scholarly journals Tropical Cyclone Formation within Strong Northeasterly Environments in the South China Sea

Atmosphere ◽  
2021 ◽  
Vol 12 (9) ◽  
pp. 1147
Author(s):  
Yung-Lan Lin ◽  
Hsu-Feng Teng ◽  
Yi-Huan Hsieh ◽  
Cheng-Shang Lee
Keyword(s):  
South China Sea ◽  
South China ◽  
Convective Instability ◽  
Wrf Model ◽  
The South China Sea ◽  
Weather Research And Forecasting ◽  
The South ◽  
Late Season ◽  
Low Level ◽  
China Sea

In the South China Sea (SCS), 17% of tropical cyclones (TCs) formed in the late season (November−January) were associated with a strong northeasterly monsoon. This study explores the effects of northeasterly strength on TC formation over the SCS. The Weather Research and Forecasting (WRF) model is used to simulate the disturbances that develop into TCs (formation cases) and those that do not (non-formation cases). Two formation (29W on 18 November 2001 and Vamei on 26 December 2001) and two non-formation (30 December 2002 and 9 January 2003) cases are simulated. To address the importance of upstream low-level northeasterly strength to TC formation, two types of sensitivity experiments are performed: formation cases with increased northeasterly flow and non-formation cases with decreased northeasterly flow. If the strength of the northeasterly is increased for the formation case, the stronger cold advection reduces the convective instability around the disturbance center, leading to the weakening of the simulated disturbance. If the strength of the northeasterly is decreased for the non-formation case, the simulated disturbance can develop further into a TC. In summary, strength of the upstream low-level northeasterly flow does affect the environmental conditions around the disturbance center, resulting in the change of TC formation probability over the SCS in the late season.

scholarly journals An Analysis of Tropical Cyclone Formations in the South China Sea during the Late Season

2011 ◽  
Vol 139 (9) ◽  
pp. 2748-2760 ◽  
Author(s):  
Yung-Lan Lin ◽  
Cheng-Shang Lee
Keyword(s):  
Tropical Cyclone ◽  
South China Sea ◽  
South China ◽  
Vertical Wind Shear ◽  
The South China Sea ◽  
The South ◽  
Late Season ◽  
Low Level ◽  
China Sea ◽  
The North

This study examines tropical cyclone (TC) formations in the South China Sea (SCS) associated with the strong northeasterly monsoon during the late season. Results show that the percentage of all vortices that develop to TC intensity is lower in the late season when compared to that associated with the mei-yu front in May and June. But the average formation time for the late-season cases is significantly shorter than that for the mei-yu frontal cases. Composite analyses show that the formation cases in the late season have larger low-level vorticity and upper-level divergence as well as higher 700-hPa relative humidity when compared to the nonformation cases. Another major difference between the formation and the nonformation cases is the low-level northeasterlies to the north (or upstream) of the incipient disturbance, which weakens right before the pre-TC disturbance reaching 25 kt (~13 m s−1). The weakening of the northeasterlies may be important to TC formation because it decreases the vertical wind shear and prevents the shift of the disturbance center from over the southern SCS to near the Borneo landmass. Furthermore, it reduces the stabilizing effect associated with the cold- and dry-air intrusion. Previous studies have shown that stronger equatorial westerlies during the active Madden–Julian oscillation (MJO) period may produce stronger cyclonic shear vorticity; thus, favorable for triggering more convection activities and more vortex formations. However, more vortices or cloud clusters are not necessarily more favorable for an individual vortex to organize into a TC. Nonetheless the initial setup of favorable synoptic environment appears to be important for the further development of the incipient vortex.

