A Numerical Study on the Characteristics of High-frequency Oscillations in the Eyewall of Tropical Cyclones

Abstract:

The characteristics of high-frequency oscillations in the eyewall of tropical cyclones (TCs) are studied through analysis of numerical model simulations. Results from the power spectrum analysis of the maximum 10-m wind speeds (MWS) show that this oscillation is significant in all of the simulated TCs, with the range of the periods in the South China Sea (SCS) is smaller than the open oceans in the western north Pacific (WNP). Sensitivity experiment shows that smaller terrain effects in the open ocean may enlarge the range of the periods. Sequences of the high-frequency oscillations are figured out by the high resolution simulation of Typhoon Hagupit (200814). In a typical cycle, the drop of density in the planetary boundary layer (PBL) is followed by an increase of convergence in PBL, which causes an increase of density, then the weakening of the convergence. The increase in convergence in the PBL causes an increase of updraft, followed by high vertical velocity (w) at high altitudes of 8-10 km, and then an increase of the MWS, and vice versa. The first three modes of the extended empirical orthogonal function (EEOF) analysis of w within the TC updraft show that the evolution of the eyewall in the high-frequency oscillations is a composite of a process similar to the classical “eyewall replacement cycles”, which fluctuate the TC intensity, and a standing wave. The comparison to the vortex Rossby waves (VRWs) and inertial gravity waves (IGWs) shows that they are different from the high-frequency oscillations in details.