Characteristics Analysis on Short-Time Heavy Rainfall during the Flood Season in Shanxi Province, China

In order to provide a reference for the correct forecasting of short-term heavy rainfall and better disaster prevention and mitigation services in Shanxi Province, China, it is very important to carry out systematic research on short-term heavy precipitation events in Shanxi Province. Based on hourly precipitation data during the flood season (May to September) from 109 meteorological stations in Shanxi, China in 1980-2015, the temporal and spatial variation characteristics of short-time heavy rainfall during the flood season are analyzed by using wavelet analysis and Mann-Kendall test. The results show that the short-time heavy rainfall in the flood season in Shanxi Province is mainly at the grade of 20 - 30 mm/h, with an average of 97 stations having short-time heavy rainfall each year, accounting for 89% of the total stations. The short-time heavy rainfall mainly concentrated in July and August, and the maximal rain intensity in history appeared at 23 - 24 on June 17, 1991 in Yongji, Shanxi is 91.7 mm/h. During the flood season, the short-time heavy rainfalls always occur at 16 - 18 pm, and have slightly different concentrated time in different months. The main peaks of June, July and August are at 16, 17 and 18 respectively, postponed for one hour. Short-time heavy rainfall overall has the distribution that the south is more than the north and the east less than the west in Shanxi area. In the last 36 years, short-time heavy rainfall has a slight increasing trend in Shanxi, but not significant. There is a clear 4-year period of oscillation and inter-decadal variation. It has a good correlation between the total precipitation and times of short-time heavy rainfall during the flood season.

Analysis on Radar Characteristics of a Short-time Heavy Rainfall Event in Wuchuan County

Using minute rainfall data of automatic ground station and a variety of products from new generation Doppler weather radar in Wuchuan, the characteristics of a short-time heavy precipitation process on April 23, 2022 were analyzed. The results showed that the appearance of differential reflectivity(ZDR) column and big-value zone of high-elevation ZDR had better indication on short-term heavy rainfall process in Shichao station. Ice phase process played a very important role in particle growth. The storm tracking information product can predict the path of the storm 15 min in advance. The storm stayed and moved less or even turned back to more than two to three scanning volumes in one place, indicating the occurrence of short-term heavy rainfall. One-hour accumulated precipitation(OHP) had a good effect on the estimation of continuous precipitation in a large area where the hourly rainfall exceeded 50 mm for more than two stations. It had the ability to estimate short-term heavy precipitation in areas lacking automatic stations.

The Roles of Low-level Jets in “21·7” Henan Extremely Persistent Heavy Rainfall Event

An extremely heavy rainfall event lasting from 17 to 22 July 2021 occurred in Henan Province of China, with accumulated precipitation of more than 1000 mm over a 6-day period that exceeded its mean annual precipitation. The present study examines the roles of persistent low-level jets(LLJs) in maintaining the precipitation using surface station observations and reanalysis datasets. The LLJs triggered strong ascending motions and carried moisture mainly from the outflow of Typhoon In-fa(2021). The varying directions of the LLJs well corresponded to the meridional shifts of the rainfall. The precipitation rate reached a maximum during 20-21 July as the LLJs strengthened and expanded vertically into double LLJs, including synoptic-weather-system-related LLJs(SLLJs) at 850–700 hPa and boundary-layer jets(BLJs)at ~950 hPa. The coupling of the SLLJ and BLJ provided strong mid-and low-level convergence on 20 July, whereas the SLLJ produced mid-level divergence at its entrance that coupled with low-level convergence at the terminus of the BLJ on21 July. The formation mechanisms of the two types of LLJs are further examined. The SLLJs and the low-pressure vortex(or inverted trough) varied synchronously as a whole and were affected by the southwestward movement of the WPSH in the rainiest period. The persistent large total pressure gradient force at low levels also maintained the strength of low-level geostrophic winds, thus sustaining the BLJs on the synoptic scale. The results based on a Du-Rotunno 1D model show that the Blackadar and Holton mechanisms jointly governed the BLJ dynamics on the diurnal scale.

