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Comparisons of HRD and SLOSH Surface Wind Fields in Hurricanes: Implications for Modeling

SAMUEL H. HOUSTON Hurricane Research Division, NOAA/AOML, Miami,

WILSON A. SHAFFER Techniques Development Laboratory, NOAA/NWS/OSD, Silver Spring,

MARK D. POWELL Hurricane Research Division, NOAA/AOML, Miami, Florida

JYE CHEN Techniques Development Laboratory, NOAA/NWS/OSD, Silver Spring, Maryland

(Manuscript received 10 October 1998, in ®nal form 4 March 1999)

ABSTRACT Surface wind observations analyzed by the Hurricane Research Division (HRD) were compared to those computed by the parametric wind model used in the Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model's storm surge computations for seven cases in ®ve recent hurricanes. In six cases, the differences between the SLOSH and HRD surface peak wind speeds were 6% or less, but in one case (Hurricane Emily of 1993) the SLOSH computed peak wind speeds were 15% less than the HRD. In all seven cases, statistics for the modeled and analyzed wind ®elds showed that for the region of strongest winds, the mean SLOSH wind speed was 14% greater than that of the HRD and the mean in¯ow angle for SLOSH was 19Њ less than that of the HRD. The radii beyond the region of strongest winds in the seven cases had mean wind speed and in¯ow angle differences that were very small. The SLOSH computed peak storm surges usually compared closely to the observed values of storm surge in the region of the maximum wind speeds, except Hurricane Emily where SLOSH underestimated the peak surge. HRD's observation-based wind ®elds were input to SLOSH for storm surge hindcasts of Hurricanes Emily and Opal (1995). In Opal, the HRD input produced nearly the same computed storm surges as those computed from the SLOSH parametric wind model, and the calculated surge was insensitive to perturbations in the HRD wind ®eld. For Emily, observation-based winds produced a computed storm surge that was closer to the peak observed surge, con®rming that the computed surge in was sensitive to atmospheric forcing. Using real-time, observation-based winds in SLOSH would likely improve storm surge computations in landfalling hurricanes affected by synoptic and mesoscale factors that are not accounted for in parametric models (e.g., a strongly sheared environment, con- vective asymmetries, and stably strati®ed boundary layers). An accurate diagnosis of storm surge ¯ooding, based on the actual track and wind ®elds could be supplied to emergency management agencies, government of®cials, and utilities to help with damage assessment and recovery efforts.

1. Introduction by the National Weather Service's (NWS) Techniques Development Laboratory (TDL), provides the primary As the populations of coastal areas grow, more lead guidance used by emergency management of®cials to time is required to safely evacuate people from the dan- develop and carry out coastal evacuation plans. Jarvinen gers posed by hurricane storm surge (Jarrell et al. 1992). and Lawrence (1985) compared 523 storm surge height The Sea, Lake, and Overland Surges from Hurricanes observations (i.e., gauge measurements and high (SLOSH) model (Jelesnianski et al. 1992), developed water marks from the interior of buildings) to SLOSH hindcast values in 10 hurricanes, ®nding a 0.43-m mean absolute error with a standard deviation of 0.61 m and Corresponding author address: Sam Houston, HRD/AOML/ a bias of Ϫ0.09 m. This level of documented perfor- NOAA, 4301 Rickenbacker Causeway, Miami, FL 33149. mance has made SLOSH the primary tool for the Federal E-mail: [email protected] Emergency Management Agency's (FEMA) evacuation

