Southwesterly Monsoon Flow Research Articles

Correction to: Effects of Long-term Climate Change on Typhoon Rainfall Associated with Southwesterly Monsoon Flow near Taiwan: Mindulle (2004) and Morakot (2009)

Correction to: Effects of Long-term Climate Change on Typhoon Rainfall Associated with Southwesterly Monsoon Flow near Taiwan: Mindulle (2004) and Morakot (2009)

Effects of Long-term Climate Change on Typhoon Rainfall Associated with Southwesterly Monsoon Flow near Taiwan: Mindulle (2004) and Morakot (2009)

Effects of Long-term Climate Change on Typhoon Rainfall Associated with Southwesterly Monsoon Flow near Taiwan: Mindulle (2004) and Morakot (2009)

Factors Leading to Heavy Rainfall in Southern Taiwan in the Early Mei-Yu Season of 2020

Abstract This paper examines the meteorological factors that led to the record-breaking heavy precipitation event in Taiwan in the early 2020 mei-yu season (15–31 May). The extreme amount of rainfall (an average of 135.9 mm per station) during the 36-h period around 22 May (hereafter Y20R) also set a record. Compared to climatology, the Pacific subtropical high was stronger and the southwesterly monsoonal flow was more intense during the first half of the 2020 mei-yu season, resulting in a stronger moisture conveyor belt over the northern Indo-China Peninsula. The record-breaking precipitation in Y20R was mainly caused by the eastward movement of a southwest vortex (SWV) generated in southwestern China. When the eastern portion of the SWV touched northern Taiwan, its associated west-southwesterly winds and the large-scale southwesterly monsoonal flow transported moisture toward the Taiwan Strait. The moisture-laden southwesterly flow was lifted by the stationary mei-yu front, leading to the heavy rainfall in southern Taiwan. When the SWV passed through northern Taiwan, it became the dominant weather system that enhanced the west-southwesterly winds and transported moisture from South China to Taiwan. The front moved southward through the Taiwan Strait during this period, with its location greatly determining the pattern of rainfall in southern Taiwan. In summary, the most critical factors leading to heavy rainfall in southern Taiwan are the strong 850-hPa southwesterly winds and moisture fluxes associated with the SWV. The other key factors include, in order of sensitivity to rainfall, the distance of the front, the distance of the SWV, the frontal speed, and the intensity of the SWV.

An Overview of Low-Level Jets (LLJs) and Their Roles in Heavy Rainfall over the Taiwan Area during the Early Summer Rainy Season

During the early summer rainy season over Taiwan, three types of low-level jets are observed, including a synoptic low-level jet (SLLJ) situated in the 850–700 hPa layer in the frontal zone, a marine boundary layer jet (MBLJ) embedded within the southwesterly monsoon flow over the northern South China Sea at approximately the 925 hPa level, and an orographically induced jet at approximately the 1 km level off the northwestern Taiwan coast (e.g., barrier jet (BJ)). The characteristics and physical processes of the formation of these three types of low-level jets are reviewed, and their roles in the development of heavy rainfall are discussed.

Classification of synoptic weather clusters associated with dust accumulation over southeastern areas of the Caspian Sea (Northeast Iran and Karakum desert)

