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2023 Vol. 40, No. 3

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2023-3 Contents
2023, 40(3): 1-1.
Editorial Notes
Preface to the Special Collection on the July 2021 Zhengzhou, Henan Extreme Rainfall Event
Ming XUE
2023, 40(3): 335-336. doi: 10.1007/s00376-022-2018-1
Original Paper
On the Key Dynamical Processes Supporting the 21.7 Zhengzhou Record-breaking Hourly Rainfall in China
Peng WEI, Xin XU, Ming XUE, Chenyue ZHANG, Yuan WANG, Kun ZHAO, Ang ZHOU, Shushi ZHANG, Kefeng ZHU
2023, 40(3): 337-349. doi: 10.1007/s00376-022-2061-y
An extremely heavy rainfall event occurred in Zhengzhou, China, on 20 July 2021 and produced an hourly rainfall rate of 201.9 mm, which broke the station record for the Chinese mainland. Based on radar observations and a convection-permitting simulation using the WRF-ARW model, this paper investigates the multiscale processes, especially those at the mesoscale, that support the extreme observed hourly rainfall. Results show that the extreme rainfall occurred in an environment characteristic of warm-sector heavy rainfall, with abundant warm moist air transported from the ocean by an abnormally northward-displaced western Pacific subtropical high and Typhoon In-Fa (2021). However, rather than through back building and echo training of convective cells often found in warm-sector heavy rainfall events, this extreme hourly rainfall event was caused by a single, quasi-stationary storm in Zhengzhou. Scale separation analysis reveals that the extreme-rain-producing storm was supported and maintained by the dynamic lifting of low-level converging flows from the north, south, and east of the storm. The low-level northerly flow originated from a mesoscale barrier jet on the eastern slope of the Taihang Mountain due to terrain blocking of large-scale easterly flows, which reached an overall balance with the southerly winds in association with a low-level meso-β-scale vortex located to the west of Zhengzhou. The large-scale easterly inflows that fed the deep convection via transport of thermodynamically unstable air into the storm prevented the eastward propagation of the weak, shallow cold pool. As a result, the convective storm was nearly stationary over Zhengzhou, resulting in record-breaking hourly precipitation.
The Roles of Low-level Jets in “21·7” Henan Extremely Persistent Heavy Rainfall Event
Yuhan LUO, Yu DU
2023, 40(3): 350-373. doi: 10.1007/s00376-022-2026-1
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 on 21 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.
Analysis on Precipitation Efficiency of the “21.7” Henan Extremely Heavy Rainfall Event
Lei YIN, Fan PING, Jiahua MAO, Shuanggen JIN
2023, 40(3): 374-392. doi: 10.1007/s00376-022-2054-x
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.
On the Influences of Urbanization on the Extreme Rainfall over Zhengzhou on 20 July 2021: A Convection-Permitting Ensemble Modeling Study
Yali LUO, Jiahua ZHANG, Miao YU, Xudong LIANG, Rudi XIA, Yanyu GAO, Xiaoyu GAO, Jinfang YIN
2023, 40(3): 393-409. doi: 10.1007/s00376-022-2048-8
This study investigates the influences of urban land cover on the extreme rainfall event over the Zhengzhou city in central China on 20 July 2021 using the Weather Research and Forecasting model at a convection-permitting scale [1-km resolution in the innermost domain (d3)]. Two ensembles of simulation (CTRL, NURB), each consisting of 11 members with a multi-layer urban canopy model and various combinations of physics schemes, were conducted using different land cover scenarios: (i) the real urban land cover, (ii) all cities in d3 being replaced with natural land cover. The results suggest that CTRL reasonably reproduces the spatiotemporal evolution of rainstorms and the 24-h rainfall accumulation over the key region, although the maximum hourly rainfall is underestimated and displaced to the west or southwest by most members. The ensemble mean 24-h rainfall accumulation over the key region of heavy rainfall is reduced by 13%, and the maximum hourly rainfall simulated by each member is reduced by 15–70 mm in CTRL relative to NURB. The reduction in the simulated rainfall by urbanization is closely associated with numerous cities/towns to the south, southeast, and east of Zhengzhou. Their heating effects jointly lead to formation of anomalous upward motions in and above the planetary boundary layer (PBL), which exaggerates the PBL drying effect due to reduced evapotranspiration and also enhances the wind stilling effect due to increased surface friction in urban areas. As a result, the lateral inflows of moisture and high-θe (equivalent potential temperature) air from south and east to Zhengzhou are reduced.
