Investigating the Environmental Characteristics of Intense Tropical Cyclones with Concentric Eyewalls over the Western North Pacific

Abstract Although it is well established that concentric eyewall (CE) formation in tropical cyclones (TCs) is closely related to their intensity, environmental factors also play nonnegligible roles. The environmental characteristics that differentiate CE TCs from non-CE TCs with intensities equal to or greater than category 4 (i.e., intense TCs) over the western North Pacific from 1999 to 2020 are investigated. CE TCs tend to move westward and experience northerly vertical wind shear (VWS), whereas non-CE TCs move slightly northward and are predominantly faced with southerly VWS. The variations in VWS direction stem from the fact that the non-CE TCs are exposed to stronger upper-tropospheric anticyclones on the eastern side. Consequently, for CE TCs, the midtropospheric storm-relative inflow at the left-of-shear side coincides with the relatively moister environmental air from the southeast side, which facilitates the development and upshear propagation of outer-core precipitation in CE TCs. Such precipitation patterns contribute to enhanced low-level tangential winds and surface fluxes in the outer-core area and foster the CE formation. Conversely, the storm-relative inflow coincides with the relatively drier environmental air from the northwest side in non-CE TCs, which suppresses the upshear propagation of outer-core precipitation. The storm motion and low-level mean flow are more likely to point to the upshear side of CE TCs than that of non-CE TCs, which also compensates for the shear-induced convective asymmetries. These findings corroborate that the development from downshear-left and upshear propagation of the outer-core spiral rainbands is crucial for CE formation.