One of the most controversial debates from recent observational analyses concerns the development of supergradient flows in the eyewall of hurricanes (e.g., Willoughby 1990, 1991; Gray 1991). In this study, the radial momentum and the absolute angular momentum (AAM) budgets are calculated using a dynamically consistent dataset from a 72-h, high-resolution (Dx = 6 km), explicit simulation of Hurricane Andrew (1992) (see Liu et al. 1997).
It is found from the radial momentum budget that the centrifugal force overcompensates the radial pressure gradient force in the sloping eyewall, leading to systematic supercyclostrophic acceleration. Adding the Coriolis force modifies little the outward (i.e., supergradient) acceleration, because this term is one order of magnitude smaller than the centrifugal force in the eyewall. The most intense supergradient acceleration exceeds 250 m s-1 h-1 and amounts to about 1/3 of the centrifugal force. It coincides with a pronounced radial outflow of greater than 9 m s-1 near the radius of maximum wind (RMW) at an altitude of 800 m. This result confirms the significant supergradient flows observed inside and near the RMW by Gray and Shea (1973), although our magnitudes are about half of theirs. Our result is also consistent with the fact that the radial outflow accelerates outward with height in the sloping eyewall until reaching the upper outflow layer and then the supergradient acceleration decreases outward. This implies that air parcels will tend to accelerate outward as they ascend in the eyewall with its maximum acceleration in the core of sloping updrafts.
To gain insight into the generation of supergradient flow, the AAM budgets are performed to examine the dominance of the centrifugal force in the eyewall. It is found that the inward horizontal advection by the low-level inflow increases the AAM, particularly so toward the RMW due to increases in both the radial and tangential winds. The magnitude of this advective process exceeds that of frictional dissipation. The horizontal advection of AAM accounts for the development of the maximum tangential wind (> 80 m s-1) near the top of the boundary layer. The vertical advection by the updrafts then transports the AAM upwards to spin up the tangential winds in the eyewall. The local centrifugal force in turn increases to cause a supergradient imbalance and the development of radial outflow in the eyewall. In the present case, the supergradient tendency could be as large as the radial pressure gradient force at the RMW.
References
Gray, W.M., 1991: Comments on ?Gradient balance in tropical cyclones?. J. Atmos. Sci., 48, 1201-1208.
Gray, W.M. and D. Shea, 1973: The hurricane?s inner-core region. Part II: Thermal stability and dynamical characteristics. J. Atmos. Sci., 30, 1544-1564.
Liu, Y., D.-L. Zhang and M.K. Yau, 1997: A multiscale numerical study of Hurricane Andrew (1992). Part I: Explicit simulation and verification. Mon. Wea. Rev., 125, 3073-3093.
Willoughby, H.E., 1990: Gradient balance in tropical cyclones. J. Atmos. Sci., 47, 265-274.
Willoughby, H.E., 1991: Reply. J. Atmos. Sci., 48, 1209-1212.