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Pao-Liang Chang and Pin-Fang Lin

Abstract

In this study, unusual radar anomalous propagation (AP) phenomena associated with foehn winds induced by Typhoon Krosa (2007) were documented by using observations from radar, surface stations, and soundings. The AP echoes embedded within rainband areas and exhibited inward motions toward the radar site within 2–3 h prior to the occurrences of foehn winds at the radar site, which would interfere with the interpretation of radar data and associated downstream applications. As Typhoon Krosa appeared in the vicinity of the northeastern coast of Taiwan, foehn winds with significant subsidence warming and drying generated by downslope winds were observed in southeastern Taiwan. The foehn winds continuously moved northward within confined areas from the southeastern to eastern–central parts of Taiwan. Before the foehn winds penetrated to the surface, the subsidence warming introduced a temperature inversion layer above the surface and caused the ducting of radar beams. Analyses of refractive index and ray tracing suggested that the occurrence and evolution of the AP echoes during Typhoon Krosa were closely related to the varying inversion heights induced by downslope winds.

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Pao-Liang Chang, Wei-Ting Fang, Pin-Fang Lin, and Ming-Jen Yang

Abstract

In this study, a vortex-based Doppler velocity dealiasing (VDVD) algorithm for tropical cyclones (TCs) is proposed. The algorithm uses a Rankine combined vortex model as a reference field for dealiasing based on an inner–outer iterative procedure. The structure of the reference vortex is adjusted in an inner iterative procedure of VDVD that applies the ground-based velocity track display (GBVTD) technique. The outer loop of the VDVD based on the GBVTD-simplex algorithm is used for center correction. The VDVD is able to recover not only the aliased Doppler velocities from a simulated symmetric vortex but also those superimposed with wavenumber-1 asymmetry, radial wind, or mean flow. For real cases, the VDVD provides dealiased Doppler velocity with 99.4% accuracy for all pixels, based on 472 elevation sweeps from a typhoon without landfall. It is suggested that the VDVD algorithm can improve the quality of downstream applications such as Doppler wind retrievals and radar data assimilations of TCs and other storms, such as tornadoes and mesocyclones, with vortex signatures.

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Pao-Liang Chang, Wei-Ting Fang, Pin-Fang Lin, and Yu-Shuang Tang

Abstract

As Typhoon Goni (2015) passed over Ishigaki Island, a maximum gust speed of 71 m s−1 was observed by a surface weather station. During Typhoon Goni’s passage, mountaintop radar recorded antenna elevation angle oscillations, with a maximum amplitude of ~0.2° at an elevation angle of 0.2°. This oscillation phenomenon was reflected in the reflectivity and Doppler velocity fields as Typhoon Goni’s eyewall encompassed Ishigaki Island. The main antenna oscillation period was approximately 0.21–0.38 s under an antenna rotational speed of ~4 rpm. The estimated fundamental vibration period of the radar tower is approximately 0.25–0.44 s, which is comparable to the predominant antenna oscillation period and agrees with the expected wind-induced vibrations of buildings. The reflectivity field at the 0.2° elevation angle exhibited a phase shift signature and a negative correlation of −0.5 with the antenna oscillation, associated with the negative vertical gradient of reflectivity. FFT analysis revealed two antenna oscillation periods at 0955–1205 and 1335–1445 UTC 23 August 2015. The oscillation phenomenon ceased between these two periods because Typhoon Goni’s eye moved over the radar site. The VAD analysis-estimated wind speeds at a range of 1 km for these two antenna oscillation periods exceeded 45 m s−1, with a maximum value of approximately 70 m s−1. A bandpass filter QC procedure is proposed to filter out the predominant wavenumbers (between 40 and 70) for the reflectivity and Doppler velocity fields. The proposed QC procedure is indicated to be capable of mitigating the major signals resulting from antenna oscillations.

