Rainfall In China Research Articles

Variations in the Establishment Time and Eastern Boundary Position of the Somali Jet and Correlations with Summer Rainfall in China

Shi, W.; Dai, W.; Cheng, W.; Xiao, Z.; Li, S.; Gu, C., and Zhao, L., 2020. Variations in the establishment time and eastern boundary position of the Somali jet and correlations with summer rainfall in China. In: Zheng, C.W.; Wang, Q.; Zhan, C., and Yang, S.B. (eds.), Air-Sea Interaction and Coastal Environments of the Maritime and Polar Silk Roads.Journal of Coastal Research, Special Issue No. 99, pp. 31-40. Coconut Creek (Florida), ISSN 0749-0208.The establishment time index and eastern boundary position index were set up to describe the characteristics of the Somali jet (SMJ) activity, and relationships between the indices and Chinese summer rainfall were also explored used NCEP/NCAR reanalysis data. The Somali jet established in the 24th pentad and retreated in the 62nd pentad, and the eastern boundary position was maintained at 45-65°E. The establishment time sequence was characterized by an earlier tendency, and the eastern boundary position sequence exhibited the strong interannual variability. The intensity, establishment time and eastern boundary position indices were verified to describe the characteristics of the Somali jet activity from different angle and flank. The connection between these three indices and the Asian monsoon index seemed to be quite different. Both the intensity and establishment time were positively correlated with the third and second pattern of China summer rainfall, respectively. The eastern boundary position was negatively related to the first pattern of the summer rainfall in China. Additionally, the impact of the establishment time and eastern boundary position of the Somali jet on China summer precipitation were closely related to the Lake Baikal high and the western Pacific subtropical high as well as the equator warm wet airflow.

Impact of rapid urbanization on temporal and spatial pattern change of heavy rainfall in China during the past 60 years

Through the study of the daily heavy rainfall observation records of 659 meteorological stations in China from 1951 to 2010, we find that rapid urbanization may trigger the significant increase of large-scale heavy rainfall in China. The main conclusions are as follows: Decadal heavy rainfall amounts (HRA), heavy rainfall days (HRD) and heavy rainfall intensity (HRI) in China have increased significantly, with an increase of 68.71%, 60.15% and 11.52% respectively, and the increase of station number is 84.22%, 84.22% and 54.48% respectively, showing a time change process of “rapid slow re rapid increase” and a spatial change from Southeast Coast to Central, Southwest, North and Northeast China. The rapid urbanization factors, including the output value of the second industry (GDP2), the proportion of urban population (UP) and the annual average haze days (HD), are likely to be the main reasons for the increase of heavy rainfall in China. Their variance explanations for the HRA, HRD and HRI in China are 61.54%, 58.48% and 65.54% respectively, of which the variance explanations for the heavy rainfall rainfall, rainy day and rain intensity are respectively high only by haze 25.93%, 22.98% and 26.64% respectively, while the variance interpretation of climate factors, including WPSH, ENSO, AMO and AAO, is only 24.30%, 26.23% and 21.92% respectively. Compared with the forcing factors of rapid urbanization, the impact of these climate factors is only 1/3 of the former. The panel data of the annual mean of the total population and visibility days at the county level in China are significantly related to the HRA, HRD and HRI in China. The spatial correlation coefficient gradually increased from 1950s to 2000s, that is, the total population at the county level increased from 0.35, 0.36 and 0.40 to 0.54, 0.55 and 0.58 respectively, and the annual mean of visibility days increased from 0.36, 0.38 and 0.48 to 0.55, 0.57 and 0.58. It is further indicated that the rapid urbanization triggered the significant increase of heavy rainfall in China.

