Convection Initiation of a Heavy Rainfall Event in the Coastal Metropolitan Region of Shanghai on the South Side of the Meiyu Front

Accurate prediction of the convection initiation (CI) in urban areas is still a challenge. A heavy rainfall event, missed by the 9-km regional operational modeling system, occurred in the coastal urban area of the Shanghai metropolitan region (SMR) in the late morning on 28 July 2020 on the warm side to the south of the Meiyu front. In this study, observational analyses and convection-permitting simulations with a resolution of 3 km were conducted to investigate the CI mechanism of this rainfall event. The results showed that the CI was due to the interaction of urban heat island (UHI), northwesterly outflows from the Meiyu front precipitation system (MFPS), and northeasterly sea winds. First, the UHI created a lifting condition producing adiabatic cooling and the vertical moisture transport in the urban region. Then, the mesolow generated by the UHI induced and enhanced local low-level convergence near the CI region and accelerated the northwesterly outflows and the northeasterly sea winds as they converged to the UHI. The convection was triggered as a result of the strengthened low-level convergence when the enhanced northwesterly outflows and northeasterly sea winds approached the updraft zone caused by the UHI center. Sensitivity experiments with either the urban area of the SMR removed or the MFPS suppressed further revealed that the enhancement of the low-level convergence was mainly contributed by the UHI. The outflows and sea winds transported cold and moist air to the CI region and partly offset the negative contribution of the urban drying effect to the low-level relative humidity to facilitate the development of the deep moist absolute unstable layer during the CI. In addition, the MFPS also contributed to the enhancement of the northeasterly sea winds by influencing the land-sea pressure contrast on the north of the SMR.