Extreme Rainstorm Events Research Articles

Characteristics of Water Vapor Transport during the “7·20” Extraordinary Heavy Rain Process in Zhengzhou City Simulated by the HYSPLIT Model

Water vapor transport is an important foundation and prerequisite for the occurrence of rainstorms. Consequently, the understanding of water vapor transport as well as the sources of water vapor during rainstorm processes should be considered as essential to study the formation mechanism of rainstorms. In this study, the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model is adopted for backward tracking of water vapor transport trajectories and sources during the “7·20” extraordinary heavy rain process in Zhengzhou City of China that occurred on 20 July 2021. On this basis, the trajectory clustering method is applied to quantitatively analyze the contributions of water vapor sources, aiming to provide a basis for exploring the maintenance mechanism of this extreme rainstorm event. The spatio-temporal characteristics of this rainstorm event show that there are 4 consecutive days with the precipitation reaching or exceeding the rainstorm level across the whole Zhengzhou City, with the daily rainfall amounts at eight national meteorological stations all breaking their respective historical extreme values. The regional-averaged rainfall amount in Zhengzhou City is 527.4 mm, while the maximum accumulated rainfall amount reaches 985.2 mm at Xinmi station and the maximum hourly rainfall amount at Zhengzhou national meteorological station reaches 201.9 mm h−1. The water vapor sources for this rainfall process, ranked in descending order of contribution, are the Western Pacific, inland areas of Northwest China and South China, and South China Sea. The water vapor at lower levels is mainly transported from the Western Pacific and the South China Sea, while those from the inland areas of Northwest China and South China provide a supply of water vapor at upper levels to a certain extent. The water vapor at 950 hPa is mainly sourced from the Western Pacific and South China Sea, accounting for 56% and 44%, respectively. The water vapor at 850 hPa mainly derives from the Western Pacific and the inland areas of South China, contributing 58% and 34% of the total, respectively. The water vapor at 700 hPa mainly comes from the inland areas of Northwest China and South China Sea. Specifically, the water vapor from inland Northwest China contributes 44% of the total, acting as the primary source. The water vapor at 500 hPa is mainly transported from the inland areas of South China and Northwest China, with that from the inland South China (56%) being more prominent. The water vapor at all levels is mainly transported to the rainstorm region through the eastern and southern regions of China from the source areas. Additionally, there are some differences in the water vapor trajectories at a 6 h interval.

Regional difference and prevention strategy of farmland erosion induced by extreme rainstorms in the Loess area and Rocky mountain area of Northern China

AbstractElucidating the mechanism of soil erosion caused by extreme rainstorms could improve agroecosystems' resilience to the extreme climate. However, less attention has been paid to the regional difference of soil erosion during extreme rainstorms through multiple field investigations. Using six field surveys of extreme rainstorms, soil erosion and its driving mechanism were investigated on farmland in the Loess area and the Rocky mountain area. Conclusions are as follows: (1) Farmland was severely eroded in comparison with abandoned land and forest–grass land. The erosion modulus of farmland in the Loess area (51 606.51 t/km2) was greater than that in the Rocky mountain area (2582.75 t/km2). (2) In both the Loess area and the Rocky mountain area, the main erosion types on sloped cropland and terraced land were rills/gullies and embankment failure, respectively. Terracing is an appropriate measure that can effectively control farmland erosion in rainstorm events. (3) Concentrated flow is a major driver of soil erosion, with nearly all rills/gullies occurring in the flow paths. Drainage measures can prevent the formation of concentrated flow, weakening rills/gullies erosion in farmland. (4) Building bio‐embankment, planting high‐coverage crops, and implementing local protection measures could reduce the farmland erosion influenced by extremely heavy rainfall. Combining drainage and agronomic measures is the common strategy to combat extreme rainstorm events for reducing farmland erosion in the two regions. These results can be applied to improve the ability of agricultural systems to cope with extreme rainstorms.