Spatiotemporal Characteristics of Rainfall over Different Terrain Features in the Middle Reaches of the Yangtze River Basin during the Warm Seasons of 2016–20

Based on hourly rain gauge data during May–September of 2016–20,we analyze the spatiotemporal distributions of total rainfall(TR)and short-duration heavy rainfall(SDHR;hourly rainfall≥20 mm)and their diurnal variations over the middle reaches of the Yangtze River basin.For all three types of terrain(i.e.,mountain,foothill,and plain),the amount of TR and SDHR both maximize in June/July,and the contribution of SDHR to TR(CST)peaks in August(amount:23%;frequency:1.74%).Foothill rainfall is characterized by a high TR amount and a high CST(in amount);mountain rainfall is characterized by a high TR frequency but a small CST(in amount);and plain rainfall shows a low TR amount and frequency,but a high CST(in amount).Overall,stations with high TR(amount and frequency)are mainly located over the mountains and in the foothills,while those with high SDHR(amount and frequency)are mainly concentrated in the foothills and plains close to mountainous areas.For all three types of terrain,the diurnal variations of both TR and SDHR exhibit a double peak(weak early morning and strong late afternoon)and a phase shift from the early-morning peak to the late-afternoon peak from May to August.Around the late-afternoon peak,the amount of TR and SDHR in the foothills is larger than over the mountains and plains.The TR intensity in the foothills increases significantly from midnight to afternoon,suggesting that thermal instability may play an important role in this process.

Assimilation of the FY-4A AGRI Clear-Sky Radiance Data in a Regional Numerical Model and Its Impact on the Forecast of the“21·7”Henan Extremely Persistent Heavy Rainfall

Assimilation of the Advanced Geostationary Radiance Imager(AGRI)clear-sky radiance in a regional model is performed.The forecasting effectiveness of the assimilation of two water vapor(WV)channels with conventional observations for the“21·7”Henan extremely heavy rainfall is analyzed and compared with a baseline test that assimilates only conventional observations in this study.The results show that the 24-h cumulative precipitation forecast by the assimilation experiment with the addition of the AGRI exceeds 500 mm,compared to a maximum value of 532.6 mm measured by the national meteorological stations,and that the location of the maximum precipitation is consistent with the observations.The results for the short periods of intense precipitation processes are that the simulation of the location and intensity of the 3-h cumulative precipitation is also relatively accurate.The analysis increment shows that the main difference between the two sets of assimilation experiments is over the ocean due to the additional ocean observations provided by FY-4A,which compensates for the lack of ocean observations.The assimilation of satellite data adjusts the vertical and horizontal wind fields over the ocean by adjusting the atmospheric temperature and humidity,which ultimately results in a narrower and stronger WV transport path to the center of heavy precipitation in Zhengzhou in the lower troposphere.Conversely,the WV convergence and upward motion in the control experiment are more dispersed;therefore,the precipitation centers are also correspondingly more dispersed.

Automated machine learning for rainfall-induced landslide hazard mapping in Luhe County of Guangdong Province,China

Landslide hazard mapping is essential for regional landslide hazard management.The main objective of this study is to construct a rainfall-induced landslide hazard map of Luhe County,China based on an automated machine learning framework(AutoGluon).A total of 2241 landslides were identified from satellite images before and after the rainfall event,and 10 impact factors including elevation,slope,aspect,normalized difference vegetation index(NDVI),topographic wetness index(TWI),lithology,land cover,distance to roads,distance to rivers,and rainfall were selected as indicators.The WeightedEnsemble model,which is an ensemble of 13 basic machine learning models weighted together,was used to output the landslide hazard assessment results.The results indicate that landslides mainly occurred in the central part of the study area,especially in Hetian and Shanghu.Totally 102.44 s were spent to train all the models,and the ensemble model WeightedEnsemble has an Area Under the Curve(AUC)value of92.36%in the test set.In addition,14.95%of the study area was determined to be at very high hazard,with a landslide density of 12.02 per square kilometer.This study serves as a significant reference for the prevention and mitigation of geological hazards and land use planning in Luhe County.