Impact of late spring Siberian snow on summer rainfall in South-Central China

Located in the Yangtze River Valley and surrounded by mountains, South-Central China (SCC) frequently suffered from natural disasters such as torrential precipitation, landslide and debris flow. Here we provide corroborative evidence for a link between the late spring (May) snow water equivalent (SWE) over Siberia and the summer (July–August, abbr. JA) rainfall in SCC. We show that, in May, anomalously low SWE over Siberia is robustly related to a large warming from the surface to the mid-troposphere, and to a stationary Rossby wave train from Siberia eastward toward the North Atlantic. On the one hand, over the North Atlantic there exhibits a tripole pattern response of sea surface temperature anomalies in May. It persists to some extent in JA and in turn triggers a wave train propagating downstream across Eurasia and along the Asian jet, as the so-called Silk Road pattern (SRP). On the other hand, over northern Siberia the drier soil occurs in JA, accompanied by an overlying anomalous anticyclone through the positive feedback. This anomalous anticyclone favors the tropospheric cooling over southern Siberia, and the meridional (northward) displacement of the Asian jet (JMD) due to the change in the meridional temperature gradient. The combination of the SRP and the JMD facilitates less water vapor transport from the tropical oceans and anomalous descending motion over SCC, and thus suppresses the precipitation. These findings indicate that May Siberian SWE can be exploited for seasonal predictability of SCC precipitation.

Differences in Pre-Flood Season Rainfall in South China between Spring and Summer El Niño Events

ABSTRACT The El Niño–Southern Oscillation (ENSO) plays an important role in pre-flood season (PFS) precipitation over South China. In this work, the analysis of observational and reanalysis data shows that PFS precipitation is closely related to the onset time of El Niño events. The PFS precipitation tended to be higher (lower) than normal for spring (summer) El Niño events during the 1979–2016 period. Our composite analyses reveal that, for spring El Niño events, the sea surface temperature (SST) anomaly in the central-east equatorial Pacific (CEEP) Ocean provided favourable large-scale circulation for abundant PFS precipitation, where the Hadley cell served as a bridge. In the year following an El Niño event, SST anomalies in the CEEP persist from January to June, while for April-May-June (AMJ) positive SST anomalies are seen offshore near China. These anomalies are associated with the AMJ–enhanced convective instability over South China through a weakened Walker circulation and a zonal teleconnection wavetrain pattern at 700 hPa in the northern hemisphere. Meanwhile, a weakened 200 hPa anticyclonic shear was seen over the Indochina Peninsula. There was a southwestward shift of the 500 hPa western Pacific subtropical high, and anomalous 850 hPa southwesterly wind-enhanced water vapour and warm advection toward South China. Therefore, the circulation-induced moisture environment and dynamical conditions both facilitated enhanced PFS precipitation over South China. For summer El Niño events, the moisture environment and dynamical conditions were unfavourable for producing precipitation, which resulted in below-normal PFS precipitation levels. Categorizing El Niño events by the onset time is very important because it provides useful information for predicting PFS precipitation with lead times of two or three seasons.

Assessing the Effects of Microphysical Scheme on Convective and Stratiform Characteristics in a Mei-Yu Rainfall Combining WRF Simulation and Field Campaign Observations

Microphysics parameterization becomes increasingly important as the model grid spacing increases toward convection-resolving scales. Using observations from a field campaign for Mei-Yu rainfall in China, four bulk cloud microphysics schemes in the Weather Research and Forecasting (WRF) model were evaluated with respect to their ability to simulate precipitation, structure, and cloud microphysical properties over convective and stratiform regimes. These are the Thompson (THOM), Morrison graupel/hail (MOR_G/H), Stony Brook University (SBU_YLIN), and WRF double-moment six-class microphysics graupel/hail (WDM6_G/H). All schemes were able to predict the rain band but underestimated the total precipitation by 23%–35%. This is mainly attributed to the underestimation of stratiform precipitation and overestimation of convective rain. For the vertical distribution of radar reflectivity, many problems remain, such as lower reflectivity values aloft in both convective and stratiform regions and higher reflectivity values at middle level. Each bulk scheme has its advantages and shortcomings for different cloud regimes. Overall, the discrepancies between model output and observations mostly exist in the midlevel to upper level, which results from the inability of the model to accurately represent the particle size distribution, ice processes, and storm dynamics. Further observations from major field campaigns and more detailed evaluation are still necessary.