• Determination of weather clusters associated with high dust-AODs over northeast Iran/Karakum desert. • Six identified weather clusters from PCA with highest frequency in spring. • The dust hotpot areas are attributed to convergence of northerly and southwesterly winds over NE Iran. • The Siberian High and Qibla wind in central Iran modulate the dust accumulation. • Changes in position, intensity and expansion of the upper-level subtropical jet modulate the dust distribution. In this study, daily-mean total and dust aerosol optical depth (TAOD, DAOD, respectively) obtained from the Monitoring Atmospheric Composition and Climate (MACC) and meteorological fields from ERA-Interim reanalysis are used to identify spatial patterns of dust accumulation over northeast Iran and Karakum Desert during 2003–2012. The most dust-affected area is defined using highest variances of DAOD by S-mode and weather clusters by T-mode principal component analysis (PCA). Six weather clusters are classified via the statistical analysis, associated with high DAOD values, with larger frequency in spring, while PC1 (41 cases) dominates in summer. The results show that changes in the intensity and expansion of the Siberian/European high-pressures in spring modulate strong northeasterlies or northwesterlies over Central Asia, which are associated with frontal dust storms over the desert areas (Aralkum, Karakum). In addition, dynamic conditions associated with the sub-tropical jet stream and the Iranian trough, and combined with convective conditions at areas of thermal lows in east Iran, create a strong southwesterly wind – called Qibla – over the Iranian Plateau. These two contrasting wind regimes converge over northeast Iran/Karakum Desert, facilitating dust accumulation over the area. In summer, northerly winds dominate over Central Asia, but the absence of Qibla flow allows them to traverse till the north coast of the Arabian Sea, where they converge with the southwest monsoon flow. The accumulation of dust over northeast Iran/Karakum is lesser than that over Pakistan and Thar desert. Furthermore, the upper-level sub-tropical jet stream moves northward in summer, with core over Turkmenistan/Uzbekistan.

The Crucial Role of Synoptic Pattern in Determining the Spatial Distribution and Diurnal Cycle of Heavy Rainfall over the South China Coast

AbstractThe South China coast suffers frequent heavy rainfall every warm season. Based on the objective classification method of principal components analysis, the key role of the synoptic pattern in determining the heavy rainfall processes that occurred over the South China coast in the warm season during 2008–18 is examined in this study. We found that heavy rainfall occurs most frequently under three typical synoptic patterns (P1–P3 hereafter) characterized by strong low-level onshore winds. P1 and P3 feature a prevailing southwesterly monsoonal flow in the lower troposphere, with heavy rainfall frequently occurring over the inland windward region in the afternoon associated with the orographic lifting and solar heating. The onshore wind of P3 is stronger than P1 as the western Pacific subtropical high extends more westward to 122°E, which induces stronger low-level convergence along the coastline than P1 when the ageostrophic wind veers from the offshore to onshore direction in the early morning. Hence, a secondary early morning rainfall peak can be found along the coastline. P2 is characterized by a low-level vortex located over the southwest portion of south China. Heavy rainfall under P2 usually initiates over the western part of the coastal region in the morning and then propagates inland in the afternoon. Overall, the synoptic patterns strongly determine the spatial distribution and diurnal cycle of heavy rainfall over the South China coast. This heavy rainfall is closely related to the diurnally varying low-level onshore winds rather than the low-level jets, as well as the different interactions between the low-level onshore winds and the local orography, coastline, and land–sea breeze circulations under different synoptic patterns.

Generation and enhancement mechanisms for extreme orographic rainfall associated with Typhoon Morakot (2009) over the Central Mountain Range of Taiwan

The generation and enhancement mechanisms and essential ingredients for the extreme rainfall associated with Typhoon Morakot (2009) passing over Taiwan's Central Mountain Range (CMR) were investigated by using the Advanced Research Weather Research and Forecasting (WRF) model. First, we found that the extreme rainfall was due to the essential orographic rain ingredients: high precipitation efficiency, strong low-level jet, steep terrain, high moisture upstream, large convective system, and slower movement of the typhoon. Second, the Orographic Rain Index (ORI) proposed by Rostom (2015) was modified and tested by twelve (12) TC cases and found it has a good correlation with daily rainfall. Thus it can be used to help the prediction of extreme orographic TC rainfall. Third, we found there are two types of orographic rainfall mechanisms associated with Morakot, namely initiation and enhancement. These rainfall mechanisms may occur simultaneously at different locations, such as in northeast and southwest CMR. The orographic rain in the northeastern CMR was initiated by the high and steep CMR while the typhoon was still 200 km away. The rainfall was enhanced gradually and continually by the CMR when Morakot was approaching it, which produced a maximum rainfall of 265 mm/3 h starting at 1800 UTC August 7. The rain then decreased when the typhoon's eyewall structure and convection were damaged and weakened, respectively, when it invaded the high and steep CMR. Orographic rain was generated in the southwestern CMR initially when the conditionally unstable, southwesterly monsoonal flow impinged on the southern CMR. It was enhanced afterward by the merged monsoonal flow and Morakot's rainband leading to an extreme maximum rainfall of 311 mm/3 h starting 0000 UTC August 8. Later, this region of orographically enhanced TC rain moved northward along the western flank of the CMR. At last, we presented a conceptual model summarizing four key processes associated with the generation and enhancement of orographic TC rain over the southwest of CMR during the passage of typhoon Morakot (2009).

