Relative vorticity is the major environmental factor controlling tropical cyclone intensification over the Western North Pacific

The forecasting skill of tropical cyclone (TC) intensity has improved slowly over recent decades. The focus of this study was environmental vorticity, which is one of the dominant atmospheric factors controlling TC intensity, because few previous studies have investigated the quantitative contribution of environmental vorticity to the intensification of the majority of TCs. Stepwise multiple regression was applied to correlations between TC intensity and various environmental factors during the intensification stages of 181 TCs, active over the western North Pacific during July-October of 2003-2017. Results showed that almost 71% of TCs were significantly affected by various environmental factors, with more than half (approximately 53%) being dominantly affected by environmental relative vorticity at 850 hPa (hereafter classified as VOR TCs). Comparison of the average minimum sea level pressures showed that VOR TCs were relatively stronger than all TCs significantly affected by other environmental factors, which were in turn stronger than those not significantly affected by environmental factors. Average enhancement of environmental vorticity in VOR TCs, corresponding to TC intensification, was relatively large compared with TCs significantly affected by other environmental factors. A vorticity diagnostic analysis showed that the Coriolis divergence term and vorticity divergence term were the main sources of enhancement of environmental vorticity in VOR TCs, having much larger values than other TC types. The additional enhancement of VOR TCs was generated by environmental convergence that was stronger than that in all other TCs of this study.