Persistent Heavy Rainfall Events Research Articles

Comparison of kinetic energy conversion characteristics of two extreme precipitation episodes during persistent unusually heavy rainfall on 17–23 July 2021 in Henan, China

A persistent heavy rainfall event, contributed by two extreme episodes (P1 and P2), occurred in Henan province of China from 17 to 23 July 2021. An active mesoscale vortex was present in the lower troposphere, along with a shallow trough moving eastward in the upper troposphere. The budgets of rotational kinetic energy (KR) and divergent kinetic energy (KD) at distinct stages are investigated. The lower-level KR anomaly, related to the mesoscale vortex growth therein, was responsible for P1. However, the upper-troposphere KR was dominant and strong during the P2 stage, associated with an eastward-developing upper-level trough. Prior to both P1 and P2, active inner interaction between rotational and divergent winds presented, manifested as a rapid increase of KVRVD signals. KVRVD and precipitation had similar pattern distribution and consistent evolution tendency. Therefore, KVRVD might be a useful indicator for the occurrence and intensification of rainfall. Expect for two components of winds interaction, the external source feeding kinetic energy growth is also explored. For P1, APE (available potential energy)-> KD-> KR was the key path towards rotational kinetic energy, providing energy for mesoscale vortex growth in lower troposphere, which was favorable for the extreme episode of P1. Nevertheless, for P2 stage, APE acted as a direct energy source towards KR in upper troposphere, with higher efficiency (via APE-> KR) and thus stronger KR. This kind of difference in conversion path depended on the vertically-coupled degree between the lower-level mesoscale vortex and the upper-level trough, in addition to the rotational and divergent wind speed. Their tilting/upright configuration produced strong baroclinic/barotropic process, accelerating energy conversion of APE-> KD/APE-> KR via GD/GR (i.e., generating KD/KR by divergent/rotational winds crossing geopotential height contours, usually as a generator resulting from barotropic/baroclinic process). Therefore, kinetic energy budget may be a valuable tool for identifying key weather systems development and forecasting extreme rainfall episodes.

Thermodynamic and dynamic effects of anomalous dragon boat water over South China in 2022

In 2022, persistent extreme precipitation events occurred during the Dragon Boat Water (DBW) period (May to June) over South China, with rainfall-breaking historical extremes at many stations. In this study, the relative importance of the thermodynamic and dynamic effects on this anomalous event was explored through budget equations of moisture and moist static energy (MSE). The moisture budget analysis suggests that the thermodynamic effect contributes to the precipitation anomaly by less than 10% through the advection of abnormal moisture by mean horizontal and vertical motion. However, the dynamic effect, associated with the mean moisture advection by anomalous vertical motion, makes the largest contribution. This seems to imply that the increase in water vapor due to global warming does not play a critical role in this extreme event. Nevertheless, further analysis of the MSE budget equation reveals that the anomalous vertical motion is governed by both the dynamical effect of atmospheric circulation changes and the thermodynamic effect of atmospheric thermal state changes. The thermodynamic effect, which accounts for 47.21% of the total effect (sum of the thermodynamic and dynamic effects), is caused by anomalous moist enthalpy (mainly dry enthalpy) advection due to the mean horizontal motion. In contrast, the dynamical effect is related to the warm and humid air advection caused by the anomalous southwest wind caused by the anomalous anticyclone over the northwest Pacific. Simultaneously, the anomalous northeast wind brings dry cold air, resulting in the convergence of cold and warm air over the Pearl River basin and persistent heavy rainfall events. The results of this study provide valuable insights into the direct and indirect physical mechanisms leading to anomalous DBW period over South China in 2022.

