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From 17–22 July 2021, Henan Province experienced the most severe torrential rainfall event since 1975 with a maximum hourly precipitation of 201.9 mm appeared in Zhengzhou, which was the largest hourly rainfall thus far observed by meteorological observation stations over the Chinese Mainland. The appearance of a long‐lived (21‐hr) northwestward‐moving mesoscale convective vortex (MCV) and its interaction with its parent mesoscale convective system (MCS) was crucial to produce the extremely strong heavy rainfall in Zhengzhou. The backward trajectory analysis indicates that air particles in the lower troposphere beneath the MCS over Henan contributed mostly to the MCV's formation. These air particles experienced notable ascending motions and condensation with their strong cyclonic vorticity mostly produced 1‐hr before the MCV's formation. Vorticity budget denotes that strong upward transport of cyclonic vorticity and convergence‐related vertical stretching, both of which were mainly due to convection associated with the parent MCS, acted as dominant factors for the MCV's formation. After formation, the MCV first coupled with its parent MCS, during which its intensity, thickness, and precipitation were all maximized; then, it moved northwestward and decoupled from the MCS, during which it weakened rapidly and finally dissipated. Convection‐related upward cyclonic vorticity transport and inward horizontal advection of cyclonic vorticity associated with an inverted trough over the Henan Province dominated the vortex's development/maintenance in the coupling stage; whereas outward horizontal advection of cyclonic vorticity dominated the MCV's dissipation after it completely decoupled from its parent MCS. These differ notably from the findings documented in previous MCV‐related literature. Key Points: A long‐lived mesoscale vortex was crucial to produce the hourly precipitation of 201.9 mm that appeared in Zhengzhou on 20 July 2021The extremely strong hourly precipitation appeared in the stage when the vortex interacted with its parent mesoscale convective systemConvection‐related upward and net‐import horizontal transport of cyclonic vorticity dominated the vortex's development [ABSTRACT FROM AUTHOR]

Explosive extratropical cyclone (EEC) is the main disastrous weather system over the ocean and offshore areas in the cold season. As a type of vertically deep system, after decades of studies, key features of EECs' vertical extents still remain vague. Based on a reasonably simulated entire‐troposphere‐thick EEC, this study analyzes variation of the EEC's vertical extent and investigates governing mechanisms for its vertical extending. Main findings are as follows: (a) the EEC's vertical extent showed consistent variation features with its central sea level pressure and lower‐level vorticity (correlation coefficients were ~0.9), whereas its relationship with EEC's maximum surface wind was not significant; (b) EEC's upward extending featured strong ascending motion and rapid cyclonic‐vorticity enhancement at the top level of the cyclone and obvious inflow (convergence) in the lower troposphere. (c) vorticity budget at the EEC's top level shows that net import transport of cyclonic vorticity (by westerly and northwesterly winds) from the trough west of the cyclone dominated its upward extending, and upward transport of cyclonic vorticity from levels below the cyclone's top level acted as the second dominant factor. In contrast, divergence‐related vertical shrinking around the EEC's top level was the most detrimental factor for the cyclone's upward extending. [ABSTRACT FROM AUTHOR]

Based on hourly geostationary satellite temperature‐of‐black‐body data, 9,754 mesoscale convective systems (MCSs) are found to form over the Tibetan Plateau (TP) during 16 warm seasons. In the whole study period, neither the occurrence numbers of these MCSs nor their other key characteristics are found to have a significant trend of increasing/decreasing. The MCSs can form anywhere over the TP, and ~6.6% of them can move eastward and vacate the TP (defined as the eastward‐moving type [EMT]). The EMTs' mean occurrence frequency and lifespan are ~0.3 per day and ~12.0 hr, respectively. Compared to the MCSs that do not vacate the TP, the EMTs usually have stronger intensity, longer lifespan, and develop more rapidly but generate in a drier environment. The vacating stage of an EMT usually begins in the latter half of its lifespan, and an EMT tends to reach its maximum cloud area when it is about to vacate the TP. After vacating the TP, an EMT usually weakens at first and then enhances again. Vorticity budget indicates that the convergence‐related horizontal shrinking and the convection‐related vertical vorticity transport govern the cyclonic‐vorticity increase/maintenance associated with the longer‐lived EMTs. Of all the EMTs, only ~8% are associated with southwest vortices (SWVs), and the precipitation related to these EMTs contributes ~20% to the local hourly heavy precipitation. Compared to the EMTs that are not related to SWVs, those that are generally vacate the TP sooner, last longer having vacated the TP, and have longer whole lifespans. [ABSTRACT FROM AUTHOR]

A rarely seen entire‐troposphere‐thick (ETT) extratropical cyclone that induced a record‐breaking catastrophic rainstorm in Beijing (the largest since 1963) on 20 July 2016 was reproduced reasonably by a convection‐permitting Weather Research and Forecasting Model. This cyclone was the result of a vertical coupling of a lower, a middle, and an upper tropospheric cyclone, which formed at different times and locations. Dry air descending from the stratosphere through a tropopause folding process capped moist air to the middle and lower troposphere, making convection associated with the cyclone relatively shallow. Overall, cyclonic vorticity associated with the ETT cyclone enhanced much rapider than its wind kinetic energy (KE), and formation mechanisms of the ETT cyclone were characterized by layer‐wise features: (i) The formation of the middle and upper tropospheric cyclones was dominated by cyclonic vorticity transport, instead of cyclonic vorticity production, whereas the lower tropospheric cyclone was generated through both cyclonic vorticity transport and convergence‐related cyclonic vorticity production. (ii) Consistent with the cyclonic vorticity intensification, the formation of the lower and middle tropospheric cyclones featured an enhancement of rotational wind KE (in western section of the cyclone, this was mainly due to the work done by the pressure gradient force, but in eastern section, it was mainly due to the inward transport of wind KE by rotational wind), whereas the formation of the upper tropospheric cyclone featured a decrease in the rotational wind KE (particularly in its eastern section), which was in contrast to its increasing cyclonic vorticity. The export of wind KE by a southwesterly wind within the eastern section of the cyclone governed the rotational wind attenuation. Key Points: An entire‐troposphere‐thick extratropical cyclone that induced the largest rainstorm in Beijing since 1963 was reproduced reasonably by WRFInward transport of cyclonic vorticity was a dominant factor for the formation of the lower, middle, and upper tropospheric cyclonesWork done by pressure gradient force governed enhancement of rotational wind kinetic energy of the lower and middle tropospheric cyclone [ABSTRACT FROM AUTHOR]

