Impacts of vortex intensity and outer winds on tropical cyclone size

Abstract

The present study seeks to understand how the initial vortex intensity and outer winds influence tropical cyclone (TC) size, which is defined as the azimuthally-averaged radius of the 10-m 17 m s-1 wind from the TC centre (R17), using a full baroclinic model in a quiescent f-plane environment. The initial vortex intensity is found to influence the size growth rate in the developing phase of the vortex life cycle. However, when the vortex comes to the mature phase and/or decaying phase of the vortex life cycle, the initial vortex intensity (ranges between 20 and 40 m s-1 in this study) does not strongly affect TC size. On the other hand, vortex intensification or re-intensification resulting from inner-core dynamics is apparently favourable for size growth in most instances. In addition, the lower-tropospheric outer winds of a vortex (i.e. winds beyond R17; e.g. the environmental flow around the TC) are found to be an important factor governing size change. The outer winds closer to R17 are more effective and can influence the vortex size at an earlier stage, especially if the winds are strong. The impact of the initial lower-tropospheric outer winds on the TC size evolution appears to be more prominent than that of the initial vortex intensity. The size change is much more sensitive to the outer-core, rather than inner-core, dynamics. The higher the angular momentum (AM) beyond R17, the more the AM can be brought towards the centre and hence favours size growth, and vice versa.