Impact of 4DVAR Assimilation of Rainfall Data on the Simulation of Mesoscale Precipitation Systems in a Mei-yu Heavy Rainfall Event

The multi-scale weather systems associated with a mei-yu front and the corresponding heavy precipitation during a particular heavy rainfall event that occurred on 4 5 July 2003 in east China were successfully simulated through rainfall assimilation using the PSU/NCAR non-hydrostatic, mesoscale, numerical model （MM5） and its four-dimensional, variational, data assimilation （4DVAR） system. For this case, the improvement of the process via the 4DVAR rainfall assimilation into the simulation of mesoscale precipitation systems is investigated. With the rainfall assimilation, the convection is triggered at the right location and time, and the evolution and spatial distribution of the mesoscale convective systems （MCSs） are also more correctly simulated. Through the interactions between MCSs and the weather systems at different scales, including the low-level jet and mei-yu front, the simulation of the entire mei-yu weather system is significantly improved, both during the data assimilation window and the subsequent 12-h period. The results suggest that the rainfall assimilation first provides positive impact at the convective scale and the influences are then propagated upscale to the meso- and sub-synoptic scales. Through a set of sensitive experiments designed to evaluate the impact of different initial variables on the simulation of mei-yu heavy rainfall, it was found that the moisture field and meridional wind had the strongest effect during the convection initialization stage, however, after the convection was fully triggered, all of the variables at the initial condition seemed to have comparable importance.

Mesoscale Moist Adjoint Sensitivity Study of a Mei-yu Heavy Rainfall Event

The mesoscale moist adjoint sensitivities related to the initiation of mesoscale convective systems （MCSs） are evaluated for a mei-yu heavy rainfall event. The sensitivities were calculated on a realistic background gained from a four-dimensional variational data assimilation of precipitation experiment to make the sensitivity computation possible and reasonable within a strong moist convective event at the mesoscale. The results show that the computed sensitivities at the mesoscale were capable of capturing the factors affecting MCS initiation. The sensitivities to the initial temperature and moisture are enhanced greatly by diabatic processes, especially at lower levels, and these sensitivities are much larger than those stemming from the horizontal winds, which implies that initiation of MCSs is more sensitive to low-level temperature and moisture perturbations rather than the horizontal winds. Moreover, concentration of sensitivities at low levels reflects the characteristics of the mei-yu front. The results provide some hints about how to improve quantitative precipitation forecasts of mei-yu heavy rainfall, such as by conducting mesoscale targetted observations via the adjoint-based method to reduce the low-level errors in the initial temperature and moisture.

On the Climatology of Persistent Heavy Rainfall Events in China

Persistent heavy rainfall events （PHR events） comprise one category of weather- and climate- related extreme events. Based on daily rainfall data measured in China during the period of 1951-2004, several quantitative criteria were developed to define PHR events by means of their precipitation intensity, temporal duration, spatial extent and persistence. Then a semi-objective classification based on these criteria was applied to summer daily rainfall data to identify all PHR events. A total of 197 events were observed during the study period. All events were further classified into 5 categories according to their comprehensive intensity; into 3 types according to their circulation regime; and into 8 groups according to the geographic locations of their rainbands. Based on these different classifications, finally, the behaviors of 130 PHR events identified as the most severe, severe and moderate categories since the year of 1951, including characteristics of the spatial and temporal distributions of their frequencies, intensities, and rainbands, were investigated in order to present a comprehensive description of the PHR events. The results will be helpful to the future study of revealing and understanding the processes that govern the production of the PHR events and to the improvement of the forecasts of the PHR events.

Diagnosis of a Moist Thermodynamic Advection Parameter in Heavy-Rainfall Events

A moist thermodynamic advection parameter, defined as an absolute value of the dot product of hori- zontal gradients of three-dimensional potential temperature advection and general potential temperature, is introduced to diagnose frontal heavy rainfall events in the north of China. It is shown that the parameter is closely related to observed 6-h accumulative surface rainfall and simulated cloud hydrometeors. Since the parameter is capable of describing the typical vertical structural characteristics of dynamic, thermodynamic and water vapor fields above a strong precipitation region near the front surface, it may serve as a physical tracker to detect precipitable weather systems near to a front. A tendency equation of the parameter was derived in Cartesian coordinates and calculated with the simulation output data of a heavy rainfall event. Results revealed that the advection of the parameter by the three-dimensional velocity vector, the covariance of potential temperature advection by local change of the velocity vector and general potential temperature, and the interaction between potential temperature advection and the source or sink of general potential temperature, accounted for local change in the parameter. This indicated that the parameter was determined by a combination of dynamic processes and cloud microphysical processes.

