A Diagnostic Study on the Formation of Double Eyewalls of Typhoon Lekima (2019) Based on POES Microwave and GOES Infrared Observations

Abstract

Microwave observations of brightness temperature (TB) from multiple polar‐orbiting operational environmental satellite imagers and infrared observations of geostationary operational environmental satellite Himawari‐8 imager are used to investigate characteristic features of secondary eyewall evolution of Typhoon Lekima (2019). It is found that microwave observations showed a longer duration time of concentric double eyewalls than Advanced Himawari Imager (AHI) infrared observations, due to a stronger cloud penetration capability of microwave than infrared observations. The AHI infrared observations with high temporal and horizontal resolutions captured several actively propagating inner spiral rainbands before the secondary eyewall formation (SEF) of Lekima (2019). They are characterized by a wavenumber‐1 asymmetry propagating radially outward and wavenumbers‐2 and ‐3 asymmetries propagating anticlockwise. The wavenumber‐2 asymmetry approximates an azimuthal phase speed of wavenumber‐2 vortex Rossby wave. Later, an outer spiral rainband experienced an obvious axisymmetrization, then merged with multiple inner spiral rainbands propagating radially outward from the primary eyewall. The stagnation radius of these inner spiral rainbands coincides with the location of the highly symmetric outer spiral rainband, which accelerated the SEF. It is suggested that the SEF of Typhoon Lekima (2019) results from a concerted activity of inner and outer spiral rainbands.