Development of an Integrated Urban Flood Model and Its Application in a Concave-Down Overpass Area

Urban floods caused by extreme rainstorm events have increased in recent decades, particularly in concave-down bridge zones. To simulate urban flooding processes accurately, an integrated urban flood model (IUFM) was constructed by coupling a distributed urban surface runoff model based on the cellular automata framework (CA-DUSRM), a widely used pipe convergence module in the storm water management model (SWMM), with an inundation module that describes the overflow expansion process associated with terrain and land-cover. The IUFM was used in a case study of the Anhua Bridge (a typical concave-down overpass) study area in Beijing, China. The spatial-temporal variations in flood depth modeled by the IUFM were verified to be reliable by comparison with actual measurements and other simulations. The validated IUFM was used to obtain temporal variations in flood range, depth, and volume under four rainstorm scenarios (return periods of 3-year, 10-year, 50-year, and 100-year). The results showed that the surface runoff process, overflow from drainage networks, and overflow expansion process could affect the flooding status by changing the composition and spatial configuration of pervious or impervious patches, drainage capacity, and underlying surface characteristics (such as terrain and land-cover). Overall, although the simulation results from the IUFM contain uncertainties from the model structures and inputs, the IUFM is an effective tool that can provide accurate and timely information to prevent and control urban flood disasters and provide decision-making support for long-term storm water management and sponge city construction.

A Framework Based on LIDs and Storage Pumping Stations for Urban Waterlogging

Climate change has resulted in an increase in extreme rainstorm events, posing the challenges of urban waterlogging and runoff pollution. Low Impact Development (LID) is widely used to address the issues above, but its effectiveness is unknown in mountainous areas. Due to a flash flood and high flood peak, storage pumping stations are also needed to drain. Thus, a framework composed of storage pumping stations and Low Impact Developments (LIDs) was proposed based on the topography and the regional upstream and downstream relationships. The water quantity in this framework is applied to YI County in Hebei Province, China. The results showed that individual LIDs effectively reduced runoff volume, with the implementation area being more crucial than the location. Combining storage pumping stations with LIDs significantly reduces peak outflow and delays it by 5 to 51 min. The combined downstream implementation of storage pumping stations and LIDs yielded the most effective results. These findings offer important insights and management strategies for controlling waterlogging in mountainous cities of developing countries.

Numerical Quantification of Surface Runoff and Soil Erosion using PCRaster

Surface runoff and soil erosion of the Upper Lagawe River Sub-watershed, Ifugao Province, Philippines were quantified using Geographic Information Systems (GIS)-based numerical simulation. PCRaster software was used as a framework for the model. Manning’s and Rose, and Freebairn’s Equations were utilized to compute surface runoff velocity and sediment mass. Analysis and comparison of the measured and predicted streamflow yielded a mean Nash and Sutcliffe Efficiency Index of 0.70, a mean percent bias of -8.38%, and a mean root mean square error (RMSE) of 0.16. Based on the model, super typhoon Lando (international name: Koppu) generated an estimated surface runoff volume of 186000 m3 and an estimated sediment mass of 36587 kg for the entire sub-watershed. The mass of eroded sediments equated to 18.6 kg/ha of erosion rate just for the said super typhoon. Stable ecosystems, such as the Upper Lagawe River Sub-watershed, where forest vegetation is intact and farmers practice soil conservation techniques are still vulnerable to the effects of extreme rainstorm events, can generate voluminous surface runoff and eroded sediments.

A Comprehensive Assessment of Multiple High-Resolution Precipitation Grid Products for Monitoring Heavy Rainfall during the “7.20” Extreme Rainstorm Event in China

Precipitation products play an important role in monitoring rainstorm processes. This study takes a rare historical event of extreme, heavy precipitation that occurred in Henan Province, China, in July 2021 as a research case. By analyzing the distribution of the spatial and temporal characteristics of precipitation errors, using a probability density function of the occurrence of precipitation and the daily variation pattern, we assess the capability of a radar precipitation estimation product (RADAR), satellite precipitation products (IMERG and GSMAP), a reanalysis product (ERA5) and a precipitation fusion product (the CMPAS) to monitor an extreme rainstorm in the Henan region. The CMPAS has the best fit with the gauge observations in terms of the precipitation area, precipitation maximum and the evolution of the whole process, with a low spatial variability of errors. However, the CMPAS slightly underestimated the precipitation extremum at the peak moment (06:00–08:00). The RADAR product was prone to a spurious overestimation of the originally small rainfall, especially during peak precipitation times, with deviations concentrated in the core precipitation area. The IMERG, GSMAP and ERA5 products have similar performances, all of which failed to effectively capture heavy precipitation in excess of 60 mm/h, with negative deviations in precipitation at mountainfront locations west of northern Henan Province. There is still a need for terrain-specific error revisions for areas with large topographic relief. By merging and processing precipitation data from multiple sources, the accuracy of the CMPAS is better than any single-source precipitation product. The CMPAS has the characteristic advantage of high spatial and temporal resolutions (0.01° × 0.01°/1 h), which play a positive role in precipitation dynamic monitoring, providing early warnings of heavy rainfall processes and hydrological application research.

