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Miniature Supercells in an Offshore Outer Rainband of Hurricane Ivan (2004)

MATTHEW D. EASTIN AND M. CHRISTOPHER LINK Department of Geography and Earth Sciences, University of North Carolina at Charlotte, Charlotte, North Carolina

(Manuscript received 8 August 2008, in ﬁnal form 7 January 2009)

ABSTRACT

Airborne Doppler radar observations are used to document the structure of three miniature supercells embedded in an outer rainband of Hurricane Ivan on 15 September 2004. The cells were located more than 100 km offshore, beyond the Doppler range of coastal radars. The combination of large CAPE, large vertical wind shear, and moderate cell-relative helicity with an apparent midlevel dry air intrusion provided an offshore environment supporting rotating storms. Each shallow cell contained a ;5–7-km-diameter mesocyclonic updraft with midlevel updraft and vorticity maxima that exceeded 6 m s21 and 0.008 s21, respectively. Such offshore structures are consistent with miniature supercells observed onshore in association with tropical cyclone tornado outbreaks. The strong updrafts resulted from a combination of kinematic convergence, thermal instability, and shear-induced vertical perturbation pressure gradients. Mesocyclone production largely resulted from the tilting and subsequent stretching of low-level horizontal streamwise vorticity into the vertical by the strong updrafts. Evidence of baroclinic contributions from inﬂow along cell-generated outﬂow boundaries was minimal. The miniature supercells persisted for at least 3 h during transit from offshore to onshore. Tornadoes were reported in association with two cells soon after moving onshore. These observations build upon a growing body of evidence suggesting that miniature supercells often develop offshore in the outer rainbands of tropical cyclones.

1. Introduction Numerous studies have documented common environmental characteristics associated with tornadic min- Supercells are long-lived convective storms that con- iature supercells observed in tropical cyclones (e.g., tain a mesocyclonic updraft and downdraft while often Novlan and Gray 1974; McCaul 1991; Curtis 2004; exhibiting a distinctive ‘‘hook echo’’ in radar reflectivity Edwards and Pietrycha 2006). The majority of events imagery. In the midlatitudes, the majority of large and occurred in outer rainbands that were embedded within violent tornadoes are spawned by supercells (e.g., Davies- the onshore flow of the northeast quadrant (often the Jones et al. 2001). In landfalling tropical cyclones, most right-front quadrant relative to storm motion). Favorable tornadoes are spawned by ‘‘miniature’’ supercells (Spratt environments often contained 1) moderate low-level et al. 1997; Suzuki et al. 2000; McCaul et al. 2004). Al- storm-relative helicity, 2) moderate CAPE with maxi- though they are often shallower, less intense, and shorter- mum buoyancy below 500 hPa, 3) moist low levels with a lived than their midlatitude counterparts, such supercells low lifting condensation level, 4) relatively dry air at have been responsible for multiple deadly tornado out- midlevels, and 5) a preexisting low-level boundary (such breaks (McCaul 1991; Hagemeyer 1997). However, our as a convergence axis or baroclinic front). The numerical understanding and ability to forecast miniature supercells simulations of McCaul and Weisman (1996, 2001) further and tornadoes in tropical cyclones remains limited, in demonstrated that the formation of miniature supercells part, because available observation networks are unable was favored in environments with the buoyancy and to regularly monitor the relevant offshore environment shear maxima concentrated in the lower troposphere. and cell evolution. Previous studies also have documented the structure and evolution of miniature supercells within landfalling tropical cyclones using Doppler radar observations (e.g., Corresponding author address: Dr. Matthew D. Eastin, De- Spratt et al. 1997; McCaul et al. 2004). The tornado- partment of Geography and Earth Sciences, 9201 University City Blvd., Charlotte, NC 28223. producing cells typically were characterized by .50-dBZ E-mail: [email protected] echoes, low-level inflow notches, and echo tops ,10 km.

DOI: 10.1175/2009MWR2753.1

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Their mesocyclones were shallow (;4 km) with small hurricane. Ivan made landfall near Gulf Shores, Alabama, diameters (;2 km) and mean shear vorticity magni- at approximately 0650 UTC 16 September with maxi- tudes of ;0.005 s21 that could be tracked for 1–2 h. The mum sustained winds of 54 m s21 and a central pressure mesocyclones were often identified ;30 min prior to of 943 hPa. Over the next 36 h, the system moved toward reported tornado production. the northeast and weakened significantly as it crossed Although the vast majority of supercells have been the southeastern United States. Franklin et al. (2006) documented over land as a result of radar limitations provides a detailed account of Ivan’s history. offshore, there is a growing body of evidence suggesting The observations analyzed in this study were col- that miniature supercells in tropical cyclones often de- lected between 1400 and 2100 UTC 15 September when velop well offshore. Indeed, prior studies have demon- Ivan was located ;250 km offshore in the northern Gulf strated that offshore environments can be conducive to of Mexico (Fig. 1). During this period, Ivan was moving supercell formation (Bogner et al. 2000; Baker et al. northward at ;5.7 m s21 in response to a strong sub- 2009), documented offshore supercells using coastal tropical ridge centered east of the Bahamas and an Doppler radars (Spratt et al. 1997; Rao et al. 2005; Lee approaching midlatitude trough extending southward et al. 2008), and noted reports of waterspouts and tor- into Texas. The storm was weakening slowly (maximum nadoes within a few kilometers of the coastline (Gentry winds decreased from 59 to 56 m s21) in part because of 1983; Hagemeyer 1997). This paper contributes to such increasing vertical shear, decreasing sea surface tem- evidence by providing the first detailed observations of peratures, and the ingestion of dry air into the core. At tropical cyclone miniature supercells located more than the same time, a prominent outer rainband developed 100 km offshore and beyond the Doppler range of op- ;350 km east of the storm center and extended through erational coastal radars. the northeast (or right front) quadrant. The band was The objective of this study is to document the three- located just east (or outside) of an apparent dry air in- dimensional structure and evolution of three distinct trusion that wrapped through the southern and eastern miniature supercells embedded within an offshore outer quadrants. Examination of animated satellite and land- rainband of Hurricane Ivan (2004) using primarily air- based radar imagery revealed that the band was com- borne Doppler radar observations. Our specific goals posed of multiple long-lived convective cells that initially include the following: formed southeast of the storm center, moved along the band though the eastern quadrant, and then moved 1) Describe the offshore environment supporting the onshore into the Florida Panhandle. This paper exam- supercells. ines a subset of these cells. 2) Document the three-dimensional structure and air- Ivan produced at least 122 tornadoes as the storm flow of the offshore supercells. crossed the southeast United States [National Climatic 3) Determine the physical processes responsible for Data Center (NCDC 2004)], which is currently the supercell formation and maintenance. record for a tropical cyclone. A period of significant 4) Describe the supercells’ subsequent evolution and tornadic activity occurred on the afternoon and evening coastline tornadogenesis. of 15 September as Ivan approached the Gulf Coast. We hope that this study will enhance our understanding During this time, the cells embedded within the prom- of tropical cyclone convection and provide further in- inent outer rainband spawned 17 reported tornadoes sight as to how tornadoes develop within landfalling (5 produced F1 damage and 2 produced F2 damage) systems. across portions of the Florida Panhandle and southeast Alabama. These tornadoes were responsible for 6 deaths and 13 injuries. A cell examined herein spawned one of 2. Overview of Hurricane Ivan (2004) these deadly tornadoes. Hurricane Ivan formed in the central Atlantic on 5 September 2004 from a strong African easterly wave. 3. Data and analysis methods On 9 September, Ivan moved into the Caribbean Sea as a major hurricane. On 12 September, the hurricane ach- On 15 September 2004, the National Oceanic and ieved a maximum intensity of 75 m s21 with a central Atmospheric Administration (NOAA) Hurricane Re- pressure of 910 hPa as it passed south of Grand Cayman. search Division conducted a multiplane investigation of Soon afterward, Ivan began to move northward in re- Hurricane Ivan. Although the primary goals of the response to a weakness in the subtropical ridge. After pass- search mission were unrelated to the prominent outer ing over the western tip of Cuba, the system entered the rainband east of the storm center (see Fig. 1), consid- Gulf of Mexico on 14 September as a strong category-4 erable data were collected in and near the rainband from

