Perspective of explosive cyclones with three shapes of upper-level potential vorticity over the northern Atlantic Ocean

The characteristics of explosive cyclones (ECs) with three shapes of upper-level potential vorticity (PV), namely “streamer”, “hook”, and “treble clef” shapes were jointly examined by composite and diagnostic approaches, by using the fifth-generation global atmospheric reanalysis data (ERA5) issued by European Centre for Medium-Range Weather Forecasts (ECMWF). Totally, 48 ECs over the northern Atlantic Ocean with typical evolution from “streamer” to “hook”, and then to “treble clef” shape of PV were selected for composite analyses. It turns out that the upper-level PV exhibits “streamer”, “hook” and “treble clef” features at early, rapid deepening and mature stages of EC, respectively. The intrinsic essences of three shapes of upper-level PV associated with different stages of EC were linked. In the upper troposphere, positive PV tendency is mainly derived from positive horizontal advection of PV (HADV), which is essentially generated by high-PV air trapped in upper-level jet and cyclonic circulation. Positive HADV is the main factor driving the evolution of upper-level PV from “streamer” to “hook”, and then to “treble clef”. In the middle troposphere, positive PV tendency is primarily associated with positive vertical advection of PV (VADV). This is due to the upper-level high-PV anomalies from stratospheric intrusion and the lower-level high-PV anomalies generated by diabatic heating both extend vertically to the middle troposphere. In the lower troposphere, diabatic heating (DIAB) overwhelmingly produce positive PV tendency. These three PV anomalies are usually vertically overlain and connected to form PV “tower” that runs through the whole troposphere. The formation of PV “tower” may strengthen cyclonic circulation, which is in turn conducive to the explosive deepening of cyclone.