ALBERT

Description: During 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 (HRF) cold surge vortex (CSV) 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-Niño-Southern Oscillation (ENSO) cycle, the SCS rainfall center undergoes an interannual variation. The impact of this ENSO cycle is accomplished through the regulation of CSV(HRF) trajectories originating from the Philippines vicinity and Borneo, and propagating to different destinations. Rain-producing efficiency determined by the interannual variation of the divergent circulation accompanies the cyclonic shear flow around the near-Equator trough in response to this ENSO cycle.

Description: The Taipei basin, located in northern Taiwan, is formed at the intersection of the Tanshui River valley (~30 km) and the Keelung River valley (~60 km). Summer is the dry season in northern Taiwan, but the maximum rainfall in the Taipei basin occurs during 15 June–31 August. The majority of summer rainfall in this basin is produced by afternoon thunderstorms. Thus, the water supply, air/land traffic, and pollution for this basin can be profoundly affected by interannual variations of thunderstorm days and rainfall. Because the mechanism for these interannual variations is still unknown, a systematic analysis is made of thunderstorm days and rainfall for the past two decades (1993–2013). These two variables are found to correlate opposite interannual variations of sea surface temperature anomalies over the National Oceanic and Atmospheric Administration Niño-3.4 region. Occurrence days for afternoon thunderstorms and rainfall amounts in the Taipei basin double during the cold El Niño–Southern Oscillation (ENSO) phase relative to the warm phase. During the latter phase, a stronger cold/drier monsoon southwesterly flow caused by the Pacific–Japan Oscillation weakens the thunderstorm activity in the Taipei basin through the land–sea breeze. In contrast, the opposite condition occurs during the cold ENSO phase. The water vapor flux over the East/Southeast Asian monsoon region converges more toward Taiwan to maintain rainfall over the Taipei basin during the cold ENSO phase than during the warm ENSO phase.

Description: Summer is a dry season in northern Taiwan. By contrast, the Taipei basin, located in this region, has its maximum rainfall during summer (15 June–31 August), when 78% of this rainfall is contributed by afternoon thunderstorms. This thunderstorm activity occurs during only 20 days in summer. Because of the pronounced impacts on the well-being of three million people in the basin and the relative infrequency of occurrence, forecasting thunderstorm events is an important operational issue in the Taipei basin. The basin’s small size (30 km × 60 km), with two river exits and limited thunderstorm occurrence days, makes the development of a thunderstorm activity forecast model for this basin a great challenge. Synoptic analysis reveals a thunderstorm day may develop from morning synoptic conditions free of clouds/rain, with a NW–SE-oriented dipole located south of Taiwan and southwesterlies straddling the low and high of this dipole. The surface meteorological conditions along the two river valleys exhibit distinct diurnal variations of pressure, temperature, dewpoint depression, relative humidity, and land–sea breezes. The primary features of the synoptic conditions and timings of the diurnal cycles for the four surface variables are utilized to develop a two-step hybrid forecast advisory for thunderstorm occurrence. Step 1 validates the 24-h forecasts for the 0000 UTC (0800 LST) synoptic conditions and timings for diurnal variations for the first five surface variables on thunderstorm days. Step 2 validates the same synoptic and surface meteorological conditions (including sea-breeze onset time) observed on the thunderstorm day. The feasibility of the proposed forecast advisory is successfully demonstrated by these validations.