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FIG. 1. (a) Aqua-1 89-GHz brightness temperatures at 1850 UTC and (b) GOES-12 water vapor (WV) temperatures at 1815 UTC 15 Sep 2004 in Hurricane Ivan. Note the intense convective cells embedded within the outer rainband east of the storm center, as well as the apparent dry air intrusion between the moist outer rainband and the inner core. Black dots denote the locations of representative soundings discussed in the text. (Imagery courtesy of the U.S. Naval Research Laboratory.) a variety of platforms. The dataset employed here pri- thus structural details of individual convective cells marily consists of airborne Doppler radar observations should be well represented. used to document the rainband’s offshore convective The Doppler analysis methodology described in cell structure. The data were supplemented with flight- Gamache (1997) and Reasor et al. (2009) was used to level, dropsonde, rawinsonde, and surface observations construct the three-dimensional wind field of the off- to analyze the rainband environment and cell structure. shore rainband segment. Initially, the raw reflectivity Observations from the Tallahassee (TLH) and Tampa and Doppler velocity data from the tail radar were Bay (TBW), Florida, operational Weather Surveillance edited following standard procedures that remove spu- Radar-1988 Doppler (WSR-88D) were used to provide rious echoes (e.g., sea clutter) and incorporate appro- an overview of the offshore rainband structure, track priate navigation corrections (Bosart et al. 2002). Then, individual cells, and document the structural evolution the edited fields were interpolated to a band-centered of individual cells as they moved onshore. Figure 2 pro- 90 3 90 km Cartesian domain extending from the sur- vides a geographic overview of these data with respect to face to 18 km AGL with uniform horizontal and vertical the rainband at the time of interest. grid spacing of 1.5 and 0.5 km, respectively. The initial values at each grid point were determined through a a. Airborne radar observations Gaussian weighting of all nearby observations using a A NOAA WP-3D aircraft crossed the rainband be- cutoff radius of 3 km (1 km) and an e-folding distance of tween 1758 and 1810 UTC at ;2.5 km AGL while en 0.75 km (0.25 km) in the horizontal (vertical) direction. route to the eyewall. The aircraft was equipped with Finally, the variational methodology was used to solve a 5.5-cm wavelength lower fuselage (LF) radar and simultaneously the radar projection equations and an- a 3.2-cm wavelength tail Doppler radar. Details of the elastic mass continuity equation for the three-dimensional radars are described in Jorgensen (1984). During the wind field while incorporating an appropriate motion rainband crossing the aircraft employed the fore–aft for the target cells. In application, Gamache (1997) scanning technique (FAST) to collect Doppler velocity found this method superior to traditional methods (e.g., observations (Gamache et al. 1995). The FAST geom- Jorgensen et al. 1983), but time evolution of the wind etry (alternating conical scans ;208 fore and aft of the field, beam filling issues, and variable cell motions still plane normal to the aircraft track) permitted a dual- could make significant contributions to the error. Doppler analysis of the three-dimensional wind field for The quality of the Doppler analysis was assessed by a portion of the rainband (see Fig. 2b). This geometry comparing flight-level observations with the Doppler- also dictates a time lag between each radar observa- derived winds at 2.5 km AGL following Gamache et al. tion at a given location within the analysis domain. In (1995). Initially, the filtered flight-level data (see section 3b) the present study, the average time lag was ,2 min, and were renavigated and interpolated to the Doppler

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FIG. 2. Base-scan radar reflectivity (dBZ) at (a) 1732 and (b) 1803 UTC from TBW and at (c) 1804 and (d) 1831 UTC 15 Sep 2004 from TLH. Each panel is 300 3 300 km and tic marks are shown every 50 km. The locations of the TLH and TBW radars (black filled circles), soundings (red filled circles), surface buoy (red plus), and airborne dual- Doppler analysis region (black square) are shown. Also labeled are the three miniature supercells (A, B, and C) discussed in the text as viewed at each time from the respective radar. analysis grid. Then, a gridpoint-by-gridpoint compari- by mass continuity and the horizontal winds (Marks et al. son of each Doppler and interpolated flight-level wind 1992). The relatively weak vertical motions along much component was conducted. To facilitate direct compari- of the flight path (the aircraft passed between cells) also son with previous studies, summary statistics for the likely contributed to the low correlation. It should be zonal, meridional, tangential, radial, along-track, cross- noted, however, that the individual distributions of flight- track, and vertical wind components were computed level and Doppler-derived vertical motions were very from the 125 available grid points (see Table 1). Overall, similar to those found in previous studies. the summary statistics were similar to values previously b. Flight-level observations reported in hurricanes (Marks et al. 1992; Gamache et al. 1995; Reasor et al. 2009). Mean differences (or biases) The NOAA WP-3D flight-level sensors are described are very small and fall within the accuracy range of the in Jorgensen (1984). Initially, the 1-Hz data1 obtained flight-level observations. The low linear correlation co- efficient (0.27) for the vertical wind component resulted, in part, from the inherent spatial smoothing involved 1 The aircraft ground speed was ;125 m s21 during the rainband when Doppler-derived vertical motions are constrained crossing.

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TABLE 1. Mean differences (bias), RMS differences, and linear loss of near-surface winds (Franklin et al. 2003) nor any correlation coefficients between aircraft flight level and Doppler- questionable segments in the thermodynamic profile derived wind components for all data points. Statistics for the (Barnes 2008). zonal (U), meridional (V), vertical (W), radial (VR), tangential (VT), along-track (VA), and cross-track (VC) wind components To facilitate comparison with prior studies, the fol- are listed. A positive (negative) bias indicates that the Doppler- lowing stability and vertical shear parameters were derived wind is more positive (more negative) than the flight-level computed from each sounding: most unstable CAPE wind. (MUCAPE; Doswell and Rasmussen 1994), mean-layer Wind component CAPE (MLCAPE) for a mean parcel from the lowest 100-mb layer, surface-based CAPE (SBCAPE), surface- Statistic UVWVR VT VA VC Bias (m s21) 0.17 20.35 0.02 0.40 20.36 0.18 0.01 based convective inhibition (SBCIN), lifting condensa- RMS (m s21) 1.55 1.17 1.12 1.63 1.23 1.35 1.39 tion level (LCL), 0–3-km SBCAPE, surface ue, 0–1-km Correlation 0.93 0.64 0.27 0.89 0.77 0.99 0.99 and 0–6-km shear vector magnitude, effective bulk shear magnitude (Thompson et al. 2007), 0–1-km and 0–3-km cell-relative helicity (CRH; Davies-Jones et al. during the rainband crossing were smoothed using a run- 2001), effective CRH (Thompson et al. 2007), bulk ning Bartlett filter designed to preserve scales greater Richardson number (BRN) and BRN shear (Weisman than 1.5 km (the Doppler grid resolution) without and Klemp 1982), 0–1-km and 0–3-km energy helicity shifting significant peaks. The vertical velocity (w) data index (EHI; Rasmussen 2003), supercell composite pa- were analyzed following Eastin et al. (2005) to remove a rameter (SCP; Thompson et al. 2004), significant tor- small nonzero offset (0.12 m s21). The available tem- nado parameter (STP; Thompson et al. 2004), and the perature (T) and dewpoint (Td) data were obtained 10–500-m critical angle (Esterheld and Giuliano 2008). by the Rosemount thermometer and General Eastern In computing cell-relative2 quantities, cell motions were cooled mirror, respectively, which are susceptible to obtained by tracking multiple convective cells in close errors induced by sensor wetting in clouds and precipi- proximity (within 3 h and 50 km) to each sounding using tation (e.g., Eastin et al. 2002). During wetting, the tem- animated TBW and TLH radar imagery. Estimates of cell perature sensor behaves more like a wet-bulb ther- motion also were computed using the modified Bunkers mometer and reports erroneously low values, whereas et al. (2000) technique described in Ramsay and Doswell the chilled mirror measures erroneously high dewpoint (2005), and they exhibited a small right bias (;158–208) values that often exceed the low temperatures. Any compared to the motions obtained by cell tracking. such wetting errors were reduced following the recom- d. Surface observations mendations of Zipser et al. (1981), but error magnitudes on the order of 0.58C may still remain (Eastin Offshore surface observations were very limited. A et al. 2002). Equivalent potential temperatures (ue) single moored buoy (station 42036) maintained by the were computed following Bolton (1980) and may be National Data Buoy Center (NDBC) was located within underestimated by ;3 K during periods of instrument the Doppler analysis domain (see Fig. 2b) and thus used wetting. to provide representative surface observations near the rainband. Detailed information on platform configura- c. Sounding observations tion, sensor descriptions and accuracy, data acquisition, Two soundings were selected to represent the rain- and quality control is available at the NDBC Web site band environment. The first was the operational rawin- (available online at http://www.ndbc.noaa.gov/). Fur- sonde launched at 1800 UTC from Tampa Bay. The ther scrutiny revealed no wet-bulb contamination as second was a GPS dropsonde (Hock and Franklin 1999) defined by Cione et al. (2000). deployed at 1807 UTC by the NOAA G-IV aircraft Surface winds were available every 10 min, based on during a synoptic surveillance mission (see Fig. 2b). an 8-min averaging period. For better consistency with Their selection from the available soundings was based the other observations and prior studies of the hurricane on proximity in time and space: they were the closest boundary layer, the winds were normalized to a maxi- upwind soundings (within 1 h and 250 km) from the mum 1-min sustained averaging time using a typical rainband crossing location. The GPS sounding was overwater roughness length (Powell et al. 1996). The processed and quality controlled using the Atmospheric Sounding Processing Environment (ASPEN) program 2 Studies of midlatitude supercells regularly evaluate storm- (available online at http://www.eol.ucar.edu/rtf/facilities/ relative parameters. Here, we have adopted the term cell relative software/aspen/aspen.html) developed by NCAR. Fur- for such evaluations, while reserving the term storm relative for ther scrutiny revealed no common data issues, neither a those analyses with respect to the moving tropical cyclone center.

