Notes and Correspondence Impact of land-surface roughness on surface winds during hurricane landfall

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1 QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Q. J. R. Meteorol. Soc. 134: (28) Published online 4 June 28 in Wiley InterScience ( DOI: 1.12/qj.265 Notes and Correspondence Impact of land-surface roughness on surface winds during hurricane landfall Ping Zhu* Department of Earth Sciences, Florida International University, USA ABSTRACT: This study investigates the impact of land-surface roughness on surface winds during hurricane Wilma s landfall using high-resolution simulations by the Weather Research and Forecasting model. Based on idealized experiments with enhanced land-surface roughness, it is found that the overall storm intensity decreases with increase in land-surface roughness during landfall. However, there is no apparent trend in reducing the maximum gusting winds when land-surface roughness was increased. This result suggests that hurricane wind damage may not be assessed properly based solely on the overall storm intensity. The localized damaging winds are intimately involved with the hurricane boundary-layer secondary circulations. When the downdraughts of the circulations are in phase with the strong hurricane momentum aloft, they can effectively transport momentum downward to result in localized damaging winds. Larger land-surface roughness can strengthen boundary-layer secondary circulations by creating stronger local convergence, and thus stronger downdraughts and localized damaging winds despite a weak storm intensity. Copyright 28 Royal Meteorological Society KEY WORDS Wilma; downdraughts; WRF model Received 27 November 27; Revised 17 April 28; Accepted 21 April Introduction Hurricane damaging winds have proved to be catastrophic to coastal communities. Most of the wind damage occurs within the first few hours during a hurricane s landfall when wind fields change dramatically due to the abrupt change in surface conditions. Hurricane landfall is characterized by the rapid decay in hurricane intensity. The increase in surface friction is thought to be a factor for the decay of hurricanes. Nevertheless, the primary mechanism for weakening a hurricane vortex as a whole is due to shutdown of oceanic energy supply (e.g. Tuleya and Kurihara, 1978; Tuleya et al., 1984). Yet, in many cases, damaging winds retain their strength well inland, indicating the complication of the decaying process of landfalling hurricanes. The interaction between a landfalling hurricane and the land surface is realized through several processes that are all involved with surface roughness. In addition to the direct dynamic effect of reducing hurricane winds due to the increased surface friction, there are other processes that may temporarily strengthen damaging winds or slow down the decay of hurricanes as suggested in literature. First, increase in land-surface roughness may enhance surface evaporation when inland water is available. Wakimoto and Black (1994) argued that the observed intense * Correspondence to: Ping Zhu, Department of Earth Sciences, Florida International University, MARC 36, 112 SW 8th Street, Miami, FL 33199, USA. zhup@fiu.edu damage by the second wind after landfall of hurricane Andrew may be attributed to the fact that the surface evaporation may not have been substantially reduced owing to Andrew s path over the Everglade swamps. Indeed, simulations by Shen et al. (22) showed that inland water could noticeably reduce the landfalling hurricane decay due to the enhanced inland surface evaporation. However, since surface evaporation is also controlled by surface winds, weak surface winds may actually reduce surface evaporation even though inland water is available. Thus, the net effect may change depending on specific conditions. Second, the increased surface friction can enhance the cross-isobar inflow, which may temporarily strengthen mass convergence, and thus moist convection (Wakimoto and Black, 1994). This process is further complicated by the strong local turbulence caused by the enhanced surface friction. So far, not all these processes are well understood. In particular, we have little knowledge about which of these processes tend to dominate the others in producing damaging winds despite a significant reduction in intensity after landfall. This study aims to investigate the impact of landsurface roughness on damaging winds during the landfall of hurricane Wilma (25) using simulations by the Weather Research and Forecasting (WRF; Skamarock et al., 25) model. Wilma is chosen because of its path over a wide range of surface conditions from Everglade swamps to urban cities. Copyright 28 Royal Meteorological Society