A Comparative Study on the Offshore Intensification of Supertyphoon Rammasun (2014) and Typhoon Rumbia (2013): the Role of Summer Monsoon

The offshore intensification mechanisms of Supertyphoon Rammasun (2014) and Typhoon Rumbia (2013) are analyzed and compared based on the best track typhoon data provided by the Shanghai Typhoon Institute of China Meteorological Administration (STI/CMA), ECMWF (CMA and JMA) objective (subjective) predicted typhoon track and intensity, and reanalysis data of the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR). Results show that the offshore sea surface temperature over 28 °C was conducive to the intensification of the two typhoons in the coastal area prior to their landfall in South China. Rammasun was embedded in an extremely active phase of a 10–20-day oscillation in the summer monsoon over the South China Sea and western North Pacific prior to its landfall. The low-level jet transported warm and moist air to the inner core of Rammasun and the deepening of the southwesterly monsoon flow up to 200 hPa reduced vertical wind shear, both favorable for the offshore rapid intensification of Rammasun. In 2013, the summer monsoon over the South China Sea and western North Pacific experienced quasi-20-day and quasi-10-day oscillations. The out of phase of the two oscillations resulted in relatively weak tropical and subtropical summer monsoon flow prior to the landfall of Rumbia. As a result, the low-level monsoon flow transported relatively less moist air into Rumbia, and the shallow monsoon flow was not conducive to the weakening of vertical wind shear, resulting in a less rapid offshore intensification of Rumbia than Rammasun.

Climatology of coastal wind regimes in Benin

Using surface and lower-tropospheric wind observations at Cotonou (Benin), a comprehensive ten-year (2006–2015) climatology of coastal wind regimes at the West African Guinea Coast is presented for the first time. Four wind regimes are objectively identified: the monsoon and Harmattan winds, the nocturnal low-level jet (NLLJ) and the land-sea breeze (LSB) system. Amongst other things, these regimes are important for the occurrence of coastal squall line systems and for the redistribution of pollutants emitted from coastal cities. The seasonal cycle is split into the long (December–February) and short (July–September) dry seasons, the long (May–June) and short rainy (October–November) seasons, and the beginning of the long rainy season (March–April). In terms of the LSB, the lack of a diurnal reversal in wind direction obscures sea breeze activity at the height of the southwesterly monsoon flow in boreal summer and during a period of stronger winds in March-April related to large land-sea thermal contrasts. During these periods, the sea breeze manifests itself in a relative increase of the monsoon background flow during daytime, while early morning land breezes are infrequent. The latter expectedly, peak during the long dry season in December with a secondary peak during the long rainy season in May–June. The large–scale monsoon and Harmattan flows, mutually exclusive by definition, occur respectively, 88.2–98.4 % and 1.6–11.8 % per year with Harmattan days restricted to the long dry season and deepest monsoon flows of about 2.5 km confined to the long rainy season. The NLLJ is most prominent during the little dry season with, however, a similar and hitherto less documented peak in occurrence frequency during the beginning of the long rainy season. At this time of the year, the African Easterly Jet passes northward over Cotonou, causing the largest 3000–600 m vertical wind shear.

