Evolution Laws and Conceptual Models of MCS for Different Types of Extreme Rainstorms in the Arid Region of Northwest China

This study investigates the morphological structure, propagation, and evolution of Mesoscale Convective Systems (MCSs) associated with four historical extreme rainstorm events in Ningxia (August 21, 2016; July 22, 2018; August 9, 2022; and August 24, 2024). The analysis utilizes high-resolution observational data, including the China Severe Weather Automatic Nowcasting(SWAN) radar mosaic, three local C-band radars, and 1020 regional automatic weather stations. The results classify these extreme precipitation MCSs into two distinct categories: (1) Topography-dominated Back-Building/Quasi-Stationary (BB/QS) systems characterized by a single rainstorm center (e.g., the "8·21" and "7·22" events); and (2) Composite Multi-MCS types (Training Line/Adjoining Stratiform, TL/AS and Embedded Lines, EL) characterized by dual rainstorm centers (e.g., the "8·9" and "8·24" events). The BB/QS MCS concentrates heavy rainfall along the eastern foothills of the Helan Mountains. Driven by orographic lifting and cold pool dynamics, new convective cells initiate on the southern flank and propagate northward (back-building), resulting in a quasi-stationary system. These systems exhibit deep convection with 40 dBZ echoes reaching 12 km and the 60 dBZ centroid located around 9 km (above the 0°C level). This "high-centroid, strong ice-phase" structure yields high precipitation efficiency, producing accumulated rainfall exceeding 240 mm. In contrast, the TL/AS MCS affects the eastern banks of the Yellow River. Here, convective cell motion is highly parallel to the orientation of the convective line, leading to a significant "training effect" where cells continuously regenerate upstream and propagate downstream. This mode features a lower convective centroid, with the 60 dBZ center located approximately at 4 km (below the 0°C level), indicating a typical "low-centroid, warm-rain" process that results in accumulations exceeding 200 mm. Furthermore, the EL MCS in the arid northwest region is notably modulated by the dry low-level environment, causing a marked contraction of the stratiform cloud region. Strong echoes (>40 dBZ) are generally confined below 6 km. Due to weaker updrafts, the EL phase itself produces limited rainfall (about 20 mm); its primary disaster risk stems from Nonlinear (NL) convective systems triggered during the system's evolution. Based on these findings, a conceptual evolution model for extreme rainstorm MCSs in Ningxia is established, providing a theoretical basis for monitoring and early warning of extreme precipitation in the arid regions of Northwest China.