An advanced check‐dam sedimentation module: Laboratory validation and implementation in a distributed sediment yield model for field application

AbstractCheck‐dams are structures used extensively around the world for soil and water conservation. However, existing models for check‐dams are unable to simulate the Sediment Trap Efficiency (STE) at the catchment scale. A numerical model was developed to simulate the SEdiment Deposition upstream of Check‐Dams (SEDCD) and integrated into a distributed sediment yield model, the Digital Yellow River Model (DYRIM). Two versions of the SEDCD model were evaluated: the SV version which used the Saint–Venant equation to simulate the unsteady flow, and the BW version, which used a modified backwater equation (based on a quasi‐steady approximation) to improve computational efficiency. Sediment deposition and the associated bed profile adjustment were simulated with the sediment conservation equation and the non‐equilibrium suspended sediment transport equation. The SEDCD model was first validated in the laboratory using experimental data from a scale‐down check‐dam. The bed profiles predicted using both versions of the SEDCD model showed good agreement with the observations, with NSE values over 0.9 in most profiles. When integrated into the DYRIM and applied to the Xiaoli River Basin (818 km2) on the Loess Plateau, which has 183 active check‐dams, the SEDCD‐DYRIM combination predicted the STE for an extreme rainstorm event in 2017 with good accuracy and high computational efficiency. The SEDCD‐BW‐DYRIM simulated the hourly discharge and sediment concentration with high accuracy (NSE values of 0.79 and 0.71, respectively) and provided single‐event STEs (R2 value of 0.99) comparable to those of the SEDCD‐SV model, with an approximately 30 times faster runtime efficiency than the SEDCD‐SV model. The SEDCD‐BW model is a powerful and efficient tool to assess the effect of check dams on sediment dynamics at the catchment scale.

Effects of soil and water conservation measures of slope surfaces on flood peaks of small watersheds: A study based on three extreme rainstorm events in northern China

Flash floods resulting from extreme rainstorms seriously threaten regional socioeconomic development. However, few studies have explored the general impacts of soil and water conservation measures (SWCMs) on flood peaks during extreme rainstorms through multiple field investigations. On the basis of field surveys of three rainstorm events in Shaanxi, Shandong, and Henan provinces, the effects of vegetation and agricultural terraces on flood peaks were investigated in 75 small watersheds in northern China. Compared with watersheds dominated by sloping cropland, the flood peak modulus of watersheds dominated by forest land, grass land, and agricultural terraces decreased by 54.75%, 47.80%, and 53.65%, respectively; the peak shaving effects (PSEs) of SWCMs were stronger during minor floods. Soil and water conservation measures reduced the flood magnitudes of small watersheds and their sensitivities to rainstorm events; the PSEs were positively correlated with the area of SWCMs. In earth-rocky mountainous areas, the PSEs of forest land were stronger than those of grassland and agricultural terraces, while in the loess hilly-gully areas, the PSEs of agricultural terraces were stronger than those of other SWCMs. In loess hilly-gully areas, the flash floods demonstrated a phenomenon of small rainfall amounts but substantial flood peaks. Besides the reasonable installation of SWCMs on slopes, appropriate channel measures (e.g., check dams) are warranted to cope with extreme rainstorm events. In northern earth-rocky mountainous areas, intercropping management should be promoted to cover bare soil on orchard agricultural terraces. Our results provide a theoretical basis for combating extreme rainstorm disasters.

