A case study of marine cyclogenesis near Cape Town

Abstract

A case study of southern hemispheric marine cyclogenesis near Cape Town is examined using surface and upper air observations and satellite imagery. The mesoscale cyclone formed in the cold air mass behind a larger trough and deepened by 8 hPa over a 6-h period. Simultaneously, the system increased in speed from 12 to 24 m/s^-1 as determined by satellite tracking of the comma cloud mass. NOAA 10 and Meteosat enhanced infrared imagery provided insight into the structure of convection and indicated a dry slot being advected into the vortex from the equatorwards side of the occluding low. Cyclogenesis coincided with passage of the warm sector over the edge of the Agulhas Current. Surface heat fluxes of 200 W m^-2 were sustained across the warm sector, sharpening the upper air ridge preceding the trough. In addition to surface inputs of energy, synoptic scale and baroclinic forcing of the system was conducive to cyclogenesis. At the 250 hPa level, maximum divergence and vorticity values approached 10^-4 s^-1, just west of the area of surface cyclogenesis. At the 850 hPa level, the temperature and streamline fields aligned in a small area to the southwest of Cape Town, providing an estimated thermal wind of 20 m s^-1 over the 1000–700 hPa layer. Following the passage of the vortex, a meso-high/gust front produced surface winds of 49.1 m s^-1 along the coast. Other aspects of the mesoscale circulation are presented from a network of surface weather stations near Cape Town.