Dynamic Trigger and Moisture Source of Two Typical Meiyu Front Rainstorms Associated with Eastward-Moving Cloud Clusters from the Tibetan Plateau

Eastward-moving cloud clusters from the Tibetan Plateau (TP) often trigger heavy rainfall events in the Yangtze River basin in summer. Forecasting these events in an operational environment remains a challenging task. Here, dynamical diagnosis and a Lagrangian trajectory model are used to analyze the background atmospheric circulation, maintenance mechanism, and moisture transport of two Meiyu front rainstorms (MYFR) during 30 June–2 July 2016 and 17–19 June 2018 associated with eastward-moving cloud clusters from the TP. It is shown that in both cases heavy rainfall is characterized by semi-continuous rainbelts extending from the eastern TP to the Yangtze River valleys with eastward-spreading convective clouds weakening and strengthening alternately from the eastern TP to downstream regions. Following the track of positive water vapor advection, centers of positive vorticity propagate downstream through the Sichuan basin. The baroclinic thermodynamic—dynamical interaction and the barotropic non-equilibrium force work against each other in the development of the MYFR. Specifically, during the early stage of precipitation development, the barotropic non-equilibrium force dominates, while during the period of heavy precipitation the baroclinic thermodynamic—dynamical interaction dominates. The convergence associated with the baroclinic thermodynamic—dynamical interaction guarantees the persistence of heavy precipitation. Compared to the climate mean state (1988–2018), both MYFR events associated with eastward-moving cloud clusters from the eastern TP are characterized by increased moisture transport from the southwest. One of the major paths of moisture transport in both cases is along the south side of the TP, directly connected to the eastward movement of cloud clusters.