The Global Importance of Increasing Design Rainstorms under Specific Return Periods in China

Evaluating the correlation between changes in design rainstorms and extreme rainstorm events under climate change facilitates flood control and disaster reduction. Based on the daily rainfall data of 609 stations during 1958–2017, the Pearson-III curve, least square method, F-test, and other methods were adopted to study the changes in design rainstorms and the response to extreme rainstorms over nearly 60 years in China, and to explore the flood control capacity of engineering during climate change. The conclusions are as follows. (1) There is a general increasing trend in extreme rainstorm events in China. Most extreme rainstorm indices present upward trends in East China (EC), South China (SC), Central China (CC), Northwest China (NW), and Southwest China (SW) and downward trends in Northeast China (NE) and North China (NC). (2) The temporal series of design rainstorms shows general growth under each return period in China. In EC, SC, and SW, the design rainstorms increase significantly with rates of 3.0, 3.1, and 1.3 mm/10a, respectively, in the 20-year return period, while they decrease significantly by −2.0 mm/10a in NC (p < 0.05). (3) Design rainstorms have a positive response to extreme rainstorm events, which resulted in increasing rainstorms in a specific return period in EC, SC, NW, and SW. These results can promote the revision of engineering design standards and improve the flood control capability of engineering.

Novel insights into impacts of the “7.20” extreme rainstorm event on water supply security of Henan Province, China: Levels and health risks of tap water disinfection by-products

Spatial distributions, levels, and comprehensive assessments of post-flood tap water disinfection by-products (DBPs) were first studied in Henan Province after the “7.20” Extreme Rainstorm Event in 2021. DBPs levels and health risks in tap water were higher in areas flooded (waterlogged) by storm or upstream flood discharge (WA) and rainstorm-affected areas (RA) compared with other areas (OA), suggesting that extreme rainstorm and flooding events may somehow exacerbate DBPs contamination of tap water through disinfection. WA sites were characterized as contamination hotspots. The results revealed high haloacetic acids (HAAs) levels in WA (Avg: 57.79 μg·L−1) and RA (Avg: 32.63 μg·L−1) sites. Compared with normal period, DBPs-caused cancer risk increased by 3 times, exceeding the negligible risk level. Cancer risk came primarily from the ingestion of trihalomethanes (THMs) (>80%), children were the sensitive group. Those between 30 and 69 showed approximately 1.7 times higher disability-adjusted life yearsper person-yearthan other age groups. Apart from regulated DBPs, bromochloracetic acid (BCAA) and dibromoacetonitrile (DBAN) appear to be the main toxicity contributors in these samples. Our results provide a scientific basis for preventing and controlling health risks from tap water DBPs and for assessing the social benefits and burdens of emergency disinfection.

Evolution and prediction of the extreme rainstorm event in July 2021 in Henan province, China

AbstractThe Once‐in‐a‐Century extreme rainstorm event caused severe floods over Henan province during July 18–21, 2021, which resulted in large casualty and property losses. Although the rainstorm event occurred in Henan after July 18, the excessive rainfall had occurred to the east of Henan before July 18, with the 4‐day accumulated rainfall exceeding +130 mm during July 14–17, 2021. How the rainfall evolving westward and intensifying after July 18 remained a puzzle, which is the focus of this study. The prerainstorm stage (July 14–17) was related to the South Asian High (SAH) extending eastward and the western Pacific subtropical high (WPSH) extending northwestward, and a low vortex between the SAH and WPSH caused above‐normal rainfall to the east of Henan. The rainstorm stage (July 18–21) was associated with an inverted trough and excessive southerly and southeasterly water vapor transportation above Henan, which resulted from the combined effects of a deep trough in the upper troposphere and typhoon activities. Additionally, three subseasonal forecasting systems predicted this rainstorm event 3 days in advance, with the European Center for Medium Range Weather Forecasts (ECMWF) performing the best, which was related to a better prediction of the inverted trough and the water vapor transportation in the middle‐lower troposphere. These results advance our understanding of the extreme rainstorm event in July 2021 in Henan.

