Doppler radar analysis of triple eyewalls in typhoon usagi (2013)

The first-ever Doppler radar observations of a triple-eyewall typhoon, the formation of a tertiary eyewall, and the subsequent ERC in Usagi is studied. The tertiary eyewall began with two rainbands spiraling out from the sec?ondary eyewall and then evolved into a nearly closed radar reflectivity ring. The axisymmetric structure and evolution of the primary and secondary circulations associated with each eyewall and moat are highlighted. The inner eyewall began to dissipate after the forma?tion of the tertiary eyewall and the rapid filamenta-tion zone located outside of the eyewall axisymmetric tangential wind maximum might not be the primary mechanism for the moat formation. Although the inner eyewall remained quite steady during the analysis period, its peak axisymmetric tangential wind began to fade after 0600 UTC as the secondary eyewall continued to contract. Each of the two moats was accompanied by a weak downdraft sandwiched by two updrafts, consistent with previous aircraft in situ and radar observations.