1. INTRODUCTION
On 8 July 1996 Hurricane Bertha's eye moved across St Thomas and St John between 1:00 and 2:00 pm. The eye of Bertha passed 40 miles northeast of San Juan, Puerto Rico at 5:00 pm. Still, the WSR-88D detected mesoscale features in the eyewall of the category 1 hurricane. Archive level IV data were reviewed for this event.
Various researchers have published articles on the structures and mesoscale features of hurricane eyewalls. Black (1993) found that intensifying hurricanes need strong, large updrafts in the eyewall to evacuate a substantial amount of mass out of the eyewall region to support low central pressure. Black found an updraft on the inside edge of the eyewall of Hurricane Hugo with a maximum of 8 ms-1 and a downdraft with a peak of 6 ms-1. Powell and Houston (1993) suggested that a large vertical vorticity component can be produced in a very small area of a hurricane. They added that wind shear is large due to the radial gradient, especially on the inward side of the eyewall wind maximum. A study of eleven hurricanes by Black et al. (1995) found the strongest upward motion is usually located upwind of the eyewall reflectivity maximum while the strongest downward motion lies downwind of the maximum. Furthermore, Fujita (1993) suggested that hurricane swirls (small vortices) occurred in the vicinity of Hurricane Andrew's eyewall where sustained winds decreased rapidly toward the eye. Fujita speculated the swirls occurred beneath the active convective towers resulting in fields of large vorticity and convergence. Marks and Houze (1984b) found in Hurricane Debby at altitudes of 2 to 4 km, in a portion of the developing eyewall, two mesoscale wind speed maxima and a mesoscale vortex were superimposed in the hurricane-scale circulation. Schubert and Guinn (1993) found through numerical model simulations that the polygonal eyewalls in some hurricanes are due to the barotropic instability associated with annular regions of high potential vorticity.
This study will suggest that in Hurricane Bertha some synoptic scale energy was transferred to the mesoscale, resulting in formation of meso-vortices on the inner edge of the eyewall. Then, the meso-vortices fed energy back to the synoptic scale through intensification of convection in the eyewall.
2. REFLECTIVITY DATA
Between 1811 and 1817 UTC a meso-vortex developed just north of the coast of St Thomas. It appeared as a 20-25 dBZ echo within the eye of the hurricane and moved east toward the eyewall. Figure 1 shows echoes associated with the meso-vortex circulation, which intensified to 40-44 dBZ by 1835 UTC and 45-49 dBZ by 1840 dBZ.
Figure 1. Four-panel Base Reflectivity time series at 0.5o elevation for the period 1835UTC (upper left) - 1852UTC (lower right) on 8 July 96
The meso-vortex continued to enhance convection on the east eyewall during the period 1840-1852 UTC. During this time, the areal coverage of higher reflectivities increased and the echo bowed to the north-northeast. It is suspected that a downdraft of air from the inner edge of the eyewall may have been drawn into the circulation of the meso-vortex, producing the bow shaped echo and weak reflectivity channel. Figure 2 shows the meso-vortex enhanced convection beginning to merge with other convection on the northeast side of the eyewall. Reflectivities have increased to 50-54 dBZ in the area of rotating echoes. By 1904 UTC, the merger of echoes appears complete with enhancement of reflectivities along the northeast eyewall still present. Finally, by 1909 UTC the meso-vortex circulation appears to loose definition in the reflectivity pattern on the north eyewall.
Figure 2. Four-panel Base Reflectivity time series at 0.5o elevation for the period 1858UTC (upper left) - 1909UTC (lower right) on 8 July 96
Later, 1927-1933 UTC, the reflectivity data suggests either another meso-vortex develops on the northwest eyewall or a pre-existing meso-vortex becomes better defined. During this period it has a reflectivity of 40-44 dBZ . As the echo associated with the meso-vortex moved south away from the northwest eyewall the reflectivity decreased to 35-39 dBZ by 1956 UTC. Figure 3 shows the echo of the meso-vortex circulation decreasing from a reflectivity maximum of 35-39 dBZ at 2002 UTC to 30-34 dBZ by 2019 UTC. During the period of decreasing reflectivities rotation was very apparent in the reflectivity loop. This meso-vortex appeared to break away from the west eyewall and cross the center of the eye. Such a path would temporarily cut off the source of moisture and surround the meso-vortex with drier subsiding air.
Figure 3. Four-panel Base Reflectivity time series at 0.5o elevation for the period 2002UTC (upper left) - 2019UTC (lower right) on 8 July 96
The meso-vortex circulation can be seen moving north to northwest toward the northeast eyewall at 2025 UTC in Figure 4. Another possible meso-vortex circulation appears to be present on the northwest eyewall from 2019-2031 UTC. There are indications of a hook echo on the northwest eyewall at both 2019 and 2031 UTC. As merger of the eyewall and echo associated with the meso-vortex begins at 2031 UTC, reflectivities in the meso-circulation increase to 40-44 dBZ. Areal coverage of the higher reflectivities continues to increase through 2042 UTC as the meso-vortex reaches the north inner edge of the eyewall. As noted in the previously mentioned meso-vortex, this echo pattern appeared to bow and have a weak echo channel even when reflectivities were decreasing.
