An Observational Analysis of Environmental Effects on Tropical Cyclones Size Expansion

This study compares the wind fields and environmental conditions of Tropical cyclones (TCs) in rapid expansion (RE) and slow expansion (SE) events with similar internal features over the North Atlantic. The results indicate that the RE event occurs in an environment with stronger deep-layer vertical wind shear, higher upper-level divergence, and stronger lower-level vorticity. The analyses reveal a two-stage process in the RE event: 1) tilted vortex with downshear expansion and 2) realigned vortex with left-of-shear expansion. At 6 h after RE onset, the interaction between TCs and upper-level trough tilts the TCs and enhances upper-level divergence on the downshear side, favoring deep convection with significant inward vorticity transport there, thus causing pronounced downshear wind field expansion. By 12 h after RE onset, the TC-trough interaction shifts the maximum upper-level divergence from the downshear to left-of-shear and closer to the TC center, accompanied by the movement of precipitation towards the TC center and a reduction of vortex tilt. The vorticity transport in downshear-left and azimuthal advection of tangential wind in upshear-left jointly contribute to the left-of-shear wind field expansion. Additionally, the positive feedback among wind, surface latent heat flux, and convection facilitates the rapid size expansion in the RE event. In contrast, the weaker interaction between the TCs and upper-level westerly in the SE event accounts for a less pronounced tilt of the TCs and downshear convection, resulting in a slower expansion of the downshear wind field supported by the vorticity transport.