Impacts of Small‐Scale Terrain on the Organization of an Extreme‐Rainfall‐Producing MCS Over the Southeastern Coast of China

Abstract The southeastern coast of China is a hotspot of heavy‐rain‐producing mesoscale convective systems (MCSs) strongly modulated by complex multi‐scale terrain. How small‐scale terrain influences MCSs over this region, especially the convection organization process, remains poorly understood. Here, we investigated a record‐breaking warm‐sector heavy rainfall event over the southeastern coast on 7 May 2018, marked by well‐organized echo‐training systems, using observational analyses and terrain sensitivity numerical experiments. By quantitatively separating different terrain scales, we systematically isolated the impacts of small‐scale terrain (horizontally less than ∼100 km) on MCS organization. Without small‐scale terrain, the maximum 13‐h accumulated rainfall decreased by 73.9%, mainly due to less‐organized and shorter‐lived convective cells. Although the low‐level airflow impinged on the southeastern coast obliquely rather than near‐perpendicularly as previously documented in southern coast, the small‐scale terrain here still significantly enhanced the coastal convergence and moisture accumulation through orographic blocking, thereby promoting the upscale growth of convective cells. Additionally, by increasing the low‐level and deep‐layer vertical wind shear parallel to the convection‐initiating boundary, small‐scale terrain accelerated the merging of cold pools during the early stage of organization. Furthermore, successive funnel‐shaped small hills along the southeastern coast repeatedly trapped and strengthened cold pools, profoundly facilitating the formation of a quasi‐linear MCS. These findings reinforce the crucial role of successive small‐scale hills in coastal MCSs even when the prevailing marine warm‐moist flow is not near‐perpendicular to the coastline.