Diagnosis of Tropical Cyclone Intensification Using the Cloud-Top Winds Derived from Himawari-8 Super Rapid Scan Observations

The high temporal and spatial resolutions of Himawari-8 super rapid scan imagery obtained at intervals of 2.5 min enable the atmospheric motion vectors (AMVs) to be derived more suitable for capturing atmospheric flows even in a tropical cyclone (TC) where the winds abruptly change. This outstanding advantage of the super rapid scans is expected to contribute to more precise analyses of TC’s cyclonic circulation and secondary circulation, which are usually enhanced by the convection within the TC inner core. For an application of the AMVs to TC wind analysis, Oyama et al. (2017) showed that TC intensification and related structural changes can be diagnosed by using the tangential winds and radial outflows near the cloud top which were obtained by the Himawari-8 and MTSAT AMVs.

To find more general aspects of TC intensification and structural changes which are captured by the AMVs, the investigations targeting more TCs in 2015–17 are ongoing. Case studies using Typhoon Noru (1705) and other TCs indicated that the maximum tangential winds near the cloud top were highly correlated with the surface maximum sustained winds of the best-track data and were accompanied by the detailed fluctuations with time scale less than 1 h, which were considered to be related to the TC inner core dynamics. It was also found that the increase of the cloud-top outflow tended to precede the increase in the tangential winds near the cloud top, suggesting that the secondary circulation driven by the deep convection developed the cyclonic circulation vertically via the upward transport of the absolute angular momentum. In this presentation, the details of the investigation results and future issues will be presented.