A Multiscale Numerical Study of Hurricane Andrew (1992). Part III: Dynamically Induced Vertical Motion

Abstract

In this study, the vertical force balance in the inner-core region is examined, through the analysis of vertical momentum budgets, using a high-resolution, explicit simulation of Hurricane Andrew (1992). Three-dimensional buoyancy- and dynamically induced perturbation pressures are then obtained to gain insight into the processes leading to the subsidence warming in the eye and the vertical lifting in the eyewall in the absence of positive buoyancy. It is found from the force balance budgets that vertical acceleration in the eyewall is a small difference among the perturbation pressure gradient force (PGF), buoyancy, and water loading. The azimuthally averaged eyewall convection is found to be conditionally stable but slantwise unstable with little positive buoyancy. It is the PGF that is responsible for the upward acceleration of high-?e air in the eyewall. It is found that the vertical motion and acceleration in the eyewall are highly asymmetric and closely related to the azimuthal distribution of radial flows in conjunction with large thermal and moisture contrasts across the eyewall. For example, the radially incoming air aloft is cool and dry and tends to suppress updrafts or induce downdrafts. On the other hand, the outgoing flows are positively buoyant and tend to ascend in the eyewall unless evaporative cooling dominates. It is also found that the water loading effect has to be included into the hydrostatic equation in estimating the pressure or height field in the eyewall. The perturbation pressure inversions show that a large portion of surface perturbation pressures is caused by the moist-adiabatic warming in the eyewall and the subsidence warming in the eye. However, the associated buoyancy-induced PGF is mostly offset by the buoyancy force, and their net effect is similar in magnitude but opposite in sign to the dynamically induced PGF. Of importance is that the dynamically induced PGF points downward in the eye to account for the maintenance of the general descent. But it points upward in the outer portion of the eyewall, particularly in the north semicircle, to facilitate the lifting of high-?e air in the lower troposphere. Furthermore, this dynamic force is dominated by the radial shear of tangential winds. Based on this finding, a new theoretical explanation, different from previously reported, is advanced for the relationship among the subsidence warming in the eye, and the rotation and vertical wind shear in the eyewall.