Abstract: Most of the heavy rainfall-producing convective systems in coastal areas of South China exhibit a nonlinear echo shape, with a broad range of convective intensity. Their internal dynamical structures remain elusive. This study focuses on two nonlinear-shaped rainstorms (referred to as “Storm1” and “Storm2”) with distinctly different convective intensities during the extreme rainfall event influencing Guangzhou in the early morning of May 7, 2017. Using the WRF-Chem-Bin model with a 3 km grid spacing coupled with a multi-layer urban canopy model and a building energy model, the study successfully simulates the evolution of the two rainstorms. The dynamical and microphysical structures of the two rainstorms are analyzed using the simulation outputs. The results are as follows. (1) Storm1 presents a circulation pattern with tower-like updrafts and strong upper-level outflows. The maximum updraft velocity of 11 m·s⁻¹ occurs at a height of 7 km, with strong riming and deposition processes occurring at about 7 km and between 9 and 10 km, respectively. The boundary layer inflows from the South China Sea, passing through the urban heat island, contain a substantial amount of convective available potential energy (CAPE), which helps generate widespread positive temperature anomalies and strong thermodynamic buoyancy inside Storm1, and thus promotes convective intensification. (2) The southerly inflow fed into Storm2 has lower CAPE. The maximum vertical velocity is about 8 m·s⁻¹, but located at approximately 3 km height. The riming and deposition processes are much weaker, while the latent heat release from condensation below the melting layer is much stronger than in Storm1. This is closely related to the closed circulation formed by the mid-to-lower-level updrafts in Storm2 and the downdrafts below 2 km at its rear. The enhanced downdraft strengthens the near-surface northerly cold outflow, forming a strong horizontal convergence center with the southerly airflow in the boundary layer. This convergence promotes the enhancement of the low-level upward motion of the rainstorm, which in turn reinforces the low-level convergence, creating a positive feedback effect that facilitates the rapid convective development.