Changes in Regional Heavy Rainfall Events in China during 1961–2012

A new technique for identifying regional climate events, the Objective Identification Technique for Regional Extreme Events（OITREE）, was applied to investigate the characteristics of regional heavy rainfall events in China during the period1961–2012. In total, 373 regional heavy rainfall events（RHREs） were identified during the past 52 years. The East Asian summer monsoon（EASM） had an important influence on the annual variations of China＇s RHRE activities, with a significant relationship between the intensity of the RHREs and the intensity of the Mei-yu. Although the increase in the frequency of those RHREs was not significant, China experienced more severe and extreme regional rainfall events in the 1990 s. The middle and lower reaches of the Yangtze River and the northern part of South China were the regions in the country most susceptible to extreme precipitation events. Some stations showed significant increasing trends in the southern part of the middle and lower reaches of the Yangtze River and the northern part of South China, while parts of North China, regions between Guangxi and Guangdong, and northern Sichuan showed decreasing trends in the accumulated intensity of RHREs.The spatial distribution of the linear trends of events＇ accumulated intensity displayed a similar so-called ＂southern flooding and northern drought＂ pattern over eastern China in recent decades.

Impacts of the Thermal Effects of Sub-grid Orography on the Heavy Rainfall Events Along the Yangtze River Valley in 1991

A P - σ regional climate model using a parameterization scheme to account for the thermal effects of the sub-grid scale orography was used to simulate the three heavy rainfall events that occurred within the Yangtze River Valley during the mei-yu period of 1991. The simulation results showed that by considering the sub-grid scale topography scheme, one can significantly improve the performance of the model for simulating the rainfall distribution and intensity during these three heavy rainfall events, most especially the second and third. It was also discovered that the rainfall was mainly due to convective precipitation. The comparison between experiments, either with and without the sub-grid scale topography scheme, showed that the model using the scheme reproduced the convergence intensity and distribution at the 850 hPa level and the ascending motion and moisture convergence center located at 500 hPa over the Yangtze River valley. However, some deviations still exist in the simulation of the atmospheric moisture content, the convergence distribution and the moisture transportation route, which mainly result in lower simulated precipitation levels. Further analysis of the simulation results demonstrated that the sub-grid topography scheme modified the distribution of the surface energy budget components, especially at the south and southwest edges of the Tibetan Plateau, leading to the development and eastward propagation of the negative geopotential height difference and positive temperature-lapse rate difference at 700 hPa, which possibly led to an improved precipitation simulation over eastern China.

Main Energy Paths and Energy Cascade Processes of the Two Types of Persistent Heavy Rainfall Events over the Yangtze River–Huaihe River Basin

Two types of persistent heavy rainfall events （PHREs） over the Yangtze River-Huaihe River Basin were determined in a recent statistical study： type A, whose precipitation is mainly located to the south of the Yangtze River; and type B, whose precipitation is mainly located to the north of the river. The present study investigated these two PHRE types using a newly derived set of energy equations to show the scale interaction and main energy paths contributing to the persistence of the precipitation. The main results were as follows. The available potential energy （APE） and kinetic energy （KE） associated with both PHRE types generally increased upward in the troposphere, with the energy of the type-A PHREs stronger than that of the type-B PHREs （except for in the middle troposphere）. There were two main common and universal energy paths of the two PHRE types： （1） the baroclinic energy conversion from APE to KE was the dominant energy source for the evolution of large-scale background circulations; and （2） the downscaled energy cascade processes of KE and APE were vital for sustaining the eddy flow, which directly caused the PHREs. The significant differences between the two PHRE types mainly appeared in the lower troposphere, where the baroclinic energy conversion associated with the eddy flow in type-A PHREs was from KE to APE, which reduced the intensity of the precipitation-related eddy flow; whereas, the conversion in type-B PHREs was from APE to KE, which enhanced the eddy flow.