Diurnal Cycle of Coastal Convection in the South China Sea Region and Modulation by the BSISO

2021 ◽  
pp. 1-53
Author(s):  
Weixin Xu ◽  
Steven A. Rutledge ◽  
Kyle Chudler
Keyword(s):  
South China Sea ◽  
South China ◽  
The Philippines ◽  
The South China Sea ◽  
Boreal Summer ◽  
Total Precipitation ◽  
The South ◽  
Low Level ◽  
China Sea ◽  
Diurnal Cycles

AbstractUsing 17-yr spaceborne precipitation radar measurements, this study investigates how diurnal cycles of rainfall and convective characteristics over the South China Sea region are modulated by the Boreal Summer Intraseasonal Oscillation (BSISO). Generally, diurnal cycles change significantly between suppressed and active BSISO periods. Over the Philippines and Indochina, where the low-level monsoon flows impinge on coast lines, diurnal cycles of rainfall and many convective properties are enhanced during suppressed periods. During active periods, diurnal variation of convection is still significant over land but diminishes over water. Also, afternoon peaks of rainfall and MCS populations over land are obviously extended in active periods, mainly through the enhancement of stratiform precipitation. Over Borneo, where the prevailing low-level winds are parallel to coasts, diurnal cycles (both onshore and offshore) are actually stronger during active periods. Radar profiles also demonstrate a pronounced nocturnal offshore propagation of deep convection over western Borneo in active periods. During suppressed periods, coastal afternoon convection over Borneo is reduced, and peak convection occurs over the mountains until the convective suppression is overcome in the late afternoon or evening. A major portion (> 70%) of the total precipitation over Philippines and Indochina during suppressed periods falls from afternoon isolated to medium-sized systems (< 10,000 km2), but more than 70% of the active BSISO rainfall is contributed by nocturnal (after 18 LT) broad precipitation systems (> 10,000 km2). However, offshore total precipitation is dominated by large precipitation systems (> 10,000 km2) regardless of BSISO phases and regions.

Interdecadal Variations of Meridional Winds in the South China Sea and Their Relationship with Summer Climate in China*

2010 ◽  
Vol 23 (4) ◽  
pp. 825-841 ◽  
Author(s):  
Chunhui Li ◽  
Tim Li ◽  
Jianyin Liang ◽  
Dejun Gu ◽  
Ailan Lin ◽  
...  
Keyword(s):  
South China Sea ◽  
East Asia ◽  
South China ◽  
The South China Sea ◽  
Central China ◽  
Convective Heating ◽  
Tropospheric Temperature ◽  
The South ◽  
Low Level ◽  
China Sea

Abstract Analysis of the NCEP and 40-yr ECMWF Re-Analysis (ERA-40) data and the Xisha Island station observation indicates that the low-level meridional wind (LLMW) over the South China Sea (SCS) experienced an interdecadal variation since the late 1970s. The LLMW change is associated with the reduction of tropospheric temperature in midlatitude East Asia. A mechanism is put forward to explain the triggering and maintenance of the tropospheric cooling. The enhanced convective heating over the southern South China Sea results in a meridional vertical overturning circulation, with anomalous descending motion appearing over continental East Asia. The anomalous descending motion reduces the local humidity through both anomalous low-level divergence and dry vertical advection. The decrease of the local tropospheric humidity leads to the enhanced outgoing longwave radiation into space and thus cold temperature anomalies. The decrease of the temperature and thickness leads to anomalous low (high) pressure and convergent (divergent) flows at upper (lower) levels. This further enhances the descending motion and leads to a positive feedback loop. The fall in tropospheric temperature over continental East Asia reduces the land–sea thermal contrast and leads to the weakening of cross-equatorial flows and the LLMW over SCS. A further diagnosis indicates that the LLMW is closely linked to the summer precipitation and temperature variations in China on interdecadal time scales. A weakening of the LLMW after 1976 is associated with a “−, +, −” meridional rainfall pattern, with less rain in Guangdong Province and north China but more rain in the Yangtze and Huaihe River basins and northeast China, and a “+, −, +” temperature pattern, with increased (decreased) surface temperature in the south and north (central) China.

