Cloud characteristics and analysis of structure and mechanism on back-bent warm front in extratropical cyclone over East Asian land

Although there are many different views on the formation and evolution processes for extratropical cyclones, the classical Norwegian cyclone model and Shapiro-Keyser cyclone model are commonly accepted. Based on FY-2E satellite images, 8 extratropical cyclone cases were studied. Then, the background, the frontal structure and mechanism of cyclones were analyzed using conventional observations, and NCEP 1°×1° reanalysis data. The results indicate the following. (1)The back-bent warm fronts of extratropical cyclones were found over East Asian land from FY-2E satellite images. (2) The extratropical cyclogenesis with back-bent warm front occurred under the conditions of a trough and ridge downward had developed at 500 hPa. This background is the same as that for a classical cyclone. (3) Four stages, such as incipient frontal cyclone, frontal fracture, frontal T-bone and bent-back front, frontal T-bone and warm seclusion were observed in the extratropical cyclone that occurred on 11-13 May 2012, and its warm core extended up to the 600 hPa level. (4) During the development of the surface cyclone, the positive vorticity centers tilted rearward with height. While weakening, they were vertically overlapped at different levels. There were frontal zones in the low-middle troposphere. (5) It was also found that cyclone would develop when the differential vorticity advection between 300 hPa and 700 hPa was positive, and the divergence caused by positive vorticity advection at 300 hPa was located above the warm advection at 700 hPa. Back-bent warm front of extratropical cyclone was accompanied by strong warm advection which were conveyed westward and southward at 700 hPa.