Ontogeny of dwarf tropical cyclones in the western North Pacific

Destructive tropical cyclones (TCs) have recently been observed to develop despite strong vertical wind shear and cause catastrophic damages. This study investigates the properties of dwarf (or shallow) TCs which form under strong-shear conditions (≥ 13 m s −1 ) at the time of genesis, using 444 TCs over the western North Pacific for the period 2003–2022. The TCs are stratified by vertical wind shear magnitude to explore the climatology of TCs at genesis. The dwarf TCs generally have shorter life spans and weaker peak intensities compared to the normal TCs formed in weak-shear environments. The dynamics governing the dwarf and normal TC genesis are compared by analyzing the terms in the azimuthally-averaged tangential wind tendency equation. The difference between the dwarf and normal TCs occurs in the mean terms of the equation: the mid-tropospheric mean outflow of absolute vorticity and the vertical transport of the tangential wind gradient are stronger in the dwarf TCs than the normal TCs. Ultimately, the strong inward low-tropospheric winds in dwarf TCs enhance their tangential winds, causing the near-surface wind to reach 17 m s −1 , the TC threshold.