Mechanisms for the isolated convections triggered by the sea breeze front and the urban heat Island

Based on the Doppler weather radar data, the reanalysis data from European Centre for Medium-Range Weather Forecasts (ECMWF), the observation data from automatic weather stations and tower-based sensors, and the high spatio-temporal resolution data from Rapid-refresh Multi-scale Analysis and Prediction System-Nowcasting (RMAPS-NOW), the triggering mechanisms for the isolated convections in the afternoon of July 18th, 2019 in Tianjin, China have been investigated. The results show that the generation and development of the isolated convections are caused by the interactions between the sea breeze front and the urban heat island, which increase the gradient of the perturbed temperature around the urban area. Thus, the disturbance of the easterly wind is strengthened during the westward movement. The ascending motion associated with the convergence, which is caused by the disturbance of the easterly wind below the height of 0.6 km, is an important dynamic triggering mechanism for the isolated convections in Tianjin urban area. Meanwhile, the interactions between the high theta(se) gradient center of the sea breeze front and the high-temperature area of the urban heat island could provide favorable thermodynamic conditions for the isolated convections. When the sea breeze moves westward and strengthens, it brings water vapor that is conducive to the development of thunderstorms, forcing the water vapor to accumulate around the urban area and, thus, providing sufficient water vapor for the generation and development of isolated thunderstorms. Moreover, the convection is triggered at the moment when the gradual increase of temperature change begins, which can provide an important reference for the release of severe convection early warning.