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FIG. 3. Environment and dewpoint temperatures (thick black lines) observed by (a) the GPS dropsonde deployed ;50 km west of the rainband at 1807 UTC and (b) the TBW rawinsonde launched ;220 km east of rainband at 1800 UTC 15 Sep 2004 plotted on skew T–logp thermodynamic diagrams. The surface-based parcel (thick gray line) is shown. winds also were adjusted to the standard 10-m AGL range from the storm center in both offshore (Bogner reference height (from 5 m) following Liu et al. (1979). et al. 2000) and tornadic landfalling (McCaul 1991) These adjustments were equivalent to applying an am- hurricanes. The wind profiles (Fig. 4) exhibited south- plification factor of ;1.2 to the 8-min winds. In addition, easterly low-level winds that veered with height, re- the hourly temperature and dewpoint observations were sulting in a 0–6-km AGL vertical shear vector oriented adjusted to the 10-m height, assuming a dry adiabatic toward the northeast. Shear magnitudes and cell-relative lapse rate and a constant mixing ratio following Cione helicities (Table 2) were equivalent to or exceeded et al. (2000). mean values observed at similar ranges in hurricanes (Novlan and Gray 1974; McCaul 1991; Bogner et al. 4. Offshore rainband and supercell structure 2000). A third wind profile was constructed by combining the 1800 UTC surface wind from the moored a. Environmental characteristics buoy (BY) with the Doppler-derived (DD) winds above Figures 3 and 4 present the thermodynamic profiles the buoy location (DD/BY; see Fig. 4b). This profile, and hodographs representative of the rainband envi- located ;10–20 km east of the strongest cells, provides a ronment. The TBW sounding was taken ;220 km east- representation of the local inflow environment and ex- southeast of the rainband segment, whereas the GPS hibited remarkably similar structure (and shear pa- sounding was located ;50 km to the southwest near rameters) to the GPS wind profile. Finally, based on the apparent modest dry air intrusion (see Fig. 1). These midlatitude forecast criteria, the BRN (,50), SCP (.1), soundings and the analyzed rainband segment were and 0–3 km EHI (.1) suggest the environment was located in the northeast (or right front) quadrant (see conducive to supercell formation (and marginally con- Fig. 2b). Table 2 lists the instability and shear parame- ducive to tornadogenesis). Baker et al. (2009) should be ters computed from each sounding. consulted for a more detailed discussion of all available The rainband environment supported the potential soundings and their forecast parameters. for rotating convection. Both soundings exhibited a b. Evolution of the band and cells shallow moist layer near the surface (below ;1 km), drier air at midlevels (;2–5 km), and moist air aloft Figure 2 provides an overview of rainband structure (Fig. 3). Midlevel RH minima approached 40%–50% and evolution between 1732 and 1831 UTC as viewed near ;4–5 km AGL. CAPE values (Table 2) and their from TBW and TLH. During this period, the analyzed vertical distributions were typical for the ;300–500-km offshore rainband segment was beyond the Doppler

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imum dBZ . 40) spaced at ;20–30-km intervals. Cells south of the analyzed segment (Figs. 2a,b) exhibited no consistent structure, but cells to the north (Figs. 2c,d) tended to exhibit northeastward elongation with maximum reflectivity gradients and hooklike appendages to the southwest, indicating supercells. The band axis (or line of strongest cells) was located along the western (or inner) edge adjacent to the apparent dry air intrusion (see Fig. 1). Stratiform precipitation dominated the eastern (or outer) side with increased areal extent to the north. Although Fig. 2 depicts a variety of evolving cellular structures along the rainband, the aircraft’s LF imagery (not shown) indicated that the analyzed cells (labeled A, B, and C) remained relatively steady in shape and intensity from 1758 to 1810 UTC when the airborne Doppler data were collected. Tracking these cells via animated TLH and TBW imagery revealed a mean north-northwest motion (toward 3448 at 24.8 m s21). This earth-relative cell motion includes the storm motion (toward 3558 at 5.7 m s21) and a small outward band motion (toward 638 at 1.1 m s21). c. Horizontal structure Figures 5 and 6 depict the reflectivity, cell-relative winds, vertical velocity, and vertical vorticity at 2 km AGL for the Doppler-analyzed rainband segment. Three distinct cells (labeled A, B, and C) were located along the western edge near the apparent (echo free) dry air intrusion. Similar to cells farther north, each occupied ;600 km2 (based on the 25-dBZ contour, as in Barnes et al. 1991), exhibited a southwest–northeast orientation, 4 FIG. 4. Hodographs of wind profiles obtained at ;1800 UTC by and contained reflectivity maxima greater than 40 dBZ (a) the GPS dropsonde deployed ;40 km west of the rainband, (b) with maximum gradients along the western edge. Cells the combination of Doppler-derived and surface winds obtained at A and B each exhibited a weak hook echo and inflow the location of the NDBC moored buoy (DD/BY), and (c) the notch. Each cell also contained a closed cyclonic circu- TBW rawinsonde launched ;220 km east of rainband on 15 Sept lation (i.e., a mesocyclone) in the cell-relative Doppler- 2004. Plotted numbers represent observation altitude in km AGL. Also shown are observed cell motions (OBS; black squares) de- derived wind field, which was roughly collocated with termined from tracking proximity cells with radar and estimated the cell’s reflectivity, updraft, and vertical vorticity max- cell motions (MBK; gray squares) computed using the modified ima. Distinct downdrafts were located in cell A (cell B) Bunkers technique (Ramsay and Doswell 2005). Related param- ;5–8 km north (south) of the primary updraft along the eters are given in Table 2. cell’s forward (rear) flank. Clearly, these offshore cells in Ivan’s outer rainband exhibited signature characteristics range (;175 km) of both coastal radars (;220 km from of supercells (e.g., Lemon and Doswell 1979). the segment), but reflectivity data3 were available. The At 2 km AGL, each primary updraft was ;6kmin band was composed of multiple convective cells (max- diameter (based on the 2 m s21 contour) with a maximum greater than 6 m s21. Such characteristics are representative of the largest and strongest updrafts 3 At such range, the ability of the radar reflectivity data to re- solve low-level convective structure (such as inflow notches) is somewhat limited by the 18 beamwidth and the base-scan elevation. Spratt et al. (1997) provide a more detailed discussion of the 4 Individual sweeps from the tail radar exhibited reflectivity long-range sampling limitations of the WSR-88D radars with maxima of .50 dBZ at 3–4 km AGL, consistent with Spratt et al. specific regard to miniature supercells. (1997) and McCaul et al. (2004).

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TABLE 2. Environmental stability, wind shear, and forecast parameters for the three soundings obtained near the rainband and miniature supercells. The DD/BY sounding (winds only) was obtained by combining the Doppler-derived winds with the surface wind observed at the buoy location (cf. Fig. 2b). All cell-relative parameters were computed using observed cell motions obtained by tracking proximity cells with radar. Cell motions computed following the modiﬁed Bunkers technique (Ramsay and Doswell 2005) are only listed for comparison purposes.

