The Rapid Shrinkage in Tropical Cyclone Sizes over the Western North Pacific

This study is the first to comprehensively investigate the climatology and environmental conditions of rapid shrinkage (RS) in four tropical cyclone (TC) sizes, including radii of the outermost closed isobar (ROCI), 17.5 m s⁻¹ winds (R17), 25.7 m s⁻¹ winds (R26), and 32.9 m s⁻¹ winds (R33) over the western North Pacific (WNP) during 2004–2022. The 5th percentile of the 24-h change rate of TC size was used to define the RS in TC size, and the range between the 5th and 95th percentiles of the 24-h change rate of TC size was used to define the non-rapid change (non-RC) in TC size. The RS thresholds for the four TC sizes over the WNP were then obtained. The initial mean sizes and initial mean latitudes of the TC centers of the RS cases were larger and higher than those of the non-RC cases. RS in TC sizes mainly occurred during the decay stage, except for RS in ROCI. Composite analysis showed that environmental conditions such as sea surface temperature, temperature profile, relative humidity profile, and inward angular momentum transport of the RS cases were favorable for RS in TC size. The mean brightness temperature and mean hourly rainfall rate of RS and non-RC in R17, R26, and R33 showed that they were in a significantly more unfavorable convective environment for RS than for non-RC. The environmental conditions differed among the RS of the four TC sizes. Sea surface temperature and atmospheric conditions may cause RS in TC sizes mainly by influencing atmospheric instability, atmospheric moisture, TC precipitation, vertical motion of the TC, radial inflow, and tangential wind of the TC.