Moisture Sources and Their Contributions to Summer Precipitation in the East of Southwest China

Complex topography,special geographical location and sea-land-air interactions lead to high interannual variability of summer precipitation in the east of Southwest China(ESWC).However,the contributions,influencing factors and mechanisms of remote and local evaporation remain to be further investigated.Using clustering analysis and Hybrid Single-Particle Lagrangian Integrated Trajectory version 5 model,we analyze the contributions of remote moisture transport and local evaporation to summer precipitation in the ESWC and their causes.There are mainly five remote moisture channels in the ESWC,namely the Arabian Sea channel,Bay of Bengal channel,western Pacific channel,Northwest channel 1 and Northwest channel 2.Among the five channels,the western Pacific channel has the largest number of trajectories,while the Bay of Bengal channel has the largest contribution rate of specific humidity(33.33%)and moisture flux(33.14%).The amount of regional average precipitation is close to that of the precipitation caused by remote moisture transport,and both are considerably greater than the rainfall amount caused by local evaporation.However,on interannual time scales,precipitation recirculation rates are negatively correlated to regional average precipitation and precipitation caused by remote moisture transport but are consistent with that caused by local evaporation.An apparent"+-+"wave train can be found on the height anomaly field in East Asia,and the sea surface temperature anomalies are positive in the equatorial Middle-East Pacific,the South China Sea,the Bay of Bengal and the Arabian Sea.These phenomena cause southwest-northeast moisture transport with strong updrafts,thereby resulting in more precipitation in the ESWC.

Contributions of moisture sources to precipitation in the major drainage basins in the Tibetan Plateau

Tracking and quantifying the moisture sources of precipitation in different drainage basins in the Tibetan Plateau(TP)help to reveal basin-scale hydrological cycle characteristics under the interactions between the westerlies and Indian summer monsoon(ISM) systems and to improve our understanding on the mechanisms of water resource changes in the ‘Asian Water Tower' under climate changes. Based on a Eulerian moisture tracking model(WAM-2) and three atmospheric reanalysis products(ERA-I, MERRA-2, and JRA-55), the contributions of moisture sources to the precipitation in six major sub-basins in the TP were tracked during an approximately 35-year period(1979/1980–2015). The results showed that in the upper Indus(UI),upper Tarim River(UT), and Qaidam Basin(QB), the moisture sources mainly extended westward along the mid-latitude westerlies to the western part of the Eurasian continent. In contrast, in the Yarlung Zangbo River Basin(YB), inner TP(ITP), and the source area of three eastern rivers(TER, including the Nujiang River, Lancang River, and Yangtze River), the moisture sources extended both westward and southward, but mainly southward along the ISM. In winter and spring, all of the sub-basins were dominated by western moisture sources. In summer, the western sources migrated northward with the zonal movement of the westerlies, and simultaneously the southern sources of the YB, ITP, and TER expanded largely toward the Indian Ocean along the ISM. In autumn, the moisture sources of the UI, UT, and QB shrank to the western sources, and the moisture sources of the YB, ITP, and TER shrank to the central-southern TP and the Indian subcontinent. By quantifying the moisture contributions from multiple sources, we found that the terrestrial moisture dominated in all of the sub-basins, particularly in the UT and QB(62–73%). The oceanic contributions were relatively high in the UI(38–42%) and YB(38–41%). In winter, evaporation from the large western water bodies(such as the Mediterranean, Red Sea, and Persian Gulf) was significantly higher than that from the continental areas. This contributed to the peak(valley) values of the oceanic(terrestrial) moisture contributions to all of the subbasins. In summer, the terrestrial moisture contributions to the UI, UT, and QB reached their annual maximum, but the abundant oceanic moisture transported by the ISM restrained the appearance of land source contribution peaks in the YB, ITP, and TER,resulting in almost equal moisture contributions in the YB from the ocean and land.

