Presented on April 17, 2024, by

Cheng-Ku Yu
Professor and Chair
Department of Atmospheric Sciences
National Taiwan University
Taipei, Taiwan

ABSTRACT

Outer tropical cyclone rainbands (TCRs) are a concentrated region of heavy precipitation and hazardous weather within tropical cyclones (TCs). Outer TCRs pose considerable risk to human societies, but the origin of outer TCRs is a long-standing, unresolved topic in the TC research community. This study performs analyses on a total of 1029 outer TCRs identified during their formative stage by long-term radar observations collected near Taiwan from 2002 to 2019. The observations provide a robust foundation of our knowledge regarding the natural diversity of the outer TCR origin. One of the striking findings is that a dominant portion of the identified TCRs (97%) tend to be initiated locally in the outer TC region. This outer-origin dominance is in distinct contrast to numerous theoretical modeling studies of outer TCRs that propose inner-origin scenarios. Large discrepancies between these observed propagation characteristics and theoretically predicted propagation velocities of both gravity waves and vortex Rossby waves are found, suggesting that wave disturbances do not have a direct impact on the origin of the observed outer TCRs. Moreover, the observed outer TCRs appear to have a statistical preference for both a squall-line environment and cold pool propagation behavior. In particular, the initiation of faster, squall-line-like outer TCRs preferentially take place immediately adjacent to the outer boundary of prominent precipitation in the outer regions of TCs, and this preexisting outer precipitation may act as an initial, critical provider of cold pools to activate the operation of squall-line dynamics. Examining the causes of outer precipitation and clarifying its roles in contributing to the occurrence of cold pools during the preformation stage of rainbands would thus be a necessary task that would help improve our understanding of the initiation of outer TCRs.

Reference: Yu, C.- K., C.- Y. Lin and C.- H. Pun, 2023: Origin of outer tropical cyclone rainbands. Nat. Commun., 14, 7061 (2023). doi:10.1038/s41467-023-42896-x.

BIO

Prof. Yu’s reseach Interests include orographic precipitation, severe weather and mesoscale atmospheric phenomena, and structure and dynamics of tropical cyclone (frontal) rainbands.