Seasonal Prediction of the Record-Breaking Northward Shift of the Western Pacific Subtropical High in July 2021
Shuai HU, Tianjun ZHOU, Bo WU, Xiaolong CHEN
2023, 40(3): 410-427. doi: 10.1007/s00376-022-2151-x
The unprecedented Zhengzhou heavy rainfall in July 2021 occurred under the background of a northward shift of the western Pacific subtropical high (WPSH). Although the occurrence of this extreme event could not be captured by seasonal predictions, a skillful prediction of the WPSH variation might have warned us of the increased probability of extreme weather events in Central and Northern China. However, the mechanism for the WPSH variation in July 2021 and its seasonal predictability are still unknown. Here, the observed northward shift of the WPSH in July 2021 is shown to correspond to a meridional dipole pattern of the 850-hPa geopotential height to the east of China, the amplitude of which became the strongest since 1979. The meridional dipole pattern is two nodes of the Pacific–Japan pattern. To investigate the predictability of the WPSH variation, a 21-member ensemble of seasonal predictions initiated from the end of June 2021 was conducted. The predictable and unpredictable components of the meridional dipole pattern were identified from the ensemble simulations. Its predictable component is driven by positive precipitation anomalies over the tropical western Pacific. The positive precipitation anomalies are caused by positive horizonal advection of the mean moist enthalpy by southwesterly anomalies to the northwestern flank of anticyclonic anomalies excited by the existing La Niña, which is skillfully predicted by the model. The leading mode of the unpredictable component is associated with the atmospheric internal intraseasonal oscillations, which are not initialized in the simulations. The relative contributions of the predictable and unpredictable components to the observed northward shift of the WPSH at 850 hPa are 28.0% and 72.0%, respectively.
Synergistic Effect of the Planetary-scale Disturbance, Typhoon and Meso-β-scale Convective Vortex on the Extremely Intense Rainstorm on 20 July 2021 in Zhengzhou
Guanshun ZHANG, Jiangyu MAO, Wei HUA, Xiaofei WU, Ruizao SUN, Ziyu YAN, Yimin LIU, Guoxiong WU
2023, 40(3): 428-446. doi: 10.1007/s00376-022-2189-9
On 20 July 2021, northern Henan Province in China experienced catastrophic flooding as a result of an extremely intense rainstorm, with a record-breaking hourly rainfall of 201.9 mm during 0800–0900 UTC and daily accumulated rainfall in Zhengzhou City exceeding 600 mm (“Zhengzhou 7.20 rainstorm” for short). The multi-scale dynamical and thermodynamical mechanisms for this rainstorm are investigated based on station-observed and ERA-5 reanalysis datasets. The backward trajectory tracking shows that the warm, moist air from the northwestern Pacific was mainly transported toward Henan Province by confluent southeasterlies on the northern side of a strong typhoon In-Fa (2021), with the convergent southerlies associated with a weaker typhoon Cempaka (2021) concurrently transporting moisture northward from South China Sea, supporting the rainstorm. In the upper troposphere, two equatorward-intruding potential vorticity (PV) streamers within the planetary-scale wave train were located over northern Henan Province, forming significant divergent flow aloft to induce stronger ascending motion locally. Moreover, the converged moist air was also blocked by the mountains in western Henan Province and forced to rise so that a deep meso-β-scale convective vortex (MβCV) was induced over the west of Zhengzhou City. The PV budget analyses demonstrate that the MβCV development was attributed to the positive feedback between the rainfall-related diabatic heating and high-PV under the strong upward PV advection during the Zhengzhou 7.20 rainstorm. Importantly, the MβCV was forced by upper-level larger-scale westerlies becoming eastward-sloping, which allowed the mixtures of abundant raindrops and hydrometeors to ascend slantwise and accumulate just over Zhengzhou City, resulting in the record-breaking hourly rainfall locally.
