SEPTEMBER 2002 BLACK ET AL. 2291 Eastern Paci®c Hurricanes Jimena of 1991 and Olivia of 1994: The Effect of Vertical Shear on Structure and Intensity M. L. BLACK,J.F.GAMACHE,F.D.MARKS JR., C. E. SAMSURY,* AND H. E. WILLOUGHBY NOAA/AOML/Hurricane Research Division, Miami, Florida (Manuscript received 16 May 2001, in ®nal form 21 January 2002) ABSTRACT Shear is a key inhibitor of tropical cyclone intensi®cation. 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 s21 over sea surface temperatures (SST) .288C in easterly environmental shear. Jimena maintained constant intensity or weakened gradually for 2 days in 13±20 m s 21 easterly shear. Olivia intensi®ed in8ms21 shear on the ®rst day. Overnight, the shear diminished to reverse and became westerly. On the second day, Olivia weakened as the shear increased to .15ms21 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 ¯ow 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 intensi®ed in weak shear. When both storms encountered stronger shear, radar re¯ectivity and vertical motion had strong wavenumber-1 components. Highest re¯ectivity 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 accel- erated and re¯ectivity increased as they passed through the left-of-shear semicircle. On the upshear side, the updrafts rose through the 08C isotherm, and hydrometeors fell out or froze. Re¯ectivity 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 re¯ectivities were greatest. 1. Introduction decreased and then became stronger again from the Eastern Paci®c Hurricanes Jimena and Olivia (Fig. 1) west-northwest as Olivia moved northward over cooler constitute a controlled experiment in the effects of ver- SST. The differences in structure and intensity under tical shear and sea surface temperature (SST) on hur- these changing conditions provide insight into environ- ricane convective structure and intensity. Both occurred mental forcing of hurricanes. in late September, Jimena in 1991 and Olivia in 1994. Three factors determine tropical cyclone structure and Jimena was observed by NOAA's WP-3D research air- intensity: the cyclones' internal dynamics, the oceanic craft, N42RF and N43RF, on the ®rst of two successive energy source, and lateral forcing by the surrounding days and by N43RF alone on the second day. While atmosphere. In the convective ring model (Willoughby monitored by the aircraft, Jimena moved westward in 1990), axisymmetric convection is localized by fric- nearly constant easterly shear over warm water. The two tional convergence near the radius of maximum wind aircraft observed Olivia on both days. Olivia, too, was (RMW). Diabatically driven angular momentum con- initially in easterly shear over warm water. The shear vergence above the frictional boundary layer accelerates the swirling wind and causes contraction of the eye. * Current af®liation: The Weather Channel, Atlanta, Georgia. Pressure falls, largely concentrated inside the RMW, stem from gradient-wind adjustment of the mass to the increasing wind. This model is well described by strictly Corresponding author address: Dr. H. E. Willoughby, NOAA/ axisymmetric solutions of the Sawyer±Eliassen equation AOML/Hurricane Research Division, 4301 Rickenbacker Cswy., Mi- ami, FL 33149. (Smith 1981; Shapiro and Willoughby 1982; Schubert E-mail: [email protected] and Hack 1982). q 2002 American Meteorological Society Unauthenticated | Downloaded 09/30/21 03:20 AM UTC 2292 MONTHLY WEATHER REVIEW VOLUME 130 FIG. 2. MPI as a function of SST based upon Emanuel's thermo- dynamic argument for tropopause temperatures of 2708, 2758, and 2808C, and upon DeMaria and Kaplan's census of most-intense At- FIG. 1. The tracks of eastern North Paci®c Hurricanes Jimena (cir- lantic hurricanes observed for each SST. Superimposed on this dia- cles show 0000 UTC position on the date indicated by plain numbers) gram are climatological or preexistent SST and min sea level pres- during Sep 1991, and Olivia (diamonds, italic numbers) during Sep sures for Hurricanes Jimena and Olivia indicated as in Fig. 1 by 1994, superimposed upon isotherms of SST for 25 Sep 1994 from diamonds and circles. Arrows show the changes in central pressure the OTIS (Optimum Thermal Interpolation System) analysis prepared and SST during the time that the hurricanes were obs. Jimena re- by the U. S. Navy's Fleet Numerical Meteorology and Oceanography mained in a nearly steady state, while