On the relay propagation of deep convection leading to a heavy rainfall event in a weak‐wind urban environment

Abstract This study examines the convection initiation (CI) and the subsequent continuous propagation of storms from the Shanghai–Suzhou–Wuxi–Changzhou city belt, which produce an afternoon heavy rainfall event at a distance of 70–100 km downstream over a coastal region, using a 1‐km grid‐spacing cloud‐permitting simulation and observed radar reflectivity and surface data. The regional environment is characterized by light winds, and high convective available potential energy with dry air in the upper troposphere under the influences of lake and sea breezes. Results show the importance of urban effects interacting with the breezes in CI on the downwind side of the city belt. By tracing the CI of individual storms in relation to convectively generated cold outflows, we find the recurrences of old storms locally dissipating, after dropping light rainfall, and new storms forming at cold outflow boundaries of the old storms until reaching the coastal region. This new mode of continuous storm propagation is termed convection relays that are governed by cold‐pool dynamics in a conditionally unstable environment with weak flows. While some storms during their downstream relay propagation are merged into larger‐sized clusters, their convective entities still remain separated from each other. However, these clusters tend to have higher rainfall rates and longer lifespans than unmerged storms. It is found that the heavy rainfall event is produced by a few convective mergers within a linear mesoscale convective system interacting with sea breezes. A single merger can produce an hourly rainfall rate of 95 mm·hr −1 due to its quasi‐stationarity and ample moisture supply from sea breezes.