Unauthenticated | Downloaded 09/30/21 11:39 PM UTC 672 WEATHER AND FORECASTING VOLUME 14 studies (V. Wiggert, Hurricane Research Division, 1998, level time series for Tropical Storm Marco (1990) and personal communication). The track forecasts of the found that SLOSH reasonably represented the wind Tropical Prediction Center's (TPC) National Hurricane ®elds and storm surges in two basins (i.e., two individual Center (NHC) have a mean error of 177 km within 24 SLOSH model domains) having relatively complicated h of (McAdie and Lawrence 1993). This 177- coastlines. A similar comparison was made for Hurri- km region of uncertainty near a coastline requires is- cane Andrew's (1992) landfall in south Florida (Powell suing warnings and commencing evacuations over a and Houston 1996, hereafter called PH96), which con- large region at least 24 h in advance of landfall. As part cluded that HRD's real-time wind ®eld information of comprehensive hurricane evacuation studies funded might be useful for improving storm surge and wave by FEMA, SLOSH is used to map the storm surge ¯ood model initializations and forecasts. Boundary layer and plain in each of 40 U.S. SLOSH basins. Atlases of storm surface wind observations in the hurricane's inner core surge ¯ooding are created for each basin based on cli- have been very scarce in the past, but the advent of new matologically likely storm directions, speeds, intensi- technologies, such as the Global Positioning System ties, and sizes. One such composite is the maximum (GPS) dropwindsondes described by Hock and Franklin envelope of water (MEOW), which depicts the highest (1999), are changing this. The dropwindsondes are ca- surges from a family of individual runs, all having the pable of making measurements in the boundary layer same intensity, forward speed, and landfall direction, and at the surface in the hurricane's eyewall. The HRD but varying in landfall location. These MEOWs are wind ®elds, which can be considered state-of-the-art available in advance to emergency managers and others representations of the surface winds in hurricanes, are who need to make decisions about evacuations of coast- capable of including these and other future surface ob- lines that may potentially be impacted by hurricane servations (e.g., drifting buoys) from the hostile envi- storm surge. ronment of the hurricane's inner core. Modeling technologies for estimating storm surge This paper compares SLOSH and HRD surface wind near the coast require realistic atmospheric forcing in ®elds for Hurricanes Hugo (1989), Bob (1991), Andrew tropical cyclones. SLOSH was designed to allow storm (1992), Emily (1993), and Opal (1995). Each hurricane surge computations to be made with limited knowledge caused a signi®cant storm surge by making landfall or of the storm's structure and intensity and to avoid un- approaching the U.S. shoreline (Fig. 1). A storm history certainties associated with input ®elds derived from hur- reference for each hurricane is listed in Table 1. The ricane wind measurements. SLOSH incorporates topog- observation-based HRD winds are assumed represen- raphy, channels, and barriers, and computes overland tative of each storm's ``true'' surface wind ®eld and were ¯ooding, but it does not include wave runup, wave set- used to validate the SLOSH parametric wind modeling up, or ¯ooding from rainfall (note that initial tide levels approach. The methodology used to produce SLOSH or astronomical can be added to the model results). and HRD winds is described in section 2. Section 3 Simple parametric wind models can force storm surges compares the SLOSH and HRD winds and section 4 for generic hurricanes, but the computed winds can dif- shows the impact of HRD wind ®elds on the SLOSH fer signi®cantly from the hurricane's actual wind ®eld model for two hurricanes. Discussion and concluding when the parameters are poorly speci®ed or insuf®cient. remarks are provided in section 5. For example, the convection from Hurricane Emily's (1993) western eyewall crossed eastern Pamlico Sound in and caused very strong surface winds 2. Methodology on the left side of the northward moving hurricane (Bur- a. SLOSH parametric wind model pee et al. 1994). Therefore, SLOSH's parametric wind model could not correctly compute Emily's nearly sym- The SLOSH parametric wind model is described by metric wind ®eld. In addition, the model uses lake winds Jelesnianski et al. (1992) and Houston and Powell over bays and inland lakes to account for the greater (1994). ``Snapshots'' of the two-dimensional SLOSH friction associated with marine wind trajectories af- model wind ®elds were created for the present study. fected by land (Jelesnianski et al. 1992; Houston and Model input parameters include the storm's track (which Powell 1994). These winds have more in¯ow and weak- provides translational motion for a 72-h period), radius er wind speeds than oceanic SLOSH winds. In Emily, of maximum winds (RMW), and pressure de®cit (⌬p; e.g., SLOSH signi®cantly underestimated the surface winds the value of ⌬p is 6.1 kPa if the environmental pressure and the resulting storm surge observed on the Pamlico is 101.0 kPa and the hurricane's central pressure is 94.9 Sound side of Cape Hatteras. kPa). SLOSH was designed to allow parameters to vary Recently, the National Oceanic and Atmospheric Ad- at 1-h intervals but is usually run with a 6-h interval. ministration (NOAA) Hurricane Research Division SLOSH input parameters for the seven landfall snap- (HRD) and TDL have compared observation-based shots of surface wind ®elds are listed in Table 2. The wind ®elds to those computed by the RMW was normally based on published reports, except SLOSH model. Houston and Powell (1994) compared in the case of Emily and Bob where the values of RMW observed and simulated winds, storm surges, and water- were determined from surface and aircraft observations.

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FIG. 1. The location (open circle) and time (UTC) of each hurricane used in this study. Track segments with arrows indicate the direction of motion of each hurricane (Emily's track segment is dashed to distinguish it from Bob's near NC and Andrew's land®ll dates are shown).

The circulation center of the SLOSH model parametric the SLOSH and HRD winds, the center of circulation wind ®eld is the geometric center of the regardless was assumed to be the midpoint between their centers. of the hurricane's forward motion. When the storm is at rest, the circulation center and geometric center of the storm are theoretically in the same location (i.e., if b. HRD surface wind ®elds one ignores convection and other in¯uences). Once the HRD's surface wind ®elds were based on all available hurricane has forward motion, the air in the original calm geometric center moves at the storm speed. The surface wind observations from buoys, Coastal±Marine result is that the calm portion of the eye shifts to the Automated Network platforms, ships, and other surface left (when facing the direction of a hurricane's forward facilities. Because these data are often sparse near hur- motion). Therefore, the snapshot of the SLOSH model's ricanes, aircraft ¯ight-level observations [adjusted to the circulation center may vary slightly from the one used surface with a planetary boundary layer model (Powell in the HRD wind ®elds. For comparisons made between 1980)] were used to supplement the in situ surface mea- surements. The method used to create comprehensive datasets conforming to a common framework for ex- TABLE 1. Reference for the storm history of each hurricane used posure, height, and averaging time was described by in the study. Powell et al. (1996). These data were then objectively Hurricane Storm history analyzed with a technique based upon the spectral ap- Hugo (1989) Case and May®eld (1990) plication of ®nite element representation (SAFER) Bob (1991) Pasch and Avila (1992) method (Ooyama 1987; DeMaria et al. 1992; Franklin Andrew (1992) May®eld et al. (1994) et al. 1993). The resulting gridded wind ®elds have es- Emily (1993) Pasch and Rappaport (1995) timated errors of 10%±20%, compared to errors of 20%± Opal (1995) Lawrence et al. (1998) 40% if procedures are not followed to provide a com-

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TABLE 2. Parameters input into SLOSH in each hurricane ``snapshot'' at landfall by state is listed with the time, date, pressure de®cit (⌬p),

radius of maximum wind (RMW), translation speed (Vt), and heading.