Correlation Analysis of Persistent Heavy Rainfall Events in the Vicinity of the Yangtze River Valley and Global Outgoing Longwave Radiation in the Preceding Month

Based on the National Oceanic and Atmospheric Administration （NOAA） daily satellite dataset of global outgoing longwave radiation （OLR） for the period of 1974-2004 and the NCEP-NCAR reanalysis for 1971- 2004, the linkage between persistent heavy rainfall （PHR） events in the vicinity of the Yangtze River valley and global OLR leading up to those events （with 1- to 3O-day lag） was investigated. The results reveal that there is a significant connection between the initiation of PHR events over the study area and anomalous convective activity over the tropical Indian Ocean, maritime continent, and tropical western Pacific Ocean. During the 30-day period prior to the onset of PHR events, the major significantly anomalous convective centers have an apparent dipole structure, always with enhanced convection in the west and suppressed convection in the east. This dipole structure continuously shifts eastward with time during the 30-day lead period. The influence of the anomalous convective activity over the tropical oceans on the initiation of PHR events over the study area is achieved via an interaction between tropical and extratropical latitudes. More specifically, anomalous convective activity weakens the Walker circulation cell over the tropical Indian Ocean first. This is followed by a weakening of the Indian summer monsoon background state and the excitation and dispersion of Rossby wave activity over Eurasia. Finally, a major modulation of the large scale background circulation occurs. As a result, the condition of a phase-lock among major large scale circulation features favoring PHR events is established over the study area.

The Upstream ＂Strong Signals＂ of the Water Vapor Transport over the Tibetan Plateau during a Heavy Rainfall Event in the Yangtze River Basin

A heavy rainfall event that occurred over the middle and lower reaches of the Yangtze River Basin （YRB） during July 11-13 2000 is explored in this study. The potential/stream function is used to analyze the upstream ＂strong signals＂ of the water vapor transport in the Tibetan Plateau （TP）. The studied time period covers from 2000 LST 5 July to 2000 LST 15 July （temporal resolution： 6 hours）. By analyzing the three-dimensional structure of the water vapor flux, vorticity and divergence prior to and during the heavy rainfall event, the upstream ＂strong signals＂ related to this heavy rainfall event are revealed. A strong correlation exists between the heavy rainfall event in the YRB and the convective clouds over the TE The ＂convergence zone＂ of the water vapor transport is also identified, based on correlation analysis of the water vapor flux two days and one day prior to, and on the day of, the heavy rainfall. And this ＂convergence zone＂ coincides with the migration of the maximum rainfall over the YRB. This specific coupled structure actually plays a key role in generating heavy rainfall over the YRB. The eastward movement of the coupled system with a divergence]convergence center of the potential function at the upper/lower level resembles the spatiotemporal evolution of the heavy rainfall event over the YRB. These upstream ＂strong signals＂ are clearly traced in this study through analyzing the three-dimensional structure of the potential/stream function of upstream water vapor transport.

Typical Circulation Patterns and Associated Mechanisms for Persistent Heavy Rainfall Events over Yangtze–Huaihe River Valley during 1981–2020

Persistent heavy rainfall events(PHREs)over the Yangtze–Huaihe River Valley(YHRV)during 1981–2020 are classified into three types(type-A,type-B and type-C)according to pattern correlation.The characteristics of the synoptic systems for the PHREs and their possible development mechanisms are investigated.The anomalous cyclonic disturbance over the southern part of the YHRV during type-A events is primarily maintained and intensified by the propagation of Rossby wave energy originating from the northeast Atlantic in the mid–upper troposphere and the northward propagation of Rossby wave packets from the western Pacific in the mid–lower troposphere.The zonal propagation of Rossby wave packets and the northward propagation of Rossby wave packets during type-B events are more coherent than those for type-A events,which induces eastward propagation of stronger anomaly centers of geopotential height from the northeast Atlantic Ocean to the YHRV and a meridional anomaly in geopotential height over the Asian continent.Type-C events have“two ridges and one trough”in the high latitudes of the Eurasian continent,but the anomalous intensity of the western Pacific subtropical high(WPSH)and the trough of the YHRV region are weaker than those for type-A and type-B events.The composite synoptic circulation of four PHREs in 2020 is basically consistent with that of the corresponding PHRE type.The location of the South Asian high(SAH)in three of the PHREs in 2020 moves eastward as in the composite of the three types,but the position of the WPSH of the four PHREs is clearly westward and northward.Two water vapor conveyor belts and two cold air conveyor belts are tracked during the four PHREs in 2020,but the water vapor path from the western Pacific is not seen,which may be caused by the westward extension of the WPSH.

