TCRs) are a concentrated region of heavy precipitation and hazardous weather within tropical cyclones (TCs). Outer TCRs pose considerable risk to human societies, but their origin remains unresolved. Here, we identify a total of 1029 outer TCRs at their formative stage from 95 TCs and present a large collection of radar observations in order to establish a robust foundation of the natural diversity of rainband origin. The results show the dominance of outer origin for the observed outer TCRs, in distinct contrast to theoretical modeling works of outer TCRs, which propose inner-origin scenarios. Our analysis also suggests that squall-line dynamics are a common, but not the sole, mechanism responsible for outer TCR formation. The nature of preexisting outer precipitation is found to be an important factor to influence the squall-line and non-squall-line outer TCR initiation.

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.

This study used Doppler radar data, surface observations, and National Centers for Environmental Prediction reanalysis data to explore the statistical characteristics of Taiwan rainbands (TRs) that formed in the outer region of tropical cyclones (TCs). A comprehensive examination of the available radar measurements from 2002 to 2017 identified a total of 103 TRs from 44 TC events and showed that approximately 47% of all TCs influencing Taiwan could develop TRs. The spatial distribution of TR formation exhibited a substantial offshore extent, with the highest frequency observed ~25–100 km offshore. The TRs tended to be initiated when the northwestward-moving typhoons passed over the oceanic area northeast of Luzon Island (16°–20°N, 122°–127°E), the Philippines. This track characteristic brought stronger easterly onshore flow to the eastern coast of Taiwan and favored the development of a pronounced coastal pressure ridge. In particular, the offshore convergence caused by upstream deceleration of the onshore flow due to orographic blocking was found to be a primary contributor to the initiation of the TRs. The strength of the observed coastal pressure ridge and its high correlation with the intensity of environmental onshore flow associated with outer circulations of TCs were consistent with the theoretical prediction of pressure distributions generated as incident flow interacted dynamically with the Taiwan topography. Results from the study suggest that the typhoon location relative to the Taiwan landmass is a critical factor determining TR initiation.

On 23 July 2014, a commercial aircraft (GE222) crashed near the Ma-Gong Airport on Penghu Island off the southwestern coast of Taiwan as it struggled to land in the stormy weather that was caused by the outer tropical cyclone rainbands (OTCRs) of Typhoon Matmo. This study aims to document the detailed aspects of airflow and precipitation of OTCRs through high-resolution radar and surface observations and to identify how these observed structures contribute to aviation weather hazards. Analyses indicate that the weather at the airport was significantly influenced by the passage of three OTCRs (R1, R2, and R3), and these rainbands share common characteristics of squall-line-like airflow and precipitation structures. As GE222 descended to approach the runway and flew immediately behind and roughly parallel to the leading edge of R3, the aircraft encountered the heaviest precipitation of the rainband and the prominent crosswind that was a manifestation of the rear-to-front flow generated locally by the rainband. The heavy rain–induced poor visibility and the occurrence of strong crosswinds were primary weather hazards affecting this flight event. Momentum budget analyses suggest that the frontward pressure gradient force provided by the near-surface, convectively generated mesohigh played a major role in driving the low-level rear-to-front flow inside the band. The results from the present study imply that closely monitoring convective activities in the outer regions of tropical cyclones and their potential transformation into squall-line-like storms is crucial to complement the routine aviation alert of severe weather under the influence of tropical cyclones.

This study uses a dense rain gauge network, radar observations, and an upslope model to document the detailed aspects of precipitation over Da-Tun Mountain (DT) of northern Taiwan under prevailing northeasterly monsoonal flow. DT is one of the most concentrated areas of heavy rainfall in Taiwan, with a size of ~15 km and a terrain peak of ~1 km (MSL), and it constitutes a concave ridge with two windward ridge arms that encompass a funnel-shaped valley. Twenty-one rainfall events identified over DT from January 2011 to February 2015 are chosen for analysis. More than half of the studied cases exhibit two local maxima of rainfall over the ridge arms, and asymmetric characteristics between these two maxima are evident. The other frequent rainfall pattern, observed as upstream winds are more from the east-northeast, is characterized by a local maximum of precipitation inside the valley. Analyses from the upslope model confirm that the occurrence of the windward maxima of rainfall is primarily caused by upslope lifting, and their asymmetric characteristics are closely related to the difference in the azimuthal variations of slope steepness between the two ridge arms. The observed characteristics of rainfall intensities inside the valley, however, cannot be well described by the upslope model. It is found that the lateral flow confluence induced by the deflected flows over the ridge arms may play an essential role in intensifying upslope-forced precipitation within the valley. This effect emerges as upstream winds are roughly parallel to the central axis of funneling regions located between the ridge arms.