Parameter GPS-1807 DD/BY-1800 TBW-1800 Observed cell motion (m s21/8) 24.8/164 24.8/164 15.8/175 Modiﬁed Bunkers cell motion (m s21/8) 24.3/181 24.3/178 15.1/194 MUCAPE (J kg21) 960 2924 MLCAPE (J kg21) 618 2344 SBCAPE (J kg21) 960 2924 0–3-km SBCAPE (J kg21) 108 158 SBCIN (J kg21) 216 212 Height of the LCL (km) 520 640 Surface ue (K) 351 359 0–1-km shear (m s21/8) 15.4/190 15.3/185 6.3/165 0–6-km shear (m s21/8) 25.6/225 28.4/215 17.5/205 Effective bulk shear (m s21/8) 21.3/211 16.3/208 BRN shear (m2 s22)819862 0–1-km CRH (m2 s22) 185 181 46 0–3-km CRH (m2 s22) 233 218 98 Effective CRH (m2 s22) 230 86 10–500-m critical angle (8)636052 BRN 8 38 SCP 4.2 4.1 STP 0.6 0.7 0–3-km EHI (using MLCAPE) 0.9 1.4 0–1-km EHI (using MLCAPE) 0.7 0.7

observed at roughly the same altitude in hurricane inner- evaporative cooling, and indeed each cell showed evi- core rainbands (e.g., Black et al. 1996; Eastin et al. dence of forward-flank and rear-flank downdrafts (FFDs 2005) and other tropical oceanic convection (e.g., Lucas and RFDs, respectively). As discussed next, evidence of et al. 1994). The collocated mesocyclones were slightly any mesocyclones at this altitude was minimal because smaller (;5 km in diameter based on the 2 3 1023 s21 the mesocyclones were shallow features. vorticity contour5) with vorticity maxima greater than 8 3 1023 s21. These characteristics are consistent with d. Vertical structure those of miniature supercells observed in onshore tropical Figures 9–11 show representative cross sections of cyclone rainbands (e.g., Spratt et al. 1997; McCaul et al. reflectivity, vertical velocity, vertical vorticity, and in- 2004; Schneider and Sharp 2007). plane cell-relative flow through the mesocyclonic up- At 5 km AGL (Figs. 7, 8), each cell’s reflectivity and drafts (see Fig. 6) of each miniature supercell. Cell A updraft maxima were located northeast of their re- extended up to 8 km (based on the 25-dBZ contour) and spective low-level positions, consistent with a tilt in- exhibited a shallow weak-echo vault (Fig. 9). Cell- duced by the ambient vertical shear (see Fig. 4). Cell- relative inflow approached from the east and was largely relative flow approached from the southwest (i.e., from confined below 2 km. The primary updraft extended the apparent dry air intrusion) and penetrated across the from the boundary layer up to ;7 km and achieved a rainband axis while diverging around each primary up- maximum of ;10 m s21 in the 3–4-km layer before draft. Such injections of drier midlevel air would support detraining to the northeast. The collocated mesocyclone extended up to ;6 km and exhibited distinct low-level 23 21 5 Studies of midlatitude supercells traditionally have used ver- and midlevel vorticity maxima (;8 3 10 s at 1.5 km 2 2 tical vorticity of .1022 s21 as the threshold definition for a me- and ;12 3 10 3 s 1 at 3.5 km, respectively). Evidence socyclone (e.g., Brandes 1984). However, given the diminutive of a weak RFD (;2ms21) was confined to the 4–5-km character of miniature supercells (and thus the potential under- layer at the point where the southwesterly flow from sampling of their mesocyclone’s peak radial velocities), such an arbitrary threshold may not be appropriate. The threshold used the apparent dry air intrusion collided with the rotat- here roughly corresponds with the outer edge of the cell-relative ing updraft. The more pronounced FFD (with magni- closed cyclonic circulation. tudes of 3–4 m s21) was associated with the midlevel

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FIG. 5. Plan view of radar reflectivity (color shading) and dual-Doppler-derived cell-relative wind vectors at 2 km AGL at ;1804 UTC 15 Sep 2004. Reflectivity is shaded at the 0-, 15-, 20-, 25-, 30-, and 35-dBZ levels. Also shown is the aircraft flight track (gray line) and the moored buoy location (red plus). Black circles denote the prominent mesocyclonic circulations associated with each miniature supercell (labeled A, B, and C).

precipitation cascade. Neither downdraft penetrated Cell C contained the strongest updraft but the weakest below 0.5 km. mesocyclone (Fig. 11). Low-level cell-relative inflow Cell B was shallower and less intense (Fig. 10). Cell- was primarily from the east-southeast and thus likely relative inflow approached from the northeast and was was impacted by the small transient cell located ;10 km confined below 1.5 km. The primary updraft extended upwind (see Figs. 5–8). The updraft extended from a up to ;6.5 km and was composed of three distinct up- shallow weak-echo region up to ;9 km, attaining a draft maxima. The strongest achieved ;6ms21 at 2.5 km maximum of 11 m s21 at 5 km. The collocated mesocy- AGL and was collocated with the mesocyclone (which clone extended up to ;6 km with distinct vorticity exhibited maximum vorticity of ;11 3 1023 s21 at maxima of ;7 3 1023 s21 at ;2and;4 km. Neither the 2 km). The lack of a weak-echo vault is consistent with RFD nor the FFD (not shown) penetrated below 1.0 km. the less intense updraft. A modest RFD was present and Clearly, several notable similarities existed among extended below 0.5 km, but no appreciable FFD was these miniature supercells. Their overall diminutive char- evident. acter (cf. classic midlatitude supercells) was consistent

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FIG. 6. Plan view of radar reflectivity, vertical velocity, and vertical vorticity at 2 km AGL. Reflectivity (gray) is shaded at the 0-, 15-, 20-, 25-, 30-, and 35-dBZ levels. Upward (red) and downward (green) vertical velocity is contoured at 62, 4, 6, 8, and 10 m s21. Cyclonic (dark blue) and anticyclonic (light blue) vertical vorticity is contoured at 62, 4, 6, 8, and 10 3 1023 s21. The black circles denote the prominent mesocyclonic circulations associated with each miniature supercell (see Fig. 5). Thin black lines denote vertical cross sections. with the local thermodynamic profile (Fig. 3a); moder- section, the tilting of low-level horizontal vorticity also ate CAPE contributed to modest updraft magnitudes, contributed. whereas the vertical confinement of CAPE by the hurricane’s upper-level warm core limited updraft and mesocyclone depth (McCaul and Weisman 1996; Bogner 5. Forces responsible for the supercells et al. 2000). The combination of well-developed up- a. Updraft production drafts and mesocyclones with only a few weak downdrafts suggests that each cell was in the developing Current conceptual models of supercell evolution stage of its life cycle. Finally, the presence of multiple suggest that persistent updraft production is supported updraft maxima located ;0.5 km above their associated through a combination of mesoscale convergence and vertical vorticity maxima suggests that convergence and local thermodynamic and dynamic forcing (e.g., Davies- stretching of vertical vorticity played a role in mesocy- Jones et al. 2001). The thermodynamic forcing results clone formation. However, as discussed in the next from lifting air parcels in an environment with nonzero

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FIG. 7. As in Fig. 5, but at 5 km AGL. Black circles denote the locations of the prominent mesocyclonic circulations associated with each miniature supercell at 2 km AGL (see Fig. 5).

CAPE until they become locally buoyant and accelerate surface convergence along the rainband axis will pro- upward. The dynamic forcing results primarily from vide adequate lift to overcome the minimal convective vertical perturbation pressure gradients, induced by an inhibition (SBCIN) and elevate parcels to the local level updraft interacting with a veering environmental verti- of free convection. However, close inspection of the cal shear of the horizontal wind, that support upward soundings reveals that CAPE is limited below 2 km, accelerations on the updraft’s right flank relative to the suggesting that the low-level updrafts may be enhanced shear vector (e.g., Rotunno and Klemp 1982). As noted primarily by the shear-induced vertical pressure gradi- by McCaul and Weisman (1996, 2001), environments ents. Both the GPS and DD/BY wind profiles depict with both maximum buoyancy and vertical shear con- semicircular cell-relative hodographs composed of fined to the low and midlevels are optimal for miniature veering shear vectors that maximize in the lowest 3 km supercell production via such combined forcing. (Fig. 4). Thus, following Rotunno and Klemp (1982), Consistent with this conceptual model, a combination persistent shear-induced updraft production would of thermodynamic and shear-induced forcing appears maximize on the southeast flank of each supercell’s sufficient to explain the modest low- to midlevel up- primary updraft. It is interesting to speculate that the drafts observed in Ivan’s supercells. Both proximity shallow low-level updraft maxima just east of cell B’s soundings exhibit ample CAPE primarily confined to primary updraft (see Figs. 6, 10a) may be a manifesta- low and midlevels (Fig. 3 and Table 2). Even weak near- tion of this shear-induced forcing.