Landfall Time ⌬p RMW Vt Heading Hurricane state (UTC) Date (kPa) (km) (m sϪ1) (Њ) Hugo SC 0405 22 Sep 1989 7.6 46.0 12.3 330 Bob NC 0250 19 Aug 1991 5.4 38.0 9.8 25 Bob RI 1810 19 Aug 1991 4.6 62.8 16.1 35 Andrew FL 0830 24 Aug 1992 8.7 14.5 8.0 275 Andrew LA 0530 26 Aug 1992 6.5 37.0 4.9 325 Emily NC 2200 31 Aug 1993 5.2 43.0 5.1 15 Opal FL 2140 04 Oct 1995 6.4 86.0 10.3 20 mon observation framework (Powell et al. 1996). Since were included in Table 3. The analysis ®lter wavelength Hurricane Emily (Burpee et al. 1994), HRD surface [␭ de®ned by Powell et al. (1996)] acted as a low-pass wind ®elds have been provided to hurricane specialists ®lter to reduce observational noise associated with ex- at NHC on a real-time experimental basis as guidance posure and sampling differences, including small-scale for wind radii (17.5, 25, and 33 m sϪ1) issued in tropical wind features (e.g., turbulent and convective gusts and cyclone advisories. Nearly 100 real-time surface wind lulls) that cannot be adequately resolved by the available analyses were provided to NHC for the Atlantic basin observations. The values were also chosen to allow the each hurricane season from 1995 to 1998. resolution of mesoscale features, such as the vortex and Since the surface data collection network was sparse, rainband wind maxima. Because of the smoothing in- surface observations were collected over an extended herent in the objective analysis scheme, a method of time interval prior to landfall, which ranged from 3 h amplitude restoration was applied to the VMESO located in Emily to 7 h for Hurricanes Hugo and Bob (Table at radii within 1.5RMW to ensure that the maximum 10- 3). These observations were transformed to a hurricane- min sustained wind was contained in the ®nal analysis centered coordinate system for analysis. Implicit in this output ®eld. The values of ␭ in Table 3 were chosen to analysis was the assumption of invariant storm intensity minimize the smoothing of the observed peak wind during the data collection period. This assumption was speed by the SAFER method and ranged from 12.2 km considered valid prior to landfall, since the largest for Andrew's south Florida landfall to 28.9 km for Bob's changes in intensities observed in these storms usually landfall in New England. As an example, a ␭ of 22 km Ϫ1 occurred after the eye crossed the coastline. and a VMESO of 36 m s would result in a timescale of For oceanographic modeling, wind speed averaging over 20 min (i.e., t ϭ 2␭ Ϭ VMESO). Hence, VM10 over times of at least 10 min are normally considered to be a 20-min period would be 2% higher (i.e., G10 ϭ 1.02) Ϫ1 representative of timescales associated with oceanic re- than VMESO, or 36.7 m s . sponse to surface stress. The SLOSH model wind speeds The sensitivity of the SAFER method to the distri- are considered equivalent to 10-min means (Jelesnianski bution of wind observations and its ability to reproduce et al. 1992; Houston and Powell 1994). For comparison a known or model-generated wind ®eld is discussed in the appendix. These tests indicated that the data cov- purposes, the HRD mesoscale analysis winds (VMESO) were adjusted to produce maximum 10-min sustained erage was adequate in each case and the SAFER tech- nique correctly reproduced known surface wind ®elds winds using a gust factor relationship (G10) described by Houston and Powell (1994). The resulting winds based on the available wind observations. were maximum 10-min sustained, 10-m marine surface 3. Comparisons of SLOSH and observation-based winds (VM10). Each hurricane's analysis domain was centered on the wind ®elds storm near the time of its closest approach to the coast- Gridpoint differences between HRD's observation- line and the time ranges of the data input to the analysis based wind ®elds and SLOSH wind ®elds (hereafter

TABLE 3. Wavelength and time range (beginning and ending hours) for the data used to create the VHRD winds for each hurricane landfall. Landfall Wavelength Begin End Hurricane state (km) (UTC) (UTC) Hugo SC 16.7 2200 (21 Sep) 0500 (22 Sep) Bob NC 16.7 2300 (18 Aug) 0600 (19 Aug) Bob RI 28.9 1615 2120 Andrew FL 12.2 0410 1030 Andrew LA 22.2 0300 0830 Emily NC 27.8 2030 2330 Opal FL 21.1 2040 (4 Oct) 0300 (5 Oct)

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referred to as VHRD and VSLOSH, respectively) were ex- 1) HURRICANE BOB amined using the methods described in the appendix. In this case, the V at each grid point was subtracted The VHRD for Hurricane Bob early on 19 August 1991 SLOSH during its closest approach to Cape Hatteras, North Car- from the VHRD at the corresponding grid point. The same grid spacing was used and the statistics for the nor- olina (Fig. 3a), depicted the highest wind speeds to the right of the storm (Bob was moving north-northeast). malized wind speed difference [NWSD; i.e., (VHRD Ϫ V ) V ] and in¯ow angle difference (IAD) will The VSLOSH speeds were ϳ10% greater than VHRD speeds SLOSH Ϭ HRD Ϫ1 be examined at the end of this section. near the location of peak winds (Ͼ45ms ), while the peak VSLOSH and VHRD speeds were nearly the same (Table 4). Beyond the area of peak winds, VHRD speeds (Fig. a. Hurricane Hugo (1989) 3a) decreased less with increasing radius, resulting in speeds 10%±40% larger than the VSLOSH speeds (Fig. Hurricane Hugo's 6.1-m maximum storm surge in the 3b). The VSLOSH in the vicinity of Cape Hatteras over Bulls Bay area of (Case and May®eld Pamlico Sound, which were lake winds, were at least

1990) agreed closely with the peak SLOSH-computed 40% weaker than the VHRD. Despite these large differ- storm surge (NOAA 1990). For the Hurricane Hugo ences, the observed and SLOSH-computed storm surges

VHRD, the aircraft ¯ight-level data above 3 km were ad- for the North Carolina (including the Pam- justed to the surface as described by Powell et al. (1991). lico Sound side where the RMW on the left side of Bob Ϫ1 The VHRD greater than 50 m s covered a large area was east of Cape Hatteras) were nearly the same. The extending from Bulls Bay into the Atlantic Ocean (Fig. IADs (Fig. 3d) indicated that VSLOSH in¯ow angles were