Summer Atmospheric Water Cycle under the Transition Influence of the Westerly and Summer Monsoon over the Yarlung Zangbo River Basin in the Southern Tibetan Plateau

This study compares the summer atmospheric water cycle,including moisture sources and consumption,in the upstream,midstream,and downstream regions of the Yarlung Zangbo River Basin in the southern Tibetan Plateau.The evolutions of moisture properties under the influence of the westerly and summer southerly monsoon are examined using 5-yr multi-source measurements and ERA5 reanalysis data.Note that moisture consumption in this study is associated with clouds,precipitation,and diabatic heating.Compared to the midstream and downstream regions,the upstream region has less moisture,clouds,and precipitation,where the moisture is brought by the westerly.In early August,the vertical wet advection over this region becomes enhanced and generates more high clouds and precipitation.The midstream region has moisture carried by the westerly in June and by the southerly monsoon from July to August.The higher vertical wet advection maximum here forms more high clouds,with a precipitation peak in early July.The downstream region is mainly affected by the southerly-driven wet advection.The rich moisture and strong vertical wet advection here produce the most clouds and precipitation among the three regions,with a precipitation peak in late June.The height of the maximum moisture condensation is different between the midstream region(325 hPa)and the other two regions(375 hPa),due to the higher upward motion maximum in the midstream region.The diabatic heating structures show that stratiform clouds dominate the upstream region,stratiform clouds and deep convection co-exist in the midstream region,and deep convection systems characterize the downstream region.

Stable Isotope Signatures and Moisture Transport of a Typical Heavy Precipitation Case in the Southern Tianshan Mountains

Stable oxygen isotopes in precipitation contain meaningful environmental information on a synoptic scale and can be applied to diagnose hydrometeorological processes.A series of rainstorms occurred at the southern Tianshan Mountains during the period from May to June 2013,and the event-based precipitation was sampled along the mountain range from west to east.Based on δ18 O values in precipitation samples as well as the corresponding meteorological parameters,the moisture transport paths during the sampling period were identified.In late-May(stage 1),isotopes in precipitation collected generally showed a depleting trend.In mid-June(stage 2),there was no coherent trend of isotopes in precipitation for these stations,and only isotope values in Aksu showed a continually depleting trend.Checking other meteorological proxies during the sampling period,the event-based precipitation isotopes sensitively reflected the moisture process.In central Asia,both the westerly and monsoon moisture can be delivered to cause extreme precipitation events,and the isotopic information provides an alternative tool to investigate the atmospheric processes.

Analysis of Paths and Sources of Moisture for the South China Rainfall during the Presummer Rainy Season of 1979–2014

The paths and sources of moisture supplied to South China during two periods of the presummer rainy season （April-June） of 1979-2014, i.e., before and after the onset of the summer monsoon over the South China Sea （SCS）, are investigated by using the Hybrid Single-Particle Lagrangian Integrated Trajectory （HYSPLIT） model. During the premonsoon-onset period, the moisture transport trajectories are clustered into 6 groups, with four ocean-originating paths providing 83.9% and two continent-originating paths （originating over Lake Baikal and the Persian Gulf） con- tributing the remaining 16.1% of the total moisture. The two Pacific-originating paths, from the western Pacific Ocean and the East China Sea, combined account for about 46%, the SCS-originating path contributes about 24.3%, while the Bay of Bengal-originating path accounts for 13.6% of the total moisture over South China. The trajectories during the postmonsoon-onset period are clustered into 4 groups, with three southwesterly paths （from the Arabian Sea, the central Indian Ocean, and the western Indian Ocean, respectively） accounting for more than 76% and the sole Pacific-originating path accounting for 23.8% of the total moisture. The formation of the moisture transport tra-jectories is substantially affected by the topography, especially the Tibetan Plateau and the Indian and Indo-China Peninsulas. The SCS region contributes the most moisture during both periods （35.3% and 31.1%）. The Pacific Ocean is ranked second during the former period （about 21.0%） but its contribution is reduced to 5.0% during the lat-ter period, while the contribution from the Bay of Bengal and the Indian Ocean combined increases from 17.1% to 43.2%.