Open access
Pin-Fang Lin, Pao-Liang Chang, Ben Jong-Dao Jou, James W. Wilson, and Rita D. Roberts

Abstract

The spatial and temporal characteristics and distributions of thunderstorms in Taiwan during the warm season (May–October) from 2005 to 2008 and under weak synoptic-scale forcing are documented using radar reflectivity, lightning, radiosonde, and surface data. Average hourly rainfall amounts peaked in midafternoon (1500–1600 local solar time, LST). The maximum frequency of rain was located in a narrow strip, parallel to the orientation of the mountains, along the lower slopes of the mountains. Significant diurnal variations were found in surface wind, temperature, and dewpoint temperature between days with and without afternoon thunderstorms (TSA and non-TSA days). Before thunderstorms occurred, on TSA days, the surface temperature was warmer (about 0.5°–1.5°C) and the surface dewpoint temperature was moister (about 0.5°–2°C) than on non-TSA days. Sounding observations from northern Taiwan also showed warmer and higher moisture conditions on TSA days relative to non-TSA days. The largest average difference was in the 750–550-hPa layer where the non-TSA days averaged 2.5°–3.5°C drier. These preconvective factors associated with the occurrences of afternoon thunderstorms could be integrated into nowcasting tools to enhance warning systems and decision-making capabilities in real-time operations.

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Pin-Fang Lin, Pao-Liang Chang, Ben Jong-Dao Jou, James W. Wilson, and Rita D. Roberts

Abstract

In this study, a fuzzy logic algorithm is developed to provide objective guidance for the prediction of afternoon thunderstorms in northern Taiwan using preconvective predictors during the warm season (May–October) from 2005 to 2008. The predictors are derived from surface stations and sounding measurements. The study is limited to 277 days when synoptic forcing was weak and thermal instability produced by the solar heating is primarily responsible for thunderstorm initiation. The fuzzy algorithm contains 29 predictors and associated weights. The weights are based on the maximum of the critical success index (CSI) to forecast afternoon thunderstorms. The most important predictors illustrate that under relatively warm and moist synoptic conditions, sea-breeze transport of moisture into the Taipei Basin along with weak winds inland provide favorable conditions for the occurrence of afternoon convective storms. In addition, persistence of yesterday’s convective storm activity contributed to improving today’s forecast. Skill score comparison between the fuzzy algorithm and forecasters from the Taiwan Central Weather Bureau showed that for forecasting afternoon thunderstorms, the fuzzy logic algorithm outperformed the operational forecasters. This was the case for both the calibration and independent datasets. There was a tendency for the forecasters to overforecast the number of afternoon thunderstorm days. The fuzzy logic algorithm is able to integrate the preconvective predictors and provide probability guidance for the prediction of afternoon thunderstorms under weak synoptic-scale conditions, and could be implemented in real-time operations as a forecaster aid.

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Pao-Liang Chang, Jian Zhang, Yu-Shuang Tang, Lin Tang, Pin-Fang Lin, Carrie Langston, Brian Kaney, Chia-Rong Chen, and Kenneth Howard

Abstract

Over the last two decades, the Central Weather Bureau of Taiwan and the U.S. National Severe Storms Laboratory have been involved in a research and development collaboration to improve the monitoring and prediction of river flooding, flash floods, debris flows, and severe storms for Taiwan. The collaboration resulted in the Quantitative Precipitation Estimation and Segregation Using Multiple Sensors (QPESUMS) system. The QPESUMS system integrates observations from multiple mixed-band weather radars, rain gauges, and numerical weather prediction model fields to produce high-resolution (1 km) and rapid-update (10 min) rainfall and severe storm monitoring and prediction products. The rainfall products are widely used by government agencies and emergency managers in Taiwan for flood and mudslide warnings as well as for water resource management. The 3D reflectivity mosaic and QPE products are also used in high-resolution radar data assimilation and for the verification of numerical weather prediction model forecasts. The system facilitated collaborations with academic communities for research and development of radar applications, including quantitative precipitation estimation and nowcasting. This paper provides an overview of the operational QPE capabilities in the Taiwan QPESUMS system.

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