2a), which closely agreed with the area of VSLOSH speeds 10Њ±20Њ larger than the VHRD in¯ow angles on the north greater than 50 m sϪ1 (Fig. 2b) and the location of the side of the storm (Fig. 3d) and up to 40Њ less near the peak surge along the coast. The peak VSLOSH speed was rear of the storm's core. Ϫ1 3ms less than the peak VHRD speed (Table 4). The The peak SLOSH-computed storm surges (based on the parametric wind ®eld) were close to the observed VSLOSH speeds near the northern South Carolina coast surges along the North Carolina coast in this case. How- decreased more rapidly beyond 1.5RMW than the VHRD speeds. ever, because the VSLOSH were lower than the VHRD well The largest variations in the NWSD ®eld (Fig. 2c) east of Bob's center, a wave model using the VSLOSH as input would likely underestimate the height of large occurred in the rear of the storm where the VSLOSH speeds were over 30% stronger, while the differences were less oceanic waves generated east of Bob. than 10% in the vicinity of the eyewall near Bulls Bay. The V speeds over the northern South Carolina coast HRD 2) HURRICANE EMILY were from 10% to 20% greater than the VSLOSH speeds.

SLOSH-computed storm surges values were also less Peak VHRD speeds in Emily (Fig. 4a) were nearly the than the observed high-water levels along the coast of same strength to the left and right of the northward- northern South Carolina over 100 km from the storm moving storm during its closest approach to Cape Hat- center (NOAA 1990). This underestimate was likely due teras (Burpee et al. 1994). The VHRD were approximately Ϫ1 to the combination of weaker VSLOSH in the region and northerly at speeds greater than 40 m s over most of the lack of wave effects in the model. The IADs (Fig. eastern Pamlico Sound (Fig. 4a). The peak VHRD speed 2d) ranged from Ϫ10Њ to Ϫ20Њ near Bulls Bay, indi- for Emily was more than 7 m sϪ1 greater than the peak cating that the VHRD had greater in¯ow than the VSLOSH VSLOSH speed (Table 4) and Fig. 4b indicates the area of in the peak wind region. IADs were mostly negative strongest VSLOSH speeds was located to the right of the throughout the domain with the VHRD having 40Њ more storm center (again, the VSLOSH over Pamlico Sound were in¯ow in the rear of the storm. Insuf®cient in¯ow does lake winds). The VSLOSH speeds were from 30% to 40% not appear to be signi®cant for peak storm surge com- less than the VHRD speeds over both coasts of the Outer putations, but might be important for computing water- Banks (Fig. 4c). The IADs (Fig. 4d) indicated that VSLOSH level time series and wave computations that consider in¯ows were very weak throughout most of the eyewall ocean currents and the direction of wave motion. region. The strong winds in Emily's western eyewall over the shallow waters of Pamlico Sound and the nearly 90Њ b. Hurricanes Bob (1991) and Emily (1993) near angle shape of the sound's shoreline near Cape Hatteras North Carolina resulted in an observed peak storm surge (ϳ3.4 m) near- ly twice the SLOSH-computed storm surge (Pasch and Hurricanes Bob and Emily both passed near Diamond Rappaport 1995). Only by setting SLOSH input param-

Shoals east of the North Carolina coast (Fig. 1). Hur- eters to unrealistic values could VSLOSH be increased over ricane Bob made landfall in Rhode Island 15 h after Pamlico Sound. However, this would likely result in passing offshore from North Carolina, while Emily later overprediction of wind speeds (and accompanying surge turned out to sea. and waves) on the right side of Emily.

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FIG. 2. Wind ®elds over water for Hurricane Hugo at 0405 UTC 22 Sep 1989. The winds are displayed as streamline and isotachs (units Ϫ1 of wind speed are m s ). The range rings are 0.75RMW (inner), 1.50RMW (middle), and 158 km (outer) from the storm center. The winds in

(a) are the VHRD and the winds in (b) are the VSLOSH. The difference ®elds are (c) the NWSD ®eld (%) and (d) the IAD (Њ) where dashed contours are negative.

TABLE 4. The peak VHRD and VSLOSH wind speeds for each hurricane c. Hurricane Andrew (1992) landfall.

The VHRD, VSLOSH, SLOSH-computed storm surges, VHRD VSLOSH Hurricane (m sϪ1) (m sϪ1) and the observed high-water marks for Hurricane An- drew's landfall near Homestead, Florida, at 0830 UTC Hugo 57.7 54.4 24 August 1992 were discussed in PH96. Peak VSLOSH Bob (NC) 46.4 46.1 Ϫ1 Bob (RI) 41.2 41.0 speeds north of the eye were 2±3 m s higher than the Andrew (FL) 60.9 63.1 VHRD speeds, but covered a much larger portion of the Andrew (LA) 50.6 48.9 eyewall and were located closer to the storm center. Peak Emily 49.3 41.9 VSLOSH speeds near the southern eyewall were about 10 Opal 42.8 42.0 Ϫ1 ms larger than the VHRD speeds. The computed

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FIG. 3. Same as Fig. 2 except for Hurricane Bob at 0250 UTC 19 Aug 1991 and the outer radius was 153 km.

SLOSH storm surges were slightly less than the ob- near the lower threshold of category 3 strength at land- served peak, but values to the north were 1±1.5 m higher fall with a very asymmetric wind ®eld (Powell and than the measured values. The underestimate of the peak Houston 1998). Opal's strong wind speeds while it was surge may have been a result of using lake winds over a category 4 hurricane produced very high waves over all of Biscayne Bay and the lack of wave effects in the the Gulf of Mexico [e.g., a signi®cant wave height of model. 8.3 m was observed at National Data Buoy Center (NDBC) buoy 42001 on the left side of Opal during the hurricane's closest approach to the buoy]. These waves d. Hurricane Opal (1995) later arrived at the Florida panhandle beaches where the Hurricane Opal was a category 4 hurricane based on bottom topography offshore, coastal relief, shape of the the Saf®r±Simpson scale (Saf®r 1977; Simpson and coastline, and storm surge resulted in very large mea- Riehl 1981) less than 12 h prior to landfall in the Florida sured-outside high-water marks (B. Jarvinen, TPC, panhandle (Lawrence et al. 1998). Opal weakened to 1996, personal communication). The storm surges

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FIG. 4. Same as Fig. 2 except for Hurricane Emily at 2200 UTC 31 Aug 1993 and the outer radius was 150 km.

