Doppler-Observed Eyewall Replacement, Vortex Contraction/Intensi®Cation, and Low-Level Wind Maxima

Home , Hurricane Danny (1985), Hurricane Danny (1997), Landfall

4002 MONTHLY WEATHER REVIEW VOLUME 128

The Evolution of Hurricane Danny (1997) at Landfall: Doppler-Observed Eyewall Replacement, Vortex Contraction/Intensi®cation, and Low-Level Wind Maxima

KEITH G. BLACKWELL Department of Geology, Geography, and Meteorology, University of South Alabama, Mobile, Alabama

(Manuscript received 26 April 1999, in ®nal form 9 March 2000)

ABSTRACT Danny made landfall as a minimal hurricane on the Alabama coast on 19 July 1997 after drifting over Mobile Bay for over 10 h. Danny's unusually close proximity to the Doppler radar (WSR-88D) in Mobile provided an unprecedented view of the storm's complex and dramatic evolution during a prolonged landfall event over a 1-day period. Base re¯ectivity and velocity products were combined with aircraft reconnaissance information to detail the formation of concentric eyewalls and complete evolution of an eyewall replacement cycle. This highly symmetric hurricane then underwent a rapid asymmetric transition in Mobile Bay during which a small eyewall mesovortex developed adjacent to intense convection in the western eyewall. Radar-estimated rainfall increased dramatically during the asymmetric phase. Rates exceeded 100 mm hϪ1 for nine consecutive hours west of the center while precipitation nearly vanished to the east. Changes in the distribution of precipitation corresponded with changes in the low-level wind velocity structure. A 25-h temporal composite of WSR-88D base velocities displayed axisymmetric intensi®cation and contraction of Danny's core during the eyewall replacement cycle. Later, the asymmetric phase was dominated by further contraction and intensi®cation on the west side only. In the western eyewall, a persistent boundary layer wind maximum evolved and contracted to a radius of only 10±13 km from the center. Concurrently, eastside boundary layer winds diminished as the maximum winds rose to 1±1.5-km altitude and the radius expanded. Danny reached maximum intensity during this asymmetric phase in Mobile Bay with base velocities Ͼ44 m sϪ1 (85 kt) at 600-m elevation. Eyewall contraction in the meandering storm, combined with climatologically elevated SSTs in shallow Mobile Bay, probably played signi®cant roles in Danny's continued intensi®cation there. Discrepancies arose in determining Danny's structural patterns, intensity, and evolution during landfall. In- terpretation varied depending on the type of platform used to observe the storm. The continual sampling of the storm by the nearby WSR-88D provided detail not available from aircraft data alone. Doppler base velocities in the intense westside convection were much stronger than measured at ¯ight level. Yet, the opposite was true in the storm's drier east side. Doppler radar showed that the westside base velocity maxima were con®ned to the boundary layer (600±700 m) and represented a slightly conservative estimate of maximum surface gusts recorded at Dauphin Island during the passage of Danny's convective eyewall. Thus, winds probably were even stronger at boundary layer levels below the WSR-88D's lowest scan elevation. The shallowness and persistence of Danny's boundary layer velocity maxima stressed the need for accurate wind information in the lowest few hundred meters of a tropical cyclone's eyewall for a better indication of the storm's true intensity and near-surface wind velocities.

1. Introduction fall, and storm surge. Wind and rainfall patterns may

1 vary widely from one storm to another (Foley 1995). Landfalling tropical cyclones (TCs) produce enor- Structural changes in a storm, such as those associated mous damage from the combined effects of wind, rain- with eyewall replacement cycles, topography, or vertical wind shear, may produce intensity ¯uctuations and a spatial redistribution of strongest winds and heaviest Corresponding author address: Dr. Keith G. Blackwell, Depart- rainfall. A TC's inner core may exhibit nearly sym- ment of Geology, Geography, and Meteorology, LSCB 136, Univer- metric, or strongly asymmetric, features. Data sources, sity of South Alabama, Mobile, AL 36688. such as Doppler radar and aircraft, are important for an E-mail: [email protected]. accurate analysis of the storm (Foley 1995). Because most hurricane damage occurs in the coastal zone, the 1 Tropical cyclone usage here refers to systems of tropical depres- speci®c structure and organization of the storm landfall sion through hurricane strength. is of great importance. Unfortunately, lack of data often

᭧ 2000 American Meteorological Society

Unauthenticated | Downloaded 09/30/21 08:31 PM UTC DECEMBER 2000 BLACKWELL 4003 precludes a detailed description and analysis of the surface mesoscale structure at landfall (Powell 1987, 1990; Powell et al. 1996). Reconnaissance aircraft are a primary data source during TC landfalls in the United States. These reports of storm location and intensity are considered the most reliable data source (Foley 1995) and are cost effective (Gray et al. 1991). Aircraft are nearly always scheduled into landfalling storms; recent policy changes now allow overland ¯ights (pilot's discretion) at these times. Yet slow-moving storms, meandering near the coast for 1±2 days may present logistical problems for continuous aircraft monitoring, similar to that portrayed by Willough- by (1990). Observational gaps during landfall may re- sult. Coastal radars can provide frequent information in landfalling storms (Ellsberry 1995), such as accurate FIG. 1. Track of Hurricane Danny through the Mississippi and Alabama coastal waters, Mobile Bay, and Baldwin County, Alabama, storm positioning when the eye is well developed. Air- between 2000 UTC 18 Jul and 0900 UTC 20 Jul 1997. Hurricane borne radars are mobile and can yield additional data symbols (letter referenced) denote eye positions from aircraft recon- at this and other times. The Weather Surveillance Radar- naissance vortex reports. Spur symbols (numbered) denote the track 1988 Doppler (WSR-88D) may provide mesoscale de- of an apparent EWMV within western sections of the original eye tails of wind and precipitation structure in the cyclone and adjacent to a strong EWCS over Mobile Bay. The storm track in the bay diverges as the EWMV develops to the west while the eye core and rainbands at landfall (Foley 1995). The land- becomes ill-de®ned. See Table 1 for additional information. based WSR-88D can continuously sample the inner-core features of a storm over long periods of time under certain conditions. First, WSR-88D wind estimates are This paper documents the structural evolution of Hur- possible only Ͻ100 km from the radar site (Foley 1995). ricane Danny's inner core (Ͻ40 km from the center) Radar base beam elevations2 at a 100-km distance ex- before and during landfall on the Alabama coast. The ceed 1500 m (i.e., the 850-hPa level). Much shorter nearby WSR-88D at Mobile (KMOB) observed massive ranges (Ͻ60 km) are required to observe the detailed changes in Danny's low-level wind and precipitation boundary layer structure. Also, a multihour structural during this time, including 1) an axisymmetric eyewall evolution of boundary layer winds and low-level pre- replacement and contraction, 2) development of a large cipitation near the storm center are only observed if the convective asymmetry accompanied by extreme rainfall system moves slowly enough to remain in radar range. rates, 3) further asymmetric contraction adjacent to in- Unfortunately, this combination of circumstances is tense convection, and 4) persistent wind maxima near rare. Really close approaches of slow-moving hurri- the top of the boundary layer in the heavily convective canes to WSR-88Ds are infrequent events. Typically, eyewall. Also, this study elaborates on Doppler-ob- hurricanes have forward speeds of ϳ5msϪ1 along the served low-level wind maxima that were signi®cantly north Gulf Coast, and even higher for many other U.S. stronger than maximum winds reported by aircraft at locations (Neumann and Pryslak 1981). So, TCs passing ¯ight level. Ͻ60 km from a WSR-88D rarely remain in this range Discussion of Danny's features is limited to those for more than a few hours (see Stewart and Lyons 1996), readily identi®able in archive level II WSR-88D veloc- and most are over land and weakening. Hurricane Danny ity and re¯ectivity data, U.S. Air Force Reserve (1997) is an exception. (USAFR) C-130 aircraft reconnaissance vortex reports, Danny entered Mobile Bay as a minimal hurricane and surface observations from Dauphin Island's on 19 July 1997 (Fig. 1) and produced devastating rains (DPIA1) Coastal-Marine Automated Network over southwest Alabama (Pasch 1997). Remarkably, (C-MAN)3 site. The low-level aspects of the storm in Danny's center remained Ͻ100 km from Mobile's WSR- or close to the boundary layer are emphasized. Section 88D for Ͼ48 h, and the strongly convective west eye- 2 details data and analysis techniques. The storm history wall remained Ͻ60 km from the radar for 12 h. This prior to landfall is summarized in section 3. Section 4 unique circumstance was a rare opportunity for nearly details the structural morphology of Danny's inner core continuous WSR-88D observation of a landfalling TC's precipitation and wind ®elds at landfall. Then, section core during dramatic structural changes. 5 compares maximum winds observed by WSR-88D,

