The Tropical Transition in the Western North Pacific: The Case of Tropical Cyclone Peipah (2007)

Abstract This study examines the transition of an extratropical disturbance to a tropical cyclone (TC), Peipah (2007), in the western North Pacific (WNP), using reanalysis and geostationary satellite data. Instead of regular diurnal fluctuations of deep convection, the pre‐TC disturbance accompanies deep convection only for short durations every other day. When the pre‐TC vorticity is traced back to 7 days prior to its formation, the traced‐back vorticity indicates a strong potential vorticity (PV) trough in the subtropical upper troposphere that originated from the midlatitude lower stratosphere. The quasi‐geostrophic forcing and reduced static stability at the leading edge of the PV trough result in the formation of an extratropical disturbance. The vertical structure of the extratropical disturbance shows maximum vorticity in the upper troposphere and cold temperature anomaly within i…