THE ROLE OF OUTFLOW-LAYER INSTABILITIES IN TROPICAL CYCLONE MOTION

The paper examines the role of the development of outflow-layer instab ilities on the motion of tropical cyclones. The influence of barotropi c instability is examined by comparing the time changes in the storm t racks with the frequencies of free, unstable barotropic modes. For int ense vortices barotropic instability is shown to contribute to the slo w (periods of a few days) trochoidal motion of a cyclone. The developm ent of instability depends on the horizontal distribution and frequenc y of environmental forcing. The strongest response occurs when the fre quency of the forcing matches the frequency of an unstable mode.