A Multiple-Grid Primitive Equation Model to Simulate the Development of an Asymmetric Hurricane (Isbell, 1964)

Abstract

A four-level primitive equation model is integrated to 96 hr. A multiple-grid system is used to increase the resolution near the center of the hurricane. The convective and nonconvective release of latent heat, surface friction, transfer of sensible and latent heat from the sea surface to air, and the variation of Coriolis parameter with latitude are incorporated in the model. The initial balanced state is derived from conventional and aircraft reconnaissance data over the Gulf of Mexico on 10 October 1964, defining a weak tropical depression. Results of the integration show that the model simulates fairly well the movement, the rate of intensification, the asymmetries in the wind field in the lower and the middle troposphere, and the banded structure in the vertical motion field which were observed in Isbell 1964. The temperature lapse curve at the center of the simulated hurricane lies close to the lapse curve that was observed in the eye of Isbell. Numerical results suggest three stages in the life cycle of the simulated hurricane-formative, storm and hurricane. In the formative stage (00?48 hr), the low-level circulation becomes well marked and extends to the middle troposphere. Appreciable warming in the middle troposphere occurs. During the period 48?72 hr, the depression intensifies into a storm. Well-marked zones of convergence form in the boundary layer. Scattered bands in the vertical motion field appear in the middle and the upper troposphere and are located at considerable distance (150 km) from the center. Intense warming in the middle and the upper troposphere takes place in the hurricane stage (72?96 hr). Realistic magnitudes of the maximum surface pressure gradient (20 mb in 37 km) and the rate of intensification (21 mb during the period 84?96 hr) are simulated. Other features of hurricanes which are realistically simulated include organized bands in the vertical motion field close to and surrounding the eye, downward motion in the eye, and the cyclonic out-flow in the upper troposphere.