Late Holocene monsoon precipitation changes in southern China and their linkage to Northern Hemisphere temperature

Observations and models suggest a latitudinal redistribution of precipitation as a result of ongoing global climatic warming. However, the dynamic link between temperature and regional precipitation in the course of natural climatic variability during the current interglacial period remains unclear, especially in the monsoon regions of China. Here we present a reconstruction of a ∼5 year-resolution summer monsoon rainfall record for the past 3200 years, based on 622 pairs of δ18O and δ13C and 29 230Th dates from a stalagmite (Wu37) from Wulu Cave in southern China. A close similarity between the δ18O and δ13C records on centennial to decadal timescales suggests that calcite δ18O is most likely a proxy associated with monsoon precipitation. The record reveals that the Little Ice Age (650 - 100 yr B.P.) was a relatively humid period, with two centennial-scale wet events, compared with the Medieval Warm Period (1050 - 650 yr B.P.). In contrast with previous observations in India and northern China, monsoon precipitation variations indicated by our record and other cave records nearby were inversely related to changes in Northern Hemisphere (NH) temperature over the past two millennia, in particular to variations in the thermal gradient between the Northern and Southern hemispheres. This suggests that NH temperature has an important influence on summer precipitation in southern China, via the interhemispheric thermal gradient and land-ocean thermal contrast. In addition, we find that variations of monsoon rainfall in southern China on multi-centennial to centennial timescales are inversely correlated with observed rainfall in the core monsoon region of India and in the northern fringe of the Asian summer monsoon region in China. This spatial pattern of rainfall variability can be interpreted as a result of the migration of the intertropical convergence zone that is likely dominated by the interhemispheric thermal gradient via cross-equatorial airflows.

The evolution process of warm season intense regional rainfall events in Yaan

Accurate simulation and prediction of intense precipitation events require better understanding of their physical mechanisms. This study uses Yaan—a place with regional maximum rainfall in central China—to investigate the cause and process of intense precipitation. Hourly rain gauge records and the new ERA5 reanalysis are used to characterize the evolution process of warm season intense regional rainfall events (RREs) in Yaan and its associated three-dimensional circulation. Results show that before the start of the Yaan intense RREs, moderate rainfall amount (frequency) appears northeast of the key region. The rainfall then moves southward in the following several hours along the eastern periphery of the Tibetan Plateau where it reaches peak. It then moves to and end up in the south and southeast Sichuan Basin. The progression of the RREs is found to be associated with a counter-clockwise rotation of anomalous surface winds associated with a developing mesoscale surface low-pressure center, which is further associated with the southeastward progression of a large-scale synoptic scale wave. The easterly phase of the winds in the counter-clockwise rotation causes upslope motion perpendicularly toward the terrain that leads to maximum rainfall. The findings illustrate how large-scale circulations, mesoscale systems, and specific topographic features interact to create the RREs evolution in Yaan.

Study on the contribution rate of urbanization and air-sea circulation factors to the increase of rainstorm in China

More and more attention has been paid to the serious disasters caused by extreme precipitation events. The relationship between climate change and extreme precipitation has become the hottest science foreland issue. Based on the study of daily rainstorm observation records of 659 meteorological stations in China from 1951 to 2010, this paper shows that rapid urbanization may trigger a significant increase in large-scale rainstorm in China. The main conclusions are as follows: (1) In terms of time, China’s rainstorm rainfall, rainy days and rainfall intensity increased significantly, reaching 68.71%, 60.15% and 11.52% respectively. In space, the torrential rains in China gradually migrated from the southeastern coast to the spatial transition process of gradient expansion in Central, Southwest, North, and Northeast China. (2) The variance explanations of the rapid urbanization factors to the rainstorm rainfall, rainy days and rainfall intensity in China are 61.54%, 58.48% and 65.54% respectively, while the variance explanations of the climatic factors are only 24.30%, 26.23% and 21.92% respectively. Rapid urbanization factors are likely to trigger a significant increase in rainstorm rainfall in China. (3) The panel data of China’s county-level total population and the annual average of visibility days are significantly spatially correlated with rainstorm rainfall, rainy days and rainfall intensity. The correlation coefficient gradually increases with age, further indicating that rapid urbanization has triggered a significant increase in large-scale inter-decadal rainstorm in China.

Study on temporal and spatial differences of multi-attribute characteristics of torrential rain in different months in China

