An observational analysis of tropical cyclogenesis in the Western North Pacific

Abstract

High-resolution satellite data are used to analyze 34 tropical storms in the Western North Pacific (WNP) during the 2000 and 2001 storm seasons. Three scenarios: Tropical Cyclone Energy Dispersion (TCED), Synoptic Wavetrain/Mixed Rossby-Gravity wave (SWT/MRG) and Easterly Wave (EW) forcing, are identified as dominant synoptic-scale triggers for TC genesis. Among these 34 cases, 6 cases are associated with TCED, 11 cases are associated with SWTIMRG and 7 cases are associated with EW forcing. For the remaining cases, three presumable scenarios are proposed. Our analyses suggest that TCED has a close relationship with TC intensity and the background wind field. Not all mature TCs produce Rossby wavetrains at their wakes; and not all wavetrains lead to the formation of new TCs. The vertical divergence profile of the Rossby wavetrain has a baroclinic structure, while the vorticity profile shows an equivalent barotropic structure, penetrating from the surface to 200mb. The large-scale environmental flow plays an important role in determining whether a wavetrain can further develop into a TC. Evolution characteristics and vertical structures of easterly waves and synoptic-scale wavetrains and their roles in cyclogenesis are also investigated. Our results also show that tropical Intraseasonal Oscillation (ISO) has a significant modulation on TC formation, especially in 2000.