Comparison of two types of persistent heavy rainfall events during sixteen warm seasons in the Sichuan Basin

Based on hourly precipitation from national surface stations,persistent heavy rainfall events(PHREs)over the Sichuan Basin(SCB)are explored during the warm season(May to September)from 2000 to 2015 to compare synoptic circulations and maintenance mechanisms between different PHRE types.There are two main types of PHREs:one is characterized by a rain belt west of 106°E over the SCB(WSB-PHREs),and the other features a rain belt east of 106°E over the SCB(ESB-PHREs).In total,there are 18 ESB-PHREs and 10 WSB-PHREs during the study period.Overall,the rain belts of WSB-PHREs are along the terrain distribution east of the Tibetan Plateau,while the precipitation intensity of ESB-PHREs is stronger.For the two types of PHREs,the shortwave trough over the SCB and the western Pacific subtropical high act as their favorable background environments,particularly for ESB-PHREs.The water vapor of WSB-PHREs is mainly transported from the South China Sea,whereas for ESB-PHREs the South China Sea and Bay of Bengal are their main moisture sources.The composite vorticity budgets of southwest vortices during their mature stage indicate that the convergence effect is a dominant factor for maintaining the two types of PHREs,and the strong vertical vorticity advection is also favorable,but the relative contribution of vertical advection is larger for WSB-PHREs.

Moist Potential Vorticity Vector for Diagnosis of Heavy Rainfall Events in Tanzania

In this paper, we modify the convective vorticity vector (CVV) defined as a cross product of absolute vorticity and gradient of equivalent potential temperature to moist potential vorticity vector (MPVV) defined as a cross product of absolute vorticity () and the gradient of the moist-air entropy potential temperature (). The patterns of (MPVV) are compared with the patterns of heavy rainfall events that occurred over different regions in Tanzania on 20th to 22nd December, 2011 and on 5th to 8th May, 2015. Moreover, the article aimed at assessing the relative contributions of the magnitude, horizontal and vertical components of (MPVV) detecting on the observed patterns of rainfall events. Dynamic and thermodynamic variables: wind speed, temperature, atmospheric pressure and relative humidity from numerical output generated by the Weather Research and Forecasting (WRF) model running at Tanzania Meteorological Agency (TMA) were used to compute MPVV. It is found that MPVV provide accurate tracking of locations received heavy rainfall, suggesting its potential use as a dynamic tracer for heavy rainfall events in Tanzania. Finally it is found that the first and second components of MPVV contribute almost equally in tracing locations received heavy rainfall events. The magnitude of MPVV described the locations received heavy rainfall events better than the components.

A Numerical Study on Forecasting the Henan Extraordinarily Heavy Rainfall Event in August 1975

This study is essentially an experiment on the control experiment in the August 1975 catastrophe which was the heaviest rainfall in China's Mainland with a maximum 24-h rainfall of 1060.3 mm, and it significantly demonstrates that the limited area model can still skillfully give reasonable results even only the conventional data are available. For such a heavy rainfall event, a grid length of 90 km is too large while 45 km seems acceptable. Under these two grid sizes, the cumulus parameterization scheme is evidently superior to the explicit scheme since it restricts instabilities such as CISK to limited extent. The high resolution scheme for the boundary treatment does not improve forecasts significantly.The experiments also revealed some interesting phenomena such as the forecast rainfall being too small while affecting synoptic system so deep as compared with observations. Another example is the severe deformation of synoptic systems both in initial conditions and forecast fields in the presence of complicated topography. Besides, the fixed boundary condition utilized in the experiments along with current domain coverage set some limitations to the model performances.

