Based on the best-track data from Japan Meteorological Agency (JMA) and the reanalysis dataset of the European Centre of Medium-Range Weather Forecasts (ECMWF) in the period between 1980 and 2013, three categories of rapid intensification (RI) processes are identified in the tropical cyclones (TCs) in the Northwestern Pacific Ocean in this study, i.e., i) both the maximum surface wind speed () and central sea-level pressure () vary dramatically in 24 hours so that and ; ii) strengthened significantly while the decrease of is comparatively slow ( and ); and iii) the increase of is less evident while decreased abruptly ( and ). Categories 1-3 occupied 60.6%, 23.1% and 16.3% of the rapid intensification cases in the Northwestern Pacific in the past 33 years, respectively.
The composite analysis shows that the Category-1 RI usually occurs under the conditions that the tropical upper-tropospheric trough locates in the near east of the storm and the western Pacific subtropical high is strong and locates in the north of the storm in the middle troposphere. The former favors the development of the upper-level "dual-channel outflow" while the later protects the storm from the intrusion of cold and dry air from the northwest of the storm. Furthermore, the low-to-mid troposphere is very humid and exhibits evident cyclonic rotational tendency in the cases of Category-1 RI. As a contrast, the large-scale background in the cases of Category-2 RI is featured with less low-to-mid-level moisture and weak cyclonic vorticity, relatively weak western Pacific subtropical high in the north and less evident tropical upper-tropospheric trough in the near east of the storm. In the storm scale, the TC experienced Category-1 RI is usually relatively strong and accompanied with nearly symmetric inner-core convection at the beginning of RI. It is hypothesized that the differences between Category-1 and 2 RI are basically ascribed to the azimuthal distribution of inner-core deep convection, which is deeply affected by the TC's large-scale environment. This hypothesis is partly approved by the idealized simulations, which show that the asymmetric inner-core diabatic heating can causes the decrease of out of step with increase of in a TC-like vortex.