Details of hurricane structure revealed in satellite photographs

Abstract

The satellite pictures of tropical cyclones suggest pronounced peripheral· subsidence. In many instances a clear channel or annular zone of varying width is formed as a result of this subsiding motion, at or near the rim of the high cloud shield of the storm. Outer convective bands or pre-hurricane squall lines are sometimes visible along the outer edge of the annular zones. Upper wind and time section analyses indicate that shear zones, formed or strengthened by outflow effects, are closely associated with the subsident areas. The shear zones appear to be formed as relatively warmer air diverging in the anticyclone above the storm converges with relatively cooler surrounding air. Individual stations in this study were found to have experienced cold air advection aloft prior to passage of the shear line at the 200 mb level, followed by warm air advection in the storm outflow. This resulted in the concentration of the horizontal thermal gradient near the shear zone and apparently aided in the production of an anticyclonic wind maximum in the same area. The thermal gradient may have been further strengthened due to warming of air parcels as they subsided in the annular zone.