An Easterly Wave Generated Heavy Rainfall Event over South India—A Case Study

Easterly waves are one of the rain-bearing systems of northeast monsoon and produce massive rainfall events over south India. In the present case study, an attempt is made to identify extreme heavy rainfall event over south India on 26th October, 2006 due to the passage of the easterly wave. Satellite images provide an inverted v-shape easterly wave. Next, circulation features at different levels clearly indicate the location, movement and speed of the easterly wave. Strong north-easterlies with a magnitude of 9.9 m/s are maintained at the surface. The convergence is mainly occupied between 12°N - 16°N, while the divergence is 5°N - 12°N on 26th October, 2006 at the surface level. On 25th, easterly wave is advected north of trough with a magnitude of 0.2 m/s and increased during the remaining days. There are two divergence cells along 5°N and 16°N before and after the event at 700 hPa level. Thus this study helps to bring out the essential characteristics of the easterly wave during northeast monsoon. The highlight of this study is that the easterly wave creates floods in the absence of tropical cyclones over south India.

Assessing the Variability of Heavy Rainfall during October to December Rainfall Season in Tanzania

Heavy rainfall is one of the primary causes of flood during rainy season in Tanzania leading to severe socio-economic impacts. The study aimed at assessing and characterizing the variability of Heavy Rainfall Events (HREs) using Empirical Orthogonal Function (EOF), Mann-Kendal (MK) trend test, Correlation and Composite analysis methods. Based on the daily-observed precipitation and reanalysis data sets for the October to December (OND) rainfall season of 35 years (1981-2015), the spatial and temporal characteristics of HREs in Tanzania are studied. The relationship between heavy rainfall (HR) and large-scale circulation anomalies including the Indian Ocean dipole (IOD) and El Ni?o southern oscillation (ENSO) indices was assessed. The study found that, approximately 590 HREs were concentrated over northern sector and coastal belt of Tanzania. The monthly variability indicates that HREs are more pronounced in December followed by November while October being the least affected. The occurrence of HREs over the Lake Victoria, Kigoma and Tabora is largely attributed to low-level convergence of westerlies and enhanced moisture from Congo basin accompanied by a pronounced rising limb of Indian Walker circulation cell. A time-series analysis of HRE exhibits an inter-annual variation characterized by a slightly increasing trend, though the computed trends were not statistically significant at 95% confidence level. In most part of Tanzania HREs were positively correlated with both ENSO and IOD indices, underscoring the critical role of ENSO and Indian Ocean dynamic in modulating rainfall variability over the region. In general, it has been found that most of the HREs are generally triggered or amplified by large-scale circulation patterns such as ENSO and IOD.

Analysis of the Characteristics of the Low-level Jets in the Middle Reaches of the Yangtze River during the Mei-yu Season

Here,we analyze the characteristics and the formation mechanisms of low-level jets(LLJs)in the middle reaches of the Yangtze River during the 2010 mei-yu season using Wuhan station radiosonde data and the fifth generation of the European Centre for Medium-Range Weather Forecasts(ERA5)reanalysis dataset.Our results show that the vertical structure of LLJs is characterized by a predominance of boundary layer jets(BLJs)concentrated at heights of 900-1200 m.The BLJs occur most frequently at 2300 LST(LST=UTC+8 hours)but are strongest at 0200 LST,with composite wind velocities>14 m s^(-1).Synoptic-system-related LLJs(SLLJs)occur most frequently at 0800 LST but are strongest at 1100LST,with composite wind velocities>12 m s^(-1).Both BLJs and SLLJs are characterized by a southwesterly wind direction,although the wind direction of SLLJs is more westerly,and northeasterly SLLJs occur more frequently than northeasterly BLJs.When Wuhan is south of the mei-yu front,the westward extension of the northwest Pacific subtropical high intensifies,and the low-pressure system in the eastern Tibetan Plateau strengthens,favoring the formation of LLJs,which are closely related to precipitation.The wind speeds on rainstorm days are greater than those on LLJ days.Our analysis of four typical heavy precipitation events shows the presence of LLJs at the center of the precipitation and on its southern side before the onset of heavy precipitation.BLJs were shown to develop earlier than SLLJs.

