Why does typhoon Noru (2022) possess severe rapid intensification as explored by the extended Sawyer–Eliassen equation?

Abstract

Severe rapid intensification (RI) of typhoon Noru (2022) occurs at a rate more than triple the conventional RI threshold. The weather research and forecasting (WRF) simulation reasonably captures the observed severe RI with pronounced inner‐core structural features, including a compact radius of maximum tangential wind (RMW) and strong eyewall diabatic heating. Solutions of the extended Sawyer–Eliassen (SE) equation indicate that the severe RI of Noru is attributed to strong boundary layer spinup by mean radial advection and strong spinup above the boundary layer by mean vertical advection in the tangential velocity tendency budget. Contributions to the partitioned SE solutions from different forcing sources indicate that the vortex spinup can be hastened, primarily resulting from rapidly increasing diabatic heating in the eyewall. The SE solutions with reconfigured diabatic heating show that the vortex spinup is enhanced at low levels when the diabatic heating is shifted slightly inward inside the RMW, and when the heating magnitude is increased. As inferred from the SE analyses, the intense diabatic heating located slightly inside the RMW associated with a compact RMW is favorable for the severe RI of Noru . In contrast, the weaker RI of typhoon Goni (2020), despite its similar RMW, can be attributed mainly to its weaker diabatic heating. The severe RI of Noru can be largely reduced as its associated diabatic heating is replaced with that of typhoon Yutu (2018) that possesses similar intense diabatic heating but at much larger radii as compared to Noru . For the larger Yutu , the spinup at low‐level eyewall, due to the inward heating shift, has resulted in a large spindown at upper levels. Severe RI thus can be more favorably induced in the presence of sufficient diabatic heating when reconfigured slightly inward inside the RMW for a smaller intense tropical cyclone as Noru .