Regionality of long-term trends and interannual variation of seasonal precipitation over India

This study investigated the long-term trends and interannual variations (IAVs) of seasonal precipitation during two different periods in India and their relation to atmospheric circulation. To focus on the relation between the regional characteristics of precipitation and the large-scale circulation system, we defined homogeneous regions based on pentad rainfall seasonality using hierarchical cluster analysis. Seven obtained clusters (regions) were used to analyze the long-term trends, IAVs, and their relation to atmospheric circulation. Seasonal precipitation showed no remarkable trend in most regions and seasons, which implies the effect of IAV in seasonal precipitation is greater than the long-term trend. Based on regression analysis, the IAV of seasonal precipitation and its relation to atmospheric circulation over the ocean surrounding India were detected for each season. Enhancement of the southwesterly monsoon flow over the Indian Ocean and the strength of the southeasterly wind related to the activity of the monsoon trough around the Gangetic Plain both affected the increase of summer monsoon precipitation in regions CL3 (central and eastern India) and CL6 (western coast of the Arabian Sea in southern India). In contrast, neither region CL5 (eastern coastal region of southern India) nor CL7 (northeastern India and part of the western coastal region of southern India) showed a relationship between IAV of summer monsoon precipitation and the southwesterly flow or the southeasterly flow over the Indian Ocean or the Gangetic Plain, respectively. Region CL5 showed strong correlation with the northeasterly monsoon flow (strength of the southeasterly flow over the Bay of Bengal) during the post-monsoon season. Other regions showed no relation with the northeasterly flow during the post-monsoon season. Furthermore, IAV of precipitation in region CL7 showed no correlation with either the southwesterly monsoon during the summer monsoon season or the northeasterly monsoon during the post-monsoon season.

Effects of Monsoon, Shamal and Levar winds on dust accumulation over the Arabian Sea during summer – The July 2016 case

Abstract This study examines the atmospheric circulation patterns that are associated with contrasting wind regimes, desert dust outflows and accumulation of dust over the Arabian Sea in the summer season. The examined period is 11–16 July 2016, when several dust plumes of various intensity and from different sources (Arabian Peninsula, Somalia and Iran), affected the Arabian Sea. During summer, south Asia is under the influence of the southwest monsoon, with main characteristics being the thermal low-pressure system centered over northwest India/Pakistan with secondary lows over south Iran and southeast Arabian Peninsula, and the Arabian ridge combined with the Indian trough in the mid troposphere. Apart from the dominant southwest monsoon flow over the Arabian Sea, strong local winds are developed over the desert areas, such as the northwest Shamal along the east Arabian Peninsula and northern Levar in southeast Iran. These wind regimes initiate several dust outbreaks, while their convergence over the Arabian Sea results in accumulation of large amounts of dust aerosols (AOD > 2.0), as observed by satellite (MODIS, MISR, CALIPSO) observations and reanalysis (MERRA-2). The dust-accumulation area (central/north part of the Arabian Sea) is characterized by weak winds, while the position of the Inter-Tropical Discontinuity (ITD) plays an important role in merging the contrasting dust plumes. The vertical profiles of the monsoon, Shamal and Levar winds are especially important for the dust accumulation, since the low-altitude dust plumes come from the southeastern Arabian Peninsula and Somalia, while those above 3000 m from northern directions (Iran, Pakistan).

The East Asian Atmospheric Water Cycle and Monsoon Circulation in the Met Office Unified Model

AbstractIn this study the low‐level monsoon circulation and observed sources of moisture responsible for the maintenance and seasonal evolution of the East Asian monsoon are examined, studying the detailed water budget components. These observational estimates are contrasted with the Met Office Unified Model (MetUM) climate simulation performance in capturing the circulation and water cycle at a variety of model horizontal resolutions and in fully coupled ocean‐atmosphere simulations. We study the role of large‐scale circulation in determining the hydrological cycle by analyzing key systematic errors in the model simulations. MetUM climate simulations exhibit robust circulation errors, including a weakening of the summer west Pacific Subtropical High, which leads to an underestimation of the southwesterly monsoon flow over the region. Precipitation and implied diabatic heating biases in the South Asian monsoon and Maritime Continent region are shown, via nudging sensitivity experiments, to have an impact on the East Asian monsoon circulation. By inference, the improvement of these tropical biases with increased model horizontal resolution is hypothesized to be a factor in improvements seen over East Asia with increased resolution. Results from the annual cycle of the hydrological budget components in five domains show a good agreement between MetUM simulations and ERA‐Interim reanalysis in northern and Tibetan domains. In simulations, the contribution from moisture convergence is larger than in reanalysis, and they display less precipitation recycling over land. The errors are closely linked to monsoon circulation biases.

