The roles of vertical wind shear and topography in formation of convective asymmetries in Typhoon Nanmadol (2011)

  • The vertical wind shear dominates the formation of TC convective asymmetry
  • Topography and model physics exert little influence on TC convective asymmetry
  • Shear modulates the storm’s secondary circulation thus leads to TC asymmetry

The effects of terrain and environmental vertical wind shear on the intensity, structure, and asymmetric convection of Typhoon Nanmadol (2011) were investigated using a high-resolution numerical model. Terrain-removed sensitivity experiments were conducted to elucidate the relative role of terrain in the formation of the storm’s asymmetric convection. Several sensitivity experiments were also employed to examine whether convective asymmetry formed in the simulated storm was influenced by model physics or existence of Typhoon Talas (2011). The control experiment shows that the simulations of the overall track and intensity evolution and asymmetric convection of Nanmadol were reasonably close to observations. Storm-relative composited analyses prove that environmental vertical wind shear enhances the storm’s secondary circulation (low-level inflow, upward motion, and upper-level outflow) over the downshear side, but suppresses secondary circulation over the upshear side, thus inducing asymmetric secondary circulation within the storm, the dynamical pattern of which can be explained by the superposition effect of environmental vertical wind shear. The results of sensitivity experiments indicate that the underlying terrain, the model physics, and the circulation of the Talas didn’t exert any obvious influence on the asymmetric convection and secondary circulation of the simulated storm. Therefore, the results presented here not only indicate that environmental vertical wind shear played a dominant role in forming the asymmetric convective pattern of Nanmadol, but also demonstrate that the proposed shearinduced dynamic pattern of the asymmetric secondary circulation is robust.

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