WSR-88D OBSERVATIONS OF BOW ECHOES IN OUTER RAINBANDS OF HURRICANE BERTHA

John E. Wright* and Shawn P. Bennett
National Weather Service, Carolina, Puerto Rico

1. INTRODUCTION

Numerous severe weather and bow echo events have been studied and documented in the mid-latitudes. Occurrences of bow echoes in a tropical environment, specifically the Caribbean are not well documented. On July 8, 1996 Hurricane Bertha moved west to northwest at 10 to 15 mph across the Caribbean. During the morning hours a couple of outer rainbands moved across Puerto Rico. This paper will demonstrate that the phenomena of bow echoes embedded in an outer rainband of a hurricane was: 1) observed, and 2) occurred in an environment of high storm relative helicity values, but marginal Convective Available Potential Energy (CAPE), reminiscent of mid-latitude bow echo events.

Przybylinski et al (1993) found that cool season bow echo events possessed the following features: 1) high precipitation (HP) storms frequently embedded within large rapidly moving squall lines, 2) HP storms characterized by shallow, but still strong rotating updraft centers with 50 dBZ cores only extended up to altitudes of 4-5 km.

Structures of rainbands in tropical cyclones studied by Jorgensen (1984) and Korotky et al. (1995) were found to have weak updrafts and reflectivities decreasing rapidly with height above the freezing level.

2. ENVIRONMENTAL CONDITIONS

The 1200 UTC sounding data for San Juan, Puerto Rico (SJU) were analyzed with the SHARP program. Convective indices indicated only a marginally unstable air mass with a CAPE of 681 Jkg^-¹. A previous study by McCaul (1991) on tornado environments in tropical cyclones that affected the United States found mean CAPE values of 299.8 Jkg^-¹ and Bulk Richardson Numbers (BRN) shear values of 8.4 ms^-¹ in the left front quadrant. Puerto Rico was in the left front quadrant of Hurricane Bertha, but exhibited much greater values of CAPE. Computation of BRN yielded a value of 12, which fell within the range of 10 to 40 found by Weisman and Klemp (1982, 1984) to most likely favor supercell type storms.

The hodograph for 1200 UTC 8 July 1996 was analyzed using the SHARP program. SHARP estimate of sr helicity (0-3 km, AGL) for the observed sounding was 218 m²s^-². A modified SHARP estimate of sr helicity was produced using the storm motion of 066 degrees at 34 kts. This yielded a sr helicity of 244 m²s^-². This fell within the range of 150-299 m²s^-² found by Davies-Jones et al. (1990) to favor weak tornadoes. A right moving storm in the outer rainband produced a weak (F0) tornado which caused damage as it passed over Mayaguez.

The VAD wind profile is shown in Fig. 1a-c. Examination of the wind profile shows an increase in depth of the 40 kt isotach from 1108 to 1114 UTC, but a much greater total depth (13,000 ft) increase from 1142 to 1206 UTC. Maximum estimated VAD wind speeds also increased to 55 knots at 10,000 ft by 1200 UTC. The increase in depth of the 40 kt isotach and maximum wind speeds to 55 kts nearly coincided with the passage of the leading edge of the outer rainband at San Juan. The momentum of the winds aloft was carried down to the surface. At 1213 UTC the visibility dropped to 1 3/4 nautical miles in moderate rain and the wind shifted from 330 degrees at 16 knots to 040 degrees at 26 kts with gusts to 36 kts at San Juan's Luis Munoz Marin International Airport. Later, the ASOS remark of PRESRR at 1234 UTC indicates a mesohigh was likely located on the east side of the rainband.

Figure 1a

Figure 1b

Figure 1a

Figure 1a-c. VAD Wind Profile 8 July 1996 a) 1102-1200 UTC b) 1206-1304 UTC c) 1304-1402 UTC

3. DISCUSSION

At 1200 UTC an outer rainband was over the east coast of Puerto Rico. The rainband had bow echoes embedded in it similar to mid-latitude mesoscale convective complexes. The band had sharp low-level reflectivity gradients along the leading edge and maximum reflectivities 50-54 dBZ. Rotating heads and rear inflow notches (RINs) were visible in a large broad bow. At least three rear inflow notches (RINs) appeared in the reflectivity data over the ocean north of Puerto Rico in the rainband. Rotating heads appeared to be present with the smaller scale bows and suggested the locations of mesoanticyclones. VIL values were 12 kgm-2 or less in the rainband and maximum echo tops 20,000-25,000 ft. Maximum reflectivities of 50-54 dBZ were confined to 9,000 ft or less. Maximum echo top data suggested the convection producing the bow echoes was low top in nature, generally less than 30,000 ft. A trailing band of stratiform precipitation was observed east of the bow echo.

