83rd Annual

Thursday, 13 February 2003
Observational evidence that "hot towers" are a significant and integral part of intense hurricane eyewalls
Matthew D. Eastin, Colorado State University, Fort Collins, CO
Flight-level data from 14 intense hurricanes are used to statistically examine total buoyancy (including the water-loading effects) in convective-scale updraft cores at the lower and middle levels of the eyewall region. A background reference state is, first, defined to effectively separate the symmetric and slowly-evolving low-wavenumber balanced mesoscale structures from any transient unbalanced convective-scale structures in profiles of vertical velocity, virtual potential temperature, pressure and liquid water content. Convective-scale updraft cores are then defined as significant perturbations from the background state vertical velocity.

Updraft core kinematic and buoyancy properties are summarized as distributions at four altitudes between 1.5 and 5.5 km. Median values of each distribution indicate that typical convective-scale eyewall updrafts are positively buoyant at the lower two levels and experience an approximately 35 percent increase in magnitude with height. Use of a simple updraft model reveals that observed vertical motions are much smaller than one would expect from pure buoyancy forcing, suggesting the vertical perturbation pressure gradient force acts to weaken the updrafts. Buoyant updraft cores are found to occupy less than 5 percent of the eyewall region but accomplish 20-40 percent of the total net vertical mass transport. This percentage (as well as a positive linear correlation coefficient between updraft magnitude and total buoyancy) tends to increase with height, suggesting that buoyant convective-scale updraft cores (i.e., hot towers) play a fundamental role in transporting mass and energy to upper levels as proposed by Malkus (1958), Riehl and Malkus (1960), and Malkus and Riehl (1961).

Case studies indicate that many of the buoyant updraft cores, observed at mid-levels may have originated in the low-level eye; not in the low-level inflow from outside the eyewall. These results further suggest that dynamical eye/eyewall interactions are required to generate the buoyant updrafts through outward mixing of the warm and moist low-level eye air into the relatively cool eyewall base.

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