A Study on Local-Scale Thermal and Dynamical Mechanisms in the Initiation of a Squall Line Under Weak Forcing

A convective system was initiated under weak forcing on the plains near Beijing on the afternoon of 26 June 2009 and developed into a squall line that resulted in heavy precipitation. Prediction of the convective initiation (CI) of the system was challenging due to the lack of precursors detectable by conventional observation networks. In this study, we investigated the CI mechanism using high-resolution analyses obtained from the Variational Doppler Radar Analysis System via the assimilation of radar and dense surface observations. It was found that the CI was resulted from the interactions of a local cold air mass and the outflows from a weakened storm propagating from the nearby mountains. The cold air mass built up an unstable condition and veered air flow to form a local convergence zone in which moisture was accumulated. New convection was triggered as a result of strengthened convergence when the outflows from the dissipating storm approached the existing convergence zone. To confirm the critical role played by the local cold air mass, sensitivity experiments were conducted using WRF-FDDA data assimilation and forecast system. The results suggested that the local cold air mass along with the southerly environmental wind and storm outflows exerted a dominant influence on the CI.