“23.7”华北特大暴雨过程小时强降水时空分布特征

由于西北太平洋副热带高压、2023年第5号台风“杜苏芮”、第6号台风“卡努”等共同作用,2023年7月29日08时—8月2日08时(北京时)华北地区发生极端特大暴雨,造成了重大社会影响。利用中国气象局国家气象信息中心提供的国家级气象站逐小时降水资料,分析了此次特大暴雨过程中的小时强降水(Hourly Heavy Rainfall,HHR)时空分布特征,并对不同历时类型强降水事件(Heavy Rainfall Event,HRE)的统计特征进行了对比。结果表明:1)此次特大暴雨HHR强度高、局地性明显,其对总降水量的贡献超过20%,北京西部、河北中部和西南部等太行山东麓为HHR降水量大值区和降水频次活跃区,双台风将水汽源源不断地输送到华北平原,受到太行山等山脉阻挡抬升,利于HHR增幅和持续。2)HHR降水量、降水频次经历6次峰值后减弱,其中第3次峰值时段的HHR降水量最大、降水频次最多、持续时间最长;而在“杜苏芮”残涡螺旋影响、暖式切变线和偏东风影响以及偏南或西南急流影响的3个主要降水阶段中,“杜苏芮”残涡螺旋影响阶段HHR最为活跃,共发生257次HHR,HHR最大降水量达73.5 mm。3)在3种类型HRE中,长历时(>12 h)最多,占比54.5%,短历时(1~6 h)次之,占比32.9%,中历时(7~12 h)最少,占比12.6%;长历时HRE降水量多>180 mm,短历时、中历时HRE的降水量多为[20,60)mm;HRE的最大降水量表现为短历时<中历时<长历时,而最大降水强度表现正相反,3种历时HRE的最大小时降水量多为[20,30)mm。4)不同类型HRE降水量、频次和平均降水强度的空间分布显示,长历时HRE因频次高于短历时和中历时,加之历时长,其降水量也高于后两种类型,北京、河北等地是长历时HRE降水量和频次大值区。

石家庄市夏季短时强降水特征及其影响因子

利用2013—2020年6—8月石家庄市215个地面加密自动气象站逐小时降水资料,采用数理统计学方法对其夏季短时强降水事件(flash heavy rainfall events,FHR events)和暴雨的精细化分布特征及影响因子进行研究。结果发现,石家庄夏季FHR events具有西部山区发生频次少,高频区分散且局地性强的特点。[200,600)m自动气象站分布在FHR events高频区的比例最大,600 m以上山地发生频次减少。山地高频区与山体走向和地形坡度有关。山地FHR events较平原持续时长长,但小时降水强度小。63.6%的暴雨日伴随FHR events发生,随着地形高度增加和暴雨日发生频次减少,有FHR events发生的暴雨日比例下降。短时强降水逐时发生站次随降水强度增大而骤减,60 mm·h^(-1)以上短时强降水日变化不明显。短时强降水平均小时降水强度日变化呈多峰型。石家庄FHR events高频区局地性强,发生于弱天气尺度强迫型下的FHR events为预报的难点,其地理环境及本地特征的影响有待深入分析。

