Environmental Vertical Wind Shear with Hurricane Bertha (1996)

Abstract

Hurricane Bertha (1996) was influenced by vertical wind shear with highly variable direction and magnitude. The paper describes a unique method for determining the vertical tilt of a tropical cyclone vortex using satellite and aircraft data. Hurricane Bertha's vortex tracks at three levels are shown during a period of intensification just prior to landfall. During this period, the hurricane vortex becomes more closely aligned in the vertical. Changes in asymmetries of satellite infrared (IR) cold cloud areas are shown to be related to the vortex alignment. Environmental vertical shear measurements throughout Hurricane Bertha's life cycle are presented using IR cloud asymmetries and numerical model analyses. Intensification periods are associated with more symmetric IR cloud measurements. The directions of the IR cloud asymmetric orientations are compared with numerical-model-derived vertical shear directions. The changes in the vertical shear analyses are discussed with respect to observed intensity change, and other potential influences on intensity change. A trough interaction and the warm ocean influence of the Gulf Stream were observed with Hurricane Bertha. Correlation coefficients indicate weak correlations among vertical shear quantities and intensity change. Slightly higher correlations are shown with IR cloud asymmetry measurements versus numerical-model-derived vertical shear quantities.