2 This base elevation represents the lowest elevation within the 3 DPIA1 is the only surface observation platform with archive ca- volume scan and denotes a 0.5Њ beam angle. pability to experience Danny's landfalling eyewall.

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2. Data and analysis The WSR-88D system and product characteristics are detailed in OFCM (1992). The National Weather Service (NWS) WSR-88D at KMOB operates in a volume-scan mode with 6-min intervals between scans, and with a wavelength of 10±11 cm. The beam pulse duration is 1.66 ␮s, providing a pulse length of approximately 500 m. Pulse repetition frequencies of 1014 and 1181 sϪ1 are used in Doppler velocity calculations. The horizontal and vertical beamwidth is 0.95Њ (0.83 km at 50-km range). The WSR-88D archive level II digital data are ma- nipulated using the WSR-88D Algorithm Testing and Display System (WATADS) (NSSL 1997). The KMOB NWS of®ce is located in hills west of the city and has a station elevation of 64 m above mean sea level (MSL) (NCDC 1990). The WSR-88D antenna is adjacent to the of®ce on a 24-m tower (J. Garmon, KMOB NWS, 1999, personal communication). Thus, an 88-m antenna height is added to WSR-88D beam elevations from WA- TADS to obtain elevations above MSL. The archive level II data are complete for 25 h between 2300 UTC 18 July and 0000 UTC 20 July 1997, except for a 1-h, 20-min gap between 0909 and 1029 UTC 19 July. Conventional surface wind observations from DPIA1 FIG. 2. Hodographs produced from 1) Slidell, LA, rawinsonde in- are used to augment and verify the radar and USAFR formation at 1200 UTC 15 Jul 1997, and 2) storm-centered combination of coastal WSR-88D and research aircraft Doppler winds for reconnaissance information. The DPIA1 wind sensor is 1915±2006 UTC 18 Jul 1997. (Aircraft hodograph produced by P. located 17.4 m above MSL (NDBC 1998) and provides, Dodge, courtesy of F. Marks, NOAA/AOML/HRD.) Numbers adja- among other things, consecutive 10-min wind averages cent to symbols represent the height (km) above MSL of the wind and the hourly maximum 5-s wind gust (Meindl and measurements. Hamilton 1992). Two hodographs (Fig. 2) were generated near Danny outward 1) from the zero isodop5 through the storm's from 1) a Slidell, Louisiana, rawinsonde sounding be- center, and 2) through the inbound or outbound maxi- fore Danny reached TC status, and 2) a storm-centered mum base velocity (MBV). This radial, normal to the location where the hurricane's circulation was factored 4 radar beam at the MBV location, extended Ͼ65 km out. For the latter, the Hurricane Research Division outward (Fig. 3). Digital base velocity information and (HRD) averaged environmental winds from airborne its distance from the storm's central zero isodop were Doppler and from coastal WSR-88Ds in a 90-km circle extracted outward at 3 m sϪ1 intervals. The point where around Danny over multiple layers for 1915±2006 UTC the radial through the MBV intersects the central zero 18 July. Both hodographs depict vertical wind shear and isodop was assumed to approximate the storm's wind possible steering currents in Danny's immediate envi- center. The Doppler winds presented only an incomplete ronment. two-dimensional picture of inner-core velocities and The KMOB WSR-88D provided near-surface re¯ec- failed to provide meaningful winds where the ¯ow was tivity and Doppler radial winds in Danny's core. To more perpendicular to the radar beam. These MBVs facilitate evaluation of the storm's structural evolution, were manually extracted using WATADS for each vol- radial snapshots of inbound and outbound base veloc- ume scan between 2300 UTC 18 July and 2400 UTC ities were generated that represent instantaneous, nearly 19 July. They were subjected to manual quality control horizontal wind structure on each side of the hurricane. and were required to maintain reasonable continuity in These snapshots were produced along radials extending

5 Isodops represent contours of constant Doppler velocity; the zero 4 Thanks to P. Dodge and F. Marks, NOAA/AOML/HRD. isodop bisects the wind center in Danny's eye.

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FIG. 4. GOES visible satellite image of Hurricane Danny south of the Mississippi coast at 2025 UTC 18 Jul 1997. The storm is well organized and nearly axisymmetric within a low-shear environment and is in the process of forming concentric eyewalls.

FIG. 3. Illustration of technique for extracting inbound and outbound pro®les of WSR-88D base velocities. Each linear pro®le orig- Hourly isohyetal analyses were completed for 1100 inates from the 0 m sϪ1 isodop within the storm's eye and extends radially outward through the MBV on either side of the storm. Tri- UTC 19 July±0300 UTC 20 July and encompassed the angles represent the inbound (eastern) and outbound (western) lo- life cycle of a large eyewall convective system (EWCS). cations of the MBV. These west- and eastside radials are normal to An EWCS was de®ned as analogous to a mesoscale the radar beam at the MBV in each respective semicircle. convective system, but one that is embedded in highly rotational background ¯ow of a TC's eyewall (H. E. direction and speed with surrounding patterns in both Willoughby 1999, personal communication). One-hour space and time. Observations were deleted when sus- precipitation (OHP) products from the WSR-88D were pected velocity aliasing produced unlikely or absurd used to compute the areal coverage of selected OHP wind patterns, a condition most common in pixels ad- rates. jacent to range folding. But the limited extent of aliasing along a radial had a negligible impact. Fewer than 0.5% 3. Storm history prior to 19 July 1997 of MBVs were deleted over the 25-h archive period. Thus, a history of Danny's radial base velocity patterns Hurricane Danny originated from a weak cold-core during landfall was produced. upper-tropospheric trough that migrated into the Gulf Eye positions were identi®ed both by WSR-88D and of Mexico from the eastern United States (Pasch 1997). reconnaissance vortex information. Core feature inter- A tropical depression began forming south of Louisiana pretations in TCs from different WSR-88D products can between 14 and 16 July 1997. Early on 15 July, north- differ. Stewart and Lyons (1996), after Wood and Brown erly vertical wind shear of 23.7 ϫ 10Ϫ4 sϪ1 from 0 to (1992), discussed different center location interpreta- 10 km (see Fig. 2) slowed intensi®cation. A weak upper- tions in WSR-88D re¯ectivity and velocity products, level trough north of Tropical Storm Danny steered the including differences between the apparent and actual system slowly northeastward on 17 July. By 0000 UTC wind centers in WSR-88D velocity ®elds due to parallax 18 July, Slidell rawinsonde data (not shown) revealed errors and elliptical eyes. Given Danny's unusually a relaxation of the wind shear as the TC strengthened small core diameter (Da), combined with a 40±100-km to a minimal hurricane and crossed the mouth of the distance (Ra) between the eye and radar site, a ratio of Mississippi River. Ra/Da Ն 1 generally was satis®ed and any correction Danny, a relatively small storm, developed an axi- for parallax was small. A distortion of the eye caused symmetric inner-core cloud pattern on 18 July 1997 by asymmetrical convection (discussed in section 4) (Fig. 4) within weak 0±10-km vertical wind shear of could promote some center location discrepancies. Thus, 14.1 ϫ 10Ϫ4 sϪ1 (see Fig. 2). Strengthening continued the WSR-88D-derived radius of MBV (RMBV) may not and Danny's central pressure fell Ͼ6 hPa during the day exactly match Danny's actual radius of maximum wind (Fig. 5); still the well-organized storm failed to intensify (RMW) derived from other sources. However, WSR- beyond a minimal category 1 hurricane. 88D RMBVs appeared to be a good indicator of the Weak westerly steering currents of 0.5±3.0 m sϪ1 be- true RMW and seemed to capture signi®cant aspects of tween 5- and 9-km altitude guided the storm on a slow Danny's evolution. eastward course on 18 July. The current study period