Analysis on Precipitation Efficiency of the “21.7” Henan Extremely Heavy Rainfall Event

A record-breaking heavy rainfall event that occurred in Zhengzhou,Henan province during 19–21 July 2021 is simulated using the Weather Research and Forecasting Model,and the large-scale precipitation efficiency(LSPE)and cloud-microphysical precipitation efficiency(CMPE)of the rainfall are analyzed based on the model results.Then,the key physical factors that influenced LSPE and CMPE,and the possible mechanisms for the extreme rainfall over Zhengzhou are explored.Results show that water vapor flux convergence was the key factor that influenced LSPE.Water vapor was transported by the southeasterly winds between Typhoon In-Fa(2021)and the subtropical high,and the southerly flow of Typhoon Cempaka(2021),and converged in Zhengzhou due to the blocking by the Taihang and Funiu Mountains in western Henan province.Strong moisture convergence centers were formed on the windward slope of the mountains,which led to high LSPE in Zhengzhou.From the perspective of CMPE,the net consumption of water vapor by microphysical processes was the key factor that influenced CMPE.Quantitative budget analysis suggests that water vapor was mainly converted to cloud water and ice-phase particles and then transformed to raindrops through melting of graupel and accretion of cloud water by rainwater during the heavy precipitation stage.The dry intrusion in the middle and upper levels over Zhengzhou made the high potential vorticity descend from the upper troposphere and enhanced the convective instability.Moreover,the intrusion of cold and dry air resulted in the supersaturation and condensation of water vapor,which contributed to the heavy rainfall in Zhengzhou.

An Extreme Monsoonal Heavy Rainfall Event over Inland South China in June 2022: A Synoptic Causes Analysis

An extreme monsoonal heavy rainfall event lasted for nine days and recurred in the interior of northern south China from June 13 to 21, 2022. Using regional meteorological stations and ERA5 reanalysis data, the causes of this extreme monsoonal rainfall event in south China were analyzed and diagnosed. The results are shown as follows. A dominant South Asian high tended to be stable near the Qinghai-Tibet Plateau, providing favorable upper-level dispersion conditions for the occurrence of heavy rainfall in south China. A western Pacific subtropical high dominated the eastern part of the South China Sea, favoring stronger and more northward transport of water vapor to the northern part of south China at lower latitudes than normal. The continuous heavy precipitation event can be divided into two stages. The first stage(June 13-15) was the frontal heavy rainfall caused by cold air(brought by an East Asian trough)from the mid-latitudes that converged with a monsoonal airflow. The heavy rains occurred mostly in the area near a shear in front of the center of a synoptic-system-related low-level jet(SLLJ), and the jet stream and precipitation were strongest in the daytime. The second stage(June 16-21) was the warm-sector heavy rainfall caused by a South China Sea monsoonal low-level jet penetrating inland. The heavy rainfall occurred on the windward slope of the Nanling Mountains and in the northern part of a boundary layer jet(BLJ). The BLJ experienced five nighttime enhancements, corresponding well with the enhancement of the rainfall center, showing significant nighttime heavy rainfall characteristics. Finally, a conceptual diagram of inland-type warm-sector heavy rainfall in south China is summarized.

Comparison of Microphysical Characteristics of Warm-sector,Frontal and Shear-line Heavy Rainfall During the Pre-summer Rainy Season in South China

Warm-sector heavy rainfall(WR),shear-line heavy rainfall(SR),and frontal heavy rainfall(FR)are three types of rainfall that frequently occur during the pre-summer rainy season in south China.In this research,we investigated the differences in microphysical characteristics of heavy rainfall events during the period of 10-15 May 2022 based on the combined observations from 11 S-band polarimetric radars in south China.The conclusions are as follows:(1)WR has the highest radar echo top height,the strongest radar echo at all altitudes,the highest lightning density,and the most active ice-phase process,which suggests that the convection is the most vigorous in the WR,moderate in the FR,and the weakest in the SR.(2)Three types of rainfall are all marine-type precipitation,the massweighted mean diameter(Dm,mm)and the intercept parameter(Nw,mm^(-1) m^(-3))of the raindrops in the WR are the largest.(3)The WR possesses the highest proportion of graupel compared with the FR and SR,and stronger updrafts and more abundant water vapor supply may lead to larger raindrops during the melting and collision-coalescence processes.(4)Over all the heights,liquid and ice water content in the WR are higher than those in the SR and FR,the ratio of ice to liquid water content in the WR is as high as 27%when ZH exceeds 50 dBZ,definitely higher than that in the SR and FR,indicating that the active ice-phase process existing in the WR is conducive to the formation of heavy rainfall.

