Abstract
Two ground-based Doppler radars have been used to examine the wind fields and the internal structure of the rainband of Typhoon 8305. The rainband is located 300km to the northeast of the storm center and is embedded in a broad stratiform precipitation region.
The air flow around the rainband is nearly two-dimensional along the rainband. Composite crosssections in the radial direction from the storm center reveal the secondary circulation associated with the rainband. A convergence zone with a large outward tilt exists from the inner edge of the rainband (the edge near the storm center) at lower levels to the outer edge at middle levels. Frictional inflow air at lower levels rises at the inner edge of the rainband and a mesoscale updraft of 2m s-1 forms. A mesoscale downdraft less than 1m s-1 exists in the maximum reflectivity zone outside the updraft zone. The downdraft is thought to be produced by the drag forces and evaporation of raindrops. The convergence between the relatively cold air associated with the downdraft and the low-level warm inflow relative to the storm center produces the updraft. This cloud dynamic mechanism is thought to play the main role in maintaining the rainband.
Taking into account the band-relative air flow, the air with large θe value near the cloud base approaches from the outer region of the typhoon toward the storm center, and enters the rainband through the inner edge due to its large crossing angle of 25°. The warm air reaches the convergence zone and rises without passing through the intense rainfall area at the center of the rainband.
Detailed analysis of air flow shows that convective-scale updrafts exist in the rainband at an interval of about 5km despite the relatively uniform reflectivity pattern. One of the interesting features is the circulation center embedded within the maximum reflectivity core in the rainband. A relative vorticity within this circulation exceeds 3×10-3s-1.
The internal structure of the rainband has been compared with other observations and numerical modeling results. The propagation speed of the rainband and the surface pressure change associated with the passage of the rainband do not suggest a relationship between internal gravity waves and the rainband.
