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A Multiscale Numerical Study of Hurricane Andrew (1992). Part I: Explicit Simulation and Veri®Cation''

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A Multiscale Numerical Study of Hurricane Andrew (1992). Part I: Explicit Simulation and Veri®Cation'' 1706 MONTHLY WEATHER REVIEW VOLUME 127 Comments on ``A Multiscale Numerical Study of Hurricane Andrew (1992). Part I: Explicit Simulation and Veri®cation'' MARK D. POWELL AND SAMUEL H. HOUSTON NOAA/Hurricane Research Division, Miami, Florida 18 February 1998 and 11 June 1998 1. Introduction The purpose of this comment is to discuss differences between the observed (PH) and simulated (LZY) surface In their paper ``A multiscale numerical study of Hur- wind distributions at landfall and to compare the mod- ricane Andrew (1992). Part I: Explicit simulation and eled and observed wind maxima using a standard frame- veri®cation,'' Liu et al. (1997, hereafter referred to as work. LZY) present an impressive simulation of a mature trop- ical cyclone initialized with environmental data and op- erational model analyses obtained during Hurricane An- 2. Distribution of surface winds at landfall drew. The simulated tropical cyclone makes landfall The only prelandfall simulated wind ®elds shown by near Palm Beach, Florida, with an eye diameter about LZY (in their Fig. 16, valid for 2000 UTC 23 August, twice that observed. The minimum central sea level about 11 h before simulated landfall) show a wind max- pressure at landfall is similar to that observed when imum in the northern semicircle at the 95.0- and 85.0- Hurricane Andrew made landfall about 105 km farther kPa levels. The LZY landfall wind ®eld (Fig. 7 of LZY) south near Homestead, Florida. According to Fig. 2 of contained maximum winds on the north side of the storm LZY, the simulated storm begins ®lling at landfall and with maximum winds over land of about the same mag- its pressure increases to about 944 hPa in 5 h, similar nitude as adjacent winds over the water with little in- to that observed in Andrew (Powell et al. 1996; Powell dication of a discontinuity associated with the land±sea and Houston 1996, hereafter referred to as PHR and roughness change. While location of the wind maximum PH). LZY compared the surface wind ®eld of the sim- on the north side compared well with the observed wind ulated storm to that observed in Hurricane Andrew by ®eld described in PH, the lack of any discontinuity along PH and suggested that the coastal discontinuities pub- the coastline was suspect. Coastal wind discontinuities lished in PH were the product of reducing ¯ight-level have been observed in numerous storms (Graham and winds to the surface with a planetary boundary layer Hudson 1960)Ðthe Lake Okeechobee hurricanes of (PBL) model, analyzing land and marine observations 1949 and 1950 (Myers 1954), Hurricanes Belle of 1976 separately, and then merging them at the coastline (SethuRaman 1979), Frederic of 1979 (Powell 1982), ``without including full dynamic and physics interac- Alicia of 1983 (Powell 1987), Hugo of 1989 (Powell et tions.'' LZY provided little information on the preland- al. 1991), and Andrew (PHR; PH)Ðas well as in nu- fall wind distribution and land surface properties to sub- merical simulations of tropical cyclone landfall wind stantiate the landfall wind ®eld produced by their model. ®elds (Moss and Jones 1978; Tuleya and Kurihara 1978; The reply to this comment (Zhang et al. 1999, hereafter Tuleya et al. 1984; Tuleya 1994). As described in PH referred to as ZLY) indicates that the landfall winds (section 3a), this discontinuity actually corresponds to described in LZY correspond to a completely different a transition zone where the ¯ow is in the process of framework for exposure, height, and averaging time adjusting to a new underlying surface. than that used by PH. The lack of coastal discontinuities In their reply, Fig. 3c of ZLY describes the simulated in LZY's landfall wind ®eld corresponds to a distinct wind ®eld 2 h prior to landfall showing strongest winds prelandfall wind distribution that differs both from the in a semicircle on the west (landward) side of the storm observations and earlier hours of the simulation. at 40 m. The landward wind maxima is quite different from the northward maximum described 11 h earlier in LZY (Fig. 16) and helps explain the lack of a coastal Corresponding author address: Dr. Mark D. Powell, NOAA-HRD- wind discontinuity. With this type of maximum wind AOML, 4301 Rickenbacker Cswy., Miami, FL 33149. distribution the frictional effects at landfall would re- E-mail: [email protected] duce the west semicircle wind maximum such that the JULY 1999 NOTES AND CORRESPONDENCE 1707 FIG. 1. (a) Hurricane Andrew wind speed distribution for 0000 UTC 23 August 1992 at the 0.5-km level derived from NOAA P3 airborne Doppler radar wind measurements using the extended velocity track display method. (b) Hodograph created from Doppler wind measurements using EVTD. East±west (U) and north±south (V) component winds in m s21, numbers adjacent to curve mark altitudes in km. differences between maximum winds over land