A Wide Rainband in a Developing Tropical Cyclone

Abstract

On 18 January 1987 the three aircraft from the Equatorial Mesoscale Experiment (EMEX) completed a mission designed to identify the mesoscale reflectivity, kinematic, and thermodynamic structure of a convectively active rainband on the leading edge of developing Tropical Cyclone Irma. The reflectivity structure, determined from land-based and aircraft radars, reveals that the band consisted of an unusually wide (80 km) region of convective clouds that did not maintain a linear organization. High-?e air entered from both sides in the subcloud and lower cloud layer. Thermodynamically, the rainband was benign in the sense that it did not produce cold downdrafts or large regions of low-?e air that would limit the energy of the low-level inflow to the developing eyewall region. Kinematically, the band was a preferred location for the enhancement of the wind component parallel to the band. The low-level flow was along rather than across the major axis of the band. Above 4.5 km flow was from the inner to the outer side of the band, but this air was also converging, which contributed to the widespread heavy rain encountered by the aircraft. The chaotic reflectivity, minor thermodynamic modification of the low-level inflow, and the tendency of the flow to be along the band are characteristics that contrast sharply with those of convectively active bands observed on the leading edge of mature tropical cyclones.