A Statistical Analysis of the Effects of Vertical Wind Shear on Tropical Cyclone Intensity Change over the Western North Pacific

Abstract

he effect of vertical wind shear (VWS) between different pressure levels on TC intensity change is statistically analyzed based on the best track data of tropical cyclones (TCs) in the western North Pacific (WNP) from the Joint Typhoon Warning Center (JTWC) and the ECMWF interim reanalysis (ERA-Interim) data during 1981?2013. Results show that the commonly used VWS measure between 200 and 850 hPa is less representative of the attenuating deep-layer shear effect than that between 300 and 1000 hPa. Moreover, the authors find that the low-level shear between 850 (or 700) and 1000 hPa is more negatively correlated with TC intensity change than any deep-layer shear during the active typhoon season, whereas deep-layer shear turns out to be more influential than low-level shear during the remaining less active seasons. Further analysis covering all seasons exhibits that a TC has a better chance to intensify than to decay when the deep-layer shear is lower than 7?9 m s?1 and the low-level shear is below 2.5 m s?1. The probability for TCs to intensify and undergo rapid intensification (RI) increases with decreasing VWS and increasing sea surface temperature (SST). TCs moving at slow translational speeds (less than 3 m s?1) intensify under relatively weaker VWS than TCs moving at intermediate translational speeds (3?8 m s?1). The probability of RI becomes lower than that of rapid decaying (RD) when the translational speed is larger than 8 m s?1. Most TCs tend to decay when the translational speed is larger than 12 m s?1 regardless of the shear condition.