Olivia strengthened and then Center. Shaded boxes indicate the times of detailed aircraft obser- weakened in response to evolving environmental shear and SST. vation. The 268C isotherm marks the Cabo San Lucas oceanic front. Contemporary Reynolds SST for Jimena have a similar pattern of isotherms, but are a fraction of a degree cooler. nearly in equilibrium with the sea surface at ;300 K and the cold reservoir is near the tropical tropopause at ;200 K. The thermodynamically determined equilib- The mirror image of convective-ring intensi®cation rium central pressure, or maximum potential intensity is vortex spindown (Eliassen and Lystad 1977) in which (MPI), is thus a function of both SST and tropopause frictionally driven convergence causes ascent at the top temperature. Tabulation of the most intense Atlantic hur- of the boundary layer, which compresses the vortex ricanes observed for a given SST (DeMaria and Kaplan tubes just above the boundary layer and induces out¯ow 1994) yields MPIs consistent with the thermodynamic that reduces the low-level swirling wind. Reasor et al. formulation (Fig. 2)Ðparticularly so when one consid- (2000) apply this idea successfully to the second day of ers the climatological correlation of a higher and colder Olivia observations. Recent results suggest that axisym- tropopause with warmer SST. metric response to heating, or lack of heating, is not the Most hurricanes fail to reach their MPI, primarily whole story. Potential vorticity production by heating because of storm-induced cooling of the sea or the in- in individual convective cells may excite vortex Rossby hibiting effects of shear. Shears greater than a threshold waves. In adiabatic models, the waves support a low- of 12.5 m s21 prevent development of tropical cyclones level eddy convergence of angular momentum that in the western North Paci®c (Zehr 1992). Conventional would reinforce the heating-induced axisymmetric con- wisdom points to vertical shear of the environmental vergence in a diabatic model (Montgomery and Kal- wind as the most widespread atmospheric in¯uence on lenbach 1997; Montgomery and Enagonio 1998). Al- tropical cyclone intensity. A contrary view accepts shear though Reasor et al. (2000) also examined the second as an inhibitor of formation, but hypothesizes weak- day of Olivia in this context with encouraging results, ening of mature tropical cyclones primarily when shear this effect does not appear to be dominant. combines with weak thermodynamic forcing (e.g., Hol- In contrast with the Sawyer±Eliassen treatment of land 1997). In the earliest explanation (Simpson and convective heating as an imposed buoyancy source, a Riehl 1958), shear was thought to inhibit intensi®cation more thermodynamic approach (Emanuel 1986) models because differential advection separated the upper warm tropical cyclones as heat engines in which moist en- anomaly from the low-level circulation. Theoretical thalpy and angular momentum are state variables linked work (Jones 1995) suggests that the vortex initially tilts through the thermal-wind relation. In a later version of downshear after a shear ¯ow is imposed. The downward this theory (Emanuel 1988, 1991), work done against projection of the upper circulation and the upward pro- surface friction balances the energy released as humid jection of the lower circulation cause the centers to circle boundary layer air rises moist adiabatically to the cold cyclonically around the midlevel center. Eventually, the upper troposphere. The heat engine's warm reservoir is rotation of the tilted axis stops, but the magnitude of Unauthenticated | Downloaded 09/30/21 03:20 AM UTC SEPTEMBER 2002 BLACK ET AL. 2293 the tilt continues to increase. It is argued that these life cycles of convection elements relative to the shear motions act to keep the vortex together through can- vector (Frank and Ritchie 2001). The simulated hurri- cellation of vertical differences of environmental ad- cane reached steady-state potential intensities weaker vection. In adiabatic vortex ¯ow, the mean axisymmetric than the MPI for the ®xed 28.58C SST. circulation follows the sloping environmental potential Another potential atmospheric interaction is intensi- temperature surfaces that maintain the environmental ®cation triggered by upper-level angular momentum shear in thermal-wind balance. This effect results in ¯uxes (e.g., Molinari et al.