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does not occur ®rst. [Willoughby (1979), Willoughby et al. (1982), and Shapiro and Willoughby (1982) dis- cuss the physics of the eyewall cycle.] Unlike Danny, most minimal hurricanes with concentric eyewalls exhibit little intensity ¯uctuation because the outer eyewall becomes as strong as the inner by the time the replacement is complete. Thus, Danny's weakening during eyewall replacement represented an unusual situation. Figure 6 shows the evolution of Danny's precipitation pattern during landfall. Concentric eyewalls (Fig. 6a) ®rst developed in Danny near point B at 2300 UTC 18 July, indicating the storm was at or near maximum intensity (Jordan and Schatzle 1961; Holliday 1977; Wil- loughby et al. 1982). Danny obtained a 984-hPa pressure minimum (the ®rst of two) near point B shortly after the end of a Tropical Cyclone Wind®elds at Landfall experiment conducted by HRD (Dodge et al. 1999). Danny's outer eyewall formed as the storm approached the coast, consistent with Hawkins (1983) and Wil- loughby (1990). A USAFR reconnaissance aircraft pen- etrated Danny at 2325 UTC 18 July, but did not report concentric eyewalls until subsequent penetrations at FIG. 5. A time series of minimum sea level pressure (hPa) in Danny's eye as reported by USAFR reconnaissance aircraft between 0455 and 0612 UTC 19 July (see Table 1, rows B, C, 1200 UTC 18 Jul and 2400 UTC 19 Jul 1997. Various events during and D). Concentric eyewalls in hurricanes are some- Danny's eyewall cycle and landfall are annotated in the ®gure. times not identi®ed by aircraft, even though postanalysis of radar loops indicates their presence (Willoughby et al. 1982); this was the case in Danny's situation. began as Danny approached point B in Fig. 1 and moved Concentric eyewalls also were found in WSR-88D within 100 km of the WSR-88D at KMOB. velocities. A temporal pro®le of manually derived radials of inbound and outbound base velocities was cre- 4. Structure at landfall ated from 2300 UTC 18 July to 2400 UTC 19 July (Fig. 7). Both of Danny's eyewalls displayed separate wind Danny sequentially underwent a symmetric, then maxima between 2300 UTC 18 July and 0200 UTC 19 asymmetric, contraction of its inner core over a 19-h July, consistent with Willoughby et al. (1982). period before landfall. The classical symmetric contraction culminated with an intensifying single-eyewall storm 12 h later, followed by an asymmetric, highly b. Eyewall replacement morphology convective contraction and further intensi®cation over Convectively driven contracting maxima of the swirl- Mobile Bay. Data from aircraft and the KMOB WSR- ing wind constitute the primary mechanism for inten- 88D chronicled the changes in precipitation and wind si®cation of hurricanes (Willoughby 1990). This type structure during this eyewall replacement and contrac- of intensi®cation occurs in axially symmetric storms and tion. may be manifest by eyewall replacement and contraction cycles (Willoughby et al. 1982). Danny experienced a. Concentric eyewalls in a minimal hurricane such a cycle during the 13-h period between 2300 UTC 18 July and 1200 UTC 19 July as it approached and Eyewall replacement cycles, discussed by Willough- entered Mobile Bay. Dramatic changes in precipitation by (1990), begin with the formation of an outer con- and low-level wind structure were observed in WSR- vective ring (eyewall) around a preexisting inner eye- 88D images during this time. wall. This outer eyewall, resulting from a coalescence Early on 19 July near point C, the storm turned slowly of outer bands, eventually contracts and forces dissi- northward. The central pressure started rising and max- pation of the inner eyewall. During a classical axisym- imum MBVs began oscillating between western and metric contraction, a major hurricane initially may lose eastern eyewalls with very little correlation to observed intensity as the inner eyewall collapses, pressure rises, ¯uctuations in broad sea level pressure (SLP) gradients RMW expands, and a ¯at wind pro®le develops (Wil- (Ϫ١p) between the eye and the surrounding nonstorm loughby et al. 1982, 1984; Willoughby 1990; Marks and environment. Westside MBVs decreased most notice- Dodge 1997). However, renewed strengthening often ably in both eyewalls after 0200 UTC 19 July, while occurs shortly thereafter as the outer eyewall continues eastside velocities increased above 31 m sϪ1 during this its contraction over a 12±36-h period, provided landfall time, even though Danny's central pressure was rising.

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FIG. 6. Base re¯ectivity of Hurricane Danny from the KMOB WSR-88D at (a) 2303 UTC 18 Jul, (b) 1039 UTC 19 Jul, (c) 1649 UTC 19 Jul, and (d) 2014 UTC 19 Jul 1997. A re¯ectivity scale (dBZ) is depicted at the bottom of each image. Re¯ectivities less than (a) 21 dBZ, (b) 22 dBZ, and (c), (d) 14 dBZ are not displayed. Maximum re¯ectivities within Danny's inner core (Ͻ40 km from Danny's center) are (a) Ͼ46 dBZ to the northwest, (b) Ͼ50 dBZ to the southwest, (c) Ͼ60 dBZ to the west and southwest, and (d) Ͼ60 dBZ to the west and northwest. Composite re¯ectivites (not shown), coincident with (d), exceed 65 dBZ to the west and northwest of Danny's center. Range rings extend concentrically outward at 50-km intervals from the KMOB WSR-88D.

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TABLE 1. Listing of storm characteristics to accompany Figs. 1, 7, eye's center or outside the storm. Willoughby (1990) and 10. Letters and numbers correspond to storm eye positions. Letters shows that the acceleration of the swirling wind is con- represent USAFR aircraft reconnaissance positions while numbers represent Doppler-estimated eyewall mesovortex positions: N/A ϭ not centrated just inside the RMW and that wind pro®les available or not accomplished (sometimes denoted by ``Ð''), NE ϭ inside the eye become more U shaped during intensi- no eye (less than 50% of the eye circumference contains an eyewall), ®cation. Thus, conceivably, pressures can fall between UNK ϭ unknown, C ϭ circular eye, CO ϭ concentric eyewalls, and the eyewall and the center while staying constant or E ϭ elliptical eye. The max wind heading contains three columns: RA ϭ radar-derived (from KMOB WSR-88D) maximum base velocity even rising at the center itself. Also, low-level winds within the storm's eyewall, AFL ϭ aircraft maximum 10-s (1-km spatial converging into the eyewall's diabatically induced up- average) ¯ight-level wind, and ASF ϭ aircraft maximum estimated drafts can become supergradient, and become stronger .surface wind. than expected from Ϫ١p alone Central Max wind (m sϪ1) After 0900 UTC 19 July, larger-scale gradients in- Date/time pressure Eye diameter creased as SLP rose 3.5 hPa in 6 h west of the center ID (UTC) (hPa) (km) RA AFL ASF and coincided with a decrease of 1 hPa in the eye at Aircraft reconnaissance vortex reports the beginning of Danny's asymmetric transition and A 2012 18 Jul 986 C28 Ð 40 28 steady strengthening of westside winds. A simultaneous B 2325 18 Jul 984 C19 32 36 39 2-hPa SLP increase to the east, however, was accom- C 0455 19 Jul 987 CO22±56 37 40 Ð panied by a 5 m sϪ1 decrease in MBVs there (opposite D 0612 19 Jul 987 CO28±56 36 32 Ð of expectations). E 0726 19 Jul 986 E11/37/19 37 31 Ð F 0854 19 Jul 985 C22 35 34 Ð Between 0455 and 0612 UTC 19 July, aircraft re-