(without waves) recorded by tide gauges in the area were for all grid points at radii ranging from 0.75RMW to close to the SLOSH-computed storm surges. There was 1.50RMW were used to determine variations in the vi- Ϫ1 only a small area of over 40 m s speeds in the VHRD cinities of hurricanes that have the highest winds, largest to the right of the northward moving storm at landfall storm surges, and the largest waves. The VSLOSH speeds

(Fig. 5a). Opal's VSLOSH (Fig. 5b) contained a large area were greater than the VHRD speeds (i.e., NWSD were of over 40 m sϪ1 speeds east of the storm center. The positive) in all cases, except for Emily, which had a Ϫ1 area of VHRD speeds over 40 m s was much larger prior mean (␮)ofϪ16%. The NWSD values of ␮ for the to landfall at 1830 UTC (Fig. 5c). other hurricanes ranged from ϩ3% for Bob (NC) to ϩ29% for Bob (RI). Negative IADs con®rm that the e. Statistics for difference ®elds VHRD have between 11Њ and 25Њ more in¯ow than VSLOSH

Gridpoint differences between the VSLOSH and VHRD within the region of highest winds. were examined for all seven cases. In Table 5, statistics Although the peak surge, winds, and waves are not

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FIG. 5. Same as Figs. 2a and 2b except for Hurricane Opal at 2140 UTC 4 Oct 1995 and the outer radius was 153 km; (c) the

1830 UTC VHRD (without range rings).

contained in the outer region of the storm (radius Ͼ

1.5RMW), damaging storm effects still occur here. In TABLE 5. Statistics [means and standard deviations (shown in pa- Table 6, the VHRD for Bob (NC) and Emily were 15% rentheses)] for NWSD and IAD for radii over water from 0.75 R MW and 10%, respectively, larger than the VSLOSH in the outer to 1.50 RMW. region. The VSLOSH were from 3% to 27% greater than NWSD IAD the VHRD for the other ®ve cases. The magnitudes of the Hurricane Vicinity n (%) (Њ) IAD for the VSLOSH and VHRD in the outer region were Hugo 203 ϩ24 (18) Ϫ24 (10) less than 10Њ for all seven cases. Bob NC 302 ϩ3 (9) Ϫ11 (12) Bob RI 354 ϩ29 (15) Ϫ22 (12) Andrew FL 50 ϩ10 (9) Ϫ17 (7) 4. SLOSH hindcasts in Emily and Opal using VHRD Andrew LA 198 ϩ19 (13) Ϫ14 (8) The V from Emily and Opal were put into SLOSH Emily 389 Ϫ16 (10) Ϫ12 (7) HRD Opal 841 ϩ18 (12) Ϫ25 (13) to test the feasibility of making storm surge computa- tions using observation-based wind ®elds. Opal's broad All cases 2337 ϩ14 (18) Ϫ19 (13) wind ®eld and large RMW prior to and during landfall

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TABLE 6. Same as Table 5 except for radii from 1.50 RMW to the would require a time-interpolation scheme for SLOSH outer radius. to allow for input at intervals as small as 1 h). The NWSD IAD steady-state assumption was reasonable for Hurricane Hurricane n (%) (Њ) Emily (Fig. 4a), but Hurricane Opal was rapidly weak- Hugo 1208 ϩ5 (17) Ϫ9 (6) ening prior to landfall, invalidating the invariant wind Bob (NC) 2217 Ϫ15 (13) ϩ1 (10) ®eld assumption in this case. The 1830 UTC VHRD was Bob (RI) 1045 ϩ27 (17) Ϫ3 (9) used for Opal, since it was considered more represen- Andrew (FL) 543 ϩ3 (10) Ϫ5 (13) tative of the hurricane as it moved over the continental Andrew (LA) 857 ϩ20 (13) Ϫ3 (9) Emily 1958 Ϫ10 (8) ϩ5 (5) shelf prior to landfall. Opal 411 ϩ16 (10) Ϫ9 (9) SLOSH model runs with the VHRD and VSLOSH for Opal All cases 8239 ϩ1 (20) Ϫ1 (10) at landfall (Fig. 6) showed good agreement. The largest storm surge for the VHRD was ϳ2.9 m, but it was dis- placed nearly 40 km closer to Opal's track than the (Powell and Houston 1998) indicated large asymmetries SLOSH-computed peak surge height. This likely re¯ects and the strongest winds were primarily located on the the stronger winds in the HRD wind ®elds nearer the right-hand side of the storm as it moved rapidly north- center. The model-computed storm surges compared ward (Figs. 5a and 5c). Also, Opal's track was nearly well to the observed values at three tide gauges at the perpendicular to the coastline at landfall (Fig. 1). In coast. contrast, Hurricane Emily's slow forward motion and For Emily, the oceanic VSLOSH over eastern Pamlico Ϫ1 distribution of eyewall convection resulted in a nearly Sound near Cape Hatteras were at least 10 m s weaker symmetric surface wind ®eld [see Fig. 4a and Burpee than the VHRD speeds (Fig. 7), resulting in the peak com- et al. (1994)] with very strong winds over Pamlico puted storm surge at Cape Hatteras being 1.2 m less Sound as the hurricane traveled northward nearly par- than the observed. The peak storm surge computed us- allel to the coast east of Cape Hatteras. ing the VHRD in SLOSH was only 0.4 m less than the observed. The VHRD were input into the SLOSH model to com- pute maximum surges by assuming the wind and surface pressure ®elds were steady state for 28 h in Emily and 5. Discussion and conclusions 30 h for Opal. This required a redesign of SLOSH to use a full nonparametric invariant hurricane wind ®eld In general, SLOSH wind ®elds were very similar to as input (note that the use of time-varying VHRD as input the HRD observation-based wind ®elds in the region of

FIG. 6. Hurricane Opal's computed peak storm surges along the Gulf Coast between 85Њ and

89ЊW longitude using the VSLOSH and VHRD (1830 UTC) winds as input. The observed peak storm surges from tide gauges (large open squares) in the area of Opal's landfall are shown.

