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

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Doppler-Observed Eyewall Replacement, Vortex Contraction/Intensi®Cation, and Low-Level Wind Maxima 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 h21 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 .44 m s21 (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 q 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 sur- face 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 air- craft 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 ,100 km from the radar site (Foley 1995). ricane Danny's inner core (,40 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 (,60 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 ;5ms21 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. ,60 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 ,100 km from Mobile's WSR- or close to the boundary layer are emphasized. Section 88D for .48 h, and the strongly convective west eye- 2 details data and analysis techniques. The storm history wall remained ,60 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 0.58 beam angle. pability to experience Danny's landfalling eyewall. Unauthenticated | Downloaded 09/30/21 08:31 PM UTC 4004 MONTHLY WEATHER REVIEW VOLUME 128 aircraft, and DPIA1 in Danny's eyewall. Finally, section 6 contains a summary of Danny's structural evolution and a discussion of future research. 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.
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