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.
Based on hourly rain gauge data during May–September of 2016–20, we analyze the spatiotemporal distributions of total rainfall (TR) and short-duration heavy rainfall (SDHR; hourly rainfall ⩾ 20 mm) ...and their diurnal variations over the middle reaches of the Yangtze River basin. For all three types of terrain (i.e., mountain, foothill, and plain), the amount of TR and SDHR both maximize in June/July, and the contribution of SDHR to TR (CST) peaks in August (amount: 23%; frequency: 1.74%). Foothill rainfall is characterized by a high TR amount and a high CST (in amount); mountain rainfall is characterized by a high TR frequency but a small CST (in amount); and plain rainfall shows a low TR amount and frequency, but a high CST (in amount). Overall, stations with high TR (amount and frequency) are mainly located over the mountains and in the foothills, while those with high SDHR (amount and frequency) are mainly concentrated in the foothills and plains close to mountainous areas. For all three types of terrain, the diurnal variations of both TR and SDHR exhibit a double peak (weak early morning and strong late afternoon) and a phase shift from the early-morning peak to the late-afternoon peak from May to August. Around the late-afternoon peak, the amount of TR and SDHR in the foothills is larger than over the mountains and plains. The TR intensity in the foothills increases significantly from midnight to afternoon, suggesting that thermal instability may play an important role in this process.
The dominant frequency modes of pre-summer extreme precipitation events (EPEs) over South China (SC) between 1998 and 2018 were investigated. The 67 identified EPEs were all characterized by the ...3–8-d (synoptic) frequency band. However, multiscale combined modes of the synoptic and three low-frequency bands 10–20-d (quasi-biweekly, QBW); 15–40-d (quasi-monthly, QM); and 20–60-d (intraseasonal) accounted for the majority (63%) of the EPEs, and the precipitation intensity on the peak wet day was larger than that of the single synoptic mode. It was found that EPEs form within strong southwesterly anomalous flows characterized by either lower-level cyclonic circulation over SC or a deep trough over eastern China. Bandpass-filtered disturbances revealed the direct precipitating systems and their life cycles. Synoptic-scale disturbances are dominated by mid-high latitude troughs, and the cyclonic anomalies originate from downstream of the Tibetan Plateau (TP). Given the warm and moist climate state, synoptic-scale northeasterly flows can even induce EPEs. At the QBW and QM scales, the disturbances originate from the tropical Pacific, downstream of the TP, or mid-high latitudes (QBW only). Each is characterized by cyclonic-anticyclonic wave trains and intense southwesterly flows between them within a region of large horizontal pressure gradient. The intraseasonal disturbances are confined to tropical regions and influence SC by marginal southwesterly flows. It is concluded that low-frequency disturbances provide favorable background conditions for EPEs over SC and synoptic-scale disturbances ultimately induce EPEs on the peak wet days. Both should be simultaneously considered for EPE predictions over SC.
Predicting warm-sector torrential rainfall over South China, which is famous for its destructive power, is one of the most challenging issues of the current numerical forecast field. Insufficient ...understanding of the key mechanisms underlying this type of event is the root cause. Since understanding the energetics is crucial to understanding the evolutions of various types of weather systems, a general methodology for investigating energetics of torrential rainfall is provided in this study. By applying this methodology to a persistent torrential rainfall event which had concurrent frontal and warm-sector precipitation, the first physical image on the energetics of the warm-sector torrential rainfall is established. This clarifies the energy sources for producing the warm-sector rainfall during this event. For the first time, fundamental similarities and differences between the warm-sector and frontal torrential rainfall are shown in terms of energetics. It is found that these two types of rainfall mainly differed from each other in the lower-tropospheric dynamical features, and their key differences lay in energy sources. Scale interactions (mainly through downscale energy cascade and transport) were a dominant factor for the warm-sector torrential rainfall during this event, whereas, for the frontal torrential rainfall, they were only of secondary importance. Three typical signals in the background environment are found to have supplied energy to the warm-sector torrential rainfall, with the quasi-biweekly oscillation having contributed the most.
