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Yuanchun Zhang, Fuqing Zhang, Christopher A. Davis, and Jianhua Sun

Abstract

The structure and diurnal evolution of long-lived, eastward-propagating mesoscale convective vortices (MCVs) along typical summertime mei-yu fronts over the east China plains are investigated through composite analysis of a 30-day semi-idealized simulation. The simulation uses lateral boundary conditions that vary only diurnally in time using analyses of recurring MCV events during 1–10 July 2007. Hence, the behavior of convection and vorticity follows a closely repeating diurnal cycle for each day during the simulation. Assisted by the eastward extension of enhanced vorticity anomalies from the Sichuan basin, the incipient MCV forms in the morning hours over the immediate lee (east) of the central China mountain ranges (stage 1). From local afternoon to early evening, as the MCV moves over the plains, convection weakens in the daytime downward branch of the mountain–plains solenoid. This allows the upper-level and lower-level portions of the vortex to partially decouple, and for convection to shift to the east-southeast side of the surface vortex (stage 2). Immediately after sunset, convection reinvigorates above the low-level MCV center as a result of moistening and destabilization from a combination of radiative forcing and an intensified low-level jet. This intensifies the MCV to maturity (stage 3). The mature MCV eventually evolves into an occluding subsynoptic cyclone with strong convection across all sectors of the low-level vorticity center during the subsequent day’s morning hours along the east China coastal plains before it moves offshore (stage 4).

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Yanan Meng, Jianhua Sun, Yuanchun Zhang, and Shenming Fu

Abstract

Hourly blackbody temperature data from the warm seasons (May–September) of 2009–18 were used to detect mesoscale convective systems (MCSs) generated in the southwest mountain area (elevation ≥ 500 m) of China. A total of 3059 MCSs were grouped into four categories (C1, C2, C3, and C4) according to their generation positions using K-means clustering. Major characteristics of the four types of MCSs and their synoptic environmental conditions were investigated. The MCSs had a peak in July and a minimum in May, and usually lasted from 3 to 21 h. The C1 MCSs generated in the northeast of the Tibetan Plateau developed faster, were largest, and had a longer lifespan. The C2 and C4 MCSs had greater intensity and were initiated in the southeast of the Tibetan Plateau and the west of the Yungui Plateau, and near the Wuling and Xuefeng Mountains, respectively. The C3 MCSs initiated in the Qinling, Ta-pa, and Wushan Mountains were smallest. The C1 and C2 MCSs contributed more than 30% to total precipitation, which was more than the C3 and C4 MCSs (<25%), and the contribution rate of MCSs to short-duration heavy rainfall affected by local MCSs was over 60%. Composite synoptic circulations of the four types of MCSs showed several factors, including the locations and intensities of cyclones in the Bay of Bengal and high pressure in the Indochina Peninsula in the low-to-middle troposphere, and vortexes or southwesterly winds in the low-level troposphere, regulate the location and intensity of convection.

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Shen-Ming Fu, Jian-Hua Sun, Ya-Li Luo, and Yuan-Chun Zhang

Abstract

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.

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