Effects of Vertical Shear on the Structure and Intensity of Hurricanes

Numerical simulations of idealized hurricanes in specified large-scale flows are performed using the Penn State/NCAR Mesoscale Model (MM5). The model is configured with a fine mesh core and explicit representation of moist processes. The storms are embedded in flows with different magnitudes and configurations of vertical wind shear, and the results are analyzed to determine how the shear affects storm structure and intensity. Particular emphasis is placed upon the manner in which shear creates asymmetric circulations within the eyewall region and upon the weakening of the storm that results when these asymmetries alter the structure of the storm's core at upper levels. Results indicate that even weak shear can have a large influence on storm intensity, though the intensity change may lag the onset of the shear by a day or more.