G1 1142 19 Jul 984 C11 41 33 21 ported inner and outer eye diameters of 25 and 56 km. G2 1259 19 Jul 984 C11 40 35 Ð During this time, inner and outer wind maxima merged H 1410 19 Jul 984 C19 42 31 21 as Ͼ30msϪ1 base velocities fused across the earlier Radar estimated eyewall mesovortex moat between the eyewalls, and the overall wind ®eld 1 1504 19 Jul UNK Ð 44 Ð Ð intensi®ed. 1558 19 Jul UNK Ð 42 Ð Ð Reconnaissance reports (Table 1) show that the inner 1704 19 Jul UNK Ð 41 Ð Ð 2 1804 19 Jul UNK Ð 44 Ð Ð eye expanded from 19 to 28 km between 2325 UTC 18 3 1903 19 Jul UNK Ð 38 Ð Ð July and 0612 UTC 19 July (Fig. 8). The expansion of 4 2013 19 Jul UNK Ð 39 Ð Ð the inner eye as the outer succeeds is not unusual. A Aircraft reconnaissance vortex reports similar eye expansion was not re¯ected in WSR-88D I 2003 19 Jul 987 NE 39 34 23 base velocities of the across-eye diameter of inner out- 2028 19 Jul 988 NE 37 28 Ð bound-to-inbound 26 m sϪ1 isodops (26I) (Fig. 8). This J 2147 19 Jul 989 NE 39 30 26 isodop diameter remained close to 24 km between 2300 K 2302 19 Jul 990 NE 36 29 26 UTC 18 July and 0400 UTC 19 July, then contracted 0034 20 Jul 993 NE Ð 27 26 L 0205 20 Jul 994 NE Ð 25 Ð to 20 km by 0600 UTC 19 July. The east- and westside M 0534 20 Jul N/A NE Ð 24 Ð portions of the isodop diameter contracted in concert N 0704 20 Jul N/A NE Ð 24 Ð with the strengthening of respective outer-eyewall O 0838 20 Jul N/A NE Ð 27 Ð MBVs. The eastside 26I (see Fig. 8) contracted after 0100 UTC 19 July, followed by the westside 26I nearly 3 h later, coincident with the base velocity increases. Thus, Danny's inner winds experienced some strength- By 0400 UTC 19 July, westside velocities once again ening and contraction between 0455 and 0612 UTC 19 increased and were Ͼ36 m sϪ1 in the outer eyewall 1 July even while the aircraft-reported inner-eye diameter h later. Central pressure falls were not observed until continued to expand. The storm's central pressure during after 0600 UTC 19 July; thus, wind increases in Danny's this time remained constant at 987 hPa, after rising from outer eyewall preceded pressure falls in the eye itself its minimum 6 h earlier. by 2±4 h. By 0700 UTC 19 July, MBVs once again By 0726 UTC 19 July, aircraft reported an elliptical increased to 36 m sϪ1 on Danny's east side, while the eye 19 km by 36 km wide. Only traces of the inner eye west side weakened. This oscillation continued past remained in re¯ectivity images (not shown) as it merged 0830 UTC 19 July as winds once again decreased (in- with the contracting outer eyewall. The western (east- creased) on the east (west) side. ern) RMBV contracted from 30 to 11 km (28 to 22 km) Outer eyewall MBV increases several hours before between 0600 and 1130 UTC 19 July (Fig. 7). During central pressure falls and the oscillation of maximum this time, the eastside RMBV remained larger than its winds from one side of the storm to the other deserves western counterpart; however, the inner 26I radii dis- further discussion. Changes in Ϫ١p may be local to the played a pronounced symmetric contraction on both inner edge of the eyewall, and so not observed in coarser sides of the storm over the same period. gradient measurements between the eye and environ- Danny developed a spectacular axisymmetric, single -ment. The local Ϫ١p in the eyewall may tighten or relax eyewall by 1039 UTC 19 July, evident in the precipi to a greater degree than re¯ected by SLP changes in the tation and velocity patterns in Figs. 6b, 7, and 8. Heavy

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FIG. 7. A temporal composite of Danny's inbound and outbound base velocities (m sϪ1) from the KMOB WSR-88D between 2300 UTC 18 Jul and 2400 UTC 19 Jul 1997. Contour (shading) intervals are every 2 m sϪ1. Only velocities Ն 14msϪ1 are analyzed (see key to right of image). See Fig. 3 for an illustration of velocity radial construction for a single volume scan. Outbound (inbound) velocities are to the left (right) and are in the western (eastern) semicircle of the storm. Distances are speci®ed in km and represent radials extending outward from the eye's 0 m sϪ1 isodop and normal to the radar beam at the point of MBV. Maximum 10-s (1-km spatial average) ¯ight- level winds (white stars) from USAFR reconnaissance aircraft are plotted in parentheses and are positioned at the ¯ight-level RMW according to which semicircle (outbound or inbound) of the storm they were observed. Note: the maximum aircraft ¯ight-level wind likely resides at a different azimuth angle (in a storm-centered reference frame) than the velocity radials used to construct Fig. 7. Hurricane symbols represent sequential positions of the0msϪ1 isodop in the eye. Letters and numbers along the storm center's path correspond with positions in Table 1 and in Figs. 1 and 10. convection (Ն50 dBZ re¯ectivity) was limited only to continued on the west side only and coincided with a a small region in the southwest quadrant of the storm dramatic increase in convection there. Re¯ectivities rap- near DPIA1, while the rest of the storm's precipitation idly diminished on the stratiform east side. Westside was primarily stratiform. Contraction of the wind and MBVs consistently exceeded 38 m sϪ1 and eventually precipitation pattern continued until a second 984-hPa reached Ͼ40 m sϪ1. This was opposite to the pattern of pressure minimum occurred shortly before 1200 UTC bands normally favoring a TC's east side (Willoughby 19 July. Aircraft reported a small circular eye only 11 et al. 1984). km in diameter and both the 26I radii were symmetric Surface streamline analyses (not shown) using local at ഠ7 km either side of the center (ഠ15 km diameter). observing stations showed strong (very weak) con¯u- The eye was near point G and was almost completely ence of northerly (southerly) winds into the western enclosed in Mobile Bay, a warm tidal estuary 48 km by (eastern) eyewall. The strongest convection persisted 37 km. along the meridional extent of Mobile Bay. Also, an elevated low-level jet (see section 5a) occupied the rain- free east side of Danny at this time and provided a c. Asymmetric displacement and contraction of conveyor belt of Gulf air to the intense westside con- Danny's center vection. The environment contained only weak vertical A second episode in Danny's prolonged landfall be- wind shear and probably was not a factor in Danny's gan after 1200 UTC 19 July 1997. A dramatic asym- asymmetry. But the origin of this asymmetry requires metry developed in both base re¯ectivity and velocity additional study and is beyond the scope of this paper. as the storm changed from a stratiform precipitation Precipitation rates and their spatial distributions ex- pattern to a heavily convective one. The eye contraction perienced signi®cant changes. Areal coverages of var-

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FIG. 8. Temporal evolution of Danny's eye and inner wind con- tractions/expansions, as represented by (i) the inner radius of 26 m sϪ1 (50 kt) base velocities (scallops) on both the west and east sides of the eye's0msϪ1 isodop, (ii) the distance (diameter) across the FIG. 9. Isohyetal analysis summary of Hurricane Danny's areal eye (dots) between inner 26 m sϪ1 isodops, and (iii) eye diameters coverage of selected OHP rates (mm hϪ1) derived from the KMOB (hurricane symbols) reported by aircraft (the last eye diameter re- WSR-88D between 1100 UTC 19 Jul and 0300 UTC 20 Jul 1997. ported by reconnaissance was at 1410 UTC 19 Jul 1997). The period The OHP products for the 1600±1700 UTC and 1800±1900 UTC spans 25 h between 2300 UTC 18 Jul and 2400 UTC 19 Jul 1997. periods on 19 Jul 1997 are not available. (A 25 mm hϪ1 rate of rainfall Base velocity radials and diameters are extracted from KMOB WSR- corresponds to ഠ1 in. hϪ1.) 88D information, as portrayed in Fig. 7.