Using the daily precipitation data of 545 meteorological stations in China from 1961 to 2016, the temporal and spatial characteristics of torrential rainfall and rain days in different months in China were detected from the three aspects of climatic state characteristics, change trend and interannual variations. The results showed that: (1) The torrential rainfall and rain days in different months in China from 1961 to 2016 had similar spatial differentiation characteristics in the corresponding months. The high-value areas of torrential rainfall and rain days gradually expanded from the southeast coast to the northwest inland from January to July, but mainly distributed in the east of the Hu Huanyong Line, and reduced from the northwest area to the southeast coastal area from August to December; The torrential rainfall and rain days were less distributed in the west of the Hu Huanyong Line in different months. (2) The temporal and spatial differentiation characteristics of torrential rainfall and rain days in different months in China from 1961 to 2016 were basically the same. The torrential rainfall and rain days in China had the most significant change trend from May to August, mainly distributed in the southeast monsoon region with an increasing trend. The torrential rainfall and rain days in the northwest have little changes in different months. (3) The interannual variations of the torrential rainfall and rain days in different months in China from 1961 to 2016 were similar. Thereinto, there was large fluctuation in the northern region from April to October, while the variations in the southern region fluctuated greatly from January to March and from November to December. The high-value areas of the torrential rainfall and rain days with large fluctuation gradually expanded from southeast to northwest, northeast and southwest with the development of month, with the fluctuation tending to decrease in the southeast, and then shrank from the northwest, northeast and southwest to the southeast, coupled with an increasing fluctuation in the southeast. The research has certain reference significance for flood control, disaster reduction, and the planning and utilization of water resources.

Modulation of the impacts of Madden–Julian Oscillation on winter rainfall in China by El Niño‐Southern Oscillation

AbstractThis study revealed the modulation of the impacts of Madden–Julian Oscillation (MJO) on winter rainfall in China by El Niño‐Southern Oscillation (ENSO) using reanalysis data and statistical analysis. When MJO is active over the Indian Ocean, its impacts are strengthened by El Niño, leading to enhanced positive rainfall anomalies in eastern China. However, the effects of MJO are counteracted by La Niña; thus, significant positive rainfall anomalies exist only in Zhejiang Province and adjacent areas. MJO activity over the western Pacific induces opposite rainfall anomalies in South China between El Niño and La Niña winters; rainfall in South China increases (decreases) in El Niño (La Niña) winters. Such differences in rainfall anomalies are closely related to anomalous circulation and moisture transport. Anomalous southwesterlies in eastern China are enhanced in El Niño winters when MJO is active over the Indian Ocean, which leads to increased moisture transport towards China and stronger ascending motion in eastern China. But the anomalous southwesterlies are weakened in La Niña winters, thereby resulting in less moisture transport and convergence. When MJO is active over the western Pacific, anomalous northeasterlies in mainland China are enhanced (weakened) in La Niña (El Niño) winters, which strengthens (weakens) the moisture divergence and descending motion in China. Moisture convergence anomalies that explain the rainfall anomalies in ENSO winters are mainly attributed to the effects of low‐frequency background state (LFBS) moisture and meridional winds. It was found that ENSO modulates the impacts of MJO by two main ways. First, ENSO can regulate the intensity and distribution of MJO activity. Therefore, MJO in the same phase will lead to different circulation and moisture transport anomalies in China. Second, ENSO‐induced LFBS circulation and moisture anomalies can also influence the moisture convergence in China, which will strengthen or weaken the effects of MJO.

Effect of Warm SST in the Subtropical Eastern North Pacific on Triggering the Abrupt Change of the Mei-Yu Rainfall over South China in the Early 1990s

AbstractIn the early 1990s, the mei-yu rainfall over South China in early boreal summer exhibited an abrupt change and northward extension. This change altered the pattern of East Asian summer rainfall from a dipole-like to a monopole-like pattern; that is, the out-of-phase relationship between the rainfall in the south and that in the north of the Yangtze and Huaihe River valley changed to an in-phase relationship. The physical processes potentially responsible for triggering this abrupt change were analyzed in this study. Our observations revealed that the western North Pacific subtropical high (WNPSH), sea surface temperature (SST) in the subtropical eastern North Pacific (SENP), and the mei-yu rainfall in South China exhibited an abrupt increase in the early 1990s, suggesting that these factors are correlated. From the observations and results of numerical experiments, we proposed that the abrupt SST warming in the SENP in the early 1990s generated an east–west overturning circulation anomaly in the Pacific Ocean and that the anomalous downward motion in the western North Pacific consequently triggered the abrupt increase and westward extension of the WNPSH in the early 1990s. The enhanced and westward extension of WNPSH created a low-level southeasterly anomaly that transported considerable humid and warm air into East Asia and sequentially triggered the abrupt increase of mei-yu rainfall in the South China in the early 1990s.

