Jenq-Dar Tsay

Synoptic Development of the Hanoi Heavy Rainfall Event of 30–31 October 2008: Multiple-Scale Processes

AbstractThe 30–31 October 2008 Hanoi, Vietnam, heavy rainfall–flood (HRF) event occurred unusually farther north than other Vietnam events. The cause of this event is explored with multiple-scale processes in the context of the midlatitude–tropical interaction. In the midlatitudes, the cold surge linked to the Hanoi event can be traced westward to the leeside cyclogenesis between the Altai Mountains and Tianshan. This cyclone developed into a Bering Sea explosive cyclone later, simultaneously with the occurrence of the Hanoi HRF event. In the tropics, a cold surge vortex formed on 26 October, south of the Philippines, through the interaction of an easterly disturbance, an already existing small surface vortex in the Celebes Sea, and the eastern Asian cold surge flow. This cold surge vortex developed into a cyclone, juxtaposed with the surface high of the cold surge flow, and established a strong moist southeasterly flow from the South China Sea to Hanoi, which helped maintain the HRF event. Spectral analy...

Forecast Advisory for the Late Fall Heavy Rainfall/Flood Event in Central Vietnam Developed from Diagnostic Analysis

AbstractThe formations of heavy rainfall/flood (HRF) events in Vietnam are studied from diagnostic analyses of 31 events during the period 1979–2009. HRF events develop from the cold surge vortices formed around the Philippines. These vortices’ speed, size, and rainfall, which evolve into HRF events, are enhanced distinguishably from non-HRF vortices, as they reach Vietnam. The HRF cyclone, the North Pacific anticyclone, and the northwestern Pacific explosive cyclone simultaneously reach their maximum intensities when the HRF event occurs. An HRF cyclone attains its maximum intensity by the in-phase constructive interference of three monsoon (30–60, 12–24, and 5 days) modes identified by the spectral analysis of zonal winds. The rainfall center of an HRF event is formed and maintained by the in-phase constructive interference of rainfall and convergence of water vapor flux anomalies, respectively, from three monsoon modes. Forecast times of regional models are dependent and constrained on the scale of the...

Impact of Afternoon Thunderstorms on the Land–Sea Breeze in the Taipei Basin during Summer: An Experiment*

AbstractEnvironmental conditions for the roughly three million people living in the Taipei basin of Taiwan are greatly affected by the land–sea breeze and afternoon thunderstorm activities. A new perspective on the land–sea breeze life cycle and how it is affected by afternoon thunderstorm activity in the Taipei basin during the dry season is provided. During the summer monsoon break–revival phase, about 75% of rainfall in the Taipei basin is produced by afternoon thunderstorms triggered by sea-breeze interactions with the mountains to the south of this basin. Because the basic characteristics of the land–sea breeze and the changes it undergoes through the influence of afternoon thunderstorms have not been comprehensively analyzed/documented, a mini–field experiment was conducted during the summers of 2004 and 2005 to explore these aspects of the land–sea breeze in this basin. Thunderstorm rainfall is found to change not only the basin’s land–sea-breeze life cycle, but also its ventilation mechanism. On t...

Formation of the Philippine Twin Tropical Cyclones during the 2008 Summer Monsoon Onset

Abstract Stretched from Indochina, across the South China Sea, to the Philippine Sea, a monsoon cyclonic shear flow was formed by easterlies of the cold surge-like flow in the north and monsoon westerlies in the south before the onset of the tropical Southeast Asian monsoon on 12 May 2008. On this date, two named tropical cyclones (Halong and Matmo) evolved with a 12-h lag from a closed vortex adjacent to the coast of central Vietnam and another closed vortex near Palawan Island (Philippines) within this shear flow. These two cyclones, named the twin Philippine tropical cyclones, moved almost on the same track, along the anomalous shear line (departure from the climatological one) across the Philippines, and turned northeastward to the ocean south of Japan. It was revealed from synoptic analysis that the cold surge-like flow was coupled with the midlatitude eastward-propagating short wave in northeast Asia, and part of the monsoon westerlies were fed by the cross-equatorial flow, the downstream flow of ea...

Impact of the Summer Monsoon Westerlies on the South China Sea Tropical Cyclone Genesis in May

AbstractAfter the onset of the Southeast Asian summer monsoon in mid-May, the South China Sea (SCS) trough is deepened by the intensified monsoon westerlies to facilitate the development of a synoptic cyclonic shear flow. This shear flow forms an environment favorable for the SCS tropical storm (TS)/typhoon (TY) genesis triggered by the surge of this monsoon circulation. This genesis mechanism has not been well documented. Seventeen named SCS TS/TY geneses in May over 1979–2016 occurred under the following environmental conditions/processes: 1) with its maximum located south of 15°N, the intensified monsoon westerlies are extended eastward beyond 120°E, 2) the synoptic SCS cyclonic shear flow is developed by the tropical easterlies fed by a northeast Asian cold surge (or a North Pacific cold-air outbreak) and the intensified monsoon westerlies, and 3) SCS TS/TY genesis is triggered by the surge of monsoon flow. The accuracy of the monthly mean forecasts is limited. However, it is found that SCS TS/TY gene...