1995). Willoughby et al. (1984) de®ne a storm's dynamic center as a region of calm winds encircled by ious WSR-88D OHP rates6 appear in Fig. 9 immediately preceding the development of westside convective closed streamlines. Sometimes the dynamic center mi- asymmetry. Precipitation rates and coverages were low grates across the eye toward intense asymmetric con- as the storm entered the bay. The area bounded by the vection in the eyewall. Also, mesovortices can form adjacent to persistent heavy rain in a sector of the eye- 25±50 mm hϪ1 (1±2 in. hϪ1) isohyet was only 100±200 wall (Gamache et al. 1997). In Danny's case, the dy- km2 around 1100±1200 UTC 19 July and coincided with namic center became very diffuse after 1200 UTC as the storm's greatest axial symmetry. Only a 25 km2 area the intense westside EWCS developed. An eyewall me- experienced OHP rates over 50 mm hϪ1 at this time. As the storm became asymmetric and the intense sovortex (EWMV) developed in western sections of the EWCS developed in the west eyewall, these rates and eye after 1400 UTC 19 July, adjacent to the EWCS. coverages steadily increased. By 1400 UTC, the 25±50 This small EWMV is clearly visible in Fig. 6c and continued to contract. The EWMV's track between 1504 mm hϪ1 isohyet encompassed 580 km2 and maximum and 2013 UTC, as estimated from WSR-88D data, is rates exceeded 100 mm hϪ1 (4 in. hϪ1) over a small area in the southwest eyewall. The area bounded by the 25± represented in Fig. 10 by a series of storm positions labeled 1±4. This EWMV may have originated 1) from 50 mm hϪ1 isohyet continued increasing and ultimately a further contraction of the 11-km-wide eye reported by reached 1400 km2 by 2100±2200 UTC 19 July, shortly reconnaissance aircraft near point G (see G and G in after the eye made landfall. Precipitation rates in the 1 2 Table 1), or 2) as a separate entity within the eye. It is west eyewall were Ͼ100 mm hϪ1 from 1300 to 2200 also unclear as to whether the EWMV represented a UTC and expanded to a maximum coverage of 130 km2 shortly after Fig. 6c. Composite re¯ectivities (not single vortex, or a cyclonic sequence of edge-of-the- shown) at times exceeded 65 dBZ here. Danny's western eye vortices that ampli®ed (weakened) upon approach- convective asymmetry (Fig. 6d) persisted for at least ing (leaving) the westside EWCS. another 24 h and produced disastrous ¯ooding as the As the EWMV developed, the eye to the east near storm drifted slowly inland over southwest Alabama. point H (see Fig. 10) became more diffuse. Aircraft The strong convection probably in¯uenced the be- observations at 1410 UTC indicated that although the havior of Danny's eye over Mobile Bay. In a well-or- pressure remained at 984 hPa, the eye had expanded by ganized TC, the dynamic (wind) center nearly always 7kminഠ1 h. A corresponding expansion of the 26I lies within the eye depicted by radar re¯ectivity (Foley diameter also was evident in Danny's core. On the east side of the eye, a broad and diffuse expansion of the RMBV continued for at least another 10 h, whereas the convective west side's RMBV quickly resumed a mul- 6 WSR-88D precipitation rates were derived from the tropical Z±R tihour contraction after 1500 UTC. rainfall algorithm. The eye remained recognizable in the base velocity

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storm. Thus, the westside EWCS remained over Mobile Bay for several more hours. Prior to Danny's arrival, sea surface temperatures (SSTs) in the northern Gulf (0000 UTC 18 Jul) were 29Њ±30ЊC at 1-m depth (buoys 42040 and 42007), and 30ЊC at the mouth of Mobile Bay [DPIA1 at 0.5 m below mean low water (MLW)].7 Yet, DPIA1's SSTs in sum- mer are more representative of Gulf SSTs and are generally cooler than SSTs in the bay's interior.8 For in- stance, interior bay SSTs were not available during Dan- ny, but a recent comparison on 6 August 1999 provides an example of warmer waters in the bay. Here, the 24-h average SST was 33.2ЊC near Cedar Point9 (8 km northwest of DPIA1), compared to 31.5ЊC at Gulf buoy 42040 (119 km south of DPIA1).10 Bay SSTs reached 35ЊC that day, compared to 32.5ЊC in the Gulf. Cooler Gulf water failed to invade the bay during Danny's landfall FIG. 10. As in Fig. 1 except a close-up of Danny's path through because the eye crossed east of the bay's mouth and Mobile Bay, and subsequent landfall near Mullet Point, AL, between north winds produced a 0.6-m tide de®cit (Pasch 1997) 0800 and 2200 UTC 19 Jul 1997. at the northern end. The bay averages 3 m in depth, except for a 12-m- pattern, but became dif®cult to recognize in re¯ectivity deep ship channel extending the bay's length, so sub- images as eastside precipitation eroded. The EWMV surface vertical mixing under Danny's eyewall probably was most apparent in the re¯ectivity images and con- failed to cool the water. However, surface heat transfer tracted to a diameter of only 4±6 km (2±3 n mi) before and cold rain (ϳ1000 mm) helped reduce DPIA1 SSTs landfall on the bay's eastern shore. to 24ЊC by 0100 UTC 20 July, a 6ЊC decline during The EWMV was not so distinct in the base velocity Danny's passage. Possibly as the bay cooled, westward pro®les; however, the WSR-88D mesocyclone algo- vortex regeneration succumbed to eastward steering and rithm repeatedly indicated small mesocyclone and 3D Danny moved slowly onshore. correlated shear symbols in the vicinity of the EWMV's The westside MBVs exceeded 39 m sϪ1 almost con- re¯ectivity center. But the algorithm often lost the tinuously for 9 h beginning around 1130 UTC near point EWMV signature possibly because of its superposition G in Mobile Bay. Danny reached peak intensity between upon the general rotation around Danny's diffuse eye. 1804 and 1824 UTC, when numerous MBVs exceeded The algorithm required the convective-scale rotation to 44msϪ1 and the westside 26I contracted to a minimum be symmetric, and Danny's was not. Some volume scans of 6 km (3 n mi).11 No corroborating aircraft observa- indicated overlapping large (eye) and small (EWMV) tions were available from 1500 to 2000 UTC 19 July; mesocyclone symbols. Operationally, forecasters should however, Danny's central pressure probably was Ͻ984 closely investigate small (possibly sporadic) Doppler- hPa prior to ®nal landfall. indicated mesocyclones near the eyewall for the possible The EWMV crossed the coast near Mullet Point, Al- existence of EWMVs offset from the eye center. abama, on the bay's eastern shore (see point 4), between Danny strengthened during the ongoing contraction of the EWMV over Mobile Bay. H. E. Willoughby (1999, personal communication) suggests that once 7 Sensor information supplied by P. Newkirk, National Data Buoy Danny's strong eyewall winds moved over shallow and Center, Stennis Space Center, Mississippi. warm Mobile Bay, increased surface heat ¯ux (Emanuel 1995, 1997) probably sustained the EWCS. Air con- 8 Dr. W. Schroeder at the Dauphin Island Sea Lab concurs with verged into the EWCS from both the concave and con- this statement based on his own studies. vex sides in response to convective heating. Surface observations adjacent to Mobile Bay, combined with WSR-88D base velocities, clearly showed pronounced 9 SSTs taken every 15 min at MLW, courtesy C. Yanny, U.S. Army con¯uence of winds here. Willoughby speculates that Corps of Engineers, Mobile, Alabama. vorticity stretching in the EWCS updraft and PV gen- eration below the level of maximum condensational 10 SSTs at DPIA1 were unavailable in 1999 due to a damaged heating may have forced the vortex to reform or prop- sensor. agate westward. A weak eastward steering current over the central Gulf Coast, south of a weak upper-tropo- 11 Some of this wind increase may be attributable to lower radar spheric trough (at 1200 UTC 19 Jul), opposed the west- 0.5Њ beam elevations while the storm is in the bay compared to when ward vortex propagation, resulting in a nearly stationary the storm is in the Gulf.