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FIG. 8. As in Fig. 6, but at 5 km AGL. Black circles denote the locations of the prominent mesocyclonic circulations associated with each miniature supercell at 2 km AGL (see Fig. 5). b. Vorticity production where j, h, and z are the vorticity components (see Holton 1992 for individual definitions) and i, j, and k are The high-resolution Doppler winds provide an op- the unit vectors in the x, y, and z directions, respectively. portunity to evaluate the three-dimensional vorticity Standard manipulation of the horizontal momentum field near each miniature supercell with a focus on ver- equations while neglecting contributions from friction, tical vorticity production conducive to mesocyclone de- the Coriolis parameter, and the solenoidal term yields velopment and maintenance. In Cartesian coordinates, the following equation describing the time rate of the three-dimensional relative vorticity (v) is defined as change of vertical vorticity [›z/›t or the local generation v 5 j i 1 h j 1 z k, (1) (GENR)]:

›z ›z ›z ›z ›u ›x ›w ›y ›w ›u 5 u 1 y w z 1 , (2) ›t ›x ›y ›z ›x ›y ›x ›z ›y ›z GENR HADV VADV CONV TILT

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FIG. 9. Vertical cross sections of radar reflectivity, cell-relative wind vectors, vertical velocity, and vertical vorticity along (a) west–east through cell A (AW–AE) and (b) south–north through cell A (AS–AN; see Fig. 6). Reflectivity (gray) is shaded at the 0-, 15-, 20-, 25-, 30-, and 35-dBZ levels. Upward (red) and downward (green) vertical velocity is contoured at 62, 4, 6, 8, and 10 m s21. Cyclonic (dark blue) and anticyclonic (light blue) vertical vorticity is contoured at 62, 4, 6, 8 and 10 3 1023 s21. where the first two terms on the right-hand side are the stretching of horizontal vorticity associated with the horizontal (HADV) and vertical (VADV) advection of environmental vertical shear (e.g., McCaul and Weisman vertical vorticity followed by the convergence (CONV, 1996; Davies-Jones et al. 2001). Optimal production or stretching) and tilting (TILT) terms. Horizontal and occurs when the low-level horizontal vorticity vector vertical derivatives were computed from the Doppler- is largely aligned with the cell-relative wind vector derived winds using center difference calculations (e.g., (Davies-Jones 1984). In such cases, the streamwise hor- Pielke 2002). Terms involving vertical derivatives (j, h, izontal vorticity is tilted into the vertical by an updraft, VADV, and TILT) could not be computed at the lowest yielding a positive correlation between vertical velocity grid levels (below 1 km for cells A and B; below 1.5 km and vertical vorticity. for cell C) because of limited Doppler observations. Figure 12 shows the low-level horizontal vorticity and cell-relative wind vectors in the vicinity of each minia- 1) HORIZONTAL VORTICITY ture supercell updraft. Based on the environmental Current conceptual models of supercell evolution hodographs (Fig. 4), the shear vectors in the lowest 2 km suggest that a primary mechanism for low to midlevel were oriented toward the north-northeast, and thus vertical vorticity production is the vertical tilting and the ambient horizontal vorticity vector in the low-level

FIG. 10. As in Fig. 9, but for cross sections along (a) BW–BE and (b) BS–BN through cell B.

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FIG. 11. As in Fig. 9, but for cross sections along (a) CW–CE and (b) CS–CN through cell C. inflow pointed west-northwest. As seen in Fig. 12, the lent in magnitude to the low-level CONV and TILT vorticity vector orientations within 10 km of each up- terms, occur in the 2–5-km layer. Finally, the HADV draft were consistent despite some local convective term exhibits a positive (negative) maximum on the modulation. Moreover, the vorticity and storm-relative west (east) flank of the mesocyclone at low levels con- wind vectors immediately southeast of each updraft sistent with simple advection by the environmental cell- were largely aligned, indicating that streamwise vortic- relative winds. This couplet rotates 908 anticyclonically ity dominated the inflow. Similar vector orientations with height to a north–south orientation at 5 km AGL were observed at all altitudes below 2 km (not shown). (not shown), which, in time, would produce a mesocy- Horizontal vorticity magnitudes on the order of 5 3 clone tilted to the northeast with height (as observed). 1023 s21 were prevalent throughout the inflow layer and Overall, low-level vorticity production within cell A was are consistent with the vertical vorticity magnitudes dominated by tilting and stretching, whereas midlevel observed with each mesocyclone (Figs. 9–11). There- vorticity production resulted from the vertical redistri- fore, in a manner consistent with the aforementioned bution of cyclonic vorticity from lower levels. conceptual models, the existence of low-level stream- The vertical vorticity production terms in the vicinity wise vorticity in the presence of preexisting midlevel of cell B’s mesocyclone are shown in Fig. 14 at similar mesocyclones supports the continued enhancement of altitudes. As observed with cell A, the CONV term vertical vorticity via tilting. exhibits positive maxima below 2 km collocated with the primary mesocyclone (at x ’ 340 km). Maximum 2) VERTICAL VORTICITY CONV exceeds 25 3 1026 s22 at 1.5 km AGL. Contri- Figure 13 shows the horizontal distribution of the butions above 2 km are negative. At low levels, the vertical vorticity production terms [rhs of Eq. (2)] at 1.0, TILT term has positive contributions on the south- 1.5, and 2.0 km AGL in the vicinity of cell A’s meso- eastern (upwind) flank but magnitudes are roughly half cyclone. Below 2 km, the CONV term is positive, of those from the CONV term. Positive tilting contri- reaching a maximum value of 20 3 1026 s22 at 1.5 km butions also diminish above 2 km. As with cell A, the within the mesocyclone. Above 2 km, the term becomes VADV contributions within the mesocyclone are pri- increasingly negative because of weak divergence. The marily negative below 2 km (i.e., below the vorticity TILT term at low levels is comparable in magnitude to maximum) but increasingly positive aloft with magni- the CONV term, with positive contributions on the tudes comparable to the CONV term at low levels. The southern and eastern flanks of the mesocyclone where HADV term also exhibits the modest positive–negative the inflow encounters the updraft. Positive tilting con- couplet (with maxima on the mesocyclone flanks) that tributions extend up to 3.5 km along the southern flank rotates anticyclonically with height in a manner con- (not shown) but diminish along the eastern flank above sistent with the veering environmental winds. Interest- 2 km. The VADV term is primarily negative below 2 km ingly, the shallow vorticity maximum (at x ’ 346 km), because of the increase in vertical vorticity with height. although likely a small-scale transient feature, exhibits However, increasingly positive VADV contributions, qualitatively similar vorticity production structure as centered within the mesocyclone and roughly equiva- observed with the primary mesocyclone. Thus, low-level

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vorticity production in cell B was dominated by stretching (with modest contributions from tilting), whereas midlevel production resulted primarily from vertical advection. Despite complexity added by the more pronounced horizontal separation of the low- and midlevel mesocyclones, the vorticity production distributions in the vicinity of cell C (Fig. 15) are largely similar to those observed with the other cells. However, a few differences are notable. In particular, positive CONV maxima, collocated with the midlevel mesocyclone (at y ’ 45 km), extend up to 4 km. Positive tilting contributions also extend up to 5 km on the southeastern flank with a maximum of ;25 3 1026 s22 at 2.5 km AGL. Such structure implies a deeper inflow layer for cell C, but midlevel outflow associated the small transient cell ;10 km to the southeast (see Fig. 7) also may have contributed. Multiple Doppler analyses are not available to document supercell evolution. However, an indication of mesocyclone evolution can be deduced from the vorticity budget by examining the GENR term in (2) as a residual. Shown in Fig. 16 are the layer mean GENR terms for each cell (averaged through the altitudes shown in Figs. 13–15). Although the GENR maxima are not always collocated with their mesocyclone, GENR values of ;10–15 3 1026 s22 overlap with each mesocyclone’s vertical vorticity maximum. Such local production rates would double the current mean vorticity values (5–8 3 1023 s21) within 10–14 min, implying mesocyclone and supercell intensification in the near future. As will be shown later, these supercells were indeed long lived with mesocyclones detected by the TLH radar as they approached the coast.

3) POTENTIAL BAROCLINIC CONTRIBUTIONS Another mechanism believed responsible for the production of low-level vertical vorticity is the vertical tilting of baroclinically generated horizontal vorticity along cold outﬂow boundaries (Klemp and Rotunno 1983; Davies-Jones et al. 2001). The primary source region of such horizontal vorticity is typically down- shear of the updraft, where low-level cell-relative inﬂow

passes along the horizontal temperature (or ue) gradient associated with the FFD outﬂow; signiﬁcant baroclinic

(b) cell B at 1.0 km AGL, and (c) cell C at 1.5 km AGL. Re- flectivity is shaded at the 0-, 15-, 20-, 25-, 30-, and 35-dBZ levels and the upward vertical velocity is contoured at 2, 4, 6, and 8 m s21. FIG. 12. Plan view of radar reflectivity (gray), upward vertical Note the nearly aligned wind and vorticity vectors to the east- velocity (red), cell-relative wind vectors (black), and horizontal southeast of each updraft, an indication that the low-level inflow vorticity vectors (blue) in the vicinity of (a) cell A at 1.0 km AGL, contains considerable streamwise vorticity.

