Abstract: Based on the conventional high-level and surface observational data and the FNL data with spatial resolution of 1°×1° from the National Centers for Environmental Prediction/National Center (NECP),the heavy rainfall event caused by an eastward-moving northwest vortex on August 17-18,2016 was diagnostically analyzed.The evolution of the northwest vortex and the physical mechanism of this heavy rainfall event were discussed in this study.The results showed that the northwest vortex is the direct system influencing on the heavy rainfall event.The intensity of this northwest vortex increases and decreases twice during its eastward movement,and the two peaks correspond to two heavy rainfall centers.The long-range transport of water vapor and energy from the typhoon "Dianmu" is an important reason for the enhancement of northwest vortex for the initial phase,and its second phase enhancement is due to the declination of isentropic surface as the northwest vortex moves from the Inner Mongolia plateau to the North China Plain,which leads to the development of slantwise vorticity after the increase in vertical wind shear and wet baroclinic zone.The low-level vortex shifts down the Yanshan Mountain,which is favorable for the enhancement of vertical vorticity.The zonal northwest vortex can trigger heavy rainfall in Hebei region under specific conditions.

Key words: Northwest vortex, Enhancement mechanism, Typhoon, Slantwise vorticity development