Doppler radar investigations of the inner core of Typhoon Songda (2004): Polygonal/ elliptical eyewalls, eye contraction, and small-scale spiral bands

Inner core is one of the most important components of tropical cyclone. The most severe convective activity and devastating winds are concentrated in the inner-core region. Furthermore, the major energy generation and conversion processes take place within this region to drive the entire system. While the vorticity dynamics of inner core have been the subject of recent numerical and theoretical studies, it has yet to be explored in great observational detail. This paper documents the structural evolution of the inner core of Typhoon Songda (2004) before and during landfall on Okinawa Island. The Doppler radar for Airport Weather (DRAW) on Okinawa Island observed around the inner core over long periods because the typhoon center moves slowly enough to remain in the radar coverage. Therefore, the observations provided unique data set of high spatial and temporal resolution that has been used for analyzing the detailed inner-core structures. The major features of inner core evolution of Typhoon Songda are as follows. 1) The perturbation reflectivity field and Doppler filed reveals many small-scale spiral structures spiraling outward from the eyewall, which are approximately similar to those shown by Gall et al. (1998) for Hurricanes Andrew, Hugo, and Erin. These bands are with a cross band width of perhaps 1-2 km and length of over 100 km. Associated with the passage of spiral bands, the surface pressure perturbations and surface wind speed perturbations are identified. The trough (ridge) of the surface pressure perturbation is roughly an 180deg phase shift from the trough (ridge) of the surface wind speed perturbation. 2) Another interesting feature that has been observed in the eyewall region is the presence of polygonal and elliptical patterns in radar reflectivity. Songda revealed polygonal eyewall shapes ranging from squares to hexagons offshore, followed by an elliptical eye near the Okinawa Island. These eyewalls had rotational periods of approximately 60-70% of maximum tangential velocities. 3) Radar reflectivity increased and a large asymmetry appeared in the inner-core during the transition from polygonal to elliptical eyes. Around the same time, the tangential velocities at 917m peaked at 56.7m s-1 and the eye began to contract to 11.0 km.