Effects of a Dry-Mass Conserving Dynamical Core on the Simulation of Tropical Cyclones
Shaoying LI, Jun PENG, Weimin ZHANG, Jianping WU, Qiang YAO, Xiangrong YANG, Tengling LUO
2023, 40(3): 464-482. doi: 10.1007/s00376-022-2085-3
The accurate forecasting of tropical cyclones (TCs) is a challenging task. The purpose of this study was to investigate the effects of a dry-mass conserving (DMC) hydrostatic global spectral dynamical core on TC simulation. Experiments were conducted with DMC and total (moist) mass conserving (TMC) dynamical cores. The TC forecast performance was first evaluated considering 20 TCs in the West Pacific region observed during the 2020 typhoon season. The impacts of the DMC dynamical core on forecasts of individual TCs were then estimated. The DMC dynamical core improved both the track and intensity forecasts, and the TC intensity forecast improvement was much greater than the TC track forecast improvement. Sensitivity simulations indicated that the DMC dynamical core-simulated TC intensity was stronger regardless of the forecast lead time. In the DMC dynamical core experiments, three-dimensional winds and warm and moist cores were consistently enhanced with the TC intensity. Drier air in the boundary inflow layer was found in the DMC dynamical core experiments at the early simulation times. Water vapor mixing ratio budget analysis indicated that this mainly depended on the simulated vertical velocity. Higher updraft above the boundary layer yielded a drier boundary layer, resulting in surface latent heat flux (SLHF) enhancement, the major energy source of TC intensification. The higher DMC dynamical core-simulated updraft in the inner core caused a higher net surface rain rate, producing higher net internal atmospheric diabatic heating and increasing the TC intensity. These results indicate that the stronger DMC dynamical core-simulated TCs are mainly related to the higher DMC vertical velocity.
The Regularized WSM6 Microphysical Scheme and Its Validation in WRF 4D-Var
Sen YANG, Deqin LI, Liqiang CHEN, Zhiquan LIU, Xiang-Yu HUANG, Xiao PAN
2023, 40(3): 483-500. doi: 10.1007/s00376-022-2058-6
A cold cloud assimilation scheme was developed that fully considers the water substances, i.e., water vapor, cloud water, rain, ice, snow, and graupel, based on the single-moment WSM6 microphysical scheme and four-dimensional variational (4D-Var) data assimilation in the Weather Research and Forecasting data assimilation (WRFDA) system. The verification of the regularized WSM6 and its tangent linearity model (TLM) and adjoint mode model (ADM) was proven successful. Two groups of single observation and real sounding data assimilation experiments were set up to further verify the correctness of the assimilation scheme. The results showed that the consideration of ice, snow, and graupel in the assimilation system of the 4D-Var, as opposed to their omission in the warm rain Kessler scheme, allowed the water substances to be reasonably updated, further improving the forecast. Before it can be further applied in the assimilation of observational data, radar reflectivities, and satellite radiances, the cold cloud assimilation scheme needs additional verification, including using conventional ground and sounding observations in the 4D-Var assimilation system.