Comparison of Microphysical Characteristics Between Warm-sector and Frontal Heavy Rainfall in the South of China

During the April-June raining season,warm-sector heavy rainfall(WR) and frontal heavy rainfall(FR) often occur in the south of China,causing natural disasters.In this study,the microphysical characteristics of WR and FR events from 2016 to 2022 are analyzed by using 2-dimensional video disdrometer(2DVD) data in the south of China.The microphysical characteristics of WR and FR events are quite different.Compared with FR events,WR events have higher concentration of D<5.3 mm(especially D <1 mm),leading to higher rain rates.The mean values of Dmand lgNwof WR events are higher than that of FR events.The microphysical characteristics in different rain rate classes(C1:R~5-20 mm h-1,C2:R~20-50 mm h-1,C3:R~50-100 mm h^(-1),and C4:R> 100 mm h^(-1)) for WR and FR events are also different.Raindrops from C3 contribute the most to the precipitation of WR events,and raindrops from C2 contribute the most to the precipitation of FR events.For C2 and C3,compared with FR events,WR events have higher concentration of D <1 mm and D~3-4.5 mm.Moreover,the shape and slope(μ-A) relationships and the radar reflectivity and rain rate(Z-R) relationships of WR and FR events are quite different in each rain rate class.The investigation of the difference in microphysical characteristics between WR and FR events provide useful information for radar-based quantitative precipitation estimation and numerical prediction.

Effect of Different Microphysical Parameterizations on the Simulations of a South China Heavy Rainfall

A heavy rainfall event in south China was simulated by the Weather Research and Forecasting(WRF) model with three microphysics schemes, including the Morrison scheme, Thompson scheme, and Milbrandt and Yau scheme(MY), which aim to evaluate the capability to reproduce the precipitation and radar echo reflectivity features, and to evaluate evaluate their differences in microphysics and the associated thermodynamical and dynamical feedback. Results show that all simulations reproduce the main features crucial for rainfall formation. Compared with the observation, the MY scheme performed better than the other two schemes in terms of intensity and spatial distribution of rainfall. Due to abundant water vapor, the accretion of cloud droplets by raindrops was the dominant process in the growth of raindrops while the contribution of melting was a secondary effect. Riming processes, in which frozen hydrometeors collect cloud droplets mainly, contributed more to the growth of frozen hydrometeors than the Bergeron process. Extremely abundant snow and ice were produced in the Thompson and MY schemes respectively by a deposition process. The MY scheme has the highest condensation and evaporation, but the lowest deposition. As a result, in the MY scheme, the enhanced vertical gradient of condensation heating and evaporation cooling at low levels produces strong positive and weak negative potential vorticity in Guangdong, and may favor the formation of the enhanced rainfall center over there.

Observational and Mechanistic Analysis of a Nighttime Warm-Sector Heavy Rainfall Event Within the Subtropical High over the Southeastern Coast of China

In August 2021,a warm-sector heavy rainfall event under the control of the western Pacific subtropical high occurred over the southeastern coast of China.Induced by a linearly shaped mesoscale convective system(MCS),this heavy rainfall event was characterized by localized heavy rainfall,high cumulative rainfall,and extreme rainfall intensity.Using various observational data,this study first analyzed the precipitation features and radar reflectivity evolution.It then examined the role of environmental conditions and the relationship between the ambient wind field and convective initiation(CI).Furthermore,the dynamic lifting mechanism within the organization of the MCS was revealed by em-ploying multi-Doppler radar retrieval methods.Results demonstrated that the linearly shaped MCS,developed under the influence of the subtropical high,was the primary cause of the extreme rainfall event.High temperatures and humidity,coupled with the convergence of low-level southerly winds,established the environmental conditions for MCS develop-ment.The superposition of the convergence zone generated by the southerly winds in the boundary layer(925-1000 hPa)and the divergence zone in the lower layer(700-925 hPa)supplied dynamic lifting conditions for CI.Additionally,a long-term shear line(southerly southwesterly)offered favorable conditions for the organization of the linearly shaped MCS.The combined effects of strengthening low-level southerly winds and secondary circulation in mid-upper levels were influential factors in the development and maintenance of the linearly shaped MCS.