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1900 and 2000 UTC 19 July (Pasch 1997), and a rapid decrease in outbound base velocities and expansion of the RMBV occurred west of the center. Once over land, the EWMV lost identity. East of the EWMV landfall point, the elongated eye of Danny moved inland closer to Weeks Bay (see point I). The most severe tree damage in¯icted by Danny occurred near the EWMV's landfall point between Fair- hope and Weeks Bay (NCDC 1997). Strong near-surface winds, undetected by the WSR-88D, likely occurred on the north and northwest sides of the EWMV and very likely accounted for much of this tree damage. A site survey by the author found that trees between Fairhope and Weeks Bay fell primarily from the north-northeast through east-northeast, indicating a wind direction nearly perpendicular to the WSR-88D beam radials; thus, no strong base velocities were observed there at landfall. All Doppler-observed hurricane-force MBVs near the EWMV were westside outbound winds located well offshore over Mobile Bay. FIG. 11. Minimum elevations (m) above MSL for maximum 10-s The eastside wind pattern during landfall was con- (1-km spatial average) ¯ight-level winds from aircraft (dashed) and siderably weaker and less de®ned. Base velocities to 31 KMOB WSR-88D MBVs (solid) between 2300 UTC 18 Jul and 2400 msϪ1 remained in a broad 20-km-wide zone over the UTC 19 Jul 1997. Actual aircraft observations are represented by the eastern sector of the storm during the entire EWMV boxes posted along the dashed line. The aircraft ¯ight levels represent 850-hPa (700 hPa prior to 0000 UTC 19 Jul) elevations within the episode. Diffuse eastside base velocity patterns resulted center of Danny, as contained in reconnaissance vortex reports. These from lack of radar scatterers in that sector of the storm elevations represent the minimum ¯ight level encountered during the (see Fig. 6d), which precluded RMBV estimates after aircraft vortex penetration; actual ¯ight level elevations are somewhat 1748 UTC 19 July. But the eastside 26I close to the higher at the RMW. Note the multihour period between 1130 and center was observable for several more hours. 1800 UTC 19 Jul where WSR-88D MBVs are sampled near 600-m elevation while the reconnaissance ¯ight level is over twice this altitude. 5. Comparison of WSR-88D base velocities to maximum aircraft and surface winds were not exactly collocated with each other. Instead, the How intense was Danny over Mobile Bay and the implication was only that the aircraft and WSR-88D Alabama coast? Because of Danny's close proximity to winds were observed in the same half semicircle12 of the KMOB WSR-88D, base velocities were sampled at the storm and at the same radius from the center; there- very low levels within Danny's inner core. At 0.5Њ scan fore, actual azimuth angles between WSR-88D and air- elevations, the base velocities were several hundred me- craft winds differed. ters below aircraft ¯ight-level winds. Figure 11 shows Dif®culties arose when comparing ¯ight-level winds a comparison of wind sampling altitudes from aircraft to WSR-88D base velocities, partly due to temporal gaps and the KMOB WSR-88D within Danny's RMW. Also, between aircraft observations (as in Danny between some winds at 17.4-m elevation above MSL (NDBC 1410 and 2003 UTC 19 Jul). Large differences can occur 1998) were available from the DPIA1 C-MAN in Dan- between ¯ight-level and surface winds, especially when ny's eyewall. ¯ight level is above the top of the boundary layer (Pow- ell 1987). The maximum horizontal wind speed in a TC a. Maximum wind comparisons is usually at between 1000 and 1500 m; however, airborne Doppler radar data from Powell and Black (1984) Reconnaissance ¯ights by USAFR aircraft reported shows maximum winds near rainbands can be lower the maximum wind encountered at ¯ight level, and the (300±500 m). Likewise, WSR-88D maximum velocities bearing of maximum wind from Danny's center. Al- within TC Ed were at the 400±500-m level (Stewart and though stronger winds possibly resided in sections of Lyons 1996). Danny that were not observed by aircraft or radar, the Thus, aircraft ¯ying at 1500 m (850 hPa) may be aircraft's maximum 10-s (1-km spatial average) ¯ight- above the strongest winds and may underrepresent a level winds provided a quick comparison against the corresponding WSR-88D MBVs and RMBVs from 2300 UTC 18 Jul to 2400 UTC 19 July. Maximum aircraft winds were posted over the Doppler base ve- 12 Semicircles are bounded by the radial extending from the WSR- locity pro®le in Fig. 7 (and provided in Table 1), but 88D location and through the storm center.

Unauthenticated | Downloaded 09/30/21 08:31 PM UTC DECEMBER 2000 BLACKWELL 4013 storm's intensity closer to the ground. A nearby Doppler radar (if available) may provide better estimates of maximum winds if base velocities are sampled below aircraft ¯ight level, but WSR-88D base velocities below 1500 m are only possible Ͻ100 km from the radar site. Within 50 km, 0.5Њ beam elevations are Ͻ650 m. There, however, is no guarantee that WSR-88D velocities, even at these low levels, correspond to the maximum tangential winds, since stronger winds could be at lower levels, or may not be oriented directly along beam radials. None- theless, WSR-88D base velocities in Danny over Mobile Bay were stronger than winds found by aircraft and consistently lay at altitudes below ¯ight level. When a TC makes landfall, reconnaissance aircraft generally ¯y above the boundary layer because of overland ¯ight restrictions (Powell 1990; Willoughby 1990). The 850- and 700-hPa levels represent the usual overland ¯ight levels. During Danny's landfall, aircraft ¯ew at the 850-hPa level (roughly 1300±1500 m MSL), while the WSR-88D 0.5Њ beam elevations in the MBVs on either side of Danny's eye ranged from 600 to 800 m. In the present study, little attempt was made to examine radar observations at aircraft ¯ight level, or use equivalent sampling intervals for aircraft and WSR- 88D. The comparison between the two platforms was made strictly to examine differences between aircraft winds and WSR-88D base velocities. Collocation was another limitation because of sharp wind changes over small time±space increments (Powell et al. 1996). Figure 7 shows that in spite of the comparison problems just discussed, radar and aircraft winds agreed rea- sonably well in some locations of the storm. Maximum ¯ight level and WSR-88D winds compared fairly well to each when the storm was symmetric and less con- vective13 (prior to 1100 UTC). Less agreement occurred after 1200 UTC when the western convective asymmetry developed and base velocities were weaker (stronger) to the east (west) than the 850-hPa ¯ight- level winds. These differences deserve further discussion, starting with the west side. Doppler MBVs were consistently Ͼ39 m sϪ1 (75 kt), in the convective western semicircle, while aircraft there found maximum winds of only 31±35 m sϪ1 (60±67 kt). After the aircraft departed the storm at 1410 UTC, MBVs increased and consistently exceeded 41 m sϪ1 FIG. 12. (a) A vertical cross section of radial velocity (m sϪ1) Ϫ1 (80 kt) from 1500 to 1900 UTC with MBVs Ͼ44 m s through Danny's inbound and outbound MBVs at 1529 UTC 19 Jul (85 kt) on several occasions. This period corresponded 1997 from the KMOB WSR-88D. Distances (km) are displayed along with the development of the EWMV. the bottom axis of the cross section, while elevation (km) above WSR- 88D antenna height is depicted along the vertical axis A velocity A vertical cross section of radial velocities (Fig. 12a) Ϫ1 Ϫ1 scale of outbound (positive) and inbound (negative) velocities (m s ) across the EWMV (Fig. 12b) shows winds Ͼ41 m s is displayed below the image. The thin white curve extending upward were con®ned to the western eyewall's lowest levels from the Ͼ41 m sϪ1 velocity maximum represents the vertical axis (i.e., only at the 0.5Њ scan elevation) and were signi®- of the RMW. (b) A base re¯ectivity image as in Fig. 6c except for cantly weaker at higher elevations. The westside RMW 1529 UTC 19 Jul 1997. The path of the cross section in (a) is represented by a line segment from A to B. The cross section in (a) extends (left to right) from a position (A) south of Dauphin Island, northeastward through Danny's intense westside EWCS and the EWMV, then across the weaker northeast quadrant of the storm over 13 The 36 m sϪ1 aircraft reconnaissance wind at 2325 UTC was observed at the 700-hPa ¯ight level, while all successive winds were Baldwin County to a position (B) near Interstate 10. at the 850-hPa level (i.e., closer to the WSR-88D 0.5Њ scan elevation).

