Abstract

This study investigates the impact of vertical wind shear (VWS) on vortex size expansion using a series of idealized numerical experiments. A control (CTL) experiment without ambient flow is compared with the experiments with 6 m s⁻¹ (SH6) and 12 m s⁻¹ (SH12) westerly VWS imposed after a 36-h vortex spinup. In SH6, the low-level wind field expands coherently in a quasi-axisymmetric manner, similar to that in CTL. However, SH12 exhibits a 12-h stagnation in size expansion before reexpansion. This stagnation is primarily associated with outer wind deceleration in the downshear-left (DSL) quadrant, where the vortex tilt and asymmetric convection are locked. In the DSL downwind section, the stratiform precipitation develops, impeding vortex tilt precession and increasing tilt magnitude. Diabatic cooling related to the stratiform precipitation enhances midlevel descending inflow (MDI), locally strengthening tangential winds in the DSL downwind section by enhancing the radial flux of absolute vorticity. As a result, a sharp azimuthal wind gradient forms to generate negative tangential advection of tangential wind that decelerates quadrant-averaged wind speed and halts overall size expansion. Later, the low-level radial outflow in the upshear quadrants deflects the MDI-induced descending parcels outward, limiting their return to the DSL updraft region and thus weakening the convection, stratiform precipitation, and MDI. As the stratiform precipitation and MDI dissipate, the wind field asymmetry diminishes, and the vortex tilt precession and size expansion resume. These results reveal that strong VWS-induced asymmetric convection can not only impede tropical cyclone (TC) intensification but also temporarily halt size expansion.