10–30‐day moist static energy evolutions related to the persistent heavy rainfall event in different stages of flood season over South China

AbstractIn this study, the flood season in South China (SC) was divided into three stages: two first rainy seasons (FRSs) around the South China Sea summer monsoon onset and one second rainy season when Typhoon prevails, denoted as FRS1, FRS2, and SRS, respectively, and then we diagnosed the moist static energy (MSE) budget associated with the 10–30‐day persistent heavy rainfall (PHR) over SC during these periods. The results indicate that there are great differences in the recharge of PHR‐related MSE in different stages of the flood season in SC: The FRS1 MSE associated with PHR moves southeastward from midlatitude; the large MSE is maintained in SC during the FRS2; during the SRS, the MSE perturbation propagates from the tropical western North Pacific to SC. From the perspective of the local MSE budget in SC, meridional and zonal advection play a key role in the maximum MSE change in the FRS1; the FRS2 and SRS MSE tendency is mainly determined by zonal advection and meridional advection, respectively. In contrast, the 10–30‐day propagating perturbation of MSE changes during both the FRS1 and FRS2 are mainly affected by the zonal advection, while the meridional circulation is dominant in the SRS. The cumulative contribution of external forcing (including radiation and surface heat fluxes) during the SRS to the propagation of PHR‐related MSE perturbation can reach more than 30%, and the closer to the land, the stronger the external forcing. During the FRS (including FRS1 and FRS2), however, the external forcing contributes little, even negatively.

Mechanism Analysis of a Wintertime Extreme Persistent Heavy Rainfall Event in Hainan Island

AbstractExtremely persistent heavy rainfall (EPHR) events frequently occur in southern China, but its mechanisms of initiation and maintenance are not well understood, leading to difficulties in accurately forecasting. Thus, we investigated a typical EPHR case that occurred on Hainan Island on 14 December 2013 (UTC) using convection‐resolving WRF simulation and multi‐source observations. Results revealed that the intrusion of the cold surge, the formation of the quasi‐stationary mesoscale perturbed vortex (MPV) caused by deflected southerly winds and northeasterly winds, and local topographic forcing are three crucial factors for this EPHR event. Firstly, the invasion of cold surge provides a favorable thermal condition for strong convection, as the CAPE around Wanning city substantially increases during the invasion, greatly enhancing atmospheric instability. Secondly, the event is triggered around 06:00 UTC due to the low‐level convergence of easterly and northeasterly winds. The confrontation of easterly and northeasterly winds forms an MPV in the middle boundary layer, which plays an important role in the maintenance of strong convection. In the following hours, the easterly airflow is continuously deflected northward, but its southerly wind components are largely balanced by the northerly counterpart from the cold surge, so the MPV remained quasi‐stationary until 18:00 UTC, resulting in persistent heavy rainfall in Wanning city. Finally, the sensitivity experiments show that topographic blocking and friction effects also play a decisive role in this EPHR process. Overall, this study fully illustrates the special formation mechanism of a wintertime EPHR event, deepening the mechanistic understanding of EPHR events.

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

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

Origins of Intraseasonal Precipitation Variability over North China in the Rainy Season

Abstract We investigated the characteristics and mechanisms of subseasonal precipitation variability in North China during the rainy season (June–September). Two dominant intraseasonal modes with periods of 8–20 and 30–60 days were identified via spectral analysis. Together, they explain 62.8% of the total precipitation variability. Nearly all persistent heavy rainfall events in North China were observed concurrently with the enhanced positive phases of biweekly or/and 30–60-day precipitation modes. To elucidate the origins of these two intraseasonal precipitation variabilities, we performed moisture and vertical motion analyses. The moisture diagnosis results show that the anomalous lower-level southerly perturbations, which transport the background summer-mean moisture from the tropical areas toward North China, are the key process causing abundant moisture for the anomalous precipitation occurrence. The local ascending motion anomalies associated with the occurrence of intraseasonal precipitation come mainly from the anomalous vorticity advection induced by summer-mean thermal wind. Although the key processes causing the precipitation anomalies of these two intraseasonal modes are similar, the geneses and evolutions of large-scale conditions associated with them are distinct. The biweekly circulation and convective anomalies are driven by the midlatitude wave train pattern propagating southeastward across the Eurasian continent, while the 30–60-day anomalies are determined by both upper-level perturbations along 40°N and northward-propagating intraseasonal convective activities from the tropics.

