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.

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.

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.

Sensitivity Analysis of the Super Heavy Rainfall Event in Henan on 20 July(2021)Using ECMWF Ensemble Forecasts

An unprecedented heavy rainfall event occurred in Henan Province,China,during the period of 1200 UTC 19-1200 UTC 20 July 2021 with a record of 522 mm accumulated rainfall.Zhengzhou,the capital city of Henan,received 201.9 mm of rainfall in just one hour on the day.In the present study,the sensitivity of this event to atmospheric variables is investigated using the ECMWF ensemble forecasts.The sensitivity analysis first indicates that a local YellowHuai River low vortex(YHV)in the southern part of Henan played a crucial role in this extreme event.Meanwhile,the western Pacific subtropical high(WPSH)was stronger than the long-term average and to the west of its climatological position.Moreover,the existence of a tropical cyclone(TC)In-Fa pushed into the peripheral of the WPSH and brought an enhanced easterly flow between the TC and WPSH channeling abundant moisture to inland China and feeding into the YHV.Members of the ECMWF ensemble are selected and grouped into the GOOD and the POOR groups based on their predicted maximum rainfall accumulations during the event.Some good members of ECMWF ensemble Prediction System(ECMWF-EPS)are able to capture good spatial distribution of the heavy rainfall,but still underpredict its extremity.The better prediction ability of these members comes from the better prediction of the evolution characteristics(i.e.,intensity and location)of the YHV and TC In-Fa.When the YHV was moving westward to the south of Henan,a relatively strong southerly wind in the southwestern part of Henan converged with the easterly flow from the channel wind between In-Fa and WPSH.The convergence and accompanying ascending motion induced heavy precipitation.

Synoptic-Scale Analysis on Development and Maintenance of the 19–21 July 2021 Extreme Heavy Rainfall in Henan,Central China

In this paper,synoptic-scale analyses of frontogenesis,moisture budget,and tropospheric diabatic heating are performed to reveal the development and maintenance mechanisms for the extreme heavy rainfall in Henan Province of central China from 19 to 21 July 2021,based on station observations and the ECMWF Reanalysis version 5(ERA5)data.The results demonstrate that owing to the blocking effect of local topography,low-level wind convergence in Henan appeared underneath high-level divergence,conducive to development and maintenance of a midtropospheric low-pressure system saddled by the Asian continental high and the western Pacific subtropical high(WPSH),during the extreme heavy rainfall.In the lower troposphere,frontogenesis occurred in the θ_(se) intensive region,as a result of the divergence and horizontal deformation(which play equally important roles),generating frontal secondary circulation with strong vertical motion favorable to heavy rainfall.Moisture budget analysis reveals that 1)with the continuous strengthening of the easterly wind from the north side of Typhoon In-Fa(2106),strong wind shear and orographic uplift led to abnormally strong convergence of water vapor flux in the boundary layer in Henan;2)there occurred extremely strong net inflow of moisture in the boundary layer from the east.Horizontally,both the apparent heat source and the moisture sink coincided with the area of heavy rainfall;vertically,however,Q_(1)exhibited a single peak with the heating center in the middle and upper troposphere,while large Q_(2)values evenly resided over 850–400 hPa;and Q_(1)(Q_(2))was dominated by vertical(horizontal)transport of potential temperature(moisture).These indicate that the latent heat release from condensation of initial heavy rainfall provided a positive feedback,leading to increasingly heavy precipitation.All these synoptic settings sustained the extreme rainfall process.

Antecedent snowmelt and orographic precipitation contributions to water supply of Pakistan disastrous floods,2022

In 2022,the Pakistan witnessed the hottest spring and wettest summer in history.And devastating floods inundated a large portion of Pakistan and caused enormous damages.However,the primary water source and its contributions to these unprecedented floods remain unclear.Based on the reservoir inflow measurements,Multi-Source Weighted-Ensemble Precipitation(MSWEP),the fifth generation ECMWF atmospheric reanalysis(ERA5)products,this study quantified the contributions of monsoon precipitation,antecedent snow-melts,and orographic precipitation enhancement to floods in Pakistan.We found that the Indus experienced at least four inflow up-rushes,which was mainly supplied by precipitation and snowmelt;In upper Indus,abnormally high temperature continued to influence the whole summer and lead to large amounts of snowmelts which not only was a key water supply to the flood but also provided favorable soil moisture conditions for the latter precipitation.Before July,the snowmelt has higher contributions than the precipitation to the streamflow of Indus River,with contribution value of more than 60%.Moreover,the snowmelt could still supply 20%-40%water to the lower Indus in July and August;The leading driver of 2022 mega-floods over the southern Pakistan in July and August was dominated by the precipitation,where terrain disturbance induced precipitation account to approximately 33%over the southern Pakistan.The results help to understand the mechanisms of flood formation,and to better predict future flood risks over complex terrain regions.

Uncertainties and error growth in forecasting the record-breaking rainfall in Zhengzhou,Henan on 19–20 July 2021

This study explores the controlling factors of the uncertainties and error growth at different spatial and temporal scales in forecasting the high-impact extremely heavy rainfall event that occurred in Zhengzhou,Henan Province China on 19−20 July 2021 with a record-breaking hourly rainfall exceeding 200 mm and a 24-h rainfall exceeding 600 mm.Results show that the strengths of the mid-level low-pressure system,the upper-level divergence,and the low-level jet determine both the amount of the extreme 24-h accumulated and hourly rainfall at 0800 UTC.The forecast uncertainties of the accumulated rainfall are insensitive to the magnitude and the spatial structure of the tiny,unobservable errors in the initial conditions of the ensemble forecasts generated with Global Ensemble Forecast System(GEFS)or sub-grid-scale perturbations,suggesting that the predictability of this event is intrinsically limited.The dominance of upscale rather than upamplitude error growth is demonstrated under the regime of k^(−5/3) power spectra by revealing the inability of large-scale errors to grow until the amplitude of small-scale errors has increased to an adequate amplitude,and an apparent transfer of the fastest growing scale from smaller to larger scales with a slower growth rate at larger scales.Moist convective activities play a critical role in enhancing the overall error growth rate with a larger error growth rate at smaller scales.In addition,initial perturbations with different structures have different error growth features at larger scales in different variables in a regime transitioning from the k^(−5/3) to k^(−3) power law.Error growth with conditional nonlinear optimal perturbation(CNOP)tends to be more upamplitude relative to the GEFS or sub-grid-scale perturbations possibly owing to the inherited error growth feature of CNOP,the inability of convective parameterization scheme to rebuild the k^(−5/3) power spectra at the mesoscales,and different error growth characteristics in the k^(−5/3) and k^(−3) regimes.