Development and Formation Mechanism of the Southeast Asian Winter Heavy Rainfall Events around the South China Sea. Part II: Multiple Interactions*

AbstractAbout 44% of the cold-season heavy rainfall/flood (HRF) events around the South China Sea require six days or longer to develop from the formation time of their parent cold surge vortices (CSVs). Formations for both the parent CSV and HRF event are involved with interactions of the concerned vortices with two different cold surge flows. The occurrence frequency of the East Asian cold surge flow varies from 4.5 to 6 days. The longevous CSVs enable their developments to interact with the second cold surge flows between formations of these CSVs and HRF events. Two requirements for the formation of HRF events are 1) synchronized occurrence of the HRF event and the northwestern Pacific explosive cyclone and 2) simultaneous occurrence of the maximum speeds among westerlies of the northwestern Pacific explosive cyclone and easterlies of the tropical trade winds and the HRF event. These requirements cannot be met by the CSV at its second maximum peak intensity, but the CSV at this stage plays an indispens...

A Comparison Study of Three Polar Grids

Abstract The circumference of a latitude circle decreases toward the Poles, making it difficult to present meteorological field variables on equally spaced grids with respect to latitude and longitude because of data aggregation. To identify the best method for displaying data at the Poles, three different grids are compared that have all been designed to reduce data aggregation: the reduced latitude–longitude (RL) grid, the National Snow and Ice Data Center Equal-Area Special Sensor Microwave Imager (SSM/I) Earth (EA) grid, and the National Meteorological Center octagonal (OG) grid. The merits and disadvantages of these grids are compared in terms of depictions of the Arctic summer circulation with wind vectors, streamfunction, and velocity potential at 400 hPa where maximum westerlies are located. Using geostrophy, the 400-hPa streamfunction at high latitudes can be formed from geopotential height. In comparison with this geostrophic streamfunction, the streamfunction generated from vorticity on the OG ...

Annual and Semiannual Variations of Surface Pressure in Taiwan

Corresponding author address: Tsing-Chang (Mike) Chen, Atmospheric Science Program, 3010 Agronomy Hall, Department of Geological and Atmospheric Sciences, Iowa State University, Ames, IA 50011. E-mail: tmchen@iastate.edu Recently, the ps observations of 59 stations (Fig. 1, including 26 original surface stations of the Taiwan Weather Bureau analyzed by CY) during 1993–98 were made available to us. In our analysis of the ps annual variation over Taiwan, a distinct semiannual signal emerged from the ps observations at some tall mountain stations. Although this signal at Yu-Shan was pointed out by CY, it was not discussed in detail. Several studies have examined the structure and cause of the semiannual oscillation in low latitudes (e.g., van Loon and Jenne 1969, 1970; Weickmann and Chervin 1988; Chen et al. 1996). However, as far as we know, this short-term climate signal in the low latitudes has not previously been explored with ps observations at a high-elevation station. Also, for the opposite-phase annual variation of ps between high and low elevations, CY only contrasted ps(Yu-Shan) with ps at the low-elevation stations of the Taiwan Weather Bureau. Is this ps inverse relationship common in Taiwan between tall mountains and the low plains? Since more ps observations are now available, it is of interest to examine this question as well.

Sudden Surface Warming―Drying Events Caused by Typhoon Passages across Taiwan

Abstract Typhoon passages across Taiwan can generate sudden surface warming in downslope regions. Special characteristics and mechanisms for 54 such warming events that were identified during the 1961–2007 period are examined. Preferred warming regions were identified in northwest Taiwan, where warming is generated by downslope flow from east or northeast winds in westward-moving typhoons, and in southeast Taiwan, where it is generated by downslope flow from west or northwest winds in northwestward-moving typhoons. In addition to the orographic effect, warmings occurred exclusively within nonprecipitation zones of typhoons. Most northwest (southeast) warmings occur during the day (night) with an average lifetime of 4 (5) h, which roughly corresponds to the average time a nonprecipitation zone remains over a station. During the period examined, three typhoons generated warming events in both northwest and southeast Taiwan, and only Typhoon Haitang (2005) generated warmings with comparable magnitudes (∼12-K...

Interannual Variation of the Cold-Season Rainfall Center in the South China Sea

AbstractDuring 15 November–31 December, a cold-season rainfall center appears in the southern part of the South China Sea (SCS) north of northwestern Borneo and juxtaposed along the southwest–northeast direction with rainfall centers for the Malay Peninsula and the Philippines. This SCS rainfall center also coincides geographically with the SCS surface trough. An effort is made to explore the formation mechanism of this rainfall center. It is primarily formed by the second intensification of heavy rainfall/flood cold surge vortex [CSV(HRF)] through its interaction with a cold surge flow over the SCS trough. Both the SCS rainfall center and the SCS surface trough are located at the easterly flow north of the near-equator trough. Modulated by the interannual variation of the cyclonic shear flow along the near-equator trough in concert with the El Nino–Southern Oscillation (ENSO) cycle, the SCS rainfall center undergoes an interannual variation. The impact of this ENSO cycle is accomplished through the regul...