Fall Persistence Barrier of Sea Surface Temperature in the South China Sea Associated with ENSO*

2007 ◽  
Vol 20 (2) ◽  
pp. 158-172 ◽  
Author(s):  
Jau-Ming Chen ◽  
Tim Li ◽  
Ching-Feng Shih
Keyword(s):  
South China Sea ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
South China ◽  
The South China Sea ◽  
Sea Surface ◽  
The South ◽  
Low Level ◽  
China Sea ◽  
Philippine Sea

Abstract The authors investigate persistence characteristics of sea surface temperature (SST) in the South China Sea (SCS) in association with El Niño–Southern Oscillation (ENSO). It is found that a persistence barrier exists around October and November. This fall persistence barrier (FPB) is well recognized in the developing phase of strong ENSO cases, but becomes vague in weak ENSO and normal (non-ENSO) cases. During a strong El Niño developing year, salient features of the SCS SST anomaly (SSTA) associated with the FPB include a sign reversal between summer and winter and a rapid warming during fall. One possible cause of these SST changes, as well as the occurrence of the FPB, is the development and evolution of a low-level anomalous anticyclone (LAAC). The analyses show that the LAAC emerges in the northern Indian Ocean in early northern fall, moves eastward into the SCS during fall, and eventually anchors in the Philippine Sea in northern winter. This provides a new scenario for the generation of the anomalous Philippine Sea anticyclone previously studied. Its eastward movement appears to result from an east–west asymmetry, relative to the anticyclonic circulation center, of divergent flow and associated atmospheric vertical motion/moisture fields. The eastward passage of the LAAC across the SCS warms the underlying SST first via increased absorption of solar heating in October as it suppresses convective activities in situ, and next via decreased evaporative cooling in November and December as the total wind speed is weakened by the outer flows of the eastward-displacing LAAC. As such, the SCS SST changes quickly from a cold to a warm anomaly during fall, resulting in an abrupt change in anomaly patterns and the occurrence of the FPB. Analyses also suggest that the LAAC development during fall is relatively independent from the preceding Indian summer monsoon and the longitudinal propagation features of the ENSO-related Pacific SSTA. The aforementioned ocean–atmosphere anomalies contain an opposite polarity in a strong La Niña event. The low-level circulation anomaly weakens in intensity during weak ENSO cases and simply disappears during normal cases. As a result, the SCS FPB becomes indiscernible in these cases.

scholarly journals Sensitivity of Different Parameterizations on Simulation of Tropical Cyclone Durian over the South China Sea Using Weather Research and Forecasting (WRF) Model

2018 ◽  
Author(s):  
Worachat Wannawong ◽  
Donghai Wang ◽  
Yu Zhang ◽  
Chaiwat Ekkawatpanit
Keyword(s):  
Tropical Cyclone ◽  
South China Sea ◽  
South China ◽  
Performance Testing ◽  
The South China Sea ◽  
Sensitivity Analyses ◽  
Weather Research And Forecasting ◽  
The South ◽  
China Sea ◽  
Weather Research

Typhoon Durian forming over the Western North Pacific Ocean and entering into the South China Sea (SCS), caused extreme and widespread damages in 2006. In this research, sensitivity analyses on different physical parameterization schemes of the Weather Research and Forecasting Atmospheric Model (WRF-ATM) have been utilized to study typhoon Durian. Model accuracy and performance testing were investigated with different initial conditions during the tropical cyclone simulation in the SCS. The initial and boundary conditions (IBCs) for all experiments were derived from the European Centre for Medium Range Weather Forecasts (ECMWF), Re-Analysis Interim (ERAI), and the National Centers for Environmental Prediction (NCEP) with Final (FNL) analysis data compiled through the WRF-ATM model. The sensitivity analysis results indicated a major improvement for the cumulus scheme by using the Grell-Devenyi scheme along with the PBL scheme of Yonsei University, mixed-phase microphysics scheme of the WRF Single Moment 5-class and IBCs for ECMWF-ERAI of TC simulation under the context of Wind-Pressure Relationships. This predicted better track and intensity comparing with these of the Joint Typhoon Warning Center. The results revealed that the TC track and intensity were well simulated by the WSM5-GD combination for the WRF-ATM model with an intensity error of 1.69 hPa for minimum surface level pressure, maximum wind speed of 1.83 knots and average track error of 25 km in 72 hours. The simulations showed that the potential track and intensity error decreased with the delayed IBCs, suggesting that the model simulation is more dependable when the coast is approached by the TC.