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FIG. 13. Distributions of the HADV, VADV, CONV, and TILT terms in the vicinity of cell A at 1.0, 1.5, and 2.0 km AGL. Red (green) contours denote positive (negative) contributions to vertical vorticity production. Contours of 65, 10, 15, 20, and 25 3 1026 s22 are shown for each term. Also shown are the radar reflectivity (gray) shaded at the 15-, 20-, 25-, 30-, and 35-dBZ levels, the cyclonic vertical vorticity (blue) contoured at 2, 4, 6, 8, and 10 3 1023 s21, and the cell-relative wind vectors. contributions along the RFD outflow have been noted imum temperature deficits of ;3–48C (or ue deficits of (Adlerman et al. 1999) but often play a minimal role ;8–12 K) are necessary for significant baroclinic gen- (e.g., Markowski et al. 2002). Numerical simulations eration of horizontal vorticity (Klemp and Rotunno suggest temperature gradients of ;18Ckm21 and max- 1983; McCaul and Weisman 1996).

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FIG. 14. As in Fig. 13, but in the vicinity of cell B at 1.0, 1.5, and 2.0 km AGL.

Although the three-dimensional distribution of such the southern edge of any FFD but through the RFD baroclinic contributions could not be determined from region of cell B. Of the three prominent convective the available observations, an estimate of their mag- downdrafts (w ,21.0 m s21) encountered, two (at nitude can be inferred. Figure 17 shows the w, T, Td, ;348 and ;366 km) contained subsaturated air ;18–28C and ue observed at ﬂight level (;2.5 km) during the warmer than their surroundings. The third downdraft rainband crossing. Note that the aircraft passed along (at ;353 km) contained only a modest cold anomaly

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FIG. 15. As in Fig. 13, but in the vicinity of cell C at 1.5, 2.0, and 2.5 km AGL.

(;1.58C), some of which may be instrument wetting form precipitation and was observing the band inflow error, and a mean ue , 346 K. Figure 18 shows the wind, as well as any cold FFD outflow. Minima in T and ue T, Td, and ue observed by the moored buoy (see Fig. 5). were observed after 1800 UTC when any FFD out- The distinct wind shift at ;2100 UTC marks the passage flow associated with cell C (or subsequent cells) would of the rainband axis over the buoy. Prior to this time, have passed over the buoy. However, maximum T and the buoy was located east of the band in the strati- ue deficits were only ;18C and ;2 K, respectively, and

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FIG. 16. Distributions of the layer mean GENR term in the vicinity of each cell. The domains and graph conventions are similar to Figs. 13–15.

ue remained .356 K. Thus, only limited evidence of 6. Discussion any cold downdrafts or outflow boundaries was ob- a. Subsequent offshore evolution and coastline served. tornadogenesis In agreement with McCaul and Weisman (1996), these observations suggest that cell-induced baroclinic The airborne dual-Doppler analysis at 1804 UTC contributions to miniature supercell evolution are mini- clearly demonstrated that three distinct miniature mal, even when dry midlevel air capable of supporting supercells were embedded within Ivan’s outer rainband evaporatively driven downdrafts may be present. One .100 km offshore. Figure 19 (combined with Fig. 2) pro- plausible explanation is that the weaker updraft and vides a TLH-based perspective of each supercell’s subse- weaker upper-level shear associated with miniature quent evolution from their offshore locations at 1804 UTC supercells result in less hydrometeor transport aloft until 2100 UTC, by which time each had moved onshore and away from the cell and thus a smaller horizontal along the Florida Panhandle. During this period, the separation between the primary updraft and any FFD. cells range from ;220 km (at 1804 UTC) to ;120 km The larger directional shear common at low levels may (near 1845 UTC) from the radar, while the base-scan further limit any upwind area occupied by cold pools. elevation decreased from ;5to;2 km as the cells ap- Thus, in a hurricane, any cold FFD outflow might reg- proach the coast. ularly be advected inside a rainband axis toward At 1804 UTC (Fig. 2c), none of the cells exhibited any the storm center, limiting the necessary cold pools and distinct supercellular traits when viewed from TLH, thermodynamic gradients outside a rainband and up- because the base scan overshot any reflectivity signa- wind of any cells. Prior studies of hurricane rainbands tures associated with the shallow mesoscyclones. By and their convective cells (e.g., Powell 1990a,b; Cione 1831 UTC (Fig. 2d), cells A and B exhibited weak inflow et al. 2000) have observed near-surface cold pools but notches along their southern flanks. At the same time, a always inside the rainband axis. The cooler surface ue new cell had appeared between the two, perhaps in re- observed by the GPS sonde (by ;5–7 K; see Table 2 and sponse to cell B splitting or developing a transient Fig. 18) is consistent and supports this explanation. outflow boundary. By 1852 UTC (Fig. 19), the new cell Contributions by preexisting baroclinic boundaries was merging with cell A, and cell C exhibited a distinct were more difficult to infer but may be significant. As- inflow notch. Over the next ;40 min, the radar imagery suming near-surface cold pools were prevalent inside and suggests that cell C underwent significant structural along the rainband axis (as discussed above), the rain- changes. At 1908 UTC, it no longer exhibited the notch band itself would embody an offshore baroclinic bound- and a new transient cell had developed along its ary with a shallow zone of enhanced vorticity and local northern flank. By 1929 UTC, the new cell had propa- solenoidal circulations. The numerical simulations of gated away and dissipated while the primary cell ap- Atkins et al. (1999) suggest that any supercells pro- peared reinvigorated and began to reacquire a weak pagating along such a boundary will develop stronger inflow notch on its southern flank. Such behavior by cell mesocyclones, with the boundary being an important C would imply the presence of at least modest down- ambient vorticity source. drafts and an evolving outflow boundary. The behavior

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FIG. 17. Flight-level (a) w, (b) T and Td, and (c) ue at ;2.5 km AGL along the southern edge of cell B during aircraft passage FIG. 18. Surface (a) wind speed, (b) wind direction, (c) T and Td, across the rainband (see Fig. 5). and (d) ue observed by the moored buoy (see Figs. 2b, 5). The line of strong cells within the northeastward (outward) moving rain- also was broadly consistent with a cyclic supercell (e.g., band passed over the buoy location between 1830 and 1930 UTC. Adlerman et al. 1999), whereby the initial mesocyclonic updraft occludes (i.e., becomes cut off from its unstable nadoes were observed with cell A as it moved onshore. inflow) and dissipates while a new mesocyclonic updraft However, a weak mesocyclone was detected briefly develops just upwind (i.e., along the southern flank). Al- within cell B (at 1945 UTC) as it passed over a barrier though strong downdrafts were not present in the Dop- island, and an F0 tornado was reported at 2040 UTC as pler wind fields ;1 h earlier, such downdrafts may have the cell moved onshore south of Panama City Beach. developed subsequently because of the ingestion of mid- Moreover, a moderate mesocyclone was detected con- level dry air (see Figs. 3, 7, 11, and 17). However, no direct tinuously in cell C between 2022 and 2049 UTC while observations are available to substantiate this hypothesis. translating from ;10 km offshore to ;20 km onshore. After ;1930 UTC, all three cells exhibited persistent The cell spawned at least three tornadoes between 2040 inflow notches or hook echoes along their southern and 2055 UTC, the most damaging of which killed one flanks as they paralleled and approached the coast from person and injured seven with F1 damage in a com- ;25 km offshore. During this period, prominent weak- mercial district of Panama City Beach. echo vaults developed and midlevel rotational velocities b. Comparison to other tropical cyclones rapidly increased (Baker et al. 2009), suggesting that the mesocyclonic updrafts intensified from interaction with Observations of offshore miniature supercells in trop- the coastline. No radar-detected mesocyclones or tor- ical cyclones have been rare. Why did such supercells

