Hurricane Vortex Motion and Evolution in a Three-Layer Model

Abstract

A three-layer multinested numerical model is used to evaluate the asymmetric evolution of a hurricane and its interaction with the large-scale environment. The model uses a compressible fluid in isentropic coordinates. In 72 h the hurricane vortex on a beta plane moves northwest at an average speed of 2.4 m s?1. In the presence of a westerly zonal wind in the upper model layer, the hurricane on an f plane moves to the southeast at an average speed of 0.9 m s?1.A series of experiments establishes that the southeastward drift in the presence of westerly shear is primarily due to the southward isentropic gradient of background potential vorticity (PV) in the middle model layer that is associated with the background temperature field. The cyclonic circulation advects low PV air southward on the west side of the vortex, inducing a negative isentropic PV anomaly to the southwest. This anomaly is associated with a wind field that advects the vortex to the southeast, just as the northward isentropic gradient of PV due to the beta effect advects the hurricane to the northwest. The northward gradient of background PV in the upper layer has little effect on the motion. The westerly wind advects upper-layer low PV outside the vortex core to the east, inducing an anticyclonic anomaly that tends to advect the middle-layer vortex to the north; this tendency is secondary to the motion. The role of vertical transports of momentum due to cumulus convection on the hurricane motion is also evaluated. Results are presented that generalize the homogenization of asymmetric absolute vorticity and oscillation in relative angular momentum (RAM) found on the beta plane in a previous study with a barotropic model. Outside the vortex core and within ?350 km of the center, the asymmetries reach a near-steady state. The middle-layer asymmetry is associated with a PV gradient that neutralizes the background gradient due to planetary vorticity or environmental temperature, thereby insulating the symmetric vortex from distortion. Horizontal fluxes in the presence of the planetary vorticity gradient tend to counteract the development of strong anticyclonic total RAM within a large circle about the vortex center.