An Integrated Model of Pluvial Flood Risk and Adaptation Measure Evaluation in Shanghai City

Under climate warming, the frequency and intensity of extreme rainstorms-induced urban pluvial floods are significantly increasing, leading to severe flooding risks in megacities. An integrated model that incorporates rainfall processing, waterlogging simulation, GIS spatial analysis, risk assessment, and measure evaluation was developed, optimizing both the assessment procedure and the computing efficiency for assessing pluvial flood risk and adaptation measures. The historic “9.13” extreme rainstorm event is simulated, showing excellent consistency of inundation depth and area compared to Shanghai’s “110” alarm validation records. Three extreme rainstorm scenarios, using return periods of 200a, 500a, and 1000a, were generated to evaluate the flooding risk and measures’ performance. The results show that the low-lying areas in the solution district are the most inundated region based on the existing defense measures. The maximum inundation depth of high-extreme rainstorm scenarios is up to 1.5 m, and the low-lying areas would still be above 1 m even in low-extreme scenarios. Increasing the drainage capacity can effectively reduce the total loss by 80% in the low-extreme scenario, yet still suffering a significant loss in both moderate-extreme and high-extreme scenarios. The integrated model can provide technical support for the rapid evaluation of flood control prevention capabilities.

Assessment of the urban waterlogging resilience and identification of its driving factors: A case study of Wuhan City, China

With rapid urbanization and extreme rainstorm events associated with climate change, urban waterlogging has become one of the most frequent and severe disasters globally. In this study, a multi-dimensional and multi-process index system based on the Pressure-State-Response (PSR) framework was developed to measure the level of urban waterlogging resilience (UWR). The spatial distribution of UWR on a block scale was explored based on the entropy weight method with the natural breakpoint method (EWM-NBM) in the central district of Wuhan City, China. In addition, the effects of the runoff control facilities and early warning measures on UWR were also quantified. Further, the Geodetector was used to investigate the main driving factors of UWR and their interactions. Results showed that the constructed index system for UWR based on the PSR framework performed reasonably, and the EWM-NBM was validated to be effective in the integrated assessment. In terms of the validation results, 82.72 % of the recorded waterlogging points belonged to high and very-high risk levels. The spatial heterogeneity of UWR was significant in the study area where the higher-level UWR mainly appears in the areas near the undeveloped suburban and water bodies (lakes and rivers), and the lower-level UWR was concentrated in central urban areas with more impervious surfaces. There was a clear increasing trend in UWR after the implementation of runoff control facilities and early warning measures, but its spatial distribution remained almost invariant. Among all the indexes, the impervious surface percentage had the strongest (69.58 %) explanatory ability for the UWR, and mean annual precipitation (15.51 %), GDP (14.03 %), and population density (11.98 %) also demanded attention. Most driving factors of UWR showed nonlinear interactions. This research could provide a benchmark for urban planning to enhance UWR to mitigate the waterlogging within the main urban area.

Integrated modelling approach for flood forecasting in small Mediterranean catchment – Application to the Cagne Catchment, France

The French Mediterranean region is vulnerable to flooding caused by extreme rainstorm events. To better predict floods and anticipate consequences, the technical managing services operating on the field need to analyze the hydrological situation, receive accurate forecasts and issue alerts to the exposed population. Within this context, the various needed modelling tools are integrated within a common framework of a Decision Support System (DSS). This approach has been developed and implemented in the Cagne catchment located in the French Riviera. The Cagne catchment covers 96 km2 area and it is densely urbanized with high risks of inundation in the downstream area. The hydrological model MIKE SHE covers 96 km2 with a 20 m resolution. The river network is represented in a 1D MIKE 11 model by 18 branches and 1899 cross sections. The downstream area is covered with a high-resolution hydraulic model built with MIKE 21 FM. The model receives inputs from the hydrological model and can produce detailed inundation maps and forecasts. A major rainfall event observed from October to December 2020 is currently used as validation situation and the built modelling system has demonstrated its efficiency to reproduce the observed processes and to deliver the forecasts on due time.