Description: During May and June, the monsoon rainfall in northern Southeast Asia is primarily produced by rainstorms. At the mature stage, these storms, coupled with a midtropospheric subsynoptic-scale trough, produce rainfall ≥50 mm (6 h)−1and exhibit a cyclonic surface vortex. With a scale ~ O(102) km, rainstorms during the period of 1979–2016 are identified with station and satellite observations, along with assimilation data. Several dynamic processes of rainstorm geneses are disclosed by an extensive analysis. 1) Maximum occurrence of rainstorm geneses is located in the midtroposphere of two regions (northern Vietnam–southwestern China and the northern South China Sea), but eventually penetrates downward to the surface. 2) The environment favorable for rainstorm genesis is a southwest–northeast-oriented narrow trough formed by the confluence of the midtropospheric northeasterly around the eastern Tibetan Plateau and the lower-tropospheric monsoon southwesterlies. Because the criterion for Charney–Stern instability is met by the shear flow of this narrow trough, rainstorms are likely initiated by this instability. 3) The majority of rainstorm geneses occurs during the evening over the land and the morning at sea. This timing preference is caused by the modulation of the clockwise rotation of the East Asia continent circulation in response to the diurnal variation of the land–sea thermal contrast. These new findings from this study offer not only a new perspective for the genesis mechanism of the late spring–early summer rainstorms in northern Southeast Asia, but also a new initiative to develop medium-range forecasts for these rainstorms.

Description: A northwest–southeast-oriented summer monsoon trough exists between northern Indochina and northwestern Borneo. Ahead of this the South China Sea (SCS) trough is located at a convergent center west of the Philippines, which provides an environment favorable for rain-producing synoptic systems to produce rainfall over this center and form the SCS summer rainfall center. Revealed from the x–t diagram for rainfall, this rainfall center is developed by multiple-scale processes involved with the SCS trough (TR), tropical depression (TY), interaction of the SCS trough with the easterly wave/tropical depression (EI), and easterly wave (EW). It is found that 56% of this rainfall center is produced by the SCS trough, while 41% is generated by the other three synoptic systems combined. Apparently, the formation of the SCS summer monsoon rainfall center is contributed to by these four rain-producing synoptic systems from the SCS and the Philippines Sea. The Southeast Asian summer monsoon undergoes an interannual variation and exhibits an east–west-oriented cyclonic (anticyclonic) anomalous circulation centered at the western tropical Pacific east of the Luzon Strait. This circulation change is reflected by the deepening (filling) of the SCS summer monsoon trough, when the monsoon westerlies south of 15°N intensify (weaken). This interannual variation of the monsoon westerlies leads to the interannual variation of the SCS summer monsoon rainfall center to follow the Pacific–Japan oscillation of rainfall. The rainfall amount produced over this rainfall center during the weak monsoon season is about two-thirds of that produced during the strong monsoon season. The rain-production ratio between TR and TY + EI + EW is 60:38 during the strong monsoon season and 47:49 during the weak monsoon season.

Description: After 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 genesis only occurs after the existence of persistent, strong, monsoon westerlies lasting for at least 5 days. Forecasts from the National Centers for Environmental Prediction Global Forecast System (2004–16) and the Global Ensemble Forecast System (1985–2003) cover these 15 SCS TS/TY geneses. The requirements for SCS TS/TY genesis in May described above are met by the 5-day-mean Southeast Asian summer monsoon circulation. Based on a statistical analysis of 5-day forecasts for these TS/TY geneses, a four-step forecast advisory is introduced. The forecasts for SCS TS/TY genesis can be made 3 days prior to occurrence.

Description: The peak intensity occurrence frequency over the life cycles of parent cold-surge vortices (CSVs) for heavy rainfall/flood (HRF) events is classified into two types depending on their life cycles having two or three peak intensities, denoted as HRF2 or HRF3, respectively. The formation of an HRF2 event from its parent CSV(HRF2) formation is ≤5 days, while the formation of an HRF3 event is ≥6 days. The latter group contributes ~57% of the total number of HRF events. As a result of some model constraints, the formation and development of HRF3 events are not well forecasted by the Global Forecast System (GFS) and regional forecast models. The life cycle and second peak intensity for CSV(HRF3) allow for the introduction of a forecast advisory for HRF3 events. Identification of CSVs and two sufficient requirements for the formation and occurrence of HRF events were developed by previous studies. Nevertheless, two new necessary steps are now included in the proposed forecast advisory. The population ratio for CSV(HRF3) and the regular CSV is only about 15%. The occurrence optimum time to for the CSV(HRF3) second peak intensity from this vortex formation is about 3 days 6 h. The GFS forecast over to is utilized to identify CSV(HRF3). Then, the relay of the GFS forecast from the occurrence time of the CSV(HRF3) second peak is used to predict the formation/occurrence of HRF3 events. Six HRF3 events during cold seasons for 2013–16 are used to test the feasibility of this forecast advisory. Results clearly demonstrate this advisory is a success for the forecast of HRF3 events over the entire life cycles of their parent CSV(HRF3)s.

