Variability of Environmental Conditions for Tropical Cyclone Rapid Intensification in the Western North Pacific

How environmental conditions vary among rapidly intensifying tropical cyclones (TCs) and which factors can help offset negative factors for intensification were examined using a dataset of geostationary satellites and environmental diagnostics. The dataset contains TCs in the western North Pacific from 1995 to 2020. A cluster analysis was performed to classify different morphologies of TC cloud patterns at the onset of rapid intensification (RI). Six clusters were identified, and each cluster had a distinct set of environmental conditions. Three clusters (clusters 1, 3, and 5) had some conditions unfavorable for RI. Cluster 1 TCs were exposed to moderate vertical (850-200 hPa) shear (similar to 6 m s(-1)). Relatively high sea surface temperature, a moist environment, and movement toward environments with weak vertical shear, high equivalent potential temperature, and high ocean heat content are potential factors that resist the effects of vertical shear. Cluster 3 TCs were characterized by a large 30-kt wind radius and moderate vertical shear (1 kt approximate to 0.51 m s(-1)). Large storm size and a moist environment caused by large-scale, strong, low-level convergence are possible factors for vortex resiliency against shear. Cluster 5 TCs were located in a very dry environment. Weak vertical shear and small storm size are factors that may offset the negative effects of dry air and ocean cooling. The results suggest that in the case of RI with negative conditions for intensification, other factors can offset the negative impacts of those conditions and that suitable combinations of environmental conditions and TC structural features are important for RI.