Diagnosis of Tropical Cyclone Intensity and Structure Using Upper Tropospheric Atmospheric Motion Vectors

Abstract

 The high temporal and spatial resolutions of geostationary satellite observations achieved by recent technological advances have facilitated derivation of atmospheric motion vectors (AMVs), even in a tropical cyclone (TC) where the winds abruptly change. This study used TCs in the western North Pacific basin to investigate the ability of upper tropospheric AMVs to estimate TC intensity and structure. We first examined the relationships between the cloud-top wind fields captured by 6-hourly upper tropospheric AMVs derived from images of the Multi-functional Meteorological Satellite (MTSAT) and the surface maximum sustained wind (MSW) of the Japan Meteorological Agency best-track data for 44 TCs during 2011–2014. The correlation between the maximum tangential winds of the upper tropospheric AMVs (UMaxWinds) and MSWs was high, about 0.73, the suggestion being that the cyclonic circulation near the cloud top was intensified by the upward transport of absolute angular momentum within the TC inner core. The upper tropospheric AMVs also revealed that the mean radii of UMaxWinds and the maximum radial outflows shifted inward as the TC intensification rate became large, the implication being that low-level inflow was strong for TCs undergoing rapid intensification. We further examined the possibility of estimating the MSW by using 30-min-interval UMaxWinds derived from Himawari-8 target observations, which have been used to track TCs throughout their lifetimes. A case study using Typhoon Lionrock (1610) showed that the UMaxWinds captured changes of the cyclonic circulation near the cloud top within the inner core on a time scale shorter than one day. It was apparent that the increase of UMaxWind was associated with intensification of the TC warm core and shrinkage of UMaxWind radius. These results suggest that the Himawari-8 AMVs include useful information on TC intensification and related structural changes to support the TC intensity analysis and structure monitoring.