Description: An effort was made to search for relationships between interannual variations of population, lifetime, genesis locations, and intensity of named typhoons and numbered tropical depressions in the western North Pacific during the 1979–2002 period. To support this research task, climatological relationships of tropical cyclone characteristics were also investigated for these cyclones. Major findings of this study are summarized as follows:Climatology: Measured by the intensity scale of the Japan Meteorological Agency, three groups of tropical cyclones were identified in terms of population versus intensity: Group 1 [tropical depression (TD) + typhoon (TY)], Group 2 (strong + very strong TY), and Group 3 (catastrophic TY). This group division coincides with that formed in terms of lifetime of tropical cyclones versus intensity. Weak cyclones (Group 1) have a larger population than strong cyclones (Group 3), while the former group has shorter lifetime than the latter group. For genesis locations, the monsoon trough is established as a favorable region of tropical cyclone genesis because it provides an environment of large vorticity. Therefore, the northward latitudinal displacement of the maximum genesis frequency in the three groups of tropical cyclones follows that of the monsoon trough.Interannual variation: Any mechanism that can modulate the location and intensity of the monsoon trough affects the genesis location and frequency of tropical cyclones. In response to tropical Pacific sea surface temperature anomalies, a short wave train consisting of east–west oriented cells emanates from the Tropics and progresses along the western North Pacific rim. Population of the Group-1 tropical cyclones varies interannually in phase with the oscillation of the anomalous circulation cell northeast of Taiwan and south of Japan in this short wave train, while that of Group 3 fluctuates coherently with the tropical cell of this short wave train. Because these two anomalous circulation cells exhibit opposite polarity, the out-of-phase interannual oscillation between these two cells results in the opposite interannual variation of genesis frequency between tropical cyclones of Groups 1 and 3.

Description: Major rainfall (≥60%) in the northern part of the South China Sea (between North Vietnam and Taiwan) during May–June (the mei-yu season—the first phase of the Southeast–East Asian monsoon) is produced by rainstorms originating over the northern Vietnam–southwestern China region and the northern part of the South China Sea. As observed in this study, the occurrence frequency of rainstorms and rainfall contribution by these rainstorms undergoes a distinct interannual variation, in-phase with those of monsoon westerlies in northern Indochina and sea surface temperature (SST) anomalies over the NOAA Niño-3.4 region ΔSST (Niño-3.4). This in-phase relationship between monsoon westerlies and the ΔSST (Niño-3.4) anomalies is a result of the filling (deepening) of the subtropical Asian continental thermal low in response to the ΔSST (Niño-3.4) warm (cold) anomalies. Accompanied with this response is a slight southward (northward) shift of the North Pacific convergence zone (NPCZ), which extends from southern China to the North Pacific east of Japan. Thus, a favorable environment that meets the Charney–Stern instability criterion in initiating rainstorm genesis is enhanced (suppressed) by the intensification (weakening) of the monsoon shear flow formed by the midtropospheric northwesterly flow around the northeast periphery of the Tibetan Plateau and the monsoon westerlies. The meridional shift of the NPCZ established an elongated anomalous convergence (divergence) zone of water vapor flux along rainstorm tracks to increase (reduce) the rain-producing efficiency of rainstorms. Consequently, this interannual rainfall variation between northern Vietnam and Taiwan is primarily caused by rainstorm genesis and rain-producing efficiency.