On the flood peak distributions over China

Abstract. Here we for the first time present a nationwide characterization of flood hazards across China. Our analysis is based on an exceptional dataset of 1120 stream gauging stations with continuous records of annual flood peaks for at least 50 years across the entire country. Our results are organized by centering on various aspects of flood peak distributions, including temporal changes in flood series and their spatial variations, the statistical distribution of extreme values, and the properties of storms that lead to annual flood peaks. These aspects altogether contribute to an improved understanding of flood hydrology under a changing environment over China and promote advances in flood science at the global scale. Historical changes in annual flood peaks demonstrate frequent abrupt changes rather than slowly varying trends. The dominance of decreasing annual flood peak magnitudes indicates a weakening tendency of flood hazards over China in recent decades. We model the upper tails of flood peaks based on the generalized extreme value (GEV) distributions. The GEV shape parameter is weakly dependent on drainage area, but it shows spatial splits tied to rainfall climatology between northern and southern China. Landfalling tropical cyclones play an important role in characterizing the upper-tail properties of flood peak distributions especially in northern China and southeastern coast, while the upper tails of flood peaks are dominated by extreme monsoon rainfall in southern China. Severe flood hazards associated with landfalling tropical cyclones are characterized by complex interactions of storm circulations with synoptic environments (i.e., mid-latitude baroclinic disturbances) and regional topography.

The 10–30‐day oscillation of winter rainfall in southern China and its relationship with circulation patterns in different latitudes

AbstractIntraseasonal oscillation (ISO) of winter rainfall in southern China and its relationship with the circulation patterns in different latitudes are investigated using ERA‐Interim reanalysis data and observations. The results show that the rainfall in southern China has obvious ISO characteristics, with a dominant period of 10–30 days. The convergence of the whole layer integral water vapour flux can satisfactorily represent the rainfall rate, and the intraseasonal (10–30‐day filtered) rainfall is converted into a function of different time scale wind and water vapour. At the intraseasonal time scale, rainfall (10–30‐day filtered) is determined by both the 10–30‐day wind and the background (greater than 30 days) moisture. Nine days before the peak day of the intraseasonal rainfall in southern China, there is an abnormal low‐pressure system (10–30‐day filtered) in the polar region (east to Novaya Zemlya), and the low‐pressure system gradually strengthens because of the intraseasonal relative vorticity advection at high levels. The intraseasonal relative vorticity advection guides the intraseasonal geopotential anomaly to move from the high latitude to the low latitude, and the low‐pressure system at the surface also moves southward. There is no intraseasonal precipitation until the low‐pressure system at the surface moves to the region near southern China, because there is little water vapour in northern China and ample water vapour in the south. After the occurrence of precipitation, the vorticity advection from the high latitude no longer affects the strength of the low‐pressure system, but the intraseasonal diabatic heating accompanied by precipitation causes a second drop in pressure from −3 days to −1 day and makes the low‐pressure system strongest here. In addition, the geopotential anomaly (10–30‐day filtered) in low latitudes moves eastward along the southern foothills of the Tibetan plateau, which can guide the low‐pressure system to move southward until it reaches southern China.

Warm-Sector Heavy Rainfall in Southern China and Its WRF Simulation Evaluation: A Low-Level-Jet Perspective

Abstract Warm-sector heavy rainfall in southern China refers to the heavy rainfall that occurs within the warm sector hundreds of kilometers south of a front or without a front during April–June, characterized by poor predictability and a close relationship with low-level jets (LLJs). Based on 45 warm-sector heavy rainfall episodes in 2013 and 2014 in southern China, this study examines their general characteristics and evaluates the performance of convection-permitting WRF Model simulations from an LLJ perspective. The results show that 64% of the warm-sector heavy rainfall episodes are associated with an LLJ (LLJ type) and 36% are not (no-LLJ type). The LLJ type is distinct from the no-LLJ type, with large rainfall accumulation along the coastal area. It is more common for LLJs to occur at both 800 and 925 hPa in the LLJ type, where there is a wide 800-hPa LLJ west of Guangdong Province and two 925-hPa LLJs over Beibu Gulf and the South China Sea (SCS). The coastal convergence associated with the terminus of the LLJ on 925 hPa is conducive to the coastal rainfall. WRF generally presents lower QPF skill in the LLJ type than in the no-LLJ type, due to the severe underestimation of coastal rainfall. The QPF skill of the LLJ type is significantly correlated with the forecast accuracy of LLJs, especially at 925 hPa. The north bias of the simulated LLJ on 925 hPa over the SCS and the associated overestimation of wind speed below ~900 hPa over the inland region weaken the coastal convergence and eventually lead to the underestimation in coastal precipitation.