Unauthenticated | Downloaded 09/30/21 08:31 PM UTC 4014 MONTHLY WEATHER REVIEW VOLUME 128 and convection tilted outward from the center with The Dauphin Island C-MAN on the end of the island height, similar to Powell et al. (1996), Marks (1985), measured 10-min average winds of 65 kt (33 m sϪ1)at and Jorgensen (1984a,b). Much less tilt of the RMW 1145 UTC and a gust to 88 kt (45 m sϪ1) 21 min earlier. was observed in the nonconvective east side. Powell Interestingly, the Mobile WSR-88D radar showed that (1990) indicates that most research ¯ights into storms around these times, the strongest eyewall convection was collect data at ¯ight levels above 1500 m. Recently, occurring in this vicinity over the southwest quadrant of Global Positioning System dropsonde data collected the hurricane. At 1139 UTC, aircraft reported maximum from hurricane eyewalls have detected the existence of winds of 64 kt (33 m sϪ1) at the 850 mb ¯ight level in low-level wind maxima as low as 200 m above MSL the southwest quadrant. Thus, surface and ¯ight-level (Dodge et al. 1997, 1999), similar to these observed by winds were about the same in this highly convective radar in Danny's western eyewall. region of the hurricane. The eastside velocity maximum was characteristic of an elevated low-level jet with Ͼ31 m sϪ1 (60 kt) winds Observed winds at DPIA1 were compared to surface at 1300 m MSL above winds Ͻ28msϪ1 (55 kt) near wind estimates derived from aircraft and WSR-88D (and possibly below) 600-m elevation. Eastside winds data. First, the 33 m sϪ1 aircraft wind mentioned by from aircraft, sampled at the 1300±1500-m elevation, Pasch closely matched the 35 m sϪ1 WSR-88D velocity probably represented winds in this elevated jet core that at 1500 m over DPIA1. Here, the upper 103% value of were not observed at lower levels in the eastside base Powell et al.'s gust range14 produced surface gust esti- velocities. The elevated jet wrapped cyclonically around mates of only 34 m sϪ1 from the reported 33 m sϪ1 the east and north sides of the storm (not shown) and aircraft wind. Thus, the estimated surface gusts were 11 fed into the large westside EWCS over Mobile Bay. msϪ1 (33%) too low when compared to the maximum 5 sϪ1 gust of 45 m sϪ1 at DPIA1 (Meindl and Hamilton 1992). Second, the comparison between WSR-88D b. Boundary layer wind representativeness MBVs and the DPIA1 gust was closer. The MBVs in the southwest eyewall from 1100 to 1200 UTC 19 July Hypothetically, if aircraft ¯ew 1) along the velocity were 38±41 m sϪ1 at 600 m, only 4 m sϪ1 (10%) short cross section's path in Fig. 12b, and 2) at the typical of the DPIA1 gust. 850-hPa level in Fig. 12a, the aircraft would observe A much better aircraft estimate of DPIA1 surface stronger winds in the elevated jet core near 1500 m, but winds occurred when the maximum ¯ight-level wind (a would miss the weaker winds in the boundary layer near 1-km spatial average over 10 s) was used to estimate 600 m. Powell (1982) found that 10-m-level gusts (mean sustained surface winds (in this case, a 10-min average) winds) at coastal stations may be estimated as 80% instead of 5-s surface gusts. Powell et al. (their appendix (56%) of the ¯ight level mean winds at 500±1500 m, A) stated that the 103% of ¯ight level value also rep- and that maximum surface gusts occurred in heavy con- resents the upper end of the sustained (1 min) surface vection. In Hurricane Andrew at landfall, Powell et al. wind range (57%±103%) in Andrew: sometimes the (1996) found that surface gusts were 64%±103% of the maximum 1-min surface wind in Andrew was just as nearest ¯ight-level 10-s wind maximum. The low end strong as the maximum 10-s wind observed nearby at of this range was more appropriate for Danny's east side, ¯ight level. Danny's surface wind gusts were much so 64% of the ¯ight-level wind yielded surface gusts of stronger over Mobile Bay and Dauphin Island than those only 21±23 m sϪ1 over open ¯at terrain. Indeed, a visual measured at ¯ight level. Indeed, the NHC increased inspection by the author to the east of the landfall point Danny's maximum sustained wind estimate from 33 to found very little tree damage. Unfortunately, no cor- 36msϪ1 (gusting to 44 m sϪ1) in their 1500 UTC 19 roborating wind observations were available. July advisory after these reports were received from The velocity pro®le was quite different on the western DPIA1. Thus, the 39±44 m sϪ1 MBVs in Danny, ob- side. The same aircraft at 850 hPa would ®nd maximum served for several hours at 600 m, may have represented winds close to the 35 m sϪ1 WSR-88D velocities de- the maximum marine-exposure surface wind gusts in picted at 1500 m in Fig. 12a. Thus, Ͼ41 m sϪ1 base the western eyewall. velocities near 600-m altitude would not be sampled by Danny and Andrew were not the only landfalling aircraft. Actually, the western eyewall's MBV at 600 m storms with stronger near-surface wind gusts than those was 44 m sϪ1 at only a 10-km radius from the EWMV, indicated at ¯ight level. During Hurricane Alicia's 1983 and was 125%±130% of the maximum outbound ve- landfall in Texas, surface gusts measured by the Coast locity at ¯ight level. Guard ship Buttonwood exceeded ¯ight-level (1500 m) How representative of surface winds were these peak gusts during each of the four offshore aircraft pass- ¯ight-level and base velocity wind maxima? To the author's knowledge, DPIA1 was the only archive-capable surface-based anemometer to experience the strong west 14 The upper end of the 64%±103% gust range is used because of eyewall of Danny along the Alabama coast. Pasch strong convection over DPIA1 and stronger Doppler base velocities (1997) mentioned: at 600 m as compared to velocities at ¯ight level.

