Asymmetric Impacts of Vertical Wind Shear on Tropical Cyclone Size Expansion Over the Western North Pacific

This study statistically investigates the impact of vertical wind shear (VWS) on the asymmetric size expansion of tropical cyclones (TCs) over the western North Pacific. Analog pairs of 24‐hr weak‐ and moderate‐VWS cases are selected to isolate VWS effects, minimizing differences in other factors that can influence TC size. All results presented are based on these analog cases. The azimuthally averaged size expansion is significantly larger in moderate‐VWS cases than in weak‐VWS ones, with a slight positive correlation between expansion and VWS magnitude in the former. Composite analysis in a shear‐relative, radius‐normalized framework reveals that weak‐VWS cases primarily experience downshear wind field expansion, whereas moderate‐VWS cases exhibit expansion both downshear and upshear. The upshear expansion is key to the greater overall size increase under moderate VWS 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. Furthermore, a stronger tangential momentum gradient—caused by higher wind speeds left of shear and lower speeds right of shear—supports increased tangential momentum advection into upshear quadrants, reinforcing upshear wind field expansion. Preliminary thermodynamic analysis highlights elevated surface latent heat flux (LHF) on the left‐of‐shear side as a key factor in upshear convection development. A positive feedback among surface wind speed, LHF, and convection further supports upshear convection development in moderate‐VWS cases.