Influence of southwest monsoon flow and typhoon track on Taiwan rainfall during the exit phase: modelling study of typhoon Morakot (2009)

This paper studies the dramatic decadal increase of Taiwan rainfall in the typhoon exit phase. The exit phase is defined as the time interval between a typhoon centre leaving the Taiwan coast to reaching 100 km away from the nearest coastline. Typhoons which move across Taiwan's northern (track N), central (track C), or southern (track S) areas are selected for a statistical study. Taiwan hourly rainfall data at 21 surface gauge stations from the past 57 years are divided into two periods: 1960–1989 (P1) and 1987–2016 (P2). From P1 to P2, there are decadal increases of rainfall (∼60%) and rainfall intensity (∼30%, mm h−1) in the exit phase. There is also a decadal increase of the track C typhoons. The southwest monsoon water vapour flux (SWF) in a local region southwest of Taiwan, as computed from the JRA‐55 dataset, is substantially larger in the track C typhoons than that in the track N typhoons. Our analysis indicates that the increase of SWF leads to the increase of rainfall intensity. Moreover, both the enhanced SWF and the prolonged duration time contribute to the increased rainfall in the exit phase. Typhoon Morakot (2009), a track C typhoon with extremely slow speed in the exit phase, produced the record‐breaking rainfall. Model experiments and potential vorticity tendency diagnosis of typhoon Morakot are used to understand the dynamics of the increased duration time. The slowdown of typhoon motion is shown to be due to the asymmetric convection in the Taiwan Strait, which is produced by the interaction between typhoon circulation and southwest monsoon flow. The enhanced SWF and the prolonged duration time may explain the observation that the decadal rainfall increase is much larger than that of rainfall intensity in the exit phase.

Multilocalization data assimilation for predicting heavy precipitation associated with a multiscale weather system

AbstractHigh‐resolution numerical simulations are regularly used for severe weather forecasts. To improve model initial conditions, a single short localization is commonly applied in the ensemble Kalman filter when assimilating observations. This approach prevents large‐scale corrections from appearing in a high‐resolution analysis. To improve heavy rainfall forecasts associated with a multiscale weather system, analyses must be accurate across a range of spatial scales, a task that is difficult to accomplish using a single localization. This study is the first to apply a dual‐localization (DL) method to improve high‐resolution analyses used to forecast a real‐case heavy rainfall event associated with a Meiyu front on 16 June 2008 in Taiwan. A Meiyu front is a multiscale weather system characterized by storm‐scale convection, a mesoscale front, and large‐scale southwesterly monsoonal flow. The use of the DL method to produce the analyses was able to correct both the synoptic‐scale moisture flux transported by southwesterly monsoonal flow and the mesoscale low‐level convergence offshore of southwestern Taiwan. As a result, the forecasted amount, pattern, and temporal evolution of the heavy rainfall event were improved.

Nocturnal low-level clouds over southern West Africa analysed using high-resolution simulations

Abstract. We performed a high-resolution numerical simulation to study the development of extensive low-level clouds that frequently form over southern West Africa during the monsoon season. This study was made in preparation for a field campaign in 2016 within the Dynamics-aerosol-chemistry-cloud interactions in West Africa (DACCIWA) project and focuses on an area around the city of Savè in southern Benin. Nocturnal low-level clouds evolve a few hundred metres above the ground around the same level as a distinct low-level jet. Several processes are found to determine the spatio-temporal evolution of these clouds including (i) significant cooling of the nocturnal atmosphere caused by horizontal advection with the south-westerly monsoon flow during the first half of the night, (ii) vertical cold air advection due to gravity waves leading to clouds in the wave crests and (iii) enhanced convergence and upward motion upstream of existing clouds that trigger new clouds. The latter is caused by an upward shift of the low-level jet in cloudy areas leading to horizontal convergence in the lower part and to horizontal divergence in the upper part of the cloud layer. Although this single case study hardly allows for a generalisation of the processes found, the results added to the optimisation of the measurements strategy for the field campaign and the observations will be used to test the hypotheses for cloud formation resulting from this study.