Warm-sector heavy rainfall (WSHR) events in China have been investigated for many years. Studies have investigated the synoptic weather conditions during WSHR formation, the categories and general ...features, the triggering mechanism, and structural features of mesoscale convective systems during these rainfall events. The main results of WSHR studies in recent years are summarized in this paper. However, WSHR caused by micro- to mesoscale systems often occurs abruptly and locally, making both numerical model predictions and objective forecasts difficult. Further research is needed in three areas: (1) The mechanisms controlling WSHR events need to be understood to clarify the specific effects of various factors and indicate the influences of these factors under different synoptic background circulations. This would enable an understanding of the mechanisms of formation, maintenance, and organization of the convections in WSHR events. (2) In addition to South China, WSHR events also occur during the concentrated summer precipitation in the Yangtze River-Huaihe River Valley and North China. A high spatial and temporal resolution dataset should be used to analyze the distribution and environmental conditions, and to further compare the differences and similarities of the triggering and maintenance mechanisms of WSHR events in different regions. (3) More studies of the mechanisms are required, as well as improvements to the model initial conditions and physical processes based on multi-source observations, especially the description of the triggering process and the microphysical parameterization. This will improve the numerical prediction of WSHR events.
During January 21–24, 2016, most land areas in the Northern Hemisphere experienced extreme low temperatures. In North America, a historic snowstorm hit the northern and eastern United States. In East ...Asia, an unprecedented cold wave occurred and led to record‐breaking low temperatures in many regions. In this study, observational analyses revealed that both extreme events were triggered by a remarkable change in atmospheric circulation in the Arctic region in early January 2016, which switched from a concentric ring pattern to a dipole pattern. The dipole pattern resulted in two inverted Ω‐shaped circulation patterns that dominated the North America and East Asia. The inverted Ω‐shaped circulation patterns induced remarkable tropopause folding, which conveyed high‐potential‐vorticity cold air downwards from the lower stratosphere of Arctic to the middle and lower troposphere of North America, which increased cyclonic vorticity and negative height perturbations, and converged with moist air from the western North Atlantic and Gulf Stream, resulting in a severe snowstorm in the northern and eastern United States from 22 to 24 January. In East Asia, the tropopause folding transported high‐potential‐vorticity cold air downwards to the middle and lower troposphere of East Asia, resulting in the outbreak of a severe cold wave in East Asia from 21 to 24 January.
During January 21–24, 2016, most land areas in the Northern Hemisphere experienced unprecedented low temperatures. In North America, a historic snowstorm hit the northern and eastern United States. In East Asia, a severe cold wave occurred and led to record low temperatures in many regions. Observational analyses revealed that both extreme events were triggered by a remarkable change in atmospheric circulation in the Arctic region in early January 2016.
A new local kinetic energy (KE) budget for the Madden-Julian Oscillation (MJO) is constructed in a multi-scale framework. This energy budget framework allows us to analyze the local energy conversion ...processes of the MJO with the high-frequency disturbances and the low-frequency background state. The KE budget analysis is applied to a pronounced MJO event during the DYNAMO field campaign to investigate the KE transport path of the MJO. The work done by the pressure gradient force and the conversion of available potential energy at the MJO scale are the two dominant processes that affect the MJO KE tendency. The MJO winds transport MJO KE into the MJO convection region in the lower troposphere while it is transported away from the MJO convection region in the upper troposphere. The energy cascade process is relatively weak, but the interaction between high-frequency disturbances and the MJO plays an important role in maintaining the high-frequency disturbances within the MJO convection. The MJO KE mainly converts to interaction KE between MJO and high-frequency disturbances over the area where the MJO zonal wind is strong. This interaction KE over the MJO convection region is enhanced through its flux convergence and further transport KE to the high-frequency disturbances. This process is conducive to maintaining the MJO convection. This study highlights the importance of KE interaction between the MJO and the high-frequency disturbances in maintaining the MJO convection.
