Effects of Eddy Fluxes of Angular Momentum on Model Hurricane Development

Abstract

The nonlinear effects of asymmetries associated with synoptic-scale waves in which hurricanes usually form are simulated by introducing steady-state and time varying eddy fluxes of angular momentum in a parameterized way into Sundqvist's (1970) symmetric model for hurricane development. The equations are integrated numerically using different initial conditions and different distributions of the parameterized eddy fluxes. It is found that angular momentum flux convergences, with magnitudes comparable to those measured from atmospheric data, markedly accelerate hurricane development, and can initiate a model hurricane when the sea surface temperature is slightly subcritical such that the purely symmetric model fails to produce a vortex of hurricane intensity. Different distributions of the eddy flux of momentum produce different rates of growth, different final intensifies and different vortex sizes. The most effective distributions are those in which the vertical derivative of the angular momentum flux convergence is large near sea level, where it acts as a forcing function for the symmetric radial circulation, drawing moist boundary-layer air into the hurricane from the surroundings. In this way, it enhances the Ekman layer inflow, particularly at the early stages when the sea level vortex is weak. On the other hand, an angular momentum flux divergence produced by the eddies is found to suppress model hurricane development, even when the sea surface temperature is supercritical such that the purely symmetric model yields explosive hurricane growth. This is because it produces a radial circulation which opposes the Ekman layer inflow. The contributions of the different terms in the kinetic energy equation in the purely symmetric integration are compared with those in one of the integrations with an eddy flux convergence of angular momentum. The calculations reveal that the kinetic energy production and dissipation are both larger in the latter case than in the former, and that the production exceeds the dissipation by a greater amount in the latter case, leading to a larger kinetic energy tendency and thereby more explosive hurricane growth.