The Role of Topography and Environmental Vertical Wind Shear in Forming the Convective Asymmetric in Typhoon Nanmadol (2011)

The intensity and structure of Typhoon Nanmadol (2011) was affected by the land mass when it passed between Luzon and Taiwan. Furthermore, Nanmadol was also embedded within the environment with high vertical wind shear during that period, which induced clearly asymmetric convection around the storm. The Weather Research and Forecasting model with finest resolution of 3.3 km was applied to perform numerical experiments to understand the effects of terrain and environmental vertical wind shear on the intensity, structure and asymmetric convection of Nanmadol when it passed these two islands. The terrain-removed sensitivity experiments were also conducted to elucidate the relative role of terrain in the forming of the asymmetric convection of the storm. Based on the simulated results, despite a little difference of timing and landfall location at Taiwan, the overall track and intensity evolution of Nanmadol are well simulated. Regarding to the asymmetric convection simulation, the active down-shear-left convection relative to storm center is well captured by the model. This asymmetric convection is consistent with the observed radar images. Further analyses indicate that the convection is originally enhanced over the down-shear side and then advected to the left of the vertical wind shear by the cyclonic circulation of the storm. Based on results from several sensitivity experiments with or without the terrain of individual or both islands, it is found that the asymmetric convection is not obviously influenced by the terrain of two islands. This result indicates that the environmental vertical wind shear plays a dominate role in forming the asymmetric convective pattern of Nanmadol. However, the existence of terrain of Luzon could slightly broadens the lower inflow, vertical motion, and upper outflow area at the down-shear side, then tends to induce more asymmetric convection of the storm.