Spiral bands are mesoscale characteristics of a developing typhoon.Observational study showed that spiral bands cause strong rainfall.It is still unknown what dynamic and microphysical processes work to produce the strong rainfall within spiral bands, because it is difficult to perform an in-situ observation of a typhoon in its mature stage.Previous numerical studies have been reported the typhoon-scale distribution and structure of spiral cloud bands.Detailed three-dimensional mesoscale structure and physical processes of the typhoon spiral band, however, are little known.
In order to simulate detailed structure of a spiral band within a typhoon and to reveal its dynamic and microphysical processes, we used the Cloud Resolving Storm Simulator (CReSS)(Tsuboki and Sakakibara, 2002) in this study.CReSS is a three-dimensional cloud resolving numerical model which is formulated in the non-hydrostatic and compressible equation system with the cold rain parameterization of the microphysics.
In order to understand the mesoscale structures and physical processes of the spiral band, a numerical experiment was conducted with a fine mesh.Horizontal and minimum vertical resolutions are 1km and 100m, respectively.In the present study, Typhoon 0216 in mature stage was simulated.The result showed that the mesoscale structures of spiral bands, an eyewall and the eye were successfully simulated.We focus on the three-dimensional structure of the spiral band and its developing and maintenance processes.The maximum mixing ratio of precipitation particles was present on the outer side of the maximum of cloud mixing ratio in the radial direction and on the lee side in the tangential direction.It is shown that the structure and intensity of the spiral band were related to distributions of cloud and precipitation particles of spiral bands.For understanding developing and maintenance of the spiral band, cloud microphysical processes were examined.This showed that cold rain processes are important to intensify the rainfall.In particular, generation and growing of graupel are the most effective processes to produce the intense spiral band.