A Review of Research on Warm-Sector Heavy Rainfall in China

Warm-sector heavy rainfall (WSHR) events in China have been investigated for many years. Studies have investigated the synoptic weather conditions during WSHR formation, the categories and general features, the triggering mechanism, and structural features of mesoscale convective systems during these rainfall events. The main results of WSHR studies in recent years are summarized in this paper. However, WSHR caused by micro- to mesoscale systems often occurs abruptly and locally, making both numerical model predictions and objective forecasts difficult. Further research is needed in three areas: (1) The mechanisms controlling WSHR events need to be understood to clarify the specific effects of various factors and indicate the influences of these factors under different synoptic background circulations. This would enable an understanding of the mechanisms of formation, maintenance, and organization of the convections in WSHR events. (2) In addition to South China, WSHR events also occur during the concentrated summer precipitation in the Yangtze River-Huaihe River Valley and North China. A high spatial and temporal resolution dataset should be used to analyze the distribution and environmental conditions, and to further compare the differences and similarities of the triggering and maintenance mechanisms of WSHR events in different regions. (3) More studies of the mechanisms are required, as well as improvements to the model initial conditions and physical processes based on multi-source observations, especially the description of the triggering process and the microphysical parameterization. This will improve the numerical prediction of WSHR events.

Comparison of the effects of soil moisture and El Niño on summer precipitation in eastern China

The effects of spring soil moisture over the vast region from the lower and middle reaches of the Yangtze River valley to North China (YRNC) and El Nino on the East Asian summer monsoon (EASM) and precipitation in eastern China, as well as the relevant mechanisms, are investigated using the modified atmospheric model ECHAM5 coupled with the Common Land Model. These models are the atmospheric and land components of the climate system model developed at the Chinese Academy of Meteorological Sciences (CAMS-CSM). The simulations show that both soil moisture anomalies in eastern China and El Nino sea surface temperature (SST) anomalies have significant influences on the EASM, with the effect of soil moisture being slightly greater than that of the El Nino. However, the impacts of soil moisture on EASM and rainfall in eastern China are markedly different from those of the El Nino. Wetter (drier) soil over the YRNC corresponds to less (more) precipitation over northern and southeastern China, and more (less) precipitation over the Yangtze River basin and northeastern China, as well as a strengthened (weakened) and westward-shifted (eastward-shifted) West Pacific Subtropical High and a deepened (shallower) East Asian trough, representing a weakened (strengthened) EASM pattern. During El Nino developing summers, an anomalous anticyclone extends from northeastern to northern China, and an anomalous cyclone occupies the middle and lower reaches of the Yangtze River and southern China. Concurrently, the West Pacific Subtropical High is anomalously weaker than normal. As a result, rainfall anomalously increases over the lower reaches of the Yangtze River and southern China, and decreases over northern and northeastern China. In El Nino decaying summers, there is an anomalous cyclone over northeastern China and an anomalous anticyclone over southern China. The convergence of southerly airflow and northerly winds leads to enhanced rainfall around northern China and the middle reaches of the Yangtze River, and reduced rainfall over other regions.

Relationship between the South China Sea summer monsoon withdrawal and September–October rainfall over southern China

This study reveals a significant positive connection between the interannual variation in the South China Sea (SCS) summer monsoon (SCSSM) withdrawal and September–October rainfall over southern China. Specifically, pronounced positive rainfall anomalies can be found around southern China when the withdrawal of the SCSSM is later than normal. The occurrence of the significant SCSSM withdrawal-southern China rainfall relation may be attributed to both the atmospheric internal dynamics and the sea surface temperature (SST) anomalies over the tropical Pacific. In particular, SST warming over the tropical western North Pacific, which resembles the developing phase of a La Nina, could induce a significant anomalous low-level cyclone over the northern SCS via a Rossby wave-type atmospheric response. The southwesterly wind anomalies on the south flank of this anomalous cyclone favor late SCSSM withdrawal and contribute to more rainfall over southern China via the northward transport of water vapor. In addition, the enhanced quasi-biweekly oscillation and synoptic-scale variation (such as tropical cyclones) during late withdrawal years are also conducive to increased rainfall over southern China. Further analyses found notable interdecadal changes and asymmetry in the connection of the SCSSM withdrawal with southern China rainfall. The factors contributing to the nonstationary and asymmetric nature of this monsoon withdrawal-rainfall relationship are also briefly discussed.

