The Organization of Convection in the Rainbands of Tropical Cyclone Laurence

Abstract

A slow-moving weak tropical cyclone passed near Darwin, Australia, in December 1990. Rainbands were observed by a Doppler weather radar and a 50-MHz wind profiler for over 24 h. The principal bands were seen to be organized on two distinct scales. Bands of stratiform precipitation formed at a radius of about 100 km from the center of the storm and moved outward at about 6 m s?1. These decayed after they moved past Darwin over land. A distinct midlevel jet extended along the bands. Within the bands, convective lines formed at regular intervals, propagated against and outward with respect to the mean flow, and acted as a partial barrier to the radial inflow. Deep, active convection was confined to these lines. The vertical motion in the convection showed a distinct acceleration above the freezing level with measured updrafts of up to 10 m s?1. The convection elevated the tropopause height over the rainband. It is hypothesized that an inertia-gravity wave propagating from near the storm eye was responsible for triggering the convection within the lines. This hypothesis, although difficult to test, accounts for the propagation characteristics of the convective lines and offers an explanation of why similar features have not been seen in more intense storms.