Dynamically-Induced Vertical Motion in Hurricane's Inner-Core regions

Earlier theoretical studies suggested that the weak subsidence in the eye is caused by radially outward advection of the eye air into the eyewall as a result of supergradient flows (Malkus 1958; Kuo 1959). The existence of supergradient flows has been found by Gray and Shea (1973) from a composite analysis of many radial flight data. In contrast, Smith (1980) demonstrated with an axisymmetric vortex model that the subsidence warming in the eye is mechanically driven by the decreased vertical shear in tangential winds in the eyewall as a consequence of thermal wind balance. This implicitly implies that the warm-cored eye is a result of the development of negative vertical shear in the eyewall.

In this study, we attempt to understand the vertical motion in the hurricane eye and eyewall through vertical momentum budget. This is achieved by inverting three-dimensional (3D) perturbation pressures, following Rotunno (1982), and Klemp and Rotunno (1983), in the inner-core region of a hurricane using a 72-h, fully explicit, high-resolution (Dx=6 km) simulation of Hurricane Andrew (1992) with MM5 (see Liu et al. 1997). The model reproduces well the track and intensity, the structures of the eye, eyewall, spiral rainbands, the radius of maximum winds, and other inner-core features as compared to available observations and the results of previous hurricane studies.

It is found 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 pressure-gradient force (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 half portion of the eyewall, particularly in the northern sector, to facilitate the lifting of high-qe 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 shear in the eyewall.