Analysis of an Extensive Heavy Rainfall Weather Process during 23-24 April, 2021 in China

An extensive rainfall occurred in central and eastern China during 23-24 April, 2021. This research mainly uses the reanalysis data of NCEP/NCAR every 6 hours to analyze this heavy rainfall weather process. The results show that the main reason for this precipitation process is the shear formed between the cold air and the warm and humid air flow in the southwest. The low temperature on the ground allows the warm and humid air flow to lift up to form precipitation. The shear system is strengthened to a low vortex, which greatly strengthens the vertical ascent movement. Good water vapor and dynamic conditions form a large range of heavy rainfall.

Characteristics of Radar Echo Parameters and Microphysical Structure Simulation of a Short-Time Heavy Precipitation Supercell

By using the conventional observations, radar data, NCEP/NCAR FNL 1°×1° reanalysis data and numerical simulation data and with the construction and calculation of radar echo parameters, this paper presents the structural characteristics and physical processes of a short-time heavy precipitation supercell that occurred in the squall line process in Shanxi Province on 24 June 2020. The results show that this squall line event occurred in front of a surface cold front,combined with infiltration of low-level cold air and continuous increase of near-surface humidity in the afternoon. The surface mesoscale convergence line and mesoscale dew point front contributed to the development and systemization of the squall line by a large degree. The short-time extremely heavy precipitation in Pingshun County was caused by the development of a supercell from thunderstorm cells on the front side of the squall line. The characteristics of sharp increase in vertical integral liquid water content, persistent increase in reflectivity factor and continuous rise in the echo top height appeared about 23 min earlier than the severe precipitation, which has qualitative indicating significance for the nowcasting of short-time heavy precipitation. A quantitative analysis of the radar echo parameters suggests that the“sudden drop”of FV40was a precursor signal of cells’ coalescence and rapid development to the mature stage. The areal change of the echo core at the 6 km height was highly subject to the merging and developing of cells, the rapid change of hydrometeor particles in clouds and the precipitation intensity. Changes in the cross-sectional area of convective cells at different heights can indirectly reflect the changes of liquid particles and ice particles in clouds, which is indicatively meaningful for predicting the coalescing and developing-to-maturing of cells and heavy precipitation 30-45 min earlier.A comprehensive echo parameter prediction model constructed by the random forest principle can predict the magnitude of short-time heavy precipitation 40-50 min in advance. Numerical simulation reveals that large amounts of water vapor existed in the near-surface atmosphere, and that the cells rapidly obtained moisture from the ambient atmosphere and developed rapidly through maternal feeding. The cold cloud zone was narrow, upright and had a high stretch height. The upward motion in clouds was strong and deep, and very rich in liquid water content. The graupel particles had a large vertical distribution range, the coexistence area of graupel and snow was large, the height of raindrops was close to the surface with a wide horizontal scale, and the precipitation efficiency was high. These may be the important elements responsible for the occurrence of the short-time heavy precipitation that exceeded historical extreme values. On the basis of the above analyses, a comprehensive parameter(CP) prediction model is worked out, which can estimate the developing trend of supercells and the intensity of short-time heavy precipitation about 1 h in advance.

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.

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.

