A cloud-resolving simulation of Hurricane Bob (1991): Storm structure and eyewall buoyancy

Abstract

A numerical simulation of Hurricane Bob (1991) is conducted using the Pennsylvania State University-National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5) with a horizontal grid spacing of 1.3 km on the finest nested mesh. The model produces a realistic hurricane that intensifies slowly during the period of finescale simulation. The time-averaged structure is characterized by a wavenumber-1 asymmetry with maximum low-level vertical motions and near-surface inflow in the left-front quadrant relative to the nearly aligned storm motion and mean wind shear vectors and strong outflow just above the boundary layer collocated with the updrafts. Instantaneous distributions of radial flow, vertical motion, and precipitation are strongly modified by a wavenumber-2 asymmetry that rotates cyclonically around the center at about half the speed of the mean tangential winds, consistent with the theory for vortex Rossby waves. The time-mean asymmetric vertical motion is comprised of small-scale convective updrafts that at any given time cover only a small portion of the eyewall area, but account for a majority of the updrafts mass flux, consistent with the concept of hot towers. Calculations of buoyancy indicate that eyewall updrafts are positively buoyant with respect to an environment that includes the vortex-scale warm core structure. Air parcels entering the eyewall in the boundary layer are initially accelerated upward by vertical pressure gradient forces, but once above the boundary layer, they accelerate upward because of buoyancy forces. The buoyancy is typically achieved along outward-sloping paths in a layer characterized by conditional symmetric instability. However, the strong low-level outflow above the boundary layer in the eyewall displaces rising parcels out from under the warm core so that they become unstable to vertical displacements. Consequently, the eyewall updrafts are generally the result of convective rather than symmetric instability. A key source for the buoyancy is the energy gained from surface fluxes of moisture and heat by select parcels that originate from outside of the eyewall in the lowest part of the boundary layer, penetrate furthest into the eye, and then accelerate outward sharply while rising out of the boundary layer. Occasionally, air within the eye is drawn into the eyewall updrafts, suggesting episodic rather than continuous venting of the eye air into the eyewall.