三峡库区蓄水前后夏季小时降水变化特征

利用国家气象信息中心小时降水资料,结合三峡库区地形的多样性,分析了库区蓄水前(1992—2002年)和蓄水后(2003—2021年)夏季(6—8月)小时降水时空变化以及不同类型强降水事件(HRE)变化特征。结果表明:蓄水后小时降水的降水量和频次减少,而强度增加;降水变化存在明显的地理分布规律,降水量、频次、强度在库区中北部增加,且多位于长江以北地区;降水量、频次在库区西南部减少,强度在库区中南部和西北部减弱,且多位于31°N以南。蓄水前后小时降水的日变化峰值时间位相的空间差异程度表现为降水强度>降水量>降水频次;蓄水后降水量、频次、强度呈增加趋势的站点增多,且降水量和强度的日变化峰值时间位相在高海拔山区具有位相前移特征。蓄水前后各等级小时降水的降水概率和降水占比变化不明显,其中降水概率在(0.1,0.5)mm等级最大,≥20 mm等级最小;降水占比在(1,5)mm等级最大,(0.1,0.5)mm等级最小,≥20 mm等级约为15%。蓄水后小时降水的日变化特征更加明显,其中≥20 mm等级的小时降水量和频次的双峰结构更为突出,其他等级的小时降水量和频次的峰值时间范围有扩大趋势,而各等级小时降水强度的次日尺度波动更频繁。蓄水前后HRE均是短历时型最多,长历时型最少,且短历时型HRE多开始于下午,中历时型和长历时型HRE多开始于夜间;蓄水后有利于HRE维持,短历时型HRE在中午和下午开始的概率增加,中历时型和长历时型HRE在清晨开始的概率增加。

豫北蔬菜种植区浅层地下水硝酸盐来源及对强降雨事件的响应

我国北方蔬菜种植过程中大量施用化学肥料和人畜粪肥,引起区内浅层地下水硝酸盐(NO_(3)^(-))浓度急剧升高,但高浓度NO_(3)^(-)来源及在浅层地下水环境中转化的过程尚缺乏同位素证据,特别是强降雨事件对其影响尚不清楚。化学肥料中铵盐(NH_(4)^(+))转化为NO_(3)^(-)是造成蔬菜种植区浅层地下水中NO_(3)^(-)浓度升高的重要原因,强降雨事件导致浅层地下水水位升高,土壤和包气带中剩余NH 4+氧化为NO_(3)^(-)造成地下水NO_(3)^(-)浓度升高。为验证上述假设,选择豫北某蔬菜种植基地浅层地下水作为研究对象,通过对比分析枯水期(2021年4月)和丰水期(2021年10月)浅层地下水NO_(3)^(-)浓度及同位素组成(δ^(15)N-NO_(3)^(-)和δ^(18)O-NO_(3)^(-)),结合水化学以及水的氢氧同位素组成(δD-H_(2)O和δ^(18)O-H_(2)O),辨识浅层地下水高浓度NO_(3)^(-)来源及其对强降雨事件的响应。结果表明:(1)丰水期浅层地下水中ρ(NO_(3)^(-))范围较枯水期更大,中间值更高,分别为177.47和114.68 mg·L^(-1);(2)丰水期浅层地下水中δ^(15)N-NO_(3)^(-)和δ^(18)O-NO_(3)^(-)范围较枯水期更宽泛,δ^(15)N-NO_(3)^(-)中间值升高,丰水期和枯水期分别为7.8‰和7.3‰,但δ^(18)O-NO_(3)^(-)中间值降低,丰水期和枯水期分别为5.1‰和6.4‰;(3)丰水期浅层地下水δ^(18)O-H_(2)O范围较枯水期变窄,中间值升高,分别为-8.8‰和-9.2‰。氘盈余值(d_(e)=δD-8×δ^(18)O)范围变小,中间值降低,丰水期和枯水期分别为6.9‰和9.5‰;(4)强降雨洗脱包气带中富集15 N的NH_(4)^(+)转化为NO_(3)^(-),导致浅层地下水ρ(NO_(3)^(-))升高,δ^(15)N-NO_(3)^(-)值升高,但δ^(18)O-NO_(3)^(-)值降低;(5)贝叶斯同位素混合模型的解析结果表明,丰水期浅层地下水中NO_(3)^(-)来自于粪肥贡献率较枯水期增加,来自于土壤有机氮贡献率降低,化肥中铵态氮贡献率变化不大,但硝态氮肥贡献率增加,同时大气降水硝酸盐贡献率降低。研究结果证实,持续强降雨导致包气带中剩余的NH_(4)^(+)发生硝化作用,产生更多NO_(3)^(-)进入浅层地下水;同时降雨将地表粪肥NH_(4)^(+)转化来的NO_(3)^(-)携带进入浅层地下水。因此,需要加强蔬菜种植区肥料田间管理,做到有效使用肥料,避免过度施用肥料对浅层地下水水质的危害。