This study examines the characteristics of tropical cyclone (TC) lightning distribution and its relationship with TC intensity and environmental vertical wind shear (VWS) over the western North Pacific. It uses data from the World Wide Lightning Location Network and operational global analysis data from National Centers for Environmental Prediction Final Analysis for 230 TCs during 2005–2017. The spatial distribution of TC lightning frequency and normalized lightning rate demonstrates that the VWS dominates the azimuthal distribution of the lightning. The flashes are active in the downshear-left side of the inner core and the downshear-right side of the outer region. TC lightning distribution for various VWS strengths and TC intensities are further investigated. As VWS increases, the flashes of lightning become more asymmetric and exhibit a higher proportion at the outer region of the downshear side. Moreover, the same features occur as TC intensity decreases. A series of composite analyses indicated that stronger TCs with weaker VWS exhibit a more compact and symmetric lightning distribution, whereas weaker TCs with stronger VWS have a more asymmetric lightning distribution. Furthermore, the TC lightning distribution and its association with TC intensity changes are also examined for three lead times. Results show that among the composite analyses of five TC intensity changes, the lightning distribution for rapid intensification type exhibits more inner-core lightning and more axisymmetric than the distributions for other categories. These features result from favorable environmental conditions comprising greater upper level divergence, sea surface temperature, maximum potential intensity, and weaker vertical wind shear.

This study investigates the size changes of tropical cyclones (TCs) traversing the Philippines based on a 37-yr statistical analysis. TC size is defined by the radius of 30-kt (≈15.4 m s−1) wind speed (R30) from the best track data of the Japan Meteorological Agency. A total of 71 TCs passed the Philippines during 1979–2015. The numbers of size increase (SI; 36) and size decrease (SD; 34) cases are very similar; however, the last 15 years have seen more SI cases (17) than SD cases (11). SI and SD cases mostly occur along northerly and southerly paths, respectively, after TCs pass the Philippines. Before landfall, SI cases have small initial sizes and weak intensities, but SD cases have larger initial sizes and stronger intensities. After landfall, most SI cases are intensifying storms, and most SD cases are nonintensifying storms. Composite analyses of vertical wind shear, absolute angular momentum flux, relative humidity, and sea surface temperature between SI and SD cases are compared. All of these values are larger in SI cases than in SD cases. Furthermore, the interdecadal difference in the ratio of the numbers of SI to SD cases reveals an unusually high number of SI cases during 2001–15. The synoptic patterns between 1979–2000 and 2001–15 are analyzed. The high SI ratio in the latter period is related to strong southwesterly wind in the south of the South China Sea that raised relative humidity, warmed the sea surface, and increased import of angular momentum flux.

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.