Stable isotopes and chloride ion of precipitation events in the northeastern Tibetan Plateau, China

Stable isotopes and chloride ion of precipitation are ideal environmental tracers to explain and reveal the formation and evolution mechanisms of water bodies. It is crucial to investigate the stable isotopes and chloride in precipitation events in the northeastern part of the Tibetan Plateau(NETP) due to the limitation of available data. This study sampled each event of precipitation during the period from July 2018 to June 2019 and the monthly dustfall in the NETP to investigate the temporal changes of stable isotopes and chloride in precipitation, and to reveal the moisture source of precipitation over the NETP using a back trajectory model. Results showed that the δ^(2)H values of precipitation ranged from-183.51‰ to 17.75‰, and the δ^(18)O values ranged from-25.18‰ to 0.48‰. The slope of the Local Meteoric Water Line was slightly lower than 8 due to the effect of belowcloud secondary evaporation on the precipitation process. Most d-excess values were higher than 10‰ because moisture recycled from the continent and Qinghai Lake surface mixed with precipitation. The chloride in precipitation accounted for 86.5% of the annual total deposition mass of chloride(1329.64 mg/m2), indicating that precipitation was the main source of chloride in the NETP. The temperature and amount effects of stable isotope in the precipitation were obvious in the NETP. The precipitation was predominantly derived from the Westerly Circulation from September through May and the East Asian Monsoon from June to August, with precipitation amounts of 246.5 mm and 178.0 mm, respectively, indicating that the precipitation over the NETP brought by the Westerly Circulation was more than that brought by the East Asian Monsoon. The air mass over the NETP transited in late May and early September, and a slight change in transition period would mainly be related to the intensity of the East Asian Monsoon, which is strongly influenced by El Ni?o-Southern Oscillation. These results provide not only baseline data for hydrological and climatological studies of the NETP but also valuable insights into the hydrological process in the inland arid area of Asia.

Moisture transport and sources of an extreme rainfall event of June 2021 in southern Xinjiang, China

A highly extreme rainfall event occurred during June 15-17,2021(21·6 event)over southern Xinjiang(SXJ)of China,with daily rainfall overwhelming the climatological annual total at three national stations along the north of Kunlun Mountains.During this event,the SXJ is situated upstream of the 500 hPa trough line and close to the 200 hPa upper jet stream entrance,favouring the upper(lower)level divergence(convergence)with intensified ascending motions and prominent moisture convergence.We examined the moisture paths and sources for this significant rainfall event using the Hybrid Single-Particle Lagrangian Integrated Trajectory(HYSPLT)model.The findings indicate that the majority of the moisture sources are located in areas west of the SXJ,extending from central Asia to the North Atlantic,whereas only a small number of trajectories originate from areas south or east of the SXJ.Seven relevant moisture sources for the rainfall event have been divided.About 80.0%of the total moisture is contributed by central Asia,which ranks top in terms of moisture contribution(∼33.2%),north Eurasia(∼25.7%),and water areas(∼17.7%),including the Mediterranean,Black and Caspian Sea region,while fractional moisture contributions among the other four regions are all below 8.0%.When compared to other parts of China(such as South China and East China),where southerly and/or easterly trajectories are the predominant moisture channels,the distribution patterns of moisture trajectories during this event are noticeably different.

Distinguishing the Regional Atmospheric Controls on Precipitation Isotopic Variability in the Central-Southeast Portion of Brazil

Precipitation isotope ratios(O and H)record the history of water phase transitions and fractionation processes during moisture transport and rainfall formation.Here,we evaluated the isotopic composition of precipitation over the central-southeastern region of Brazil at different timescales.Monthly isotopic compositions were associated with classical effects(rainfall amount,seasonality,and continentality),demonstrating the importance of vapor recirculation processes and different regional atmospheric systems(South American Convergence Zone-SACZ and Cold Fronts-CF).While moisture recycling and regional atmospheric processes may also be observed on a daily timescale,classical effects such as the amount effect were not strongly correlated(δ^(18)O-precipitation rate r≤-0.37).Daily variability revealed specific climatic features,such asδ^(18)O depleted values(~-6‰to-8‰)during the wet season were associated with strong convective activity and large moisture availability.Daily isotopic analysis revealed the role of different moisture sources and transport effects.Isotope ratios combined with d-excess explain how atmospheric recirculation processes interact with convective activity during rainfall formation processes.Our findings provide a new understanding of rainfall sampling timescales and highlight the importance of water isotopes to decipher key hydrometeorological processes in a complex spatial and temporal context in central-southeastern Brazil.

