EVOLUTION OF THE INFLOW BOUNDARY-LAYER OF HURRICANE GILBERT (1988)

On 12 September 1988 the two NOAA WP-3D aircraft conducted an experime nt in and around an intense, outer rainband located 175 km southeast o f 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 theta(e) of the inflow but in a region radially outward from the rainband. Kinematic analyses that i ncorporate both in situ and pseudo-dual-Doppler data illustrate that t he inflow is only 2 km deep and strongly divergent prior to reaching t he convective core of the band. The Doppler-derived wind fields, which compare favorably with the in situ wind fields, demonstrate that ther e is a radially outward or return flow directly above the inflow. Soun dings show that this return flow is unusually moist despite being domi nated by mesoscale descent, which contrasts the dry conditions found u nder the anvil of virtually all tropical mesoscale convective systems. A one-dimensional general structure entrainment model of the inflow l ayer, initialized with a wind field derived from the pseudo-dual-Doppl er analysis, demonstrates that the overlying return flow adds substant ial energy to the inflow via entrainment. The placement of this high-t heta(e) layer directly above the inflow is due to the circulation asso ciated with the rainband. Low convective available potential energy, h igh shear of the radial wind, and a weak cold outflow at the surface a re factors that help produce the shallow return flow. The analyses dem onstrate that significant spatial variations of the flux divergence of heat and moisture exist in the inflow to a tropical cyclone, the vari ations are closely related to the secondary circulations produced by c onvectively active rainbands, and these variations produce significant asymmetries of theta(e) within the inflow. Rainbands of this type hav e thermodynamic characteristics similar to an eyewall and may be the t ype of rainband that evolves into a convective ring.