2 152 P. ZHU 2. Numerical experiments In this study, a two-stage WRF simulation of Wilma was performed. Stage 1 simulation, which aims to obtain a realistic hurricane vortex structure before landfall, was executed for 32 hours from UTC on 23 October to 8 UTC on 24 October 25. The simulation was performed with a parent domain D-1 and two moving nests (nesting ratio 1:3) following the hurricane vortex. Figure 1(a) shows the area coverage of domain D-1, which has a grid-mesh of 2 15 and a resolution of 15 km. The lateral boundary condition was supplied with the National Centers for Environmental Prediction (NCEP) Global Tropospheric Analyses with 1 degree resolution available every 6 hours. However, since NCEP reanalysis data can only provide a poorly resolved storm vortex due to its low resolution, the initial condition used in this study was taken from the Geophysical Fluid Dynamics Laboratory (GFDL) data, which contain a bogus vortex constructed using a sophisticated bogussing scheme (Kurihara et al., 1993). Many successful simulations of hurricane movement and structure have been conducted using vortex bogussing technique for hurricane model initialization (e.g. Singh et al., 25; Leslie and Holland, 1995). To obtain the right hurricane track, the WRF 3-D analysis nudging option was switched on to nudge the NCEP reanalyses in domain D-1. The detailed procedure of nudging can be found in Deng et al. (27). But nudging was not activated for all the nested domains. This 32-hour spin-up simulation was then used to initialize stage 2 simulation that consists of the parent domain D-1 and three stationary nests (D-2, D-3, and D-4) illustrated by the boxes in the inset panel of Figure 1(a). The nested domains D-2, D-3, and D-4 have a grid mesh of , , and , respectively. The nesting ratio is 1:3 so that the innermost domain D-4 has a horizontal resolution of m. The lowest level is about 4 m high and there are 12 levels below 19 m. Stage 2 simulation was executed from 8 UTC before Wilma made landfall to 18 UTC on 24 October after Wilma passed the Florida peninsula. (a) Latitude ( ) Z 13Z 9Z 1Z 5Z.6 2 D D 2 D (b) K (c) g kg (d) 1 (e) 3 W m 2 5 W m Local Sidereal Time (h) Local Sidereal Time (h) Figure 1. (a) 2 m temperature (K, shading) and 1 m winds (vectors) for Wilma at 14 UTC on 24 October 25 simulated by WRF. The black stars denote the best track of Wilma from observations. Inset panel: land-surface roughness (m) derived from the WRF land-use data. The red boxes denote the stationary two-way nested domains D-2, D-3, and D-4 in the Stage 2 simulation. Time series of (b) 2 m temperature, (c) 2 m mixing ratio, (d) sensible heat flux, and (e) latent heat flux averaged over land in domain D-3 during Wilma s landfall. This figure is available in colour online at Copyright 28 Royal Meteorological Society Q. J. R. Meteorol. Soc. 134: (28) DOI: 1.12/qj

3 LAND-SURFACE ROUGHNESS EFFECT ON HURRICANE SURFACE WINDS 153 Three experiments of stage 2 simulation were executed to investigate the sensitivity of landfalling hurricane winds to land-surface roughness. In the control simulation (CNTL), the land-surface roughness was derived directly from the WRF land-use data illustrated by the shading in the Figure 1(a) inset. In the sensitivity experiments, this land-surface roughness was increased by a fixed percentage at 9 UTC. In experiment 1 named as EXP+25, everything else was the same except that the land roughness was increased by 25%, whereas in experiment 2 (EXP+3) the land roughness was increased by a factor of 3. The key model parametrizations activated in the simulations are: the Mellor Yamada Janjić boundarylayer scheme (Janjić, 22), the Rapid Radiative Transger Model (Mlawer et al., 1997) long-wave and Dudhia (1989) short-wave radiation schemes, the Thompson microphysics scheme (Thompson et al., 24), and the Kain Fritsch (1993) deep convection scheme (activated only in domain D-1). The atmospheric model was coupled with a 5-layer thermal diffusion land-surface model. 