Synoptic Climate Settings and Moisture Supply for the Extreme Heavy Snowfall in Northern China during 6–8 November 2021

A record-breaking extreme heavy snowfall(EHS)event hit northern China during 6–8 November 2021,with two maximum snowfall centers in North China(NC)and Northeast China(NEC),which inflicted severe socioeconomic impacts.This paper compares the differences in the synoptic processes and moisture supply associated with the EHS event in NC and NEC,as well as the atmospheric circulation anomalies before the event,to provide a reference for better prediction and forecasting of EHS in northern China.Synoptic analyses show that a positively tilted,inverted 500-hPa trough channeled cold-air outbreaks into NC,while dynamic updrafts along the front below the trough promoted moisture convergence over this region.In NEC,the dynamic updraft south of the frontogenesis region firstly triggered a low-level Yellow–Bohai Sea cyclone,which then converged with the 500-hPa trough to ultimately form an NEC cold vortex.Calculation of the vorticity tendency indicates that absolute vorticity advection was a better indicator than absolute vorticity divergence for the movement of the trough/ridge at the synoptic scale.Moreover,NOAA’s HYSPLIT(Hybrid Single-Particle Lagrangian Integrated Trajectory)model results reveal that the moisture for the EHS over NC mainly originated from the mid-to-low levels over the Asian–African region and the Eurasian mid-to-high latitudes,accounting for 32%and 31%,respectively.In contrast,the source of water vapor for the EHS over NEC was mainly the Eurasian mid-to-high latitudes and East Asia,with contributions of 38%and 28%,respectively.The findings of this study shed some fresh light on the distinctive contributions of different moisture sources to local precipitation.Further analyses of the atmospheric circulation anomalies in October reveal that a phase shift in the Arctic Oscillation related to the weakening of the polar vortex could have served as a useful indicator for the cold-air outbreaks in this EHS event.

Isotopic insights on quantitative assessments of interaction of eco-hydrological processes in multi-scale karst watersheds

The dynamics of hydrological processes and the storage mechanisms of karst water resources are the most important issues in karst hydrology.The impact of environmental changes on water quantity,and the evaluation and quantification of eco-hydrological processes remain poorly addressed.In this study,high-frequency continuous monitoring in multi-scale karst watersheds in Southwest China combined the approaches of water isotopes and the hybrid single-particle lagrangian integrated trajectory(HYSPLIT)model to identify the recharge mechanisms between atmospheric vapor,rainfall,surface water,and groundwater,and to reveal the interaction of eco-hydrological processes.The dominant moisture sources in Puding(PD)County were the Indian Ocean(43-69%)and local moisture(24-33%).Theδ^(18)O and deuterium excess(d-excess)values showed a positive correlation indicating that secondary or sub-cloud evaporation was prominent in the wet seasons.Karst water line-conditioned excess(lc-excess)indicated that karst water interacted with recent precipitation,groundwater,and evaporation across seasons.Owing to its specific hydrogeological structure,surface water and rainwater have a higher contribution rate to groundwater replenishment.The Chenqi stream replenished the Houzhai River mainly in the form of groundwater,with percentages ranging from 38.1 to 93.5%in the wet season,and 47.8-80.1%in the dry season.In the Houzhai outlet,surface water and groundwater interconverted frequently with a percentage of 45.6-49.1%.We believe this is the first systematic study to quantify the supply relationship between water vapor transport,rainfall,surface water and groundwater in the Chinese karst zone,making a significant move forward in the field of karst hydrological processes and improving the efficiency of water resource evaluation and management.

A Synoptic Overview and Moisture Tra jectory Analysis of the“7.21” Heavy Rainfall Event in Beijing

The heavy rainfall in Beijing on 21 July 2012 was a high impact event. First, a synoptic overview of this event is presented based on the ECMWF reanalysis and forecast data, together with observations of hourly precipitation. It is found that an east to northeastward moving ＂low-level northwest vortex＂ caused this event. The vortex was formed under favorable circulation patterns and intensified by low-level dry intrusions. The source regions of the large volume of moisture necessary to sustain the intense rainfall are diagnosed by back-trajectory analysis. Approximately 77% of the moisture was transported from the Bay of Bengal （BOB）. The transport processes, including vertical profiles, mean humidity variations, and relative importance of the rain-paths, are further quantified by the back-trajectory analysis and cluster analysis. The results highlight the importance of the southwestern path in transporting moisture to northern China, which accounts for 88.4% of the moisture from the BoB, and 68% of the total of the ＂7.21＂ heavy rainfall event.

