Climatology of Tropical Cyclone Seed Frequency and Survival Rate in Tropical Cyclones

Abstract

The relative contribution of tropical cyclone (TC) seed frequency and survival rate to TC frequency, together with factors affecting both, are investigated by focusing on monthly or interannual variations during boreal summers in 2000–2018. In this study, TC seeds are defined as weak lower-tropospheric vortices. The results show that variation in TC frequency is controlled mainly by the survival rate interannually and horizontally. Survival rates vary with environmental fields, particularly those represented by a genesis potential index and thermodynamic variables at the initial stage of corresponding TC seeds, whereas the variation in TC seed frequency is correlated with monthly mean lower-tropospheric relative vorticity and vertical velocity. These results suggest that large-scale environmental fields affect TC frequency mainly through the survival rate, and low-level vorticity and vertical motion are related to the TC seed frequency.