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FIG. 7. Time series of (a) Hurricane Emily's VSLOSH (ocean) and VHRD (2200 UTC) and storm surge from 1400 UTC 31 Aug to 1600 UTC 1 Sep 1993 wind speeds (m sϪ1) and (b) computed peak storm surge values on the Pamlico Sound side of Cape Hatteras, NC. the highest winds for six of the seven hurricane cases ami were underestimated approximately 0.5 m by studied. Two regions of winds were considered when SLOSH, while the SLOSH-computed surges to the north computing statistics for the differences between the of the peak were overestimated by over 1.0 m (PH96).

VSLOSH and VHRD for all seven cases: an inner region The weaker VSLOSH over portions of Biscayne Bay (lake containing radii ranging from 0.75RMW to 1.50RMW and winds) may account for some of the differences in the an outer region containing radii ranging from 1.50RMW area of the peak surge in Cutler Ridge, which has a to an outer radius (which was based on the size of each mostly open exposure from the Atlantic Ocean for east- storm's domain). In the inner region, the SLOSH wind erly winds (note that not being able to include wave speeds were an average of 14% greater than the VHRD effects in SLOSH computations may have also contrib- speeds and the mean in¯ow angles for SLOSH averaged uted to the underestimate of the computed peak storm

19Њ less than the VHRD in¯ow angles. The mean differ- surge here). Discerning overwater versus overland tra- ences for the wind speeds and in¯ow angles in the outer jectories, if included, may have potentially improved region were Ϫ1% and ϩ1Њ, respectively. the SLOSH model computations for Andrew's landfall

Hurricane Emily was a major exception since the in south Florida. The VSLOSH lake winds that were ¯ow- VSLOSH speeds were much weaker than VHRD speeds in ing from Miami Beach and Key Biscayne southwest- the inner region. This underestimate resulted in a peak ward across Biscayne Bay would change to an oceanic SLOSH-computed storm surge over Pamlico Sound that trajectory in the location of the observed peak storm was only one-half of the observed height. Emily's sym- surge as the winds shifted to a more easterly direction. metric wind structure was in¯uenced by strong con- Determining ¯ow trajectories may improve the accuracy vection on the left side of the storm (Burpee et al. 1994). of SLOSH-computed storm surge near the observed For Andrew's landfall in south Florida, the observed maximum water marks in this case. Other SLOSH basins high-water marks in the Cutler Ridge area south of Mi- may bene®t from local trajectory distinctions in certain

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Differences between the VHRD and VSLOSH for the hur- Some examples include 1) Hurricane Emily, which had ricane wind ®elds examined were shown to have been intense convection on the left side according to Burpee generally small near the RMW in most cases, but the et al. (1994); 2) Hurricane Bob, which showed the ef- differences often increased at radii beyond the region fects of fast forward motion, especially evident on the of peak winds. For example, in the Bulls Bay area of left and left-rear portion of the storm's circulation (these

South Carolina, the VHRD and VSLOSH were relatively effects are mostly observed north of 30ЊN latitude); 3) close and the observed high-water marks and SLOSH- major hurricanes with concentric eyewalls such as Allen computed storm surges were nearly the same in Hur- (1980) and Gilbert (1988) described by Willoughby et ricane Hugo. However, the VSLOSH were weaker than al. (1982); 4) storms in¯uenced by strong environmental VHRD to the northeast of the center at outer radii; hence, vertical wind shear [e.g., Hurricane Norbert (1984) de- the SLOSH-computed storm surge decreased more rap- scribed by Marks et al. (1992)]; 5) cases in northern idly than the observed surges along the northern South latitudes where the hurricane's circulation causes warm Carolina coast 100 km northeast of the storm center (the air advection over cold water bodies resulting in a stably inclusion of waves in SLOSH could conceivably have strati®ed boundary layer (Powell and Black 1990); and improved the computed storm surges here). 6) cases with rapid intensi®cation where the inner core For Opal, the SLOSH model storm surge computa- and outer wind radii respond differently (provided data tions matched well with the water levels observed by coverage and timeliness are suf®cient to document such tide gauges in the area. Wave setup and runup added to changes). the surge resulted in debris line high-water marks more An accurate diagnosis of storm surge ¯ooding, based than twice as large as the tide gauge measurements and on observation-based wind ®elds, can be supplied to

