The Characteristics of Tropical Cyclone Formation in an Environment with Large Low-Level Low-Frequency (More than 10 days) Vorticity in the Western North Pacific

Abstract

AbstractA total of 14 tropical cyclones (TCs) that formed from 2008 to 2009 over the western North Pacific are simulated to examine the effects that environmental low-frequency and high-frequency vorticity (more than 10 days and less than 10 days, respectively) have on the formations of TCs [where the maximum surface wind ~25 kt (≈13 m s−1)]. Results show that all the simulations can reproduce the formation of a TC in an environment with a large 850-hPa low-frequency vorticity, even if the high-frequency parts are removed from the initial conditions. High-frequency vorticity mainly affects the timing and location of TC formation in such an environment. The 850-hPa vorticity is also analyzed in 3854 tropical cloud clusters that developed in 1981–2009 and may or may not have formed TCs; this reveals that the strength of the low-frequency vorticity is a crucial factor in TC formation. A tropical cloud cluster is expected to develop into a TC in an environment favorable for TC formation in the presence of a large 850-hPa low-frequency vorticity. The lead time for forecasting the formation of a TC can probably be extended under such conditions.