Sensitivity of the initiation of an isolated thunderstorm over the Beijing metropolitan region to urbanization, terrain morphology and cold outflows

Abstract

To gain insight into the processes leading to the convective initiation (CI) of an isolated thunderstorm near the central urban area (CUA) of the Beijing metropolitan region (BMR) on 9 August 2011, a series of numerical sensitivity experiments by removing or modifying the CUA, terrain morphology, and convectively generated cold outflows is performed. Results show that the CUA accounts for the formation of cloud condensation near the top of the mixed layer at the CI site as a result of the growing boundary‐layer depth through its urban heat island effects and urban‐induced convergence between a warm and moist southerly flow and an easterly flow on the downstream side. The northerly approaching cold outflows from the mesoscale convective system (MCS) facilitate the generation of northerly flows through ‘a northwestward‐concave valley’ such that the enhanced low‐level convergence with the southerly and easterly flows near the CUA could determine the timing of the outbreak of shallow clouds near the mixed‐layer top into an intense thunderstorm. If the concave valley to the northwest of the CUA is eliminated, little northerly flow ahead of the cold outflow boundaries into the CI site would be present. The results obtained herein appear to have significant implications for understanding and forecasting the development of afternoon thunderstorms in the BMR under weak‐gradient environments.