Impacts of Including Rain-Evaporative Cooling in the Initial Conditions on the Prediction of a Coastal Heavy Rainfall Event during TiMREX

Abstract A cycling run, which began 36 h before the model forecast, was employed to assimilate special Terrain-influenced Monsoon Rainfall Experiment (TiMREX) soundings, Global Telecommunications System (GTS) data, and Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) global positioning system (GPS) radio occultation (RO) refractivity profiles to improve the model initial conditions provided by the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) to study a coastal, heavy rainfall event over southwestern Taiwan during 15–16 June 2008. The 36-h cycling run with data assimilation (DA_ALL_DATA run) has a positive impact on the depiction of subsynoptic flow in the model initial conditions at 1200 UTC 15 June, including the warm moist tongue and southwesterly monsoon flow over the open ocean. Furthermore, the cold pool caused by the evaporative cooling of antecedent rains and orographic blocking over southwestern Taiwan are better resolved in the nested high-resolution domain in the DA_ALL_DATA run as compared to the initial conditions provided by the NCEP GFS. As a result, the heavy rainfall along the southwestern coast and afternoon localized heavy rainfall over northern Taiwan are better predicted in the DA_ALL_DATA run. Model sensitivity tests are also performed to diagnose the effects of terrain and rain-evaporative cooling on the intensity and depth of the cold pool and degree of orographic blocking on the southwesterly flow over southwestern Taiwan. It is apparent that including rain-evaporative cooling from antecedent rains and orographic effects in the model initial conditions are important to account for the predicted rainfall distribution of this coastal rainfall event.

Characteristics of the Asian–Pacific oscillation in boreal summer simulated by BCC_CSM with different horizontal resolutions

The summer Asian–Pacific Oscillation (APO) is a major teleconnection pattern that reflects the zonal thermal contrast between East Asia and the North Pacific in the upper troposphere. The performance of Beijing Climate Center Climate System Models (BCC_CSMs) with different horizontal resolutions, i.e., BCC_CSM1.1 and BCC_CSM1.1(m), in reproducing APO interannual variability, APO-related precipitation anomalies, and associated atmospheric circulation anomalies, is evaluated. The results show that BCC_CSM1.1(m) can successfully capture the interannual variability of the summer APO index. It is also more capable in reproducing the APO’s spatial pattern, compared to BCC_CSM1.1, due to its higher horizontal resolution. Associated with a positive APO index, the northward-shifted and intensified South Asian high, strengthened extratropical westerly jet, and tropical easterly jet in the upper troposphere, as well as the southwesterly monsoonal flow over North Africa and the Indian Ocean in the lower troposphere, are realistically represented by BCC_CSM1.1(m), leading to an improvement in reproducing the increased precipitation over tropical North Africa, South Asia, and East Asia, as well as the decreased precipitation over subtropical North Africa, Japan, and North America. In contrast, these features are less consistent with observations when simulated by BCC_CSM1.1. Regression analysis further indicates that surface temperature anomalies over the North Pacific and the southern and western flanks of the Tibetan Plateau are reasonably reproduced by BCC_CSM1.1(m), which contributes to the substantial improvement in the simulation of the characteristics of summer APO compared to that of BCC_CSM1.1.

