Evolution of the Inflow Boundary Layer of Hurricane Gilbert (1988)

Abstract

On 12 September 1988 the two NOAA WP-3D aircraft conducted an experiment in and around an intense, outer rainband located 175 km southeast of the center of Hurricane Gilbert. Radial-height cross sections along a constant azimuth reveal a rapid and an exceptionally large increase of the equivalent potential temperature θ_e of the inflow but in a region radially outward from the rainband. Kinematic analyses that incorporate both in situ and pseudo-dual-Doppler data illustrate that the inflow is only 2 km deep and strongly divergent prior to reaching the convective core of the band. The Doppler-derived wind fields, which compare favorably with the in situ wind fields, demonstrate that there is a radially outward or return flow directly above the inflow. Soundings show that this return flow is unusually moist despite being dominated by mesoscale descent, which contrasts the dry conditions found under the anvil of virtually all tropical mesoscale convective systems. A one-dimensional general structure entrainment model of the inflow layer, initialized with a wind field derived from the pseudo-dual-Doppler analysis, demonstrates that the overlying return flow adds substantial energy to the inflow via entrainment. The placement of this high-θ_e layer directly above the inflow is due to the circulation associated with the rainband. Low convective available potential energy, high shear of the radial wind, and a weak cold outflow at the surface are factors that help produce the shallow return flow. The analyses demonstrate that significant spatial variations of the flux divergence of heat and moisture exist in the inflow to a tropical cyclone, the variations are closely related to the secondary circulations produced by convectively active rainbands, and these variations produce significant asymmetric of θ_e within the inflow. Rainbands of this type have thermodynamic characteristics similar to an eyewall and may be the type of rainband that evolves into a convective ring.