TCRs have been traditionally considered as manifestations of atmospheric waves initiated near the eyewall or close to the TC center. In this context, the convective initiation and development of TCRs are primarily govern by wave-induced disturbances and have been usually thought to be distinctly different from those of ordinary convective rainbands such as squall lines. This long-standing, traditional belief has been shaken lately by the limited research evidence showing the possibility for TCRs to develop squall-line-like characteristics in the outer environment of TCs. However, whether this appealing similarity emerges as a common or exceptional case has not been identified nowadays because only very few outer TCRs have been thoroughly studied and reported in the literature. In this study, the degree of the prevalence for this similarity is explored by radar and surface observations from a large set of 50 outer TCRs associated with 22 TCs as they approached Taiwan. The results indicate that around 58% of outer TCRs are similar to squall lines. These outer TCRs are generally characterized by convective precipitation, an obvious convergence zone between the band-relative rear-to-front flow and front-to-rear flow at low levels, either frontward or rearward tilting updrafts, and a surface cold pool signature. The frequent similarity between the outer TCRs and squall lines documented in this study provides important insights into the formation of organized, heavy precipitation associated with TCs.

Figure caption: Schematic band-normal vertical cross sections qualitatively illustrating the band-relative kinematic structures and their associated precipitation for five different airflow patterns (APs) identified from the studied TCRs. Heavy solid arrows indicate salient airflow features observed from the dual-Doppler analyses, and color shading denotes the generalized precipitation pattern associated with the studied TCRs. The number of the TCR cases and frequencies of occurrence for each AP are also indicated.

Citation: Yu, C.- K.*, C.- Y. Lin, L.- W. Cheng, J.- S. Luo, C.- C. Wu, and Y. Chen, 2018: The degree of prevalence of similarity between outer tropical cyclone rainbands and squall lines. Sci. Rep., 8, 8247, doi: 10.1038/s41598-018-26553-8. (SCI)