Contraction of the radius of maximum symmetric rotational kinetic energy during the intensification of Tropical Cyclone Khanun (2017)

Tropical cyclone (TC) Khanun in 2017 was simulated in this study by the Weather Research and Forecasting (WRF) model. The observation-validated simulation data were used to examine how asymmetric rotational and environmental flows played the roles in determining the contraction of the radius of maximum kinetic energy of symmetric rotational flow ($K_{\mathrm{\psi s}}$). The radius of maximum $K_{\mathrm{\psi s}}$ was contracted rapidly before rapid intensification (RI) and moved inward slowly, then barely moved, and moved inward slowly again during RI.

The conversion from kinetic energy of asymmetric rotational flow ($K_{\mathrm{\psi a}}$) to $K_{\mathrm{\psi s}}$ was induced by advection of asymmetric rotational tangential wind by asymmetric divergent radial wind. The conversion and convergence of inward flux of $K_{\mathrm{\psi s}}$ led to the rapid contraction before RI. During RI, in additional to horizontal and vertical flux convergence of $K_{\mathrm{\psi s}}$, the conversion from environmental kinetic energy ($K_e$) to $K_{\mathrm{\psi s}}$ through the interaction between $K_{\mathrm{\psi s}}$ and symmetric radial environmental flow was important for the second slow contraction.