Dominance of terrestrial moisture supply for the record-breaking extreme precipitation in Hubei, China on August 12, 2021

A record-breaking heavy rainfall swept across Suizhou of Hubei province in China on August 12,2021.It was characterized by suddenness and extremeness and caused severe socio-economic losses.To deepen the understanding of such an urban rainstorm and to improve the forecasting ability,this study revealed the dominant atmospheric circulation and moisture sources for this event.We performed a Lagrangian model FLEXPART to understand this event in terms of moisture sources and transport trajectories.Three key circulation systems affecting this extreme event were identified,including the western Pacific subtropical high(WPSH),the low-level southwesterly jet and an anticyclone over northern China.The low-level jet was lifted along the northern dry and cold air mass to form a front,resulting in the heavy rain.The moisture sources located on land contributed about 64%of the moisture.The development of the low-level southwesterly jet in southern China was strengthened near the time of the precipitation occurrence,providing the main moisture supply.Southern China(23°-32.5°N,98°-122°E)was the most important source region,which contributed the most to the precipitation(43.6%).The results highlight the prominent role of the terrestrial water cycle in this extreme precipitation event over Hubei.

Why was Pakistan extreme precipitation stronger in 2022 than in 2010?

In Pakistan,five continuous extreme precipitation events in the summer(July-August)of 2022 caused disastrous floods,depriving thousands of people's lives and ruining millions of hometowns.This tremendous disaster in Pakistan also happened in 2010 but with three concentrated extreme precipitation events in the middle of summer.The amount of Pakistan heavy daily precipitation in 2022 surpasses that in 2010 record,making it the strongest precipitation event ever recorded.To comprehensively understand the causes of extreme precipitation in Pakistan,this study investigated the anomalies in atmospheric circulation and moisture contribution of 2010/2022 extreme precipitation and compared their differences.The results show that an atmospheric blocking over northern Europe in both 2022 and 2010 enhanced convection in Pakistan by transporting cold-dry air from the high-latitude region and benefiting warm-wet monsoonal air marching to Pakistan.By employing a moisture track model,the main moisture sources for summer precipitation in Pakistan were identified.It is found that moisture contributions except from Eurasia were enhanced,causing extreme precipitation.In particular,enhanced moisture contribution from the southern Indian Ocean and the northern Indian continent in 2022 are more prominent than that in 2010.The increased moisture contribution in 2010 was due to increased evaporation induced by warming sea surface in the Indian Ocean,while much richer moisture transport in 2022 was attributed to the enhanced cross-equatorial flow induced by the anomalous subtropical high in the Southern Hemisphere.Therefore,attention should be paid to the role of subtropical high in the Southern Hemisphere in addition to those in Northern Hemisphere in understanding disastrous extreme precipitation events inPakistan.

NATURE OF PRECIPITATION AND ACTIVITY OF CUMULUS CONVECTION DURING THE 1991 MEIYU SEASON OF CHANGJIANG-HUAIHE RIVER BASIN

Seasonal variability regarding the nature of precipitation and the activity of cumulus convection during the 1991 Meiyu season of Changjiang-Huaihe River Basin(Jianghuai)has been investigated by calculating apparent heat source/apparent moisture sink and analyzing TBB(cloud-top blackbody radiation temperature)data.It is found that three periods of strong ascending motion during the Meiyu season lead to three episodes of heavy rain,and the latent heat due to the precipitation is of the sole heat source of the atmosphere.The nature of precipitation shows distinct seasonal variability,from frontal precipitation of the first episode to the extremely strong convective precipitation of the third episode.TBB field of East Asia may well reflect not only the intensity of convection and rainfall,but also the movement of rain belt and convection belt.In the whole Meiyu season.convection belt mainly stays in Jianghuai.but may shift within the domain of East Asia.Its locating in Jianghuai or not determines the maintenance or break of Meiyu.In the third episode,the narrow convection belt over Jianghuai is mainly caused by southwest monsoon which takes moist and convective atmosphere from tropical ocean.