Despite considerable research, our understanding of the inner-core structures of hurricanes is very limited owing to the lack of high-resolution data. In this study, the results of a 72-h explicit simulation of Hurricane Andrew (1992) with a grid size of 6 km are examined to explore the inner-core structures of the storm during its rapid deepening stage. Based on the simulation and various diagnostic analysis, a conceptual model of the motion and structures of precipitation in the inner-core region of the storm is proposed. The thermodynamic structure and the properties of the vertical motion of the eye and their evolution during the rapid deepening stage of the simulated hurricane are analyzed.
The proposed conceptual model extends those developed in previous studies in several respects. The main circulation components include a main inflow (outflow) in the boundary layer (upper troposphere) with little radial flow in between, a divergent slantwise ascent in the eyewall, a penetrative dry downdraft at the inner edge of the eyewall, and a generally weak subsiding motion in the eye with typical warming/drying above an inversion located near an altitude of about 2-3 km. The storm deepens as the axes of these features contract, accompanied by the contraction of the eyewall and the RMW.
It is found that the inversion divides the eye of the hurricane vertically into two parts, with a deep layer of warm/ dry air above and a shallow pool of warm/moist air below. The air aloft descends at an average rate of 5 cm/s, whereas the instantaneous vertical motions tends to oscillate (with a magnitude of around 0.5 m/s) with time due to the outbreak of irregular convection in the eyewall. The air mass appears to be secluded in the eye at the early deepening stage and resides in the eye for several days. It is the slow drying/warming caused by the average weak descending motion of this air mass, partly compensated by the mixing of air detrained from the eyewall, that leads to the intensification of the storm. In contrast, the warm/moist pool consists of air from the main inflow and penetrative downdrafts, offset somewhat by the air streaming in a returning outflow into the eyewall in the lowest 2 km. It is subject to the influence of the upward heat and moisture fluxes from the underlying warm ocean. The warm/moist pool is in dynamic equilibrium in terms of both mass and energy, and appears to play an important role in supplying high-THETAe air for deep convective development in the eyewall. The penetrative dry downdraft originates from the return inflow in the upper troposphere and is enhanced by sublimative/evaporative cooling of the hydrometeors detrained from the eyewall. It penetrates to the bottom of the eye in a radially narrow zone along the slantwise inner edge of the eyewall and returns to the eyewall after mixing with warm/moist air at the bottom of the eye. Comparisons with airborne observations within other intense hurricanes and previous inner-core conceptual models confirms the above findings.