High resolution numerical simulation of typhoon Mujigae (2015) and analysis of vortex Rossby waves
Authors: Jiao Ya-Yin  , Ran Ling-Kun  , Li Na  , Gao Shou-Ting  , Zhou Guan-Bo  ,   Institude of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China;,   University of Chinese Academy of Sciences, Beijing 100049, China;,   National Meteorological Center, Beijing 100081, China
Published: December 2020

Mesoscale weather research and forecasting model with high resolution is used to investigate the landfall process of typhoon Mujigae (2015). The simulation well reproduces the path, intensity and rainfall of the typhoon, especially before and after the landfall. The fine thermal and dynamical structures of the typhoon Mujigae and its macroscopic characteristics of rain bands are examined with the simulation output. The rain band regions from the eyewall outward are composed of mixing rain band, secondary rain band, principal rain band and distant rain band. The lower-level inflow and upper-level outflow are observed in the eyewall. The maximum tangential wind, strong updraft and positive temperature anomaly are located in the eyewall and tilted outward with height. The convective systems in the eyewall with high radar reflectivity are much deeper than those in the principal rain band, secondary rain band and distant rain band.In order to analyze the vortex Rossby waves, the fast Fourier transform is performed to decompose the model output variables into perturbations with different wavenumbers. The vorticity perturbations in the wavenumbers 1 and 2 have significant features in the azimuthal and radial propagation. The amplitude of wavenumber 1 is larger than that of wavenumber 2, while the wavenumber 2 propagates much faster than the wavenumber 1 both in azimuthal and radial directions. The waves propagate with a speed less than 10 m/s, which are in consistent with the magnitudes of the radial velocities in spiral rain band. The amplitude of vortex Rossby waves decreases quickly beyond the stagnation radius which is about 90 km from the cyclone center. For the perturbations of wavenumbers 1 and 2, there are some intrinsic relations among the vertical vorticity, divergence and vertical velocity. The positive values of vertical vorticity with the two wavenumbers are associated with the strong reflectivity indicating deep convections. When the dipole patterns of positive vorticity in the upper level and negative vorticity in the lower level over the rainfall region are coupled with the pattern of divergence, the upper-level divergence and lower-level convergence are promoted. Then, updrafts are enhanced, which is favorable for the development of convective system and the increase of precipitation. On the other hand, the updrafts can be weakened in two cases: i) the vertical distribution of negative vorticity in the upper level and positive vorticity in the lower level is similar to the divergence distribution; ⅱ) the vertical distribution of vorticity is opposite to that of divergence. Consequently, the convective systems are inhibited and less rainfall is produced. The dynamical structures of vortex Rossby waves with wavenumbers 1 and 2 affect the development of deep convective system and precipitation in the typhoon Mujigae.