Regions around Dabie Mountain (DBM) in the Yangtze River basin (YRB) are the source of a mesoscale vortex: the Dabie vortex (DBV). Based on a 14-yr statistical study, 11 long-lived heavy-rain-producing DBVs were composited for convection-permitting semi-idealized simulations. A control simulation, initialized 12 h before the composite vortex formation, successfully reproduced a DBV, with all the salient characteristics of the 11 events. Sensitivity experiments were designed to understand the impacts of large-scale environmental conditions, regional topography, and latent heating on DBV formation. The main results were as follows: (i) Supposition of a 500-hPa shortwave trough with an east–west-oriented lower-level transversal trough around the DBM is crucial for the formation of vortices. A nocturnal lower-level jet on the southern side of the transversal trough accelerates DBV formation by enhancing convergence, triggering/sustaining convection, and producing cyclonic vorticity. (ii) During the simulation period, the topography east of the second-step mountain ranges, including the DBM, significantly affects nearby precipitation and convective activity, whereas this is not crucial for DBV formation. (iii) Latent heating is not required for generating DBVs, but it enhances the intensity, longevity, and eastward progression of these vortices along the shear line associated with the transversal trough. (iv) The vorticity budget suggests the convergence-related (horizontal) shrinking and vertical transport dominate the cyclonic-vorticity increase associated with DBVs, whereas tilting and horizontal transport mainly act in the opposite manner. [ABSTRACT FROM AUTHOR]

Dabie vortices (DBVs) are a type of heavy-rainfall-producing mesoscale vortices that appear with a high frequency around the Dabie Mountain over the Yangtze River Basin. For a long time, scholars have found that DBVs tend to form when a low-level jet (LLJ) appears in their neighboring regions. However, the underlying mechanisms of this phenomenon still remain vague. This study furthers the understanding of this type of event by conducting detailed analyses on a long-lived eastward-moving DBV that caused a severe flood in the 2020 summer. It is found that the LLJ in this event was belonged to a nocturnal LLJ type, with its maximum/minimum appeared around 2100/0600 UTC. The diurnal cycle of LLJ affected precipitation and intensity of the DBV notably: As the LLJ intensified, vortex's precipitation and intensity both enhanced, and vice versa. The LLJ exerted two effects on the DBV's formation that are opposite to each other. The more important effect is that the LLJ caused intense lower-level convergence around its northern terminus. This convergence directly produced cyclonic vorticity through vertical stretching, which dominates the DBV's formation and enhances the convection-related upward cyclonic vorticity transport that acted as another favorable factor. The less important effect is that (i) the LLJ induced import of anticyclonic vorticity into the vortex's central region, which decelerated the DBV's formation; and (ii) the LLJ-related to strong ascending motions tilted horizontal vorticity into negative vertical vorticity, which reduced the growth rate of cyclonic vorticity. [ABSTRACT FROM AUTHOR]

This study investigates the formation of a type of mesoscale vortex around the Sichuan Basin (i.e., the southwest vortex [SWV]) that usually induces heavy precipitation, based on a semi‐idealized simulation of eight similar SWV‐formation events. After producing a reasonable SWV that shares salient features with the events used in composite, a trajectory analysis is conducted and an along‐track vorticity budget is constructed. The results show that approximately 64.7% of the air particles related to SWV formation originate from four source regions outside the vortex. The largest contribution of cyclonic vorticity associated with the SWV formation (approximately 67.7%) comes from the lower levels southwest of the vortex and is due mainly to tilting. The second‐largest contribution (approximately 19.4%) is made by air particles that originate from the upper levels south of the SWV and is also due mainly to tilting. By contrast, for the other two sources (the upper levels west and southwest of the vortex), the convergence‐related horizontal shrinking generally dominates their cyclonic‐vorticity production. Overall, along the tracks of the air particles, vertical motions and water‐vapor phase transitions affect the vorticity variation significantly via modulating divergence‐related vorticity production/dissipation (i.e., the stretching effect), and this is vital for the SWV formation. [ABSTRACT FROM AUTHOR]

On the basis of a 14‐year statistical dataset of the southwest vortex (SWV), 10 long‐lived and heavy rain–producing SWVs are selected for a series of convection‐permitting composite semi‐idealized simulations. After confirming the simulation has reproduced the common salient features of these selected vortices, the general underlying mechanisms for the formation of this type of SWV are investigated. Results show that the SWV formation is characterized by significant unevenness, as the fundamental characteristics and dominant factors for the cyclonic‐vorticity production vary from place to place. Overall, the eastern/western section of SWV mainly features favorable/unfavorable conditions for the cyclonic‐vorticity enhancement. The vorticity budget indicates the convergence‐related horizontal shrinking and convection‐related tilting contribute to the cyclonic‐vorticity enhancement in the SWV's eastern section, whereas in the western section, the terrain‐related tilting and horizontal transport result in the cyclonic‐vorticity reduction. [ABSTRACT FROM AUTHOR]