Simulations of an extreme rainstorm event (1056.7 mm/day) along the South China coast: Cloud microphysical processes and maintenance mechanism of rainstorm

The coastal area of Guangdong Province underwent an extreme rainstorm in Aug. 2018. In this paper, using the convection-permitting WRF simulation results, a comparative analysis was given to the cloud microphysical conversion processes simulated under four cloud microphysics schemes, i.e. WSM6, WDM6, WSM7 and WDM7, as well as the accompanying latent heat budget. The maintenance mechanism of the rainstorm is discussed. In all four schemes, water vapor condensing into cloud water and accretion of cloud water by rain water were the top two conversion processes. In WSM6/WDM6 scheme, the third largest conversion process was accretion of rain water by graupel; in WSM7/WDM7, however, the third largest was cloud water evaporating to water vapor. The most important source of rain water was accretion of cloud water by rain, but the differences in hydrometeors conversion between rain water and graupel, snow and hail were the leading cause of differences in simulated rainfall under different schemes.The top three releasing latent heat processes were water vapor condensing into cloud water, water vapor deposited to cloud ice, and snow sublimation. The top two absorbing latent heat processes were cloud water and rain water evaporating to water vapor. In WSM6/WDM6/WSM7 schemes, the third major process of absorption of latent heat was snow sublimating to water vapor, in WDM7, however, it was hail melting into rain water. Compared with ice-phase processes, liquid-phase processes provide more latent heat. Differences in the release and absorption of latent heat were found in the mid-high troposphere. Hydrometeors in the troposphere maintain efficient conversion and release large amounts of latent heat that formed positive feedback with convection, which was the critical reason for the development and maintenance of the extreme rainstorm.

Impacts of FY-4A GIIRS Water Vapor Channels Data Assimilation on the Forecast of “21·7” Extreme Rainstorm in Henan, China with CMA-MESO

A record-breaking extreme rainstorm occurred in Henan Province of China on 20 July 2021. To investigate the impacts of the Geostationary Interferometric Infrared Sounder (GIIRS) data assimilation on model analysis and forecasts of this rainfall event, the high temporal resolution GIIRS water vapor (WV) channel data were assimilated in the high-resolution CMA-MESO (Mesoscale Weather Numerical Forecast System of China Meteorological Administration) in this study. The results showed that the GIIRS WV radiance assimilation could improve the model WV analysis, which in turn adjusted the distributions of hydrometeors (radar composite reflectivities) and wind field, and finally improved the precipitation forecast. Additionally, although barely any GIIRS observations were assimilated over the cloudy area, the precipitation forecast errors of “21·7” extreme rainstorm events could be reduced by improving the structure of atmospheric circulations through the assimilation of neighboring data around Henan, especially over the upstream region. With the GIIRS WV data assimilation, the location error of maximum 24-h accumulated precipitation forecasts decreased from 128.48 km to 28.97 km (improved by 77.45%) for the cold start at 0000 UTC (Universal Time Coordinated) on 19 July 2021, and it was also reduced by about 60.52% for the warm start experiment at 0600 UTC on 19 July 2021. In addition, the GIIRS assimilation experiment showed an extraordinarily heavy rainfall area (above 250 mm/24 h) around Zhengzhou station, which did not appear in the control experiment, and was closer to the observed extreme precipitation. This study demonstrates the potential value of geostationary hyperspectral infrared sounders data assimilation in extreme weather early warning and forecasting.

Identifying and Assessing the Critical Criteria for Material Selection in Storm Drainage Networks: A Stationary Analysis Approach

Recent years have seen a rise in the frequency and severity of extreme rainstorm events, which have caused widespread damage and death in numerous cities. The manufacture and use of storm drainage materials result in numerous environmental concerns in the construction industry. Green materials for storm drainage networks are environmentally friendly compared to their traditional counterparts. Identifying and assessing sustainability criteria for green materials for storm drain networks has been challenging. This study aims to determine the critical criteria for selecting green materials for storm drainage networks using a stationary analysis approach. To this end, a questionnaire survey was administered to Egyptian storm engineers to assess their importance based on a selection criteria 29 green materials. From the results obtained, “Operation and maintenance cost” and “Use of local material” were seen to be the “stationary materials”. The obtained findings in this research pave the way for the Egyptian storm industry towards becoming environmentally friendly, which will in turn improve the functioning mechanism of sewer networks.