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FIG. 19. Base-scan radar reflectivity from TLH following the three miniature supercells as they approached the coastline and moved onshore between 1852 and 2044 UTC 15 Sep 2004. Each panel is 100 3 100 km and tic marks are shown every 25 km. The black circles denote radar-detected mesocyclones. develop offshore in this case? Was the environment ex- Was the structure and organization of Ivan’s rain- ceptionally favorable? Compared to the mean offshore band significantly different? Compared to the gross and onshore soundings from McCaul (1991) and Bogner structure of other offshore outer rainbands (e.g., Powell et al. (2000), the CAPE values were typical (above av- 1990a,b; Barnes et al. 1991), many characteristics of erage) for an offshore (onshore) environment, whereas the precipitation and wind fields were similar. The the vertical shear and helicity were above average. The most notable difference was in the organization of the orientation and proximity of the rainband with respect to band and convective cells. In particular, hurricane the upwind Florida Peninsula may have produced the rainbands are often composed of multiple convective greater shear and helicity through increased exposure to line segments spaced at 10–20-km intervals in the cross- land (e.g., McCaul 1991). However, similar or greater band direction, whereas each line is composed of sev- helicity values have been observed in offshore tropical eral distinct cells irregularly spaced ;10 km apart in cyclones without the upwind exposure to land (Molinari the along-band direction. Ivan’s rainband, however, and Vollaro 2008). Likewise, various forms of dry air exhibited a single convective line with regular along- intrusions often are observed well offshore (e.g., Dunion band cell spacing of ;20–30 km (see Fig. 2). It is un- and Velden 2004; Curtis 2004). Therefore, although clear whether such organization permitted the devel- Ivan’s offshore environment was quite favorable, it does opment of multiple miniature supercells (by providing a not appear to constitute a rare event. continuous supply of warm moist air to each while

Unauthenticated | Downloaded 09/26/21 07:07 PM UTC 2102 MONTHLY WEATHER REVIEW VOLUME 137 limiting interactions with neighboring cells) or resulted genesis while still offshore. Two cells spawned re- from supercell dynamics. The numerical simulations ported tornadoes soon after moving onshore. of Bluestein and Weisman (2000) and James et al. 6) The rainband contained a single line of embedded (2005) provide support for the development hypothe- convective cells regularly spaced at ;20–30-km in- sis: cells initially spaced .20 km apart remained dis- tervals. In contrast to typical offshore outer rain- tinct and acquired supercell characteristics, whereas bands, such organization may have benefited supercell smaller cell spacings resulted in nearly continuous formation and maintenance by allowing individual convective lines. cells to evolve in relative isolation from one another. 7) These observations build upon a growing body of evidence suggesting that miniature supercells often 7. Summary and concluding remarks develop offshore in the outer rainbands of tropical cyclones. A combination of airborne and land-based observations was used to document the structure and evolution The inability of current observational networks to of three miniature supercells in an outer rainband of regularly identify and monitor offshore supercell con- Hurricane Ivan on 15 September 2004. In contrast to vection in tropical cyclones has limited our under- previous studies, the supercells were located more than standing of such events, as well as our ability to forecast 100 km offshore and beyond the Doppler range any associated tornadogenesis. As a result, many open of coastal radars. Our specific findings include the fol- questions remain. In particular, how prevalent are lowing: supercells in the offshore outer rainbands? What characteristics of the tropical cyclone, rainband, and local 1) The offshore rainband environment was conducive to environment support their formation? How does their miniature supercell formation. Local SBCAPE structure and evolution differ offshore? How critical (.900 J kg21), 0–6-km vertical shear magnitude (.20 are preexisting boundaries (e.g., coastlines, dry air in- ms21), and 0–3-km cell-relative helicity (.200 trusions, and low-level baroclinic zones)? Ongoing m2 s22) equaled or exceeded mean values observed at work involves the examination of multiple outer rain- similar radii in the right-front (or northeast) quadrant bands embedded within a variety of offshore and of hurricanes (McCaul 1991; Bogner et al. 2000), and nearshore tropical cyclone environments. Numerical convective inhibition was minimal. simulations are also needed to isolate the relative roles 2) Each offshore miniature supercell contained a mes- played by various environmental features on supercell ocyclonic updraft that extended from the boundary evolution. layer up to ;6–8 km AGL and was ;5–7 km in diameter. Midlevel vertical vorticity (updraft) maxima exceeded 8 3 1023 s21 (6 m s21). Such characteristics Acknowledgments. This research would not have been are consistent with onshore miniature supercells ob- possible without the dedicated efforts of the scientists served in association with tropical cyclone tornado and flight crews at the NOAA Hurricane Research Di- outbreaks (e.g., Spratt et al. 1997; McCaul et al. 2004). vision and Aircraft Operations Center. Discussions with 3) Updraft production and maintenance likely resulted Matthew Parker, Mark Powell, Peter Dodge, Frank from a combination of convergence, thermal insta- Marks, Rob Rogers, John Gamache, and Michael Bell bility, and shear-induced vertical perturbation pres- proved beneficial through the course of this work. Sug- sure gradients. gestions by Roger Edwards and two anonymous re- 4) At the time of the Doppler analysis, mesocyclone viewers clarified and improved the presentation. production resulted from the tilting and subsequent stretching of low-level horizontal streamwise vor- REFERENCES ticity into the vertical by the strong updrafts, as in midlatitude supercells (e.g., Davies-Jones et al. 2001). Adlerman, E. J., K. K. Droegemeier, and R. Davies-Jones, 1999: A numerical simulation of cyclic mesocyclogenesis. J. Atmos. Evidence of baroclinic contributions from inflow Sci., 56, 2045–2069. along cell-generated outflow boundaries was mini- Atkins, N. T., M. L. Weisman, and L. J. Wicker, 1999: The influ- mal. Estimates of net vertical vorticity generation ence of preexisting boundaries on supercell evolution. Mon. suggested that each mesocyclone was intensifying. Wea. Rev., 127, 2910–2927. 5) Each supercell persisted during the ;3 h between Baker, A. K., M. D. Parker, and M. D. Eastin, 2009: Environ- mental ingredients for supercells and tornadoes within Hur- their initial offshore examination and landfall. One ricane Ivan. Wea. Forecasting, 24, 223–244. cell showed signs of subsequently developing prom- Barnes, G. M., 2008: Atypical thermodynamic profiles in hurri- inent downdrafts and undergoing cyclic mesocyclo- canes. Mon. Wea. Rev., 136, 631–643.