SLOSH-computed storm surges. The VSLOSH were some- emergency management agencies and government of- times less than the VHRD wind ®elds for outer wind radii ®cials responsible for identifying areas requiring search according to Table 6, especially in Hurricanes Bob (NC) and rescue and to identify damages to transportation, and Emily. The VSLOSH in¯ow angles in the inner core communication, and utility systems. were less than the VHRD in¯ow angles, but show little The wind ®elds produced in real time and after the difference in the outer region. Future versions of the fact by HRD are now being made available on a Web SLOSH model, which might contain computations of site (http://www.aoml.noaa.gov/hrd) for researchers to wave heights, directions, and oceanic currents, would use in comparisons with model output or with remote likely be sensitive to wind direction errors. wind sensing platforms (e.g., Special Sensor Micro- The large differences between surge measurements wave/Imager and the Second European Remote Sensing and high-water marks point to the need for future models Satellite). Tests of the feasibility of using real-time HRD to incorporate both surge and waves. Possible augmen- surface winds in the SLOSH model for landfalling hur- tation of the current SLOSH model to include wave ricanes are currently being planned for future hurricane effects (e.g., wave setup, wave runup, etc.) is under seasons (possibly in 1999). investigation. Despite signi®cant improvements in wave modeling technologies during the last 10 years [Dr. Rob- Acknowledgments. B. Jarvinen (TPC) provided some ert Jensen, Coastal Hydraulic Laboratory/U.S. Army input parameters used for SLOSH and the observed Corps of Engineers (USACE), 1997, personal com- storm surges for Hurricane Opal. C. Bakker and L. Amat munication], hurricane wave simulation and validation (HRD) developed software for data visualization. J. studies have been limited because of the lack of high- Franklin and S. Feuer (HRD) assisted with the surface resolution wind ®elds and directional wave data for ver- wind analyses. R. Kohler (HRD) and R. St. Fleur i®cation. These problems make it dif®cult to distinguish (MAST Academy) assisted with the display of gridded whether absolute errors in wave height computation wind ®elds. Special thanks to these dedicated crews for were caused by wave model de®ciency or inherent errors providing ¯ight-level data, which were essential for pro- in the winds. ducing the HRD surface wind ®elds: the 53d Weather The SLOSH parametric model uses a simple wind Squadron of the U.S. Air Force Reserves and the NOAA model, designed for use in an operational forecasting Aircraft Operations Center. Other essential meteorolog- environment. The model gives a good representation of ical observations were provided by NOS, NWS (in- hurricane surface winds, based on minimal input and is cluding the NDBC and the National Climatic Data Cen- recommended when observations are insuf®cient to de- ter), and the Department of Defense. High-water marks ®ne the wind ®eld. Real-time operational storm surge were provided by the U.S. Geological Survey and USA- predictions for coastal areas threatened by landfall rely CE (with funding from FEMA). S. Rogers (NC Sea on forecasts of the storm's track, intensity (central pres- Grant) provided additional observations in Hurricane sure), size (RMW), and outer wind characteristics, which Opal. Reviews of the manuscript by V. Wiggert and J. are all subject to large errors. The use of observation- Cione of HRD and two anonymous reviewers were ex- based winds in SLOSH would likely be most bene®cial tremely helpful.

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APPENDIX point in the hurricane's domain, these model winds were used as the basis for the tests. Sensitivity Tests for the HRD Winds In the ®rst sensitivity test for the HRD analysis meth- It was decided that a method for testing the adequacy od, the known or model-generated wind ®eld was cre- of the data coverage and evaluating the amplitude res- ated for each of the seven landfall cases using the SLOSH parametric wind model to de®ne a wind every- toration used to generate the VHRD was necessary for each of the cases examined here. In addition, a deter- where in the domain. The SLOSH winds were then lin- mination of which regions of the hurricane's domain early interpolated at actual observation locations to cre- (i.e., near the maximum winds and beyond) should be ate a set of synthetic wind observations for each case used to make statistical comparisons between the VHRD (e.g., in Hurricane Bob shown in Fig. A1). The SAFER and VSLOSH was needed. Because the SLOSH parametric method was then applied to the synthetic observations wind model computes wind speed and direction at every and amplitude restoration was used on the synthetic

FIG. A1. Synthetic winds for Hurricane Bob at 0250 UTC 19 Aug 1991 where (a) shows the synthetic wind barbs used to compute the

(b) VSYN (isotachs and streamlines), (c) NWSD (%), and (d) IAD (Њ).

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TABLE A1. Sensitivity test with means and standard deviations (shown in parentheses) for VSYN minus known winds. The NWSD and IAD

are shown for radii over water from 0.75 RMW to 1.50 RMW.

0.75 RMW 1.50 RMW NWSD IAD Hurricane n (km) (km) (%) (Њ) Hugo 203 34.5 69.0 ϩ1 (2) ϩ2 (2) Bob (NC) 302 28.5 57.0 0 (4) 0 (5) Bob (RI) 354 47.3 94.5 ϩ1 (2) ϩ1 (2) Andrew (FL) 50 10.9 21.8 Ϫ3 (3) 0 (1) Andrew (LA) 198 27.8 55.5 ϩ2 (2) 0 (4) Emily 389 32.3 64.5 ϩ2 (3) 0 (3) Opal 841 64.5 129.0 Ϫ1 (4) Ϫ1 (2) All cases 2337 ϩ0.3 (3.5) ϩ0.2 (2.9)

TABLE A2. Same sensitivity test as in Table A1 except for radii ranging from 1.50 RMW to the outer radius.

1.50 RMW Outer NWSD IAD Hurricane n (km) (km) (%) (Њ) Hugo 1208 69.0 158.0 ϩ5 (4) Ϫ1 (2) Bob (NC) 2217 57.0 153.0 ϩ5 (6) Ϫ1 (2) Bob (RI) 1045 94.5 150.0 Ϫ4 (6) Ϫ1 (2) Andrew (FL) 543 21.8 69.0 ϩ4 (4) Ϫ1 (1) Andrew (LA) 857 55.5 122.0 ϩ1 (2) 0 (2) Emily 1958 64.5 150.0 ϩ1 (5) 0 (2) Opal 411 129.0 153.0 Ϫ5 (4) ϩ1 (2) All cases 8239 ϩ1.9 (6.0) Ϫ0.4 (1.9)

TABLE A3. Same as Table A1 except for all observed winds minus VHRD for radii over water from 0.75 RMW to 1.50 RMW.