Dynamic downscaling ensemble forecast of an extreme rainstorm event in South China by COSMO EPS

In this study, the high-resolution ensemble prediction system COSMO (Consortium for Small Scale) EPS is used to predict the extreme rainstorm that occurred from 27 to 31 August 2018 in Guangdong Province, China, which leads to intensities exceeding historical extreme values. COSMO EPS is run with a 2.8-km grid spacing, allowing for an explicit treatment of deep convection, and 24 members of the EPS are initialized and laterally driven by the ICON (ICOsahedral Nonhydrostatic) global model. We compare the predictions of COSMO EPS against observations derived from the global precipitation measurement (GPM) and with ensemble forecasts of both mesoscale EPS and global EPS provided by GRAPES (Global and Regional Assimilation and PrEdiction System), and with the deterministic forecasts of global models ICON and ECMWF (European Centre for Medium-Range Weather Forecasts). Model performances are evaluated both by gridpoint-based scores, such as the equitable threat score (ETS), and by the Method for Object-based Diagnostic Evaluation (MODE) for spatial verification. According to our results, COSMO EPS could perform better forecasts for the rainstorms taken place in eastern Guangdong than other models. However, the location and coverage area of its predicted rainstorm is eastward and smaller in contrast with the observations. Therefore, COSMO EPS exhibits relative high performance by object-based spatial evaluations, while it could not display evident superiority in terms of the gridpoint-based scores. The cause analysis of this extreme rainstorm shows that Guangdong Province of China is mainly affected by monsoon depression. Southwesterly and southerly winds continuously transport water vapor from the South China Sea to Guangdong Province. The southwest monsoon low-level jet advances northward over time, which promotes the occurrence and development of continuous heavy precipitation in the coastal areas of Guangdong. In an additional experiment, we investigate the benefit of assimilation of radar data, by applying the latent heat nudging (LHN) approach based on surface-based radar observations to the COSMO EPS. Subsequently, the prediction by assimilation of radar data more reasonably reproduces the spatial distribution of precipitation observations, while the coverage and intensity of the rainstorm in eastern Guangdong are still not reflected satisfactorily.

Influence of Assimilating Wind Profiling Radar Observations in Distinct Dynamic Instability Regions on the Analysis and Forecast of an Extreme Rainstorm Event in Southern China

This study quantitatively examines the contribution of assimilating observations in the regions with different dynamic instabilities to the analysis and prediction of an extreme rainstorm event in Fujian Province of China. The wind profiling radar (WPR) observations are classified into two groups, i.e., strong and weak instability areas (SIA and WIA), according to their local dynamic instability identified by the ensemble spread. Their performance of assimilation and prediction in terms of the wind and precipitation are evaluated and compared in detail. The results show that the wind analysis error by assimilating all of the WPR observations can be reduced by about 30%. In particular, the wind analysis errors by only assimilating the observations in the SIA are about 12% lower than those in the WIA. They are related to the existence of the low-level horizontal wind shear with strong instability in the SIA. The case study shows that the assimilation of observations in the SIA can effectively correct the wind fields on the two sides of the wind shear line, producing an improved precipitation forecast compared to observation assimilation in the WIA.

Analysis of drainage efficiency under extreme precipitation events based on numerical simulation

AbstractThe recent increase in extreme climatic phenomena has gradually attracted the attention of researchers regarding urban flooding. This paper used a hydrological model combined with the Computational Fluid Dynamics software to assess the drainage efficiency, predict the hydrological processes under different precipitation intensities and establish the relationship between rainfall intensity and inundation depth under extreme rainstorm events. The study proposed several thresholds such as specific rainfall intensities (that is, RI95% and RI50%), drainage efficiency, and the limited flow rate of pipes. Based on 2021 July 20, 0:00 to 24:00 precipitation monitoring data from the “7.20 Zhengzhou Rainstorm Extreme Event,” the relationship between rainfall intensity and road‐pipe overflow patterns was determined by analysing these thresholds for different drainpipe diameters and spacings of catchment wells. The results demonstrated the evaluation parameters varied with rainfall intensities and pipe characteristics and revealed the main limitation of drainage efficiency and flow rate of drainpipes. The simulation helped the drainage systems design for different precipitations and proposed several relative suggestions for drainage‐system improvements, wherein the diameter of the branch pipes plays a dominant role in coping with extreme rainstorm events.