Characteristics of a backdoor cold front and associated MCS in North China during spring

A spring rainstorm which was underestimated by subjective quantitative precipitation forecast struck southern North China on 21 April 2018. The mesoscale and large scale dynamic processes of the backdoor cold front and associated mesoscale convective system (MCS) were studied using high spatial and temporal resolution observations, the fifth generation of the European Centre for Medium-Range Weather Forecasts atmospheric reanalysis and high-resolution numerical simulations. Results showed that the rainstorm was produced by the backdoor cold front. The backdoor cold front consisted of two sections, and the western one was oriented along the south-north direction while the eastern one was oriented along the east-west direction. Cold air was concentrated below 1.5 km. With the strengthening of northeasterly or easterly winds behind the front, the front moved southward, the height of cold air dam on the east side of Taihang mountain increased, and the front intensified. The rainstorm was brought by the MCS, which occurred in the horizontal wind convergence region ahead of warm air climbing along the backdoor front. The formation and maintenance of MCS occurred near the backdoor front accompanied by frontogenesis. The enhancement of the northeasterly winds behind the front resulted in rapid southward movement of the front and higher cold air dam, while the MCS was enhanced and the convective center also moved southward. The ascending motion favorable for MCS formation was mainly contributed by the resultant force of vertical pressure gradient force and buoyancy. Diagnostic analysis showed that the large value center of the resultant force was ahead of the warm and moist air climbing along the front, corresponding to the region of large horizontal equivalent potential temperature gradient. These results explain the southward movement of the convective center and the development of MCS. This work reveals the key mesoscale dynamic processes of the MCS and backdoor cold front in spring rainstorm in North China, and sheds light on the improvement of related numerical model physical processes and forecasting techniques in the future.