Abstract
The Northwest Pacific tropical cyclone (TC) intensification is classified into rapid intensification (RI), normal intensification (NI), and slow intensification (SI) categories. The initial location and intensity, the preceding intensity change, the motion direction, the occurrence month, and the intensification duration time are all found to differ for RI cases compared with NI and SI cases. The dependence of RI, NI, and SI on environmental conditions is further examined statistically by using the intensification rates of named TCs, for the 21-year period 1995–2015, obtained from JTWC best track data, and the environmental conditions derived from the ERA-Interim reanalysis data and GODAS high-resolution global ocean analysis data. It was found that deep-layer and upper-mid vertical wind shear (VWS), upper-level outflow, sea surface temperature (SST), and ocean heat content (OHC) are statistically different among RI, NI, and SI both before and during intensification. RI is enhanced by weaker and decreasing VWS, warmer oceans, and stronger and increasing outflow. In contrast, SI typically occurs with larger and increasing VWS, cooler oceans, and weaker, static outflow. The impacts of low-level VWS and net moisture inflow are only significantly different between RI and SI and between NI and SI, but not between RI and NI. Another key finding is that increased upper-level outflow and decreased VWS are important precursors and hence are possible predictors, of RI onset. The direction of upper-level outflow affects TC intensification, with NW and NE outflow being more favorable for TC RI than SE and SW outflow.
Highlights
E Northwest Pacific tropical cyclone (TC) intensification is classified into rapid intensification (RI), normal intensification (NI), and slow intensification (SI) categories. e initial location and intensity, the preceding intensity change, the motion direction, the occurrence month, and the intensification duration time are all found to differ for RI cases compared with NI and SI cases. e dependence of RI, NI, and SI on environmental conditions is further examined statistically by using the intensification rates of named TCs, for the 21-year period 1995–2015, obtained from Joint Typhoon Warning Center (JTWC) best track data, and the environmental conditions derived from the ERA-Interim reanalysis data and GODAS high-resolution global ocean analysis data
Hendricks et al [7] classified the intensification into two categories: rapidly intensifying and intensifying and showed that the environmental conditions, including sea surface temperature (SST), and other environmental parameters associated with these two categories are similar. e present study further explores the relationship between environmental conditions and TC intensification rate (IR) and, in contrast to previous studies, classifies the IR into the three categories of RI, NI, and SI. e main motivation for this study is toward addressing the need for, and challenges of, distinguishing between the various intensification phases present during the operational forecast period
For mid-low VWS500–850, consistent significant differences exist between RI and SI, but the differences between RI and NI are significant only starting 12 hours before the onset of the intensification and during the intensification. ese e typhoon center of RI
Summary
Academic Editor: Budong Qian e Northwest Pacific tropical cyclone (TC) intensification is classified into rapid intensification (RI), normal intensification (NI), and slow intensification (SI) categories. e initial location and intensity, the preceding intensity change, the motion direction, the occurrence month, and the intensification duration time are all found to differ for RI cases compared with NI and SI cases. e dependence of RI, NI, and SI on environmental conditions is further examined statistically by using the intensification rates of named TCs, for the 21-year period 1995–2015, obtained from JTWC best track data, and the environmental conditions derived from the ERA-Interim reanalysis data and GODAS high-resolution global ocean analysis data. It was found that deep-layer and upper-mid vertical wind shear (VWS), upper-level outflow, sea surface temperature (SST), and ocean heat content (OHC) are statistically different among RI, NI, and SI both before and during intensification. E present study seeks to address an important, but as yet unanswered, question: “Do significant differences in environmental factors, including atmospheric conditions and ocean heat conditions, exist between rapidly, normally, and slowly intensifying TCs in the NWP?” In this study, a range of environmental factors is examined, including vertical wind shear (deep-layer, upper-mid, mid-low, and low-level), net moisture inflow, upper-level outflow (strength and direction), and ocean heat conditions (both SST and ocean heat content (OHC)). Earlier, limited studies of the NWP TC basin either examined different TC categories [14] or focused only on one environmental factor [12]. The VWS between different vertical levels are used to analyze the impact of VWS on the three TC intensification rates, RI, NI, and SI
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