Uplift Mechanism of Coastal Extremely Persistent Heavy Rainfall (EPHR): The Key Role of Low‐Level Jets and Ageostrophic Winds in the Boundary Layer

AbstractExtremely persistent heavy rainfall (EPHR) events frequently occur in southern China. The mechanisms underlying their generation and development are not well understood, and this leads to difficulties in accurately forecasting these events. In this paper, hourly precipitation grid data and ERA‐5 reanalysis data are used to examine the relationships between precipitation, ageostrophic winds, and low‐level jets (LLJ). The results suggest that ageostrophic winds in the boundary layer mainly exhibit confrontational and asymptotic convergence, with the former being accompanied by double LLJ, which transport more vapor and provide favorable uplift conditions, thus producing stronger precipitation and wider rainbands. The different cooperated relationships between LLJ and ageostrophic winds play an important role in the development of EPHR events, and greater account needs to be taken of these relationships when forecasting.

Reduced Probability of 2020 June–July Persistent Heavy Mei-yu Rainfall Event in the Middle to Lower Reaches of the Yangtze River Basin under Anthropogenic Forcing

"Reduced Probability of 2020 June–July Persistent Heavy Mei-yu Rainfall Event in the Middle to Lower Reaches of the Yangtze River Basin under Anthropogenic Forcing" published on Mar 2022 by American Meteorological Society.

Typical Circulation Patterns and Associated Mechanisms for Persistent Heavy Rainfall Events over Yangtze-Huaihe River Valley during 1981\u20132020

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

Two Distinct Types of 10-30-day Persistent Heavy Rainfall Events over the Yangtze River Valley

AbstractLarge-scale circulation anomalies associated with 10-30-day filtered persistent heavy rainfall events (PHREs) over the middle and lower reaches of the Yangtze River Valley (MLYV) in boreal summer for the period of 1961-2017 were investigated. Two distinct types of PHREs were identified based on configurations of anomalies in western Pacific subtropical high (WPSH) and South Asian High (SAH) during the peak wet phase. One type named as PSAH is characterized by eastward extension of the SAH while the other named as NSAH is featured by westward retreat of the SAH, and they both exhibit westward extension of the WPSH. Both types of PHREs are dominated by Mei-yu frontal systems. The lower-level circulation anomalies play a crucial role in initiating rainfall but through different processes. Prior to rainfall occurrence, a strong anticyclonic circulation anomaly is over the western North Pacific (WNP) for the PSAH events and the related southwesterly wind anomaly prevails over the south-eastern China, which advects moisture into the MLYV, moistens the boundary layer, and induces atmospheric convective instability. For the NSAH events, the WNP anticyclonic circulation is weak while a strong northerly wind is observed north of the MLYV. It brings cold air mass southward, favoring initiating frontal rainfall over the MLYV. The formation of upper-level circulation anomalies over the MLYV is primarily due to the shift of anomalous circulations from mid-high latitudes. After the rainfall generation, the precipitation would influence the lower- and upper-level circulation anomalies.

Multiscale Causes of Persistent Heavy Rainfall in the Meiyu Period over the Middle and Lower Reaches of the Yangtze River

Based on observation data supplied by the Chinese Meteorological Administration (CMA) and reanalysis datasets provided by the ECMWF, the multiscale causes of persistent heavy rainfall events (PHREs) that occurred from 1979 to 2018 during Meiyu periods over the middle and lower reaches of the Yangtze River (MLYR) are investigated. During Meiyu periods, precipitation shows obvious interannual variabilities. In PHRE years, the contribution rate of persistent heavy rainfall to the total precipitation is approximately 57%. Precipitation also shows significant synoptic-scale (less than 10 days) characteristics. Through the quantitative diagnosis of interactions among background-scale (greater than 30 days), quasi-biweekly-scale (10–30-days), and synoptic-scale variables, the possible causes of PHREs are explored. The results reveal that the difference in precipitation intensity between PHRE years and non-PHRE years is determined by the background water vapor, background wind and synoptic-scale wind conditions. In PHRE years, the prevailing background southwesterly winds from lower latitudes provide more background water vapor, and more mean kinetic energy is converted to perturbation energy. Moreover, the active synoptic-scale oscillations from higher latitudes and the convergence of Rossby wave disturbance energy over the MLYR could also cause the occurrence and maintenance of PHREs during Meiyu periods. The multiscale causes and corresponding circulation patterns in 2020 PHREs are similar to PHREs years.