Based on a 16-warm-season statistical study on the mesoscale convective systems (MCSs) that were generated over the Tibetan Plateau (TP), 11 long-lived eastward propagating MCSs of the same type were ...selected for a composite semi-idealized simulation and a corresponding no-latent-heating sensitivity run by using the Weather Research and Forecasting (WRF) model. Common evolutionary features and associated mechanisms of this type of long-lived eastward propagating MCS were investigated. Main results are as follows: (i) This type of MCS was generated in a favorable background environment which was characterized by a notable upper-tropospheric divergence south of an upper-level jet, a strong warm advection around a middle-level shortwave trough’s central area, and an instable convective stratification below the trough. Development of the MCS featured rapid increase of cyclonic vorticity in the middle and lower troposphere. The convergence-related vertical stretching and tilting were key factors for the cyclonic-vorticity’s production, and convection-related upward cyclonic-vorticity transport contributed to the upward extending of the MCS. (ii) During the vacating stage of the MCS, it first coupled with a quasi-stationary Tibetan Plateau vortex (TPV) over the TP’s eastern section, and then decoupled from the vortex. In the former stage, the MCS contributed to maintaining ascending motions and convergence associated with the TPV, which favored its persistence; whereas, in the latter stage, decoupling weakened the TPV-associated convection significantly. This reduced the upward transport of cyclonic vorticity notably, which, together with the negative tilting effect, finally led to the vortex’s dissipation. (iii) After vacating TP, the MCS first weakened due to the disappearance of strong direct sensible heating from the TP on its bottom, and then, under the favorable conditions associated with the shortwave trough over the eastern section of the TP, the MCS redeveloped rapidly. Convergence-related cyclonic-vorticity production in the middle and lower troposphere and upward transport of cyclonic vorticity due to convection governed the MCS’s redevelopment. (iv) Sensitivity simulation shows that latent heating was a necessary condition for the formation and development of the long-lived eastward propagating MCS. On the one hand, this MCS affected the TP’s eastern section and downstream regions directly by inducing precipitation; and on the other hand, it exerted effects on the precipitation over a wider range in the downstream regions by modulating large-scale circulations over and around the TP.
During mid‐July 2021, an extreme heavy rainfall event (HRE) occurred in Henan Province (hereafter “21.7” HRE), with extreme hourly precipitation of 201.9 mm appearing at Zhengzhou station. Our ...preliminary analyses of the “21.7” HRE using the observations and ECMWF (European Centre for Medium‐Range Weather Forecasts) ERA5 reanalysis data, reached the following conclusions. Favorable configurations of various synoptic weather systems (e.g., strong upper‐level high‐pressure ridge, intense middle‐level low‐pressure trough) acted as crucial background conditions for the occurrence of the “21.7” HRE. A 21‐h long‐lived mesoscale convective vortex (MCV), mainly located in the middle and lower troposphere west of Zhengzhou city, was a key system that produced the extreme hourly rainfall of 201.9 mm·h−1. The MCV's development/sustainment was dominated by the vertical transport of cyclonic vorticity and tilting, as well as the horizontal import of cyclonic vorticity to the vortex's key region. In contrast, the divergence‐related vertical shrinking was the most detrimental factor. Lagrangian moisture transport analysis showed that moisture for the extreme heavy rainfall in Zhengzhou on July 20 mainly came from levels below 2200 m, driven by airflows on the peripheries of tropical cyclones IN‐FA and CEMPAKA. To enhance the understanding of “21.7” HRE, we suggest more in‐depth investigations in the future.
China is located in the East Asian monsoon region, which experiences relatively frequent heavy rainfall events in summer. Generally, the heavy rainfall frequency and accumulated amount in South China are higher than those of North China, whereas, the intensity of heavy rainfall in North China is similar to or even higher than that of South China. In July 2021, a record‐breaking heavy rainfall event occurred in Henan Province. The maximum observed precipitation in the whole event exceeded 1000 m, and an extreme hourly rainfall of 201.9 mm occurred in Zhengzhou. The major favorable large‐scale circulation conducive to heavy rainfall in North China during this event includes an extratropical‐cyclone/low‐pressure area between the continental high pressure and intensive subtropical high. The moisture related to two tropical cyclones IN‐FA and CEMPAKA was transported from the Western Pacific and the South China Sea to Henan Province. A mesoscale convective vortex was a key system to produce the extreme hourly rainfall of 201.9 mm in Zhengzhou city on 20 July, 2022.
Based on CMORPH precipitation estimates and ERA5 reanalysis data, this study investigates the mechanisms accounting for the repeated occurrence of torrential rainfall over South Thailand in early ...January 2017, which induced the strongest floods over Ko Samui and Ko Phangan in the last almost 30 years. It is found that the maintenance of a northeastward-moving mesoscale vortex that formed southwest of the Indochina Peninsula was the direct reason for the series of torrential rainfall events. Analysis of the vorticity budget illustrates that convergence-related horizontal shrinking was the most favorable factor for the maintenance of the vortex. Tilting was the second most favorable factor, whereas horizontal and vertical transport mainly caused a net export of cyclonic vorticity from the vortex's three-dimensional range, which was detrimental for its maintenance. Further analysis indicates that tilting and vertical vorticity transport were sensitive to the vortex's displacement and the enhancement of cyclonic vorticity at lower levels around the vortex, respectively, as the two factors showed completely different effects on the persistence of the vortex during two different stages.
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