A Case Study on a Strong Tropical Disturbance and Record Heavy Rainfall in Hat Yai, Thailand during the Winter Monsoon

The evolutionary process and structural characteristics of the atmospheric circulation and synoptic situation which caused the record heavy rainfall with a precipitation amount of 550 mm in Hat Yai, Thailand from 20 to 23 November 2000 is studied. In the study, the modern three dimensional observational data were collected as completely as possible, and detailed analyses were made. It is revealed that the cold surges of the Asian winter monsoon that originate from Siberia can arrive at the lower latitudes, including South Thailand, Malaysia, Indonesia, cause strong heavy rainfall there, and interact with weather systems in the near-equatorial regions of the Southern Hemisphere. This is strongly supported by Chinese scientist's original finding in 1930s. The strong convective cloud clusters in the above areas are generated by the direct influence of the cold surges, and are related with the South China Sea disturbances in the lower troposphere. The maximum of the convergence of total moisture flux near South Thailand in the situation under study implies that the water vapour supply is abundant and very favorable to the occurrence of the heavy rainfall. The release of latent heat enhances the Hadley Circulation also. The feedback of the strong severe weather on climate indeed exists, and there are pronounced interactions between the multi-scale systems and between both hemispheres.

Mesoscale Analysis of a Heavy Rainfall Event over Hong Kong During a Pre-rainy Season in South China

During the Heavy Rainfall Experiment in South China （HUAMEX） of 1998, a record heavy rainfall event occurred in the delta of the Pearl River during the 24 hours from 1200 UTC 8 June to 1200 UTC 9 June, 1998, and a 24-hour precipitation maximum of 574 mm was reported in Hong Kong. In this paper, some mesoscale characteristics of this heavy rainfall event are studied using data from satellites, Doppler radar, wind profilers, and automatic meteorological stations collected during HUAMEX. The following conclusions are drawn： （1） During this heavy rainfall event, there existed a favorable large-scale environment, that included a front with weak baroclinity in the heavy rain area and with an upward motion branch ahead of the front. （2） Unlike most extratropical or subtropical systems, the closed low in the geopotential height field does not exited. The obvious feature was that a southerly branch trough in the westerlies existed and Hong Kong was located ahead of the trough. （3） The rainfall areas were located in the warm sector ahead of the front, rather than in the frontal zone, which is one of the characteristics of heavy rainfalls during the pre-rainy season of South China. A southerly warm and moist current contributed to the heavy rainfall formation, including the transportation of rich water vapor and the creation of strong horizontal wind convergence. （4） The observations show that the heavy rainfall in Hong Kong was directly caused by a series of meso β systems rather than a mesoscale convective complex （MCC）. These meso β systems moved with the steering current in the lower-mid troposphere, their life cycles were 3-6 hours, and their horizontal sizes were 10-100 km. （5） The disturbances in the lower and mid troposphere, especially that in the planetary boundary layer （PBL） were very shallow. However, they are a possible trigger mechanism for the occurrence and development of the mesoscale convective systems and related heavy rainfalls. Finally, a conceptual model of the heavy rainfall in the warm sector ahead of the front in South China is proposed.

A Simulation Study of a Heavy Rainfall Process over the Yangtze River Valley Using the Two-Way Nesting Approach

In this study, the major features of a heavy rainfall event in the Yangtze River region on 3-7 June 2011 and its event-related large-scale circulation and predictability were studied. Both observational analysis and model simulation were used, the latter being based on the Weather Research and Forecasting （WRF） model forced by NCEP Global Forecast System （GFS） datasets. It was found that, during 3-5 June, the western Pacific subtropical high apparently extended to the west and was much stronger, and the Indian summer monsoon trough was slightly weaker than in normal years. The east-west oriented shear line over the middle and lower reaches of the Yangtze River was favorable for the transportation and convergence of water vapor, and the precipitation band was located slightly to the south of the shear line. During 6-7 June, the western Pacific subtropical high retreated eastward, while the trough over the Okhotsk Sea deepened. The low vortex in Northeast China intensified, bringing much more cold air to the middle and lower reaches of the Yangtze River, and the shear line over this area moved slightly southward. The convection band moved southward and became weaker, so the rainfall during 6-7 June weakened and was located slightly to the south of the previous precipitation band. Many of the observed features, including background circulation and the distribution and amount of precipitation, were reproduced reasonably by the WRF, suggesting a feasibility of this model for forecasting extreme weather events in the Yangtze River region.

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.