Abstract:

We analyzed the characteristics of spring extratropical cyclones affecting Dalian and the physical mechanism of extreme weather caused by outbreak cyclones using conventional and intensive meteorological observation data from 2010 to 2019, the National Centers for Environmental Prediction (NCEP) reanalysis data, satellite cloud images, etc.The results show that the number of extratropical cyclones entering the Bohai and the northern Yellow Sea is 2.4 per month.Cyclones usually enter the Yellow Sea at first, and 84.5% of the extratropical cyclones (including entering the Yellow Sea directly or through the Bohai Sea) can enter the northern Yellow Sea.In spring, 73% of the cyclones entering the Bohai and the Yellow Sea can cause strong winds or precipitation in Dalian, and their effect probability on the eastern coast of Dalian is much higher than on the other areas.The outbreak cyclones causing severe disastrous weather occur most frequently in spring, with the paths moving from the southwest to the northeast.The occurrence of outbreak cyclones is mainly due to the quick decline of extratropical cyclones pressure in a short period after cyclones passing the Bohai and the Yellow Sea and their explosive development in the land of Dalian.The extratropical cyclones usually develop from the lower levels with significant frontogenesis and baroclinity.The outbreak development of the cyclones occurs in exit areas of the upper-level jets with the strongest positive vorticity advection and corresponds to the left front convergence of the low-level jets at lower altitudes.Strong cold and warm temperature advection are major reasons for extreme precipitation and strong winds.The formation of a rainstorm is mainly due to the continuous transport of warm and wet air brought by extratropical cyclones.Vertical transport of water vapor driven by strong ascending motion moistens a whole layer.When low-level jets develop and cold and warm air masses intersect, strong precipitation occurs at the moist baroclinic front with high humidity and temperature.Strong cold air from the north and the extratropical cyclones on the Bohai and the Yellow Sea contribute extremely strong air pressure gradient, which is the main reason for extremely strong winds.

Key words: The Bohai and the Yellow Sea, Extratropical cyclone, Front zone, Baroclinicity, Temperature advection