A diagnostic study on heavy rainfall induced by landfalling Typhoon Utor (2013) in South China: 2. Postlandfall rainfall

AbstractIn this part, mechanisms responsible for the maintenance of Typhoon Utor (2013) after landfall and its associated heavy rainfall in South China were investigated with methods including piecewise potential vorticity (PV) inversion. The focus was on the monsoonal influence and the interaction between Utor and the mesoscale convective systems (MCSs) embedded in the southwesterly monsoon flow. The results show that after landfall Utor underwent coalescence with the cyclonic monsoon gyres, related to the quasi‐biweekly oscillation (QBWO) and the Madden‐Julian oscillation (MJO). The QBWO cyclonic gyre, which better developed and coalesced with the tropical cyclone (TC) vortex, enhanced the large‐scale southwesterly monsoon flow southeast of the TC, providing favorable condition for initiation of convection and organization of MCSs in the eastern outer rainband of the TC. Latent heat release in the MCSs provided a positive feedback to enhance the TC circulation and the southwesterly monsoon flow, slowing down the decay of Utor and sustaining heavy rainfall. Piecewise PV inversion confirmed that the nonlinear balanced flow inverted from the PV anomaly associated with latent heating in MCSs led to an increase of the low‐level southwesterly flow south of the TC by over 4 m s−1, which contributed notably to the formation of the low‐level jet and moisture convergence in the eastern outer rainband. It is suggested that the positive feedback between the outer rainband MCSs and the southwesterly monsoon flow is a major mechanism responsible for the maintenance of Typhoon Utor after its landfall and the associated postlandfall heavy rainfall over South China.

A diagnostic study on heavy rainfall induced by Typhoon Utor (2013) in South China: 1. Rainfall asymmetry at landfall

AbstractAfter striking the Philippines on 11 August 2013, Typhoon Utor made its second landfall over southern South China on 14 August and brought heavy rainfall, causing widespread damages after its landfall. The heavy rainfall associated with Utor includes three main episodes: rainfall at landfall, postlandfall rainfall in the inner core region, and postlandfall rainfall induced by the intensification of southwesterly monsoon flow. In part 1 the rainfall at landfall is analyzed and diagnosed using the surface mesonet observations and the high‐resolution European Centre for Medium‐Range Weather Forecasting Interim reanalysis (ERA‐Interim) data. Results show that when Utor approached and made landfall over the southern coast of South China, it was embedded in a deep‐layer (200–850 hPa) easterly northeasterly vertical wind shear (VWS) environment with heavy rainfall mostly occurring downshear left in its southwestern quadrant. Under a favorable environment with moderate convective available potential energy and moderate low‐level VWS, a surface cold pool was found in the west‐southwestern quadrants of Utor triggered by the middle‐lower tropospheric dry air intrusion from inland to the north. Both frontogenesis and quasi‐balanced, ageostrophic Q‐vector diagnostics demonstrate that the quasi‐balanced forcing near the surface cold cool contributed to the enhancement of convection in the west‐southwestern quadrants of Utor. It is concluded that the environmental VWS and the consequent surface cold pool dynamics play an important role in rainfall asymmetry in Utor when it made landfall over the southern coast of South China.

Classification of weather patterns to study the influence of meteorological characteristics on PM2.5 concentrations in Yunlin County, Taiwan

Yunlin County is located in the central part of western Taiwan with major emissions from the Mailiao industrial park, the Taichung Power Plants and heavy traffic. In order to understand the influence of meteorological conditions on PM2.5 concentrations in Yunlin County, we applied a two-stage cluster analysis method using the daily averaged surface winds from four air quality monitoring stations in Yunlin County to classify the weather pattern. The study period includes 1095 days from Jan 2013 to December 2015. The classification results show that the low PM2.5 concentration occurs when the synoptic weather in Taiwan is affected by the strong southwesterly monsoonal flow. The high PM2.5 concentration occurs when Taiwan is under the influence of weak synoptic weather conditions and continental high-pressure peripheral circulation. A high PM2.5 event was studied and the Weather Research and Forecasting (WRF) meteorological model was performed. The result indicated that due to being blocked by the Central Mountain Range, Yunlin County, which is situated on the leeside of the mountains, exhibits low wind speed and strong subsidence behavior that favors PM2.5 accumulation.