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Shen-Ming Fu, Zi Mai, Jian-Hua Sun, Wan-Li Li, Yang Ding, and Ya-Qiang Wang

Abstract

In summer, convective activity over the Tibetan Plateau (TP) is vigorous, with some of it moving eastward and vacating the plateau [defined as the eastward-moving type (EMT)]. Although the EMT only accounts for a small proportion, it is closely related to heavy precipitation east of the TP. This study investigates EMT impacts based on a series of composite semi-idealized simulations and piecewise potential vorticity (PV) inversion. The main results are as follows. (i) An EMT begins to affect downstream precipitation before it vacates the TP. A weaker EMT tends to cause the main downstream rainband to reduce in intensity and move southward. (ii) The EMT contributes to the formation of an eastward-moving plateau vortex (PLV) by enhancing convergence-induced stretching. Over the TP, the PLV mainly enhances/maintains the EMT, whereas during the vacating stage, the PLV dissipates (since convergence decreases rapidly when sensible heating from the TP reduces), which substantially reduces the intensity of the EMT. (iii) After PLV dissipation, a southwest vortex (SWV) forms around the Sichuan basin mainly due to convergence-induced stretching, convection-related tilting, and background transport. Piecewise PV inversion indicates that an EMT can directly contribute to SWV formation via lowering geopotential height and enhancing cyclonic wind perturbations around the Sichuan basin (even before its vacating stage), while neither of them governs the SWV formation. Sensitivity runs show that an EMT is not necessary for SWV formation, but can modify the SWV formation time and location, as well as its displacement, which significantly affects downstream precipitation.

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Shen-Ming Fu, Jing-Ping Zhang, Jian-Hua Sun, and Tian-Bao Zhao

ABSTRACT

A 14-yr climatology is presented of the mesoscale vortices generated in the vicinity of the Dabie Mountains [Dabie vortices (DBVs)] in the Yangtze River valley. Analyzing these vortices using the Climate Forecast System Reanalysis (CFSR), DBVs were found to be a frequent type of summer mesoscale weather system, with a mean monthly frequency of 12.2. DBVs were mainly located in the middle and lower troposphere, and ~92% of them triggered precipitation. Most DBVs were short lived, and only 19.5% persisted for more than 12 h. Latent heat release associated with precipitation is a dominant factor for the DBV’s three-dimensional geometry features, life span, and intensity.

The long-lived DBVs, all of which triggered torrential rainfall, were analyzed using a composite analysis under the normalized polar coordinates. Results indicate that these vortices generally moved eastward and northeastward, which corresponded to the vortices’ orientation, divergence, vorticity budget, and kinetic energy budget. The evolution of long-lived DBVs featured significant unevenness: those octants located at the front and on the right side of the vortices’ moving tracks were more favorable for their development and maintenance, while those octants located at the back and on the left side acted conversely. Convergence-related shrinking was the most favorable factor for the vortices’ development and persistence, while the tilting effect was a dominant factor accounting for their attenuation. Long-lived DBVs featured strong baroclinity, and the baroclinic energy conversion acted as the main energy source for the vortices’ evolution. In contrast, the barotropic energy conversion favored the vortices’ development and maintenance at first, and later triggered their dissipation.

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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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