Ben Jong-Dao Jou

Tropical Cyclone Kinematic Structure Retrieved from Single-Doppler Radar Observations. Part I: Interpretation of Doppler Velocity Patterns and the GBVTD Technique

Abstract Deducing the three-dimensional primary circulation of landfalling tropical cyclones (TCs) from single ground-based Doppler radar data remains a difficult task. The evolution and structure of landfalling TCs and their interactions with terrain are left uncharted due to the lack of dual-Doppler radar observations. Existing ground-based single-Doppler radar TC algorithms provide only qualitative information on axisymmetric TC center location and intensity. In order to improve understanding of the wind structures of landfalling TCs using the widely available WSR-88D data along the U.S. coastal region, a single ground-based radar TC wind retrieval technique, the ground-based Velocity Track Display (GBVTD) technique, is developed. Part I of this paper presents 1) single-Doppler velocity patterns of analytic, asymmetric TCs, 2) derivation of the GBVTD technique, and 3) evaluation of the GBVTD-retrieved winds using analytic TCs. The Doppler velocity patterns of asymmetric TCs display more complex structu...

Tropical Cyclone Kinematic Structure Retrieved from Single-Doppler Radar Observations. Part III: Evolution and Structures of Typhoon Alex (1987)

Abstract This paper is the third of a series that focuses on the applications of the ground-based velocity track display (GBVTD) technique and the GBVTD-simplex center finding algorithm developed in the previous two papers to a real tropical cyclone (TC). The evolution and structure of Typhoon Alex (1987), including full tangential winds, mean radial winds, one component of the mean flow, and their derived axisymmetric angular momentum and perturbation pressure fields are reconstructed from 16 volume scans (6.5 h of data with a 2-h gap) from the Civil Aeronautic Administration (CAA) Doppler radar while Typhoon Alex moved across the mountainous area in northern Taiwan. This analysis retrieves a plausible and physically consistent three-dimensional primary circulation of a landfalling TC using a single ground-based Doppler radar. Highly asymmetric wind structures were resolved by the GBVTD technique where the maximum relative tangential wind at z = 2 km evolved from 52 m s−1 (before landfall), to less than ...

An Orography-Associated Extreme Rainfall Event during TiMREX: Initiation, Storm Evolution, and Maintenance

AbstractThis study investigates a long-duration mesoscale system with extremely heavy rainfall over southwest Taiwan during the Terrain-influenced Monsoon Rainfall Experiment (TiMREX). This mesoscale convective system develops offshore and stays quasi-stationary over the upstream ocean and southwest coast of Taiwan. New convection keeps developing upstream offshore but decays or dies after moving into the island, dropping the heaviest rain over the upstream ocean and coastal regions. Warm, moist, unstable conditions and a low-level jet (LLJ) are found only over the upstream ocean, while the island of Taiwan is under the control of a weak cold pool. The LLJ is lifted upward at the boundary between the cold pool and LLJ. Most convective clusters supporting the long-lived rainy mesoscale system are initiated and develop along that boundary. The initiation and maintenance is thought to be a “back-building–quasi-stationary” process. The cold pool forms from previous persistent precipitation with a temperature ...

Initiation, maintenance, and properties of convection in an extreme rainfall event during SCMREX: Observational analysis

A long-lived mesoscale convective system (MCS) with extreme rainfall over the western coastal region of Guangdong on 10 May 2013 during the Southern China Monsoon Rainfall Experiment (SCMREX) is studied. The environmental conditions are characterized by little convective inhibition, low-lifting condensation level, moderate convective available potential energy and precipitable water, and lack of low-level jets from the tropical ocean. Repeated convective back building and subsequent northeastward “echo training” of convective cells are found during the MCSs development stages. However, the initiation/maintenance factors and organization of convection differ significantly during the earlier and later stages. From midnight to early morning, convection is continuously initiated as southeasterly flows near the surface impinge on the east side of mesoscale mountains near the coastal lines and then moves northeastward, leading to formation of two quasi-stationary rainbands. From early morning to early afternoon, new convection is repeatedly triggered along a mesoscale boundary between precipitation-induced cold outflows and warm air from South China Sea and Gulf of Tokin, resulting in the formation of “band training” of several parallel rainbands that move eastward in a later time, i.e., two scales of “training” of convective elements are found. As the MCS dissipates, a stronger squall line moves into the region from the west and passes over within about 3.5 h, contributing about 10%–15% to the total rainfall amount. It is concluded that terrain, near-surface winds, warm advection from the upstream ocean in the boundary layer, and precipitation-generated cold outflows play important roles in initiating and maintaining the extreme rain-producing MCS.

Quality-Controlled Upper-Air Sounding Dataset for TiMREX/SoWMEX: Development and Corrections

Abstract During the Terrain-Influenced Monsoon Rainfall Experiment (TiMREX), which coincided with Taiwan’s Southwesterly Monsoon Experiment—2008 (SoWMEX-08), the upper-air sounding network over the Taiwan region was enhanced by increasing the radiosonde (“sonde”) frequency at its operational sites and by adding several additional sites (three that were land based and two that were ship based) and aircraft dropsondes. During the special observing period of TiMREX (from 15 May to 25 June 2008), 2330 radiosonde observations were successfully taken from the enhanced network. Part of the challenge of processing the data from the 13 upsonde sites is that four different sonde types (Vaisala RS80, Vaisala RS92, Meisei, and Graw) were used. Post–field phase analyses of the sonde data revealed a significant dry bias in many of the sondes—in particular, in the data from the Vaisala RS80 sondes that were used at four sites. In addition, contamination of the sonde data by the ship’s structure resulted in poor-quality ...

