A study on the mechanism for the vortex system evolution and development during the torrential rain event in North China on 20 July 2016

Using intensive auto weather station (AWS) data and Doppler radar data in Beijing, and the NCEP finally reanalysis data (1°×1°) and ERA-Interim (0.125°×0.125°) data, the structure characteristics and evolution mechanisms for the vortex that triggered the torrential rain over North China on 19-20 July 2016 are analyzed. Among the three stages of the torrential rain event, the second stage was the major one corresponding to rapid development of the vortex and heavy precipitation. Analysis of the vortex system shows that a newly formed vortex first appeared in northwestern Henan province, which was located at the northeastern area of a low pressure zone associated with the Southwest vortex in Sichun basin. The intensity of the vortex at 850 hPa and the cyclone in the surface level gradually strengthened following their movement toward North China. Meanwhile, the vortex axis tilted seriously at the beginning and gradually evolved into a nearly vertical vortex system throughout the entire troposphere. The rapid development of the low-level Vortex system was associated with the coupling of high and low level synoptic systems. Prior to the rapid development of the vortex in lower levels, the upper trough at 300-200 hPa had deepened and developed abnormally toward the south. At the initial stage, its development corresponded to the baroclinic structure caused by cold advection in its rear and warm advection in front of the trough. As the high potential vorticity (PV) in stratosphere propagated southward from higher latitude along the isentropic surface, anomalously positive PV appeared in the upper levels of the troposphere. When the abnormally positive PV overlapped with the frontal zone in the middle and lower troposphere, the vortex developed and stretched downward rapidly, and the new cyclone was induced in northwestern Henan province. The developing vortex enhanced temperature advection, and the dynamical process was favorable for the vortex or cyclone to move northeastward (the essence was the development and propagation of newborn vortex because of warm advection forcing). Vorticity in upper levels in turn strengthened. The positive feedback not only enhanced the intensity of the vortex, but also promoted its vertical structural evolution, and the vortex column evolved from obviously tilt to almost erect. The vertical velocity greatly intensified and convective activity was triggered by the strengthened vortex system. Large latent heat release accompanied with the heavy rain made the vortex to develop rapidly in the lower troposphere during the initial stage. Although convection weakened obviously after 00:00 UTC 20 July, the intensified vortex system maintained heavy rain in a large area. The positive feedback between the heavy rainfall and vortex development is not only an important mechanism for the torrential rainstorm that lasted for a long time in North China, but also a major reason for the thermal dynamic structural changes in the vortex system in the mid-troposphere with sustained latent heat release.