A Nested Grid for a Three-Dimensional Model of a Tropical Cyclone

Abstract

A system of nested grids has been designed for the National Hurricane and Experimental Meteorology Laboratory three-level, three-dimensional model of a tropical cyclone. The inner two of the system of three nested grids move with the center of the cyclone. The nested grids are fully interacting. The nested-grid system has been tested by means of several numerical experiments. In the original series of experiments the grids are stationary and no environment current is present. Results were obtained for a 2700 km ? 2700 km domain with uniform 90 km resolution, with a 30 km grid nested within the 90 km grid, and finally, with a 10 km grid nested within the 30 and 90 km grids. The vortex decayed on the 90 km grid, grew to hurricane strength of 52 m s?1 on the 30 km grid, and grew to 78 m s?1 on the 10 km grid. The moving grids were introduced with 10 km resolution to follow the motion of the vortex from 90 to 178 h. The motion is self-induced by the asymmetry of the outflow layer. The structure of this vortex is discussed. The characteristic asymmetry of the outflow layer and the outward propagating spiral gravity wave bands that appear during the rapid growth and mature stages of the 10 km resolution experiments are shown to be the result of two different manifestations of barotropic instability. Excessive outflow occurs from the eyewall at the mid-troposphere level of the 10 km resolution experiments. This seems to be the result of neglecting the vertical transports of horizontal momentum by cumulus clouds.