Extreme rainfall trends of 21 typical urban areas in China during 1998-2015 based on remotely sensed data sets.

With the increase of population, many cities are growing in size at a phenomenal rate. Urbanization changes the urban underlying surface, influences the micro-climate, and sometimes affects the local precipitation process. In this study, we investigated the trends of extreme rainfall in China's 21 typical urban areas. Based on a series of daily rainfall and "Urban/built-up" dataset from TMPA 3B42 and MCD12Q1 products in China, trends in extreme precipitation, with the threshold defined as 95th (pre95p) and 99th (pre99p) percentiles of annual rain days during 1998-2015, have been assessed in China, and especially in 21 typical urban areas from 1998 to 2015. The tendency curves in extreme rainfall of different years are presented. In this period, more than 66% regions of China covered by TMPA 3B42 have increasing trends in extreme rainfall with pre95p threshold. The 21 typical urban areas showed different trends-in over half of these areas, upward tendencies in extreme rainfall were observed, particularly in Dalian, Beijing, and Chongqing. Seventeen urban areas showed increasing tendencies in pre95p extreme rainfall days, including Shanghai, Nanjing, Hangzhou, and Suzhou in the Yangtze River Delta region. The results also illustrate that southeastern coastal urban areas of China may have experienced decreasing occurrences in extreme rainfall.

Diurnal Variations of Low-Level Winds and Precipitation Response to Large-Scale Circulations during a Heavy Rainfall Event

AbstractA succession of MCSs developed during the last week of October 2016 and produced extreme heavy rainfall in central China. The event underwent an evident shift from a mei-yu-like warm scenario to an autumn cold scenario. Diurnal cycles of rainfall and low-level winds may be modulated by the shifting of large-scale atmospheric conditions. We conducted observational analyses and numerical experiments to examine how large-scale circulations influenced rainfall systems through diurnally varying processes. The results show that, in the first half (warm) period of the event, intense rainfall mostly occurred in eastern-central China with an early morning peak. It was closely related to a nocturnal southwesterly low-level jet (NLLJ) on the flank of the western Pacific subtropical high. The NLLJ formed near midnight in southern China where ageostrophic wind rotated clockwise due to Blackadar’s inertial oscillation. The NLLJ extended downstream to central China during the predawn hours due to the horizontal advection of momentum. Both the formation and extension of the NLLJ were supported by an enhanced subtropical high that provided relatively warm conditions with surface heating for boundary layer inertial oscillation and strong background southwesterly winds for momentum transport. The NLLJ induced MCSs at its northern terminus where the low-level ascent, moisture flux convergence, and convective instability were enhanced during the predawn hours. In the second half period with an intrusion of cold air, the diurnal amplitude of low-level winds became small under relatively cold and cloudy conditions. Moderate rainfall tended to occur in western-central China with a peak after midnight, most likely due to frontogenetic processes, upslope lifting, and nighttime cloud-top cooling.

Quasi-biweekly oscillation of the Asian monsoon rainfall in late summer and autumn: different types of structure and propagation

This study contrasts two types of quasi-biweekly oscillations (QBWOs) over tropical Asia in late-summer and autumn (from August to October). Using a tracking method to calculate the frequency of QBWO events over the Asian monsoon region, two types of QBWOs in monsoon rainfall are revealed. One originates from 110° to 140°E and propagates westward to southern China with a notable impact on the regional rainfall, while the other initiates from 160°E to the dateline and does not affect southern China rainfall significantly. Analysis of the vertical structure of moisture flux shows that the moisture source for type 1 events is dominated by the zonal flux component and that for type 2 the meridional flux component. The nature of the moisture flux determines whether the oscillation can propagate across 120°E and affect rainfall over southern China. Results also show that the strength of the South Asian high and the western Pacific subtropical high differently modulate the generation of the two types of QBWOs. Specifically, mutually stronger (weaker) highs favor the first (second) type of the oscillation. A close relationship also exists between the QBWOs and western Pacific sea surface temperature (SST) anomalies, suggesting that the SST anomalies can potentially trigger the QBWOs.