Unauthenticated | Downloaded 09/30/21 08:31 PM UTC DECEMBER 2000 BLACKWELL 4015 es (Powell 1987). Danny's situation was unique among landfall. Catastrophic ¯ooding of bayous and rivers these three landfalls because a nearby WSR-88D re- adjacent to Mobile Bay occurred. corded the low-level wind maxima. 6) An EWMV formed adjacent to the westside EWCS The present discussion indicates that, while in the bay, and coincided with continued asymmetric contrac- Danny's heavily convective (nonconvective) westside tion and intensi®cation there. Westside RMBVs (eastside) eyewall contained near-surface wind gusts contracted to 11 km (6 n mi) prior to landfall as signi®cantly greater than (probably much less than) WSR-88D base velocities at 600 m peaked at 45 100% of maximum ¯ight-level winds. Damage inspec- msϪ1 (87.4 kt); this period probably coincided with tions by the author and NWS personnel over Dauphin Danny's greatest intensity and lowest central pres- Island, and between Weeks Bay and Fairhope (NCDC sure.15 The greatest tree damage occurred near the 1997), de®nitely con®rm stronger surface winds in Dan- EWMV's landfall point on Mobile Bay's eastern ny's western semicircle; surface winds and associated shore. damage appear to be much less to the east. 7) The eastside RMBV expanded and boundary layer winds weakened before landfall as precipitation there decreased and strongest winds evolved into a 6. Summary and discussion broad elevated low-level jet. 8) The WSR-88D showed that Danny's strongest The slow motion of Hurricane Danny, combined with winds persisted for several hours in the boundary its proximity to the KMOB WSR-88D, provided an ex- layer (600±700 m) within the western EWCS, and ceptional opportunity for close, low-level sampling of were observed several hundred meters below much the storm's inner core at landfall over a 1-day period. weaker ¯ight level (1300±1500 m) winds there. Similar sampling continuity would be dif®cult or im- These WSR-88D MBVs indicate that Danny was possible by aircraft, or in a faster-moving storm. Many signi®cantly stronger in the bay than aircraft ¯ight- interesting properties were observed in Danny during level winds suggested. this time. 9) Winds at ¯ight level (1300±1500 m) in the non- 1) Danny experienced concentric eyewalls and a sym- convective east side of the storm were stronger than WSR-88D winds in the boundary layer (600±700 metric eyewall replacement/contraction offshore, m). followed by an asymmetric and heavily convective 10) A 45 m sϪ1 (88 kt) gust at DPIA1 in Danny's south- contraction over Mobile Bay. west eyewall was only 10% stronger than 600-m 2) Danny weakened during its replacement cycle, a Doppler MBV values there, but was 33% stronger behavior unusual for a minimal hurricane experi- than the maximum wind measured by aircraft at encing eyewall succession. This weakening is more 1300±1500 m. Thus, the WSR-88D MBVs at characteristic of major storms. 600-m elevation probably represented slightly con- 3) Early in the symmetric replacement cycle, the cen- servative estimates of Danny's maximum near-sur- tral pressure rose and eyewall radial velocities ini- face gusts in the storm's EWCS. tially weakened. However, as the pressure continued to rise, the outer eyewall MBVs strengthened Results from this study show the importance of sam- and began oscillating between east and west sides pling hurricane eyewall boundary layers for an accurate of the storm with very little correlation to environ- assessment of surface wind speeds and overall storm mental SLP changes. The central pressure fell only intensity. Danny's heavily convective (nonconvective) after the strengthening outer eyewall winds con- westside (eastside) eyewall contained near-surface wind tracted and merged with the inner eyewall. Thus, gusts signi®cantly greater than (probably much less central pressure reduction in the eye lagged 2±4 h than) 100% of maximum ¯ight-level winds. More in- behind a general increase in central core wind investigation of eyewall boundary layer wind maxima at tensity. levels closer to the surface is warranted. 4) Eyewall contraction and a sustained 6-h pressure Danny demonstrated that, given favorable environ- fall culminated in an axisymmetric single-eyewall mental conditions, a slow-moving small storm may storm composed mainly of stratiform precipitation. maintain, or even increase, intensity for several hours 5) Danny became asymmetric and extremely convec- over a shallow con®ned tidal estuary, as long as con- tive over shallow, warm Mobile Bay. An intense vective portions of the eyewall remain over water with EWCS persisted over the bay for 10±11 h west of SSTs possibly higher than those in the open ocean. the eye. As this western asymmetry developed, 25± The origin of Danny's asymmetry, EWMV devel- 50 mm hϪ1 (1±2 in. hϪ1) precipitation rates ex- opment and evolution, and the in¯uence of the westside panded from 200 to 1400 km2. Extreme rainfall rates of Ͼ100 mm hϪ1 (4 in. hϪ1) persisted for nine consecutive hours, reached maximum coverage just 15 Independent con®rmation of central pressure by reconnaissance before landfall, and occurred for up to 4 h after aircraft was not available at that time.

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EWCS on the intensi®cation and motion of Danny while istics of mature hurricanes. Part I: General observations by re- in Mobile Bay need further investigation. Environmen- search aircraft. J. Atmos. Sci., 41, 1268±1285. , 1984b: Mesoscale and convective-scale characteristics of ma- tal in¯uences, such as coastline proximity and possible ture hurricanes. Part II: Inner core structure of Hurricane Allen low-level wind surges, on concentric eyewall initiation (1980). J. Atmos. Sci., 41, 1287±1311. warrants further study. The in¯uences of vertical shear, Marks, F. D., Jr., 1985: Evolution and structure of precipitation in boundary layer discontinuities, and coastal topography Hurricane Allen (1980). Mon. Wea. Rev., 113, 909±930. , and P. P. Dodge, 1997: Hurricane concentric eyewall charac- on Danny's structural transition, vortex maintenance, teristics as revealed by airborne Doppler radar analyses. Pre- and resulting rainfall patterns pose interesting problems prints, 22d Conf. on Hurricanes and Tropical Meteorology, Fort for further research. Further examination of Danny's Collins, CO, Amer. Meteor. Soc., 102±103. extreme rainfall is in progress. Meindl, E. A., and G. D. Hamilton, 1992: Programs of the National Data Buoy Center. Bull. Amer. Meteor. Soc., 73, 984±993. NCDC, 1990: Local Climatological Data Monthly SummaryÐMobile Acknowledgments. I wish to express my deep appre- AL, July 1990. [Available from National Climatic Data Center, ciation to Jeff Medlin and the National Weather Service 151 Patton Avenue, Asheville, NC 28801-5001.] , 1997: Storm Data. Vol. 39, No. 7, 347 pp. Of®ce in Mobile, Alabama, for assisting in data acqui- NDBC, cited 1998: At what heights are the sensors located at C-MAN sition and display capabilities. I am also indebted to Jeff sites? [Available online at http://seaboard.ndbc.noaa.gov/ for creating Fig. 9. Dr. Hugh Willoughby provided many cmanht.shtml.] helpful suggestions for improving the initial manuscript Neumann, C. J., and M. J. Pryslak, 1981: Frequency and motion of Atlantic tropical cyclones. NOAA Tech. Rep. NWS 26, NOAA/ and provided invaluable insight regarding certain as- NHC, Coral Gables, FL, 15 pp. [Available from National Tech- pects of Danny's structural evolution. Dr. Mark Powell nical Information Service, Technology Administration, U.S. De- provided some helpful comments regarding ¯ight-level partment of commerce, Spring®eld, VA 22161.] to surface wind comparisons in hurricanes. Finally, I NSSL, 1997: WATADS version 9.0: WSR-88D Algorithm Testing and Display System reference guide. [Available online at http:// thank two anonymous reviewers and Dr. Aaron Williams ftpnssl.nssl.noaa.gov/pub/watads/doc.tar.] for their constructive comments. OFCM, 1992: Federal Meteorological Handbook No. 11ÐDoppler Radar Meteorological Observations. Parts A±D, 4 vols. 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