Eastern Pacific Hurricanes Jimena of 1991 and Olivia of 1994: The Effect of Vertical Shear on Structure and Intensity

Abstract

Shear is a key inhibitor of tropical cyclone intensification. Although its signature is readily recognized in satellite imagery and theoretical or modeling studies provide some insight, detailed observations have been limited. Airborne radar and in situ observations in Hurricanes Jimena of 1991 and Olivia of 1994 are a step toward better understanding. Each storm was observed on two consecutive days. Initially, both had small eyes, 16?18-km radius, and maximum winds of ?57 m s?1 over sea surface temperatures (SST) >28°C in easterly environmental shear. Jimena maintained constant intensity or weakened gradually for 2 days in 13?20 m s?1 easterly shear. Olivia intensified in 8 m s?1 shear on the first day. Overnight, the shear diminished to reverse and became westerly. On the second day, Olivia weakened as the shear increased to >15 m s?1 from the west, the storm moved over cooler SST, and became surrounded by dryer air. As convection weakened and the outer rainbands ceased to be effective barriers, relative flow due to the environmental shear penetrated more deeply into the vortex core. In both storms, shear controlled the convective structure. Convection organized itself into axisymmetric rings as Olivia intensified in weak shear. When both storms encountered stronger shear, radar reflectivity and vertical motion had strong wavenumber-1 components. Highest reflectivity lay generally to the left of the shear. Most radar echoes and updrafts formed in the downshear quadrant of the storm and advected around the eye with 60%?80% of the swirling wind, consistent with vortex Rossby wave propagation. The buoyant updrafts accelerated and reflectivity increased as they passed through the left-of-shear semicircle. On the upshear side, the updrafts rose through the 0°C isotherm, and hydrometeors fell out or froze. Reflectivity declined as the echoes transformed into lower-tropospheric downdrafts overlain by glaciated upper-tropospheric updrafts in the right-of-shear semicircle. In relatively weak shear, clusters of echoes could be tracked completely around the eye. Each time the clusters passed through the downshear and left-of-shear quadrants, new echoes would form. In strong shear, all echoes were short lived, and none could be tracked around the eye. Echoes appeared downshear of the center and completed their life cycles on the left side of the shear vector where the composite reflectivities were greatest.