A Study On The Characteristics Of Tropical Cyclone Size Over The Western North Pacific

The observations of tropical cyclones (TCs) with at least tropical storm intensity during2001-2011over the western North Pacific are used to study the characteristics of TC size. In this study, the TC size contains inner-core and outer size, which are defined respectively as the radius of maximum wind (RMW) and the azimuthally averaged radius of gale-force wind (R17).The statistical characteristics of TC size are investigated.(1) The RMW distribution mostly ranges from20to80km and the mean RMW is47.8km, while the R17distribution mostly ranges from80to280km and the mean R17is194.2km.(2) By comparing the R17of the TCs between the western North Pacific and the North Atlantic during2004-2011, we confirm that the mean R17in the western North Pacific is larger than that in the North Atlantic.(3) The mean RMW in January is largest and then tends to decrease with month; the mean R17in October is largest and is smallest in February.(4) The smallest mean RMW (20-40km) is mostly located in the coastal waters of Taiwan, while the largest mean R17(more than250km) mostly lies in the southwest part of Japan and the Yellow Sea and Bohai Sea in China.Meanwhile, the relationship between TC size and TC intensity is studied.(1) When the TC intensity increases, the RMW tends to decrease but the R17tends to increase.(2) Comparison of the size and intensity of all TCs during their lifetime demonstrates that in the developing (weakening) stage of the TC, the RMW mostly decreases (increases) and the R17mostly increases (decreases).(3) In the developing stage of the TC, the R17is more easily affected by environmental factors than TC intensity.(4) Rapid intensification (RI) process and different intensity categories of the TC both easily affect the RMW, but they don’t easily influence the R17. Further, the relationship between TC size and environmental factors are also studied. In this study, the environmental factors include the thermodynamic factors (sea surface temperature and outflow temperature), the dynamical factors (translational speed and vertical wind shear) and the relative humidity. This paper also brings in the new empirical maximum potential intensity (MPInew) which incorporates the thermodynamic and dynamical factors of the TC. The finds are summarized as follows.(1) The thermodynamic factors hardly affect TC size.(2) The dynamical factors are negative to top90%grouped samples of the R17, but it hardly affects the RMW.(3) The MPInew are positive to top90%grouped samples of the R17, but it hardly affects the RMW, which confirms that the thermodynamic and dynamical factors have limited effect on TC size.(4) The relative humidity is a very important environmental factor and is positive to top50%grouped samples of the R17, but it hardly affects the RMW.