This study statistically investigates the impacts of vertical wind shear (VWS) on the asymmetric size expansion of tropical cyclones (TCs) over the western North Pacific. To isolate the effect of VWS, analog pairs of 24-h weak- and moderate-VWS cases are selected, minimizing differences in other factors that can influence TC size. Results show that the azimuthally averaged size expansion is significantly larger in moderate-VWS cases compared to weak-VWS ones, with a slight positive correlation between size expansion and VWS magnitude in the former. Composite analysis in a shear-relative, radius-normalized framework reveals that weak-VWS cases experience more pronounced downshear wind field expansion, whereas moderate-VWS cases exhibit expansion both downshear and upshear. The upshear expansion in moderate-VWS cases plays a key role in their greater size expansion and is linked to enhanced upshear convection and shear-induced surface wind asymmetry. Under moderate VWS, the upshear convection development is a typical response to vortex tilting and is accompanied by intensified low-level radial inflow, particularly in the upshear-left quadrant, which can facilitate radial momentum import. Additionally, a stronger tangential momentum gradient—resulting from higher wind speeds left of shear and lower speeds right of shear—can support increased tangential momentum advection across upshear quadrants, reinforcing upshear wind field expansion. Preliminary thermodynamic analysis highlights higher surface latent heat flux (LHF) on the left-of-shear side as a critical factor in enhancing upshear expansion. The positive feedback among surface wind speed, LHF, and convection promotes upshear convection development and an asymmetric wind field in moderate-VWS cases.
 

Tropical cyclone, Size expansion, Vertical wind shear