Between 1200 UTC and 1419 UTC, the outer rainband progressed past San Juan toward western Puerto Rico. Figure 2 shows a bow echo embedded in a rainband of Hurricane Bertha as it crosses the north coast of Puerto Rico on 8 July 1996. A well defined bow echo with maximum reflectivities of 50-54 dBZ, a RIN, and subtle hook echo are visible at 1310 UTC before weakening by 1414 UTC.

Figure 2. Four-panel Base Reflectivity time series at 0.5° elevation for the period 1304UTC (upper left) - 1321UTC (lower right) on 8 July 96

Another portion of the initial rainband (north of the coast) discussed, began bowing southeast from the Atlantic toward northwest Puerto Rico. By 1419 UTC an incipient bow echo was over northwest Puerto Rico and had increased in maximum reflectivity to 50-54 dBZ. VIL values were 14 kgm^-² or less. A RIN present east of the developing bow echo at 1419 UTC expanded and progressed into the bow. From 1425 to 1431 UTC a subtle hook and, or mesocyclone was suggested by the reflectivity data near Mayaguez. Unconfirmed reports of a tornado were received from the Mayaguez area in the morning. Examination of base reflectivity data for 1425 UTC indicated a shallow core to the bow echo, 35-39 dBZ limited to below 22,000 ft. VIL values in the bow echo were 12 kgm-2 or less. Maximum echo tops were only around 20,000-25,000 ft. Other storms in the rainband north of the coast exhibited subtle bow echo features also.

Figure 3. Shows the SRM data corresponding to the low-level reflectivity data. Examination of this data confirms the existence of features normally associated with bow echoes, such as a RIN, and suggests mesocirculation features were also present in the outer rainband. Further diagnosis of SRM data for the period 1304 UTC to 1414 UTC showed weak mesocyclonic and mesoanticyclonic areas of shear associated with the bow echoes. Maximum outbound velocities of 10-21 kts and maximum inbound velocities of 22-29 kts occurred between 1316 and 1321 UTC.

Figure 3. Four-panel Storm Relative Velocity map time series at 0.5° elevation for the period 1304 - 1321UTC on 8 July 1996

From 1356 to 1414 UTC, velocities and shears gradually weakened in the original bow echo which had moved over western Puerto Rico. The acceleration of the portion of the rainband north over the Atlantic Ocean continued southeast into northwest Puerto Rico. By 1414 UTC, outbound velocities east of the new bow echo had increased from 10-21 kts to 22-29 kts. Weak subtle areas of shear began to develop on the north and south ends of the concave shaped echo.

Outbound velocities peaked at 22-29 kts over western Puerto Rico immediately east of the evolving bow echo at both 1419 and 1431 UTC. Subtle areas of meso -anticyclonic shear on the north end and mesocyclonic shear on the south end of the bow echo increased. Inbound velocities at both ends of the bow echo peaked at 22-29 kts. The radar beam was likely overshooting much of the low-level outflow air with a beam height of 9,000-10,000 ft. By 1437 UTC, the bow echo had begun to dissipate.

4. SUMMARY AND CONCLUSIONS

WSR-88D data indicate that bow echoes are more frequent in hurricane rainbands over the Caribbean than previously thought. Examination of the data revealed more than one occurrence of bow echoes in the outer rainbands of Hurricane Bertha.

Bow echoes observed in Hurricane Bertha's outer rainbands were more similar in structure and environmental conditions to that found in cool season mid-latitude bow echo events than to warm season mid-latitude events. Low echo tops and VILs, and limited vertical extent of the high reflectivity cores provided strong evidence of weak updrafts in the rainbands, which is similar to earlier findings of research on tropical cyclones. These structural features are opposite of those found for warm season bow echo events at mid-latitudes. Environmental parameters, marginal instability and higher sr helicity, also conformed to the conditions found present in cool season bow echo and tornadic hurricane events.

Finally, topography and wind direction appear to be key factors in the formation and longevity of bow echoes in Puerto Rico. Mountainous terrain and wind direction on the island likely affected storm type. Most days convection over the island appears to be pulse type storms, but the bow echoes in the outer rainbands on 8 July 1996 had features more common to HP supercells.

5. ACKNOWLEDGEMENTS

The authors wish to thank Rachel Gross for reading the manuscript and suggesting helpful changes.

6. REFERENCES

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Jorgensen, D.P., 1984: Mesoscale and convective-scale characteristics of mature hurricanes. Part 1: General observations by research aircraft. J. Atmos. Sci., 41, 1268-1285.

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