Impact of Perturbation Schemes on the Ensemble Prediction in a Coupled Lorenz Model
Qian ZOU, Quanjia ZHONG, Jiangyu MAO, Ruiqiang DING, Deyu LU, Jianping LI, Xuan LI
2023, 40(3): 501-513. doi: 10.1007/s00376-022-1376-z
Based on a simple coupled Lorenz model, we investigate how to assess a suitable initial perturbation scheme for ensemble forecasting in a multiscale system involving slow dynamics and fast dynamics. Four initial perturbation approaches are used in the ensemble forecasting experiments: the random perturbation (RP), the bred vector (BV), the ensemble transform Kalman filter (ETKF), and the nonlinear local Lyapunov vector (NLLV) methods. Results show that, regardless of the method used, the ensemble averages behave indistinguishably from the control forecasts during the first few time steps. Due to different error growth in different time-scale systems, the ensemble averages perform better than the control forecast after very short lead times in a fast subsystem but after a relatively long period of time in a slow subsystem. Due to the coupled dynamic processes, the addition of perturbations to fast variables or to slow variables can contribute to an improvement in the forecasting skill for fast variables and slow variables. Regarding the initial perturbation approaches, the NLLVs show higher forecasting skill than the BVs or RPs overall. The NLLVs and ETKFs had nearly equivalent prediction skill, but NLLVs performed best by a narrow margin. In particular, when adding perturbations to slow variables, the independent perturbations (NLLVs and ETKFs) perform much better in ensemble prediction. These results are simply implied in a real coupled air–sea model. For the prediction of oceanic variables, using independent perturbations (NLLVs) and adding perturbations to oceanic variables are expected to result in better performance in the ensemble prediction.
Response of the North Pacific Storm Track Activity in the Cold Season to Multi-scale Oceanic Variations of Kuroshio Extension System: A Statistical Assessment
Peilong YU, Minghao YANG, Chao ZHANG, Yi LI, Lifeng ZHANG, Shiyao CHEN
2023, 40(3): 514-530. doi: 10.1007/s00376-022-2044-z
In this paper, a statistical method called Generalized Equilibrium Feedback Analysis (GEFA) is used to investigate the responses of the North Pacific Storm Track (NPST) in the cold season to the multi-scale oceanic variations of the Kuroshio Extension (KE) system, including its large-scale variation, oceanic front meridional shift, and mesoscale eddy activity. Results show that in the cold season from the lower to the upper troposphere, the KE large-scale variation significantly weakens the storm track activity over the central North Pacific south of 30°N. The northward shift of the KE front significantly strengthens the storm track activity over the western and central North Pacific south of 40°N, resulting in a southward shift of the NPST. In contrast, the NPST response to KE mesoscale eddy activity is not so significant and relatively shallow, which only shows some significant positive signals near the dateline in the lower and middle troposphere. Furthermore, it is found that baroclinicity and baroclinic energy conversion play an important role in the formation of the NPST response to the KE multi-scale oceanic variations.
Extended Impact of Cold Air Invasions in East Asia in Response to a Warm South China Sea and Philippine Sea
Marco Y.-T. LEUNG, Dongxiao WANG, Wen ZHOU, Yuntao JIAN
2023, 40(3): 531-540. doi: 10.1007/s00376-022-2096-0
During boreal winter, the invasion of cold air can lead to remarkable temperature drops in East Asia which can result in serious socioeconomic impacts. Here, we find that the intensity of strong synoptic cold days in the East China Sea and Indochina Peninsula are increasing. The enhanced synoptic cold days in these two regions are attributed to surface warming over the South China Sea and Philippine Sea (SCSPS). The oceanic forcing of the SCSPS on the synoptic cold days in the two regions is verified by numerical simulation. The warming of the SCSPS enhances the baroclinicity, which intensifies meridional wind and cold advection on synoptic timescales. This leads to a more extended region that is subject to the influence of cold invasion.
News & Views
Historic and Future Perspectives of Storm and Cyclone
Gang FU, Pengyuan LI, Lijia CHEN, Yongmao PENG, Jing NI
2023, 40(3): 447-463. doi: 10.1007/s00376-022-2184-1
In weather sciences, the two specific terms “storm” and “cyclone” frequently appear in literature and usually refer to the violent nature of a number of weather systems characterized by central low pressure, strong winds, large precipitation amounts in the form of rain, freezing rain, or snow, as well as thunder and lightning. But what is the connection between these two specific terms? In this paper, the historic evolutions of the terms “storm” and “cyclone” are reviewed from the perspective of weather science. The earliest recorded storms in world history are also briefly introduced. Then, the origin of the term “meteorological bomb”, which is the nickname of the “explosive cyclone” is introduced. Later, the various definitions of explosive cyclones given by several researchers are discussed. Also, the climatological features of explosive cyclones, as well as the future trends of explosive cyclones under global climate change, are discussed.