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

Based on hourly precipitation from national surface stations, persistent heavy rainfall events (PHREs) over the Sichuan Basin (SCB) are explored during the warm season (May to September) from 2000 to 2015 to compare synoptic circulations and maintenance mechanisms between different PHRE types. There are two main types of PHREs: one is characterized by a rain belt west of 106°E over the SCB (WSB-PHREs), and the other features a rain belt east of 106°E over the SCB (ESB-PHREs). In total, there are 18 ESB-PHREs and 10 WSB-PHREs during the study period. Overall, the rain belts of WSB-PHREs are along the terrain distribution east of the Tibetan Plateau, while the precipitation intensity of ESB-PHREs is stronger. For the two types of PHREs, the shortwave trough over the SCB and the western Pacific subtropical high act as their favorable background environments, particularly for ESB-PHREs. The water vapor of WSB-PHREs is mainly transported from the South China Sea, whereas for ESB-PHREs the South China Sea and Bay of Bengal are their main moisture sources. The composite vorticity budgets of southwest vortices during their mature stage indicate that the convergence effect is a dominant factor for maintaining the two types of PHREs, and the strong vertical vorticity advection is also favorable, but the relative contribution of vertical advection is larger for WSB-PHREs.摘要本文筛选出四川盆地西部 (盆西型) 和盆地东部 (盆东型) 持续性暴雨个例, 深入对比两类持续性暴雨的大气环流特征和直接造成持续性暴雨的西南低涡维持的机理. 四川盆地的短波槽和西太平洋副热带高压的配置有利于持续性暴雨的维持, 盆东型的降水强度较盆西型个例强, 高空急流位置偏南, 南亚高压的强度更强, 高层辐散更强, 对流层中层副热带高压偏东偏南. 盆西型的水汽输送主要来自南海, 而盆东型的水汽输送主要来自南海和孟加拉湾. 合成涡度收支的结果表明散度项是两类持续暴雨中西南涡维持的主要原因, 但盆西型中, 垂直平流的作用更强.

Impact of potential vorticity anomalies around the eastern Tibetan Plateau on quasi-biweekly oscillations of summer rainfall within and south of the Yangtze Basin in 2016

Six persistent heavy rainfall events that occurred mostly over Hubei–Anhui–Jiangxi (HAJ) provinces within and south of the middle–lower Yangtze Basin during the summer of 2016 were regulated by the 10–20-day quasi-biweekly oscillation (QBWO). The characteristics and mechanisms of the QBWO associated with the HAJ rainfall events were examined using ERA-Interim reanalysis data. Composite analysis shows that during a QBWO cycle, the extreme wet (dry) phase of the HAJ rainfall was characterized by the strongest ascending (descending) motion associated with an anomalous upper-tropospheric saddle-shaped circulation field resulting from an eastward-propagating middle–high latitude wave train and westward-migrating tropical potential vorticity (PV) anomalies. The wet (dry) phase was preceded by the corridor establishment of significant anomalous poleward-directed (equatorward-directed) moisture transport over southern China on the northwestern side of the western North Pacific subtropical high in the lower troposphere. The corridor-related moisture convergence (divergence) over the HAJ tended to intensify as an anomalous anticyclone (cyclone) propagated northwestward to the northern South China Sea, in conjunction with the eastward-propagating mid-latitude wave train. The QBWO of the HAJ rainfall was closely linked with upstream PV anomalies generated over the eastern slope of the Tibetan Plateau (TP) due to topographic lateral friction. The PV budget analysis demonstrates that the horizontal PV advection and subsequent topographic friction with a four-day phase-lag between them dictated the QBWO of PV anomalies around the eastern TP. The TP-generated PV anomalies then migrate downstream to facilitate the development of the anomalous circulation over the HAJ.

