Simulation of an extratropical cyclone in the Southern Hemisphere : model sensitivity

A rapidly deepening cyclone that occurred over the South Pacific on 5 September 1987 was investigated in order to assess the possible factors contributing to its development. Cyclogenesis took place when a disturbance in the subtropics merged with a wave in the polar westerlies. Analysis revealed that the evolution of the cyclone system was associated with the interaction of a potential vorticity anomaly from the subpolar region with a subtropical surface disturbance in a manner typical of ``Class B'' cyclogenesis. As the storm intensified, the subtropical jet merged with the polar jet, producing a strong poleward heat transport characteristic of baroclinic systems. However, the absence of tilt to the frontal zone, together with weak vertical wind shear, was suggestive of a significant barotropic component to the storm. The zonal average of potential vorticity over the storm displayed large regions where the meridional gradients have different signs, indicating that the system could have developed initially by internal instabilities (barotropic and/or baroclinic) without significant external forcings. Sensitivity experiments were conducted to determine the role of surface processes in the development of the storm. It was found that development was insensitive to both surface heat fluxes and the presence of South American topography, with little change in either the circulation or kinetic energy of the storm. Intensification of the storm was substantially affected by surface frictional effects, as indicated by significant increases in the vertically averaged kinetic energy when the surface roughness was reduced. The results suggest a need to reduce the roughness heights not only over sea ice, but over the ocean in areas of strong winds as well.