Radar Observations of the Diurnally Forced Offshore Convective Lines along the Southeastern Coast of Taiwan

This study documents offshore convective lines along the southeastern coast of Taiwan, a frequent but poorly understood mesoscale phenomenon that influences coastal weather during the Taiwan mei-yu season. Doppler radar and surface observations were gathered from a specially chosen period (11–15 May 1998) when the offshore convective lines were active off the southeastern coast of Taiwan. These observations were used to show the basic character, structure, and possible formative processes of offshore convective lines. The synoptic environment accompanying these events was found to be relatively undisturbed and featured uniformly prevailing southerly/south-southeasterly winds in the boundary layer with southwesterlies/westerlies aloft. Examination of radar data during the study period indicates that the lines generally occurred 10–30 km offshore and were characterized by an elongated narrow zone (5–10 km wide) of heavy precipitation. The lines were oriented roughly parallel to the coastline and generally did not move significantly. The intensity of the radar reflectivity associated with the lines exhibited a marked diurnal variation and was closely related to the coastal offshore flow developing at night. Detailed analyses of an event on 14–15 May 1998 further show the important physical link between the offshore flow and the development of the line. The offshore line was found to be located near and immediately ahead of the seaward extent of the offshore flow. Particularly, a very narrow zone (2 km) of low-level heavy precipitation (40–45 dBZ ) coincided with regions of strong updrafts and convergence, where the prevailing southerly onshore flow encountered the cool offshore flow nearshore. This offshore flow–induced convergence, given a stable thermodynamic condition in the lowest 1 km in the inflow region, was a crucial low-level forcing that provided lifting to trigger moist deep convection in this case. The line’s precipitation tilt eastward was confined primarily to the warmer inflow side rather than feeding the offshore flow to the west of the line. No consistent upshear tilt of updrafts throughout the storm layer was observed, which is consistent with the presence of a strong westerly shear in the line’s environment. Both of these observations explain a relatively strong (weak) modification of low-level onshore (offshore) flow by precipitation. Additionally, a combination of surface and Doppler radar observations indicates that the leading edge of the offshore flow moved seaward very slowly at 0.7 m s 1 and possessed a frontal character with notable discontinuities in near-surface wind and temperature (instead of pressure and dewpoint temperature).

MM5 Ensemble Mean Precipitation Forecasts in the Taiwan Area for Three Early Summer Convective (Mei-Yu) Seasons

Abstract This study presents precipitation verification of individual members and ensemble means in the Taiwan area for a real-time mesoscale ensemble prediction system, during the 2000, 2001, and 2002 early summer convective (mei-yu) seasons. The ensemble system, using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) as a forecast model, consists of six members, each run with a different combination of moisture physics schemes. Precipitation forecasts within the 15-km domain were verified against the observational data from the 342 rain gauge stations on the island. In general the model that utilized the Grell cumulus parameterization scheme (CPS) and the Reisner I microphysics scheme (the GR model) had the best forecast skill among the six members. This physics combination is, therefore, recommended for MM5 rainfall simulations in the Taiwan area during the mei-yu season. The Kain–Fritsch CPS dominated the rainfall process and generally underforecast rainfall at high rainfal...

An Application of Reflectivity Climatology in Constructing Radar Hybrid Scans over Complex Terrain

Abstract Three years’ worth of radar reflectivity data from four radars in an area of complex terrain (Taiwan) from 2005 to 2007 were analyzed and a reflectivity climatology was developed. The climatology was applied in the construction of new hybrid scans to minimize the impacts of ground clutter and beam blockages. The reflectivity climatology showed significant seasonal variations and captured distributions of ground/sea clutters, beam blockages, and anomalous propagations in addition to precipitation systems in the radar domains. By comparing the reflectivity climatology with gauge observations, it was found that 15 (20) dBZ was a good approximation for rain/no-rain segregation during cool (warm) seasons. Comparisons between the standard (i.e., based on terrain and scan strategies only with the assumption of standard propagations) and nonstandard (i.e., standard plus the clutter and blockage mitigation using the reflectivity climatology) hybrid scans showed that the former did not accurately reflect t...

Warm Season Afternoon Thunderstorm Characteristics under Weak Synoptic-Scale Forcing over Taiwan Island

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 ...

The Southern China Monsoon Rainfall Experiment (SCMREX)

AbstractDuring the presummer rainy season (April–June), southern China often experiences frequent occurrences of extreme rainfall, leading to severe flooding and inundations. To expedite the efforts in improving the quantitative precipitation forecast (QPF) of the presummer rainy season rainfall, the China Meteorological Administration (CMA) initiated a nationally coordinated research project, namely, the Southern China Monsoon Rainfall Experiment (SCMREX) that was endorsed by the World Meteorological Organization (WMO) as a research and development project (RDP) of the World Weather Research Programme (WWRP). The SCMREX RDP (2013–18) consists of four major components: field campaign, database management, studies on physical mechanisms of heavy rainfall events, and convection-permitting numerical experiments including impact of data assimilation, evaluation/improvement of model physics, and ensemble prediction. The pilot field campaigns were carried out from early May to mid-June of 2013–15. This paper: i...