Figure 4. Four-panel Base Reflectivity time series at 0.5o elevation for the period 2025UTC (upper left) - 2042UTC (lower right) on 8 July 96
3. AIR FORCE RECONNAISSANCE DATA
Figure 5 shows the maximum sustained 1-minute averaged surface winds with streamline and isotach analysis at 2200 UTC on 8 July 1996. The figure is a composite of Air Force reconnaissance data collected from 1715-2013 UTC and adjusted to the surface from 700 mb and marine surface observations from 1500-2100 UTC. The analysis time is 78 minutes after the last reflectivity panel shown in Figure 4. However, the data used in the analysis spans the entire reflectivity time series shown in Figures 1 to 4. The figure indicates that the radius of maximum winds (RMW) was located 19 nmi east-northeast of the eye center. This position of the RMW correlates well with the location of the maximum intensity of the meso-vortices (ref. Fig. 1 - Fig. 4) just before they merged with the eyewall.
Figure 5. Maximum sustained 1-minute surface winds with isotachs and streamlines at 2200 UTC 8 July 96. Courtesy NOAA/AOML Hurricane Research Division, Frank Marks, Sam Houston
4. SUMMARY AND CONCLUSIONS
At least two, and possibly four distinct cyclonically rotating meso-vortices were observed on the inner edge of the eyewall of Hurricane Bertha, while Bertha was in the process of intensification. The mesoscale circulations appeared to initiate on the north or northwest eyewall of the hurricane. Echoes associated with the meso-vortices decreased in intensity as the meso-vortices moved south along the western inner edge and east along the southern inner edge of the eyewall. Convection associated with the meso-vortices and in the eyewall was enhanced along the east and north sides of the eyewall as the circulations moved north along the eastern, and west along the northern inner edges.
Echoes associated with the meso-vortices moving east then north along the inner edge of the eyewall bore patterns similar to bow echoes. Weak echo channels were present and may have been the result of dry subsiding air being drawn into the circulation of the meso-vortices. Increases in echo intensity of the convection associated with the meso-vortices was likely due to two factors: 1) increased positive vorticity values due to the northeast trades merging with the hurricane circulation and 2) higher moisture inflow on the east and north sides of the hurricane due to inflow from the eyewall convection and the ITCZ. The loss of echo intensity by the meso-vortex which crossed the center of Bertha was probably due to the surrounding of the circulation by dry, subsiding air in the eye of Bertha. Hook echoes appeared to occur at times on the inner edge of both the north and east eyewalls. Both a tornado and an outbreak of waterspouts were reported in the U.S. Virgin islands when Bertha's eyewall was in the vicinity. Still, one must consider that the range of the eyewall of Bertha was too far away from the radar data acquisition (RDA) to detect low-level features near the storm's center. The height of the center of the radar beam in Bertha's eyewall was 8,000-10,000 ft. when the hurricane was near and north of the U.S. Virgin islands.
Nowcasts and Hurricane Local Statements may be adjusted to consider the effects of meso-vortices on the inner edge of eyewalls. The most damaging winds are likely to occur with the higher reflectivities associated with echoes of the meso-vortices and mergers. Tornadoes and waterspouts too are more likely to occur in the vicinity of the meso-vortices.
REFERENCES
Black, M. L., 1993: Comparisons of tropical cyclone intensity with eyewall vertical velocities. Preprints, 20th Conf. On Hurricanes and Tropical Meteorology, San Antonio, TX, Amer. Meteor. Soc., 520-523.
Black, M. L., R. W. Burpee, and F. D. Marks Jr., 1995: Vertical motion asymmetries in the hurricane eyewall. Preprints, 27th Conf. on Radar Meteorology. Vail, CO, Amer. Meteor. Soc., 574-576.
Fujita, T.T., 1993: Wind fields of Andrew, Omar, and Iniki, 1992. Preprints, 20th Conf. on Hurricanes and Tropical Meteorology, San Antonio, TX, Amer. Meteor. Soc., 46-49.
Guinn, T. A., and W. H. Schubert, 1993: Preprints, 20th Conf. on Hurricanes and Tropical Meteorology, San Antonio, TX, Amer. Meteor. Soc., J30-31.
Marks, F. D. Jr., and R. A. Houze, Jr., 1984b: Airborne doppler radar observations in Hurricane Debby. Bull. Amer. Meteor. Soc., 65, 569-582.
Powell, M. D., and S. Houston, 1993: Surface wind field analyses in Hurricane Andrew. Preprints, 20th Conf. on Hurricanes and Tropical Meteorology, San Antonio, TX, Amer. Meteor. Soc., 58-62.
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* Corresponding author address: John E Wright, National Weather Service
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