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.

The mesoscale southwest vortices (SWVs) over the Sichuan Basin during the summer of 2000–2013 were detected and tracked based on a high‐resolution reanalysis data and station observations. Four types of SWV were classified according to dynamical and thermodynamical standards, and their main features were compared. This study presents the first analysis of the universal evolution mechanisms and energy conversion characteristics of long‐lived SWVs by using the vorticity and kinetic energy budgets, as well as a new composite method under a normalized polar coordinate. Also, this study is the first work to reveal the three‐dimensional shape of SWVs.

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.