A COMBINED VERIFICATION METHOD FOR PREDICTABILITY OF PERSISTENT HEAVY RAINFALL EVENTS OVER EAST ASIA BASED ON ENSEMBLE FORECAST

Persistent Heavy Rainfall (PHR) is the most influential extreme weather event in Asia in summer, and thus it has attracted intensive interests of many scientists. In this study, operational global ensemble forecasts from China Meteorological Administration(CMA) are used, and a new verification method applied to evaluate the predictability of PHR is investigated. A metrics called Index of Composite Predictability (ICP) established on basic verification indicators, i. e., Equitable Threat Score(ETS) of 24h accumulated precipitation and Root Mean Square Error(RMSE) of Height at 500hPa, are selected in this study to distinguish“good”and“poor”prediction from all ensemble members. With the use of the metrics of ICP, the predictability of two typical PHR events in June 2010 and June 2011 is estimated. The results show that the“good member”and“poor member”can be identified by ICP and there is an obvious discrepancy in their ability to predict the key weather system that affects PHR.“Good member”shows a higher predictability both in synoptic scale and mesoscale weather system in their location, duration and the movement. The growth errors for “poor” members is mainly due to errors of initial conditions in northern polar region. The growth of perturbation errors and the reason for better or worse performance of ensemble member also have great value for future model improvement and further research.

Decadal and Interannual Variability of Persistent Heavy Rainfall Events over the Middle and Lower Reaches of the Yangtze River Valley

This study investigates the relationship between the anomalous atmospheric circulation pattern and summertime persistent heavy rainfall (PHR) over the middle and lower reaches of the Yangtze River valley (MLYRV) on the decadal and interannual timescales. Based on the gridded daily rainfall data of the US Climate Prediction Center, the PHR events on grid-point and the regional PHR events considering both the area of PHR and regionally averaged rainfall intensity are identified over the MLYRV during the summers of 1979–2017. A PHR index (PHRI) is defined, to describe the variability of summertime PHR event number over the MLYRV. The PHRI is then divided into the decadal and interannual components. Further analysis reveals that the decadal PHR events are closely related to anomalous precipitation, intensified vertical motion, and strengthened upper-level divergence over southern China, as well as an anomalous anticyclone over the western Pacific transporting more water vapor from the South China Sea (SCS) to southern China. As for the interannual component, the above pattern still co-exists but over a narrow region around the MLYRV. By choosing the years in which the decadal and interannual components of the PHRI are simultaneously positive (SP) or negative (SN), the features of regional PHR events in SP and SN years are analyzed, respectively. The results show that there are more regional PHR events in SP years with enhanced intensity and larger affected areas compared with those in SN years. Meanwhile, the zonal oscillations of the South Asian high (SAH) and the western Pacific subtropical high (WPSH) during the regional PHR events demonstrate a better regularity in SP years than those in SN years.

Impact of Synoptic‐Scale Factors on Rainfall Forecast in Different Stages of a Persistent Heavy Rainfall Event in South China

AbstractThis study investigates the stage‐dependent rainfall forecast skills and the associated synoptic‐scale features in a persistent heavy rainfall event in south China, Guangdong Province, during 29–31 March 2014, using operational global ensemble forecasts from the European Centre for Medium‐Range Weather Forecasts. This persistent rainfall was divided into two stages with a better precipitation forecast skill in Stage 2 (S2) than Stage 1 (S1) although S2 had a longer lead time. Using ensemble‐based sensitivity analysis, key synoptic‐scale factors that affected the rainfall were diagnosed by correlating the accumulated precipitation of each stage to atmospheric state variables in the middle of respective stage. The precipitation in both stages was found to be significantly correlated with midlevel trough, low‐level vortex, and particularly the low‐level jet on the southeast flank of the vortex and its associated moisture transport. The rainfall forecast skill was mainly determined by the forecast accuracy in the location of the low‐level jet, which was possibly related to the different juxtapositions between the direction of the movement of the low‐level vortex and the orientation of the low‐level jet. The uncertainty in rainfall forecast in S1 was mainly from the location uncertainty of the low‐level jet, while the uncertainty in rainfall forecast in S2 was mainly from the width uncertainty of the low‐level jet with the relatively accurate location of the low‐level jet.