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——, J. F. Gamache, M. A. LeMone, and G. J. Stossmeister, 1991: on Radar Meteorology, Austin, TX, Amer. Meteor. Soc., A convective cell in a hurricane rainband. Mon. Wea. Rev., 422–423. 119, 776–794. ——, F. D. Marks Jr., and F. Roux, 1995: Comparison of three Black, M. L., R. W. Burpee, and F. D. Marks Jr., 1996: Vertical airborne Doppler sampling techniques with airborne in situ motion characteristics of tropical cyclones determined with wind observations in Hurricane Gustav (1990). J. Atmos. airborne Doppler radial velocities. J. Atmos. Sci., 53, 1887– Oceanic Technol., 12, 171–181. 1909. Gentry, R. C., 1983: Genesis of tornadoes associated with hurri- Bluestein, H. B., and M. L. Weisman, 2000: The interaction of canes. Mon. Wea. Rev., 111, 1793–1805. numerically simulated supercells initiated along lines. Mon. Hagemeyer, B. C., 1997: Peninsular Florida tornado outbreaks. Wea. Rev., 128, 3128–3149. Wea. Forecasting, 12, 399–427. Bogner, P. B., G. M. Barnes, and J. L. Franklin, 2000: Conditional Hock, T. F., and J. L. Franklin, 1999: The NCAR GPS drop- instability and shear for six hurricanes over the Atlantic windsonde. Bull. Amer. Meteor. Soc., 80, 407–420. Ocean. Wea. Forecasting, 15, 192–207. Holton, J. R., 1992: An Introduction to Dynamic Meteorology. Bolton, D., 1980: The computation of equivalent potential tem- Academic Press, 511 pp. perature. Mon. Wea. Rev., 108, 1046–1053. James, R. P., J. M. Fritsch, and P. M. Markowski, 2005: Environ- Bosart, B. N., W.-C. Lee, and R. M. Wakimoto, 2002: Procedures mental distinctions between cellular and slabular convective to improve the accuracy of airborne Doppler radar data. lines. Mon. Wea. Rev., 133, 2669–2691. J. Atmos. Oceanic Technol., 19, 322–339. Jorgensen, D. P., 1984: Mesoscale and convective-scale charac- Brandes, E. A., 1984: Relationships between radar-derived ther- teristics of mature hurricanes. Part I: General observations by modynamic variables and tornadogenesis. Mon. Wea. Rev., research aircraft. J. Atmos. Sci., 41, 1268–1285. 112, 1033–1052. ——, P. H. Hildebrand, and C. L. Frusch, 1983: Feasibility test of Bunkers, M. J., B. A. Klimowski, J. W. Zeitler, R. L. Thompson, an airborne pulse-Doppler meteorological radar. J. Climate and M. L. Weisman, 2000: Predicting supercell motion using a Appl. Meteor., 22, 744–757. new hodograph technique. Wea. Forecasting, 15, 61–79. Klemp, J. B., and R. Rotunno, 1983: A study of the tornadic region Cione, J. J., P. G. Black, and S. H. Houston, 2000: Surface obser- within a supercell thunderstorm. J. Atmos. Sci., 40, 359–377. vations in the hurricane environment. Mon. Wea. Rev., 128, Lee, W.-C., M. M. Bell, and K. E. Goodman Jr., 2008: Super- 1550–1561. cells and mesocyclones in outer rainbands of Hurricane Curtis, L., 2004: Midlevel dry intrusions as a factor in tornado Katrina (2005). Geophys. Res. Lett., 35, L16803, doi:10.1029/ outbreaks associated with landfalling tropical cyclones from 2008GL034724. the Atlantic and Gulf of Mexico. Wea. Forecasting, 19, 411–427. Lemon, L. R., and C. A. Doswell III, 1979: Severe thunderstorm Davies-Jones, R., 1984: Streamwise vorticity: The origin of updraft evolution and mesocyclone structure as related to tornado- rotation in supercell storms. J. Atmos. Sci., 41, 2991–3006. genesis. Mon. Wea. Rev., 107, 1184–1197. ——, R. J. Trapp, and H. B. Bluestein, 2001: Tornadoes and tor- Liu, W. T., K. B. Katsaros, and J. A. Businger, 1979: Bulk pa- nadic storms. Severe Convective Storms, Meteor. Monogr., No. rameterizations of air–sea exchanges of heat and water vapor 50, Amer. Meteor. Soc., 167–221. including the molecular constraints of the interface. J. Atmos. Doswell, C. A., III, and E. N. Rasmussen, 1994: The effect of ne- Sci., 36, 1722–1735. glecting the virtual temperature correction on CAPE calcu- Lucas, C., E. J. Zipser, and M. A. LeMone, 1994: Vertical velocity lations. Wea. Forecasting, 9, 625–629. in oceanic convection off tropical Australia. J. Atmos. Sci., 51, Dunion, J. P., and C. S. Velden, 2004: The impact of the Saharan 3183–3193. air layer on Atlantic tropical cyclone activity. Bull. Amer. Markowski, P. M., J. M. Straka, and E. N. Rasmussen, 2002: Direct Meteor. Soc., 85, 353–365. surface thermodynamic observations within the rear-flank Eastin, M. D., P. G. Black, and W. M. Gray, 2002: Flight-level downdrafts of nontornadic and tornadic supercells. Mon. instrument wetting errors in hurricanes. Part I: Observations. Wea. Rev., 130, 1692–1721. Mon. Wea. Rev., 130, 825–841. Marks, F. D., Jr., R. A. Houze Jr., and J. F. Gamache, 1992: Dual- ——, W. M. Gray, and P. G. Black, 2005: Buoyancy of convective aircraft investigation of the inner core of Hurricane Norbert. vertical motions in the inner core of intense hurricanes. Part I: Part I: Kinematic structure. J. Atmos. Sci., 49, 919–942. General statistics. Mon. Wea. Rev., 133, 188–208. McCaul, E. W., Jr., 1991: Buoyancy and shear characteristics Edwards, R., and A. E. Pietrycha, 2006: Archetypes for surface of hurricane–tornado environments. Mon. Wea. Rev., 119, baroclinic boundaries influencing tropical cyclone tornado 1954–1978. occurrences. Preprints, 23rd Conf. on Severe Local Storms, St. ——, and M. L. Weisman, 1996: Simulation of shallow supercell Louis, MO, Amer. Meteor. Soc., P8.2. [Available online at storms in landfalling hurricane environments. Mon. Wea. http://ams.confex.com/ams/pdfpapers/114992.pdf.] Rev., 124, 408–429. Esterheld, J. M., and D. J. Giuliano, 2008: Discriminating between ——, and ——, 2001: The sensitivity of simulated supercell struc- tornadic and non-tornadic supercells: A new hodograph ture and intensity to variation in the shapes of environmental technique. Electron. J. Severe Storms Meteor., 3, 1–50. buoyancy and shear profiles. Mon. Wea. Rev., 129, 664–687. Franklin, J. L., M. L. Black, and K. Valde, 2003: GPS dropwind- ——, D. E. Buechler, S. J. Goodman, and M. Cammarata, 2004: sonde wind profiles in hurricanes and their operational im- Doppler radar and lightning network observations of a severe plications. Wea. Forecasting, 18, 32–44. outbreak of tropical cyclone tornadoes. Mon. Wea. Rev., 132, ——, R. J. Pasch, L. A. Avila, J. L. Beven II, M. B. Lawrence, S. R. 1747–1763. Stewart, and E. S. Blake, 2006: Atlantic hurricane season of Molinari, J., and D. Vollaro, 2008: Extreme helicity and intense 2004. Mon. Wea. Rev., 134, 981–1025. convective towers in Hurricane Bonnie. Mon. Wea. Rev., 136, Gamache, J. F., 1997: Evaluation of a fully three-dimensional 4355–4372. variational Doppler analysis technique. Preprints, 28th Conf. NCDC, 2004: Storm Data. Vol. 46, No. 9, 262 pp.

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Novlan, D. J., and W. M. Gray, 1974: Hurricane-spawned torna- Rotunno, R., and J. B. Klemp, 1982: The influence of the shear- does. Mon. Wea. Rev., 102, 476–488. induced pressure gradient on thunderstorm motion. Mon. Pielke, R. A., Sr., 2002: Mesoscale Meteorological Modeling. Ac- Wea. Rev., 110, 136–151. ademic Press, 676 pp. Schneider, D., and S. Sharp, 2007: Radar signatures of tropical Powell, M. D., 1990a: Boundary layer structure and dynamics in cyclone tornadoes in central North Carolina. Wea. Forecast- outer hurricane rainbands. Part I: Mesoscale rainfall and ki- ing, 22, 278–286. nematic structure. Mon. Wea. Rev., 118, 891–917. Spratt,S.M.,D.W.Sharp,P.Welsh,A.Sandrik,F.Alsheimer, ——, 1990b: Boundary layer structure and dynamics in outer and C. Paxton, 1997: A WSR-88D assessment of tropi- hurricane rainbands. Part II: Downdraft modification and cal cyclone outer rainband tornadoes. Wea. Forecasting, 12, mixed layer recovery. Mon. Wea. Rev., 118, 918–938. 479–501. ——, S. H. Houston, and T. Reinhold, 1996: Hurricane Andrew’s Suzuki, O., N. Hiroshi, O. Hisao, and N. Hiroshi, 2000: Tornado- landfall in south Florida. Part I: Standardizing measurements producing mini supercells associated with Typhoon 9019. for documentation of surface wind fields. Wea. Forecasting, Mon. Wea. Rev., 128, 1868–1882. 11, 304–328. Thompson, R. L., R. Edwards, and C. M. Mead, 2004: An update Ramsay, H. A., and C. A. Doswell III, 2005: A sensitivity study of to the supercell composite and significant tornado parameters. hodograph-based methods for estimating supercell motion. Preprints, 22nd Conf. on Severe Local Storms, Hyannis, MA, Wea. Forecasting, 20, 954–970. Amer. Meteor. Soc., P8.1. [Available online at http://ams. Rao, G. V., J. W. Scheck, R. Edwards, and J. T. Schaefer, 2005: confex.com/ams/pdfpapers/82100.pdf.] Structures of mesocirculations producing tornadoes associ- ——, C. M. Mead, and R. Edwards, 2007: Effective storm-relative ated with Tropical Cyclone Frances (1998). Pure Appl. Geo- helicity and bulk shear in supercell thunderstorm environ- phys., 162, 1627–1641. ments. Wea. Forecasting, 22, 102–115. Rasmussen, E. N., 2003: Refined supercell and tornado forecast Weisman, M. L., and J. B. Klemp, 1982: The dependence of nu- parameters. Wea. Forecasting, 18, 530–535. merically simulated convective storms on vertical wind shear Reasor, P. D., M. D. Eastin, and J. F. Gamache, 2009: Rap- and buoyancy. Mon. Wea. Rev., 110, 504–520. idly intensifying Hurricane Guillermo (1997). Part I: Low- Zipser, E. J., R. J. Meitin, and M. A. LeMone, 1981: Mesoscale wavenumber structure and evolution. Mon. Wea. Rev., 137, motion fields associated with a slowly moving GATE con- 603–631. vective band. J. Atmos. Sci., 38, 1725–1750.

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