and wa- deepening stage at 96.0 kPa, the National Oceanic and ter would be small. Effectively, this results in a rotation Atmospheric Administration (NOAA) P3 research air- of the wind maximum to just offshore on the north side craft were conducting a synoptic ¯ow experiment and (Fig. 4a of ZLY). The LZY simulation is remarkably collected wind velocity measurements with the airborne similar to a study by Moss and Jones (1978) for a hur- Doppler radar. Using the extended velocity track display ricane moving west at5ms21. In their simulation, a (EVTD) method (Roux and Marks 1996), the wind dis- low-level wind maximum initially in the right (north) tribution at 500 m (Fig. 1a) depicts maximum wind semicircle rotated to the landward side several hours speeds on the northwestern side that persisted through before landfall with a subsequent shift of the maximum 5 km. winds to the north side at landfall. A simulation for a A persistent shift in the azimuthal position of the wind storm moving west at 10 m s21 by Tuleya et al. (1984) maximum with height could be produced by vertical maintained maximum surface winds in the northern shear of the environmental ¯ow (Marks et al. 1992). A semicircle throughout the landfall process. For trans- front-side, landward wind maximum at the surface im- lation speeds similar to Andrew, tropical cyclone bound- plies a strong cross-track environmental shear associated ary layer ¯ow models (Myers and Malkin 1961; Shapiro with northerly ¯ow at the surface. A hodograph created 1983; Wang and Holland 1997) suggest a wind maxi- from the EVTD analysis (Fig. 1b) shows very little mum on the front right or right side of the storm (relative cross-track shear in the lower 3 km. Northerly shear is to the earth), but do not address landfall. suggested from 3 to 6 km, with southerly shear from 8 The physical processes associated with a shift of the to 13 km. simulation wind maximum to the landward front (west) Examination of air force reconnaissance eyewall semicircle shortly before landfall are of great interest passes at 3 km from 23 August (not shown) con®rm since the northwesterly ¯ow over land obstacles on the that the wind maximum was maintained in the right west side would contain much higher turbulent intensity (north) semicircle for the entire prelandfall period. than northeasterly ¯ow with a marine fetch on the north Highest winds were located 16±25 km north (;0330 side. An investigation relating landfall wind distribution UTC), northwest (;1200 UTC), and northeast (;1600± changes to variations in model parameters, storm speed, 2230 UTC) of the center on 23 August and then re- and environmental ¯ow speci®cations would help to un- mained north of the center through landfall. Considering derstand the model's sensitivity and suggest processes that the position of the wind maxima at the 500-m and that may be important at landfall. 3.0-km levels were well correlated from the earlier At ;0000 UTC 23 August, while Andrew was in its Doppler measurements, a landward shift of the surface 1708 MONTHLY WEATHER REVIEW VOLUME 127 wind maximum to the front semicircle is not consistent 3. Comparison of maximum winds using a with the observations. standard framework A large convective asymmetry could in¯uence the azimuthal position of the maximum surface winds as Hurricane surface winds are strongly dependent on indicated in Hurricane Emily of 1993 (Burpee et al. the averaging time attributed to the wind observations, 1994). Since Miami WSR-57 radar observations of An- the roughness of the underlying surface, and height of drew's eyewall (see Wakimoto and Black 1994) indi- the wind measurements. The identi®cation of wind cated a symmetric ring of convection at low levels about speed averaging times, heights, and local terrain is crit- 30 min before landfall, there is little evidence that the ical in the comparison of observed wind speeds to those wind maximum at lower levels shifted to the front (west) obtained from numerical models. PHR documented the semicircle just before landfall due to a convective asym- assembly and quality control of all available surface and metry. Convective asymmetries were observed during ¯ight-level wind measurements collected in south Flor- and after landfall. Willoughby and Black (1996) used ida during Hurricane Andrew, adjusting the land ob- radar re¯ectivity measurements at the 3±8-km level servations to a common ``open terrain'' exposure at from the Tampa WSR-57 to describe a process of cell 10-m level with an averaging time corresponding to the development initiated where easterly ¯ow associated maximum 1-min sustained surface wind that might oc- with the north side wind maximum in the eyewall im- cur over some longer period. In their reply, ZLY provide pinged on the coast, creating re¯ectivity maxima on the details on model grid cell wind exposure (speci®ed by west and southwest sides of the storm, but this process land use/land cover over land or de®ned by a roughness was not supported by Melbourne WSR-88D radar mea- length parameterization over the sea), averaging time (a surements discussed in PH.
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