scholarly journals Synoptic-Dynamic Analysis of Early Dry-Season Rainfall Events in the Vietnamese Central Highlands

2016 ◽  
Vol 144 (4) ◽  
pp. 1509-1527 ◽  
Author(s):  
Roderick van der Linden ◽  
Andreas H. Fink ◽  
Tan Phan-Van ◽  
Long Trinh-Tuan
Keyword(s):  
South China Sea ◽  
South China ◽  
Cold Front ◽  
Dry Season ◽  
The South China Sea ◽  
The South ◽  
Low Level ◽  
China Sea ◽  
Central Highlands ◽  
Wet Spells

Abstract The Central Highlands are Vietnam’s main coffee growing region. Unusual wet spells during the early dry season in November and December negatively affect two growing cycles in terms of yield and quality. The meteorological causes of wet spells in this region have not been thoroughly studied to date. Using daily rain gauge measurements at nine stations for the period 1981–2007 in the Central Highlands, four dynamically different early dry-season rainfall cases were investigated in depth: 1) the tail end of a cold front, 2) a tropical depression–type disturbance, 3) multiple tropical wave interactions, and 4) a cold surge with the Borneo vortex. Cases 1 and 4 are mainly extratropically forced. In case 1, moisture advection ahead of a dissipating cold front over the South China Sea led to high equivalent potential temperature in the southern highland where this air mass stalled and facilitated recurrent outbreaks of afternoon convection. In this case, the low-level northeasterly flow over the South China Sea was diverted around the southern highlands by relatively stable low layers. On the contrary, low-level flow was more orthogonal to the mountain barrier and high Froude numbers and concomitant low stability facilitated the westward extension of the rainfall zone across the mountain barrier in the other cases. In case 3, an eastward-traveling equatorial Kelvin wave might have been a factor in this westward extension, too. The results show a variety of interactions of large-scale wave forcings, synoptic-convective dynamics, and orographic effects on spatiotemporal details of the rainfall patterns.

scholarly journals Tropical Cyclone Formations in the South China Sea Associated with the Mei-Yu Front

2006 ◽  
Vol 134 (10) ◽  
pp. 2670-2687 ◽  
Author(s):  
Cheng-Shang Lee ◽  
Yung-Lan Lin ◽  
Kevin K. W. Cheung
Keyword(s):  
Tropical Cyclone ◽  
South China Sea ◽  
South China ◽  
The South China Sea ◽  
Lower Latitude ◽  
The South ◽  
Low Level ◽  
China Sea ◽  
Tropical Depression ◽  
Stationary Front

Abstract This study examines the 119 tropical cyclone (TC) formations in the South China Sea (SCS) during 1972–2002, and in particular the 20 in May and June. Eleven of these storms are associated with the weak baroclinic environment of a mei-yu front, while the remaining nine are nonfrontal. Seven of the 11 initial disturbances originated over land and have a highly similar evolution. Comparison of the frontal and nonfrontal formation shows that a nonfrontal formation usually occurs at a lower latitude, is more barotropic, develops faster, and possibly intensifies into a stronger TC. Six nonformation cases in the SCS are also identified that have similar low-level disturbances near the western end of a mei-yu front but did not develop further. In the nonformation cases, both the northeasterlies north of the front and the monsoonal southwesterlies are intermittent and weaker in magnitude so that the vorticity in the northern SCS does not spin up to tropical depression intensity. Because of the influence of a strong subtropical high, convection is suppressed in the SCS. The nonformation cases also have an average of 2–3 m s−1 larger vertical wind shear than the formation cases. A conceptual model is proposed for the typical frontal-type TC formations in the SCS that consists of three essential steps. First, an incipient low-level disturbance that originates over land moves eastward along the stationary mei-yu front. Second, the low-level circulation center with a relative vorticity maximum moves to the open ocean with the stationary front. Last, with strengthened northeasterlies, cyclonic shear vorticity continues to increase in the SCS, and after detaching from the stationary front, the system becomes a tropical depression.

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