Landfall 0.75 RMW 1.50 RMW NWSD IAD Hurricane state n (km) (km) (%) (Њ) Hugo SC 24 34.5 69.0 0 (7) 0 (4) Bob NC 29 28.5 57.0 0 (6) ϩ2 (8) Bob RI 37 47.3 94.5 Ϫ3 (8) ϩ1 (7) Andrew FL 21 10.9 21.8 ϩ4 (10) 0 (5) Andrew LA 24 27.8 55.5 ϩ2 (11) Ϫ3 (6) Emily NC 42 32.3 64.5 ϩ3 (13) ϩ1 (11) Opal FL 49 64.5 129.0 Ϫ1 (5) 0 (3) All cases 226 ϩ0.4 (9.1) ϩ0.2 (6.9)

TABLE A4. Same as Table A3 except for radii over water from 1.50 RMW to the outer radius.

Landfall 1.50 RMW Outer NWSD IAD Hurricane state n (km) (km) (%) (Њ) Hugo SC 68 69.0 158.0 Ϫ5 (10) ϩ1 (7) Bob NC 91 57.0 153.0 Ϫ1 (11) 0 (7) Bob RI 46 94.5 150.0 Ϫ3 (14) Ϫ1 (5) Andrew FL 85 21.8 69.0 Ϫ3 (12) 0 (6) Andrew LA 102 55.5 122.0 ϩ7 (9) Ϫ5 (5) Emily NC 80 64.5 150.0 Ϫ6 (8) ϩ1 (6) Opal FL 7 129.0 153.0 Ϫ5 (10) 0 (3) All cases 479 Ϫ0.2 (13.0) Ϫ0.7 (6.8)

VMESO for radii within 1.5RMW. The result was a wind to the known ®eld by computing the difference for wind ®eld based on synthetic observations (VSYN) that could speeds and in¯ow angles between the known and VSYN then be compared with the known (i.e., SLOSH in this at all grid points. The grid spacing used in each case case) wind ®eld for each hurricane (Fig. A1b). If the was 0.5Њ latitude ϫ 0.5Њ longitude, except for Hurricane two wind ®elds compared well, the HRD analysis meth- Andrew in south Florida, which had a grid spacing of od faithfully reproduced the known wind ®eld. 0.4Њ latitude ϫ 0.4Њ longitude. The wind speed differ- The analyzed synthetic observations were compared ences at each grid point were divided by the known

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TABLE A5. Same as Table A1 except for in situ surface winds only minus VHRD for radii over water from 0.50 RMW to the outer radius.

Landfall 0.50 RMW Outer NWSD IAD Hurricane state n (km) (km) (%) (Њ) Hugo SC 9 23.0 158.0 Ϫ12 (14) ϩ11 (10) Bob NC 41 19.0 153.0 Ϫ4 (13) ϩ5 (9) Bob RI 58 31.4 150.0 Ϫ7 (11) ϩ1 (7) Andrew FL 36 7.3 69.0 Ϫ7 (14) ϩ1 (6) Andrew LA 0 18.5 122.0 Ð Ð Emily NC 37 21.5 150.0 Ϫ6 (14) ϩ3 (12) Opal FL 19 43.0 153.0 Ϫ4 (7) 0 (2) All cases 200 Ϫ6.0 (13.0) ϩ2.5 (8.7) wind speed at the same grid point to produce NWSDs; the IAD ␮ was Ϫ1Њ (␴ ϭ 7Њ). This test indicated that the IAD were not normalized. Examples of these dif- the SAFER technique correctly reproduced the wind ference ®elds for Hurricane Bob are shown in Figs. A1c ®elds in the vicinity of the actual observations that were and A1d. used to generate the VHRD ®elds. Differences between the known and VSYN were largest The ®nal sensitivity test used the same technique to in the vicinity of each hurricane's eye and inner core examine the differences between only the in situ surface where gradients were largest. Therefore, error statistics observations and the VHRD (i.e., aircraft observations for known and VSYN winds were computed at marine adjusted to the surface were not used in this test). In grid points within the following radii: 1) from 0.75RMW this case, the limited number of in situ surface obser- to 1.50RMW, which contains the region of the maximum vations required that only one range of radii be used to winds for each storm; and 2) the domain beyond compute the statistics (here the radii were from 0.5RMW 1.50RMW (the outer radius ranged from 69 to 158 km to the outer radius). The value of NWSDs in Table A5 and was based on the extent of each storm's analysis were negative in all cases with a ␮ of Ϫ6.0% (␴ ϭ domain). In addition, the edges of the domain were ex- 13.0%) and the IADs were always positive with a ␮ of cluded to minimize possible effects of any noise that ϩ2.5Њ (␴ ϭ 8.7Њ). Therefore, the aircraft observations might exist at the boundaries. For inner radii (Table A1), in the SAFER method were in general increasing the the NWSDs ranged from Ϫ3% to ϩ1% with a ␮ near HRD wind speeds and the HRD in¯ow angles were zero and ␴ of 3.5%, while the IADs ranged from Ϫ1Њ slightly less than the actual observed. to ϩ2Њ with ␮ also near zero and ␴ of 3Њ. The outer annulus (Table A2) NWSD's ranged from Ϫ5% to ϩ5% with a ␮ of ϩ2% (␴ ϭ 6%), indicating that the analyzed REFERENCES synthetic wind speeds were slightly stronger on average Burpee, R. W., and Coauthors, 1994: Real-time guidance provided than the known wind speeds. The IAD ␮ was slightly by NOAA's Hurricane Research Division to forecasters during negative (␴ ϭ 2Њ). The mean differences (not shown) Emily of 1993. Bull. Amer. Meteor. Soc., 75, 1765±1783. Case, R., and M. May®eld, 1990: Atlantic hurricane season of 1989. between known and VSYN radial and tangential winds Mon. Wea. Rev., 118, 1165±1177. were near zero in both regions. The storm with the least DeMaria, M., S. M. 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