On the Scale Interactions that Dominate the Maintenance of a Persistent Heavy Rainfall Event: A Piecewise Energy Analysis

Abstract Persistent heavy rainfall events (PHREs) are the product of the combined effects of multiscale systems. A PHRE that occurred during the 2016 mei-yu season was selected to further the understanding of the scale interactions accounting for the persistence of this type of event. The scale interactions were analyzed quantitatively using a piecewise energy budget based on temporal scale separation. Results show that the strongest interactions between the precipitation-related eddy flow and its background circulation (BC) occur in the mid- to lower troposphere, where a significant downscale kinetic energy (KE) cascade alone dominates eddy flow persistence. An obvious upscale KE cascade (i.e., a feedback effect) appears in the mid- to upper troposphere but has a negligible effect on the BC. Overall, within the precipitation region, the downscale KE cascade is primarily dependent on BC signals with shorter periods, whereas the upscale KE cascade is more dependent on BC signals with longer periods. Thus, the BC has asymmetric effects on the KE cascades. The most significant BC signal as determined via wavelet analysis [i.e., quasi-biweekly (10–18 days) oscillations in this event] does not play the leading role in the downscale KE cascade. Instead, the quasi-weekly oscillations provide the maximum amount of energy for eddy flow maintenance. Semi-idealized simulations of various BC signals show similar results: precipitation and the intensities of lower-level shear lines and transversal troughs (both of which are closely related to the precipitation-related eddy flow) are more sensitive to the quasi-weekly oscillation than to the quasi-biweekly oscillation.

Subtropical westerly jet waveguide and winter persistent heavy rainfall in south China

AbstractUsing observed daily precipitation and National Centers for Environmental Prediction‐National Center for Atmospheric Research reanalysis data, what induced winter large spatial persistent heavy rainfall (PHR) events in south China was examined, based on composite analyses of 30 large spatial PHR events during 1951–2015. The results showed that wave trains within North Africa‐Asia (NAA) westerly jet existed in upper troposphere during these PHR processes. The wave trains shared the characteristic of a Rossby wave. The Rossby wave originated from northwest Europe, entered into the NAA jet through strong cold air advection to form convergence over the Mediterranean, and then propagated eastward along subtropical NAA jet. The Rossby wave propagated toward Southeast Asia and caused strong divergence in the upper troposphere. The strong divergence in the upper troposphere induced vertical convection and favored large spatial PHR events in south China. In addition, the enhanced India‐Burma trough and subtropical high in the northwestern Pacific supplied enough water vapor transportation. This mechanism would be useful to the medium‐range forecast of such winter rainfall processes over south China.

Main energy paths and energy cascade processes of the two types of persistent heavy rainfall events over the Yangtze River–Huaihe River Basin

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

Impact of multi-scale oscillations at high and low latitudes on two persistent heavy rainfall events in the middle and lower reaches of the Yangtze River

To investigate the multi-scale features in two persistent heavy rainfall (PHR) events in the middle and lower reaches of the Yangtze River (MLRYR) in June of 1982 and 1998, this study examines the impact of multi-scale oscillations in the north and south of 30°N on the PHR events by performing sensitivity experiments with the Weather Research and Forecast (WRF) model. It is found that the 60-day lowpass perturbation made a trivial contribution to the MLRYR precipitation during the PHR event in 1982. This PHR event resulted mainly from the combined effects of 30–60-day oscillation at low latitudes and 10–30-day oscillation at both high and low latitudes. The southwesterly anomalies associated with the 30–60-day anticyclonic anomaly over the northwestern Pacific facilitated moisture transport from the ocean to the MLRYR and enhanced the low-level convergence and ascending motion in the MLRYR. This similarly occurred in the 10–30-day oscillation as well. Moreover, the 10–30-day anomalies at high latitudes played a role in strengthening the large-scale low-level convergence over the MLRYR. The PHR event in 1998 was mainly related to the 60-day oscillation at both high and low latitudes and 30–60-day oscillation at low latitudes. The 60-day low-pass filtered anomalous cyclone at high latitudes in the north of 30°N contributed to the development of low-level convergence and ascending motion in northern MLRYR while the anomalous anticyclone at low latitudes in the south of 30°N not only increased the moisture in the MLRYR but also preconditioned the dynamical factors favorable for PHR over the whole area. The 30–60-day perturbations located north and south of 30°N worked together producing positive moisture anomaly in the MLRYR. In addition, the anomalous circulation in the south of 30°N tended to favor the development of ascending motion and low-level convergence in the MLRYR.