Effects of Unsaturated Moist Froude Number and Orographic Aspect Ratio on a Conditionally Unstable Flow over a Mesoscale Mountain

Chen, Shu Hua; Lin, Yuh-Lang; Zhao, Zhan

doi:10.2151/jmsj.86.353

Cited by 9 publications

(6 citation statements)

References 13 publications

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“…Since we focus on the flow in the lower levels that may be altered by the model terrain, the horizontal wind speeds have been vertically averaged over the lowest 12 model levels (to about 3.5 km above the surface, which corresponds to the tallest peaks.). Following Chen and Lin (2005) and Chen et al (2008), the moist Froude number is defined as Fr 5 U/(N w h), where U is horizontal wind speed, h is the terrain height, and the unsaturated moist Brunt-Väisälä frequency is N 2 w 5 (g/u y )(›u y /›z), with u y being the virtual potential temperature. The Froude number concept suggests that the flow can pass over the mountain barrier when and where Fr is greater than one, which is favored by higher wind speeds and/or lower vertical stabilities.…”

Section: Sensitivity Testsmentioning

confidence: 99%

On Typhoon Track Deflections near the East Coast of Taiwan

Hsu

Fovell

et al. 2018

Mon. Wea. Rev.

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Typhoons with “deflection tracks” (DTs) within a 200-km distance of the mountainous island of Taiwan are examined. We analyze 84 landfalling typhoons that compose 49 DT cases turning to the left-hand side, including 18 with very large deflection angles (DA > 20°) and another 7 having looped tracks (LTs). Most of the large DA and LT cases are “northern landfall” type, reaching Taiwan’s east coast poleward of 24°N and originally possessing relatively slow translation speeds (~4 m s−1). Their average translation speeds, however, increase by 50% in the 3 h prior to landfall. The WRF Model is used to simulate DT cases, and potential vorticity (PV) tendency diagnosis is used to interpret the contributions of the horizontal advection (HA), vertical advection (VA), and diabatic heating (DH) terms. The northern landfall tropical cyclones (TCs) possess significant cross-mountain flow to the south of the storm near the coast, resulting in vorticity stretching (the VA effect) and subsidence warming. The subsidence suppresses storm convection and produces heating asymmetries (the DH effect) that can induce significant southwestward deflections. The cross-mountain VA and DH effects are weaker for the “southern landfall” storms. The results explain well the observed increase of translation speed prior to landfall in DT cases and show that the HA effect, in general, does not contribute to the track deflection. Our results highlight the impact of topography on TC track by the vorticity stretching effect and by asymmetric diabatic heating.

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Section: Sensitivity Testsmentioning

confidence: 99%

On Typhoon Track Deflections near the East Coast of Taiwan

Hsu

Fovell

et al. 2018

Mon. Wea. Rev.

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“…Numerous papers report that heavy rainfall during Mei-Yu seasons is closely related to the topographic complexity of Taiwan (Chen 1994;Chen 2007), with studies having employed numerical analysis to determine topographic influences on rainfall (Chu and Lin 2000;Chen and Lin 2005;Chen et al 2008). Accordingly, the topographic complexity in Taiwan is likely to affect the rainfall estimation of the Global Satellite Mapping of Precipitation (GSMaP).…”

Section: Introductionmentioning

confidence: 99%

Bias Adjustment of Satellite Precipitation Estimation Using Ground-Based Observation: Mei-Yu Front Case Studies in Taiwan

Yeh

Chuang²,

Peng

et al. 2019

Asia-Pacific J Atmos Sci

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The Global Satellite Mapping of Precipitation (GSMaP) was used to estimate the accumulated rainfall in May from the Mei-Yu front in Taiwan. Rainfall estimation from GSMaP during 2002-2017 were evaluated using more than 400 local gauge observations, collected from the Taiwan Central Weather Bureau (CWB). Studies have demonstrated that the GSMaP rainfall estimation estimates can be biased, depending on the target region, elevation, and season. In this experiment, we have evaluated GSMaP over three elevation ranges. The GSMaP systemic errors for each elevation range were identified and corrected using regression analysis. The results indicated that GSMaP estimation can be improved significantly through adjustment over three elevation ranges (elevation less than 50 m, elevation of 50-100 m, and elevation higher than 100 m). For these three elevation ranges, the correlation coefficient between the GSMaP estimations and CWB rainfall data was 0.76, 0.78, and 0.59, respectively. This indicated that the GSMaP estimation was more accurate for low-elevation regions than high-elevation regions. After the proposed approaches were employed to correct the errors, the bias errors were respectively improved by 5.64(13.7%), 7.33(38.4%) and 10.52(31.2%) mm for low-, mid-and high-elevation regions. This study demonstrated that the local correction approaches can be used to improve GSMaP estimation of Mei-Yu rainfall in Taiwan.

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“…During the period from 0600 to 1200 UTC, the average moist Froude number was about 1.3872. Chen et al (2008) use idealized numerical simulations to identify four kinds of flow regions by combining effects of F w and orographic aspect ratio (H/a) of mountain height (H) to width (a). Here, the flow region is Regime III: the flow with an orographic convective precipitation system over the mountain and downstream propagating convective system.…”

Section: Sounding Datamentioning

confidence: 99%

Precipitation Characteristics of an Autumn Torrential Rainfall Event in Northern Taiwan as Determined from Dual-Polarization Radar Data

Feng

Wang

2011

Journal of the Meteorological Society of Japan

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In Taiwan, comparing with the major hazardous Mei-Yu fronts and typhoons in summer, a synoptic condition with the relatively weak high and low pressure systems at certain locations in the autumn season may bring torrential rainfall to northern Taiwan. A detailed dual-polarimetric/Doppler radar analysis was carried out for one case under this condition to reveal the mesoscale precipitation mechanisms and microphysical characteristics over terrain.The high pressure system moving eastward off China and the low pressure system over the ocean in the southeast of of south-eastern Taiwan formed a convergent zone at the low levels, resulting in a sequence of convective activities. These convective cells moved westward, and became more organized and intense in the Mt. Datun area and the estuary of Tamsui River near the lee side of Mt. Datun.Moderate intensity convective cells were embedded in the wide, long-lasting stratiform regions. In the mountain area with intense precipitation, terrain-induced upward motion of the cells enhanced condensation, significantly increased drop counts, and acted as a feeder. The older cells in this convective system continued to provide lighter hydrometeors in the upper layer and formed a widespread stratiform region as a seeder. The wider spectrum of drop size distribution set the stage for collision and coalescence process, resulted in the larger drops formed at the low level of the mountain area. Along with the increasing concentration of raindrops, the total effect finally caused heavy rainfall over the mountain area.

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Effects of Unsaturated Moist Froude Number and Orographic Aspect Ratio on a Conditionally Unstable Flow over a Mesoscale Mountain

Cited by 9 publications

References 13 publications

On Typhoon Track Deflections near the East Coast of Taiwan

On Typhoon Track Deflections near the East Coast of Taiwan

Bias Adjustment of Satellite Precipitation Estimation Using Ground-Based Observation: Mei-Yu Front Case Studies in Taiwan

Precipitation Characteristics of an Autumn Torrential Rainfall Event in Northern Taiwan as Determined from Dual-Polarization Radar Data

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