Structure of a Typhoon Rainband Observed by Two Doppler Radars

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.

The Effect of Surface Friction on the Mesoscale Organization of Cumulus Convection in Tropical Cyclones

“…In this case, the rainfall, downdraft and cold pool were located at the outside (larger radius) of the updraft. A similar situation has been observed by Ishihara et al (1986) and Tabata et al (1992). In many cases, observed rainbands were not isolated but coexisted with a few rainbands; in some cases, a merger of two rainbands was observed (Barnes et al, 1983).…”

“…Observed mesoscale components of rainbands consist of convective-scale (order of 1 km) updrafts and downdrafts (Barnes et al, 1983(Barnes et al, , 1991Ishihara et al, 1986). Such convective-scale updrafts correspond in the present experiment to each cloud (cell) generated at low-levels being advected to the upper portion of the cloud system.…”

“…Mesoscale structures with a distinct downdraft and cold pool in spiral rainbands have been reported in many observational studies (Tatehira, 1962;Barnes et al, 1983Barnes et al, , 1991Ishihara et al, 1986;Powell, 1990a,b;Tabata et al, 1992). Structures and phase velocities of the observed spiral rainbands are different from those of gravity waves.…”

“…Most observed rainbands have a vast stratiform region with significant rainfall (Barnes and Stossmeister, 1986;Barnes et al, 1991;Ishihara et al, 1986;Powell, 1990a,b;Tabata et al, 1992;Shimazu, 1997). A distinct radar reflectivity peak corresponding to a bright band is usually seen at 4-5 km height.…”

“…As mentioned above, the longer lifetime was obtained when the tangential velocity increased and frictional inflow became significant. The importance of frictional inflow on the organization of cumulus convection was also stated in observational studies (Ishihara et al, 1986;Tabata et al, 1992). However, the mechanism by which it affected the mesoscale organization has not been well understood.…”