3. Simulation results Wilma made landfall around 13 UTC, which is about 63 LST, and entered the Atlantic at about 16 UTC (12 LST). Figure 1(b,c,d,e) show the time series of thermodynamic variables averaged over land in domain D-3 during Wilma s passage over the Florida peninsula from CNTL. The surface latent and sensible heat fluxes show a typical diurnal variation, but 2 m temperature and moisture drop substantially after sunrise, indicating the strong non-local influence. As illustrated by Figure 1(a), the decrease in surface temperature and moisture is caused by the strong cold and dry advection associated with the southward hurricane winds and the cold/dry continental air mass. The decrease in surface-layer temperature suggests that the boundary layer is not convective and the shear production should remain to be the main mechanism for turbulence despite the fact that Wilma passed over the Florida peninsula during the daytime. To examine the overall effect of land-surface roughness on landfalling hurricane decay, Figure 2 compares several variables from domain D-3 between the experiments (EXP+25 and EXP+3) and CNTL. These include 1 m winds, surface convergence (defined as U(1 m)/ x V(1 m)/ y), and latent heat fluxes averaged over land in D-3, and the storm centre pressure in D-3. The hurricane decay after landfall is clearly indicated by the decrease in surface winds and convergence and rising in storm centre pressure from CNTL. Increasing land-surface roughness further enhances the dissipation of landfalling Wilma reflected by a further drop (a) 3 (e) (EXP+25) (CNTL) (EXP+3) (CNTL) (b) (s 1 ) X (f) 1 1 (c) 3 (g) 5 Wm (d) 96 (h) 2 hpa Time (UTC) Day: Time (UTC) Day: Figure 2. (a) 1 m wind speeds, (b) surface convergence, and (c) latent heat fluxes averaged over land in domain D-3 from CNTL. (d) storm centre pressure over land from CNTL. (e) (h) difference of the variables in (a) (d) between the experiments and CNTL. The dark line is (EXP+3) minus (CNTL); the light line is (EXP+25) minus (CNTL). Local standard time is UTC 4. Copyright 28 Royal Meteorological Society Q. J. R. Meteorol. Soc. 134: (28) DOI: 1.12/qj

4 154 P. ZHU of surface winds and increase of storm centre pressure (Figures 2(e) and (h)). Unlike the previously suggested mechanism of possibly fostering stronger storm-scale surface convergence and latent heat fluxes due to the increased surface friction, there is a clean decrease in latent heat fluxes in both experiments (Figure 2(g)). Such a decrease can be attributed to the overall weakened surface winds. A much more complicated variation, however, is seen in the inland surface convergence (Figure 2(f)). Overall there is a decreasing trend in surface convergence, but there are certain periods when the inland surface convergence does increase during landfall, which cannot be simply related to the change in the averaged surface winds. A detailed discussion will be provided shortly. An interesting result, however, is shown in the gust wind fields. Figure 3 compares the spatial maximum 1 m winds in domain D-4 and over land in domain D-3 between the experiments and CNTL. The change in maximum surface winds in response to the change in land surface roughness does not have a clean trend but shows a substantial fluctuation. Even in EXP+3 when land surface roughness was increased by a factor of 3, the maximum surface wind gust can still increase by more than 5 m s 1 in certain periods during Wilma s landfall. It is interesting to see that the maximum surface winds in EXP+3 appear to have a similar magnitude to those of EXP+25, despite the fact that the storm intensity in EXP+3 drops substantially compared with EXP+25, indicating that surface gusting winds in the hurricane boundary layer are controlled by a complicated interplay involving other processes. Damage surveys after hurricane landfalls often show uneven damage patterns. It has been suggested that they are caused by localized damaging winds (e.g. Wakimoto and Black, 1994; Fujita, 1992). Thus, no substantial decrease of the maximum winds shown in the sensitivity experiments implies that wind damage may not drop significantly despite the overall weakened storm intensity. Rather, wind damage could be worse at particular locations where stronger localized damaging winds occur. As an illustration of localized damaging winds produced by landfalling Wilma, Figure 4(a) shows a snapshot of surface winds from CNTL at 1428 UTC, a time when the first wind wave has passed the Florida peninsula and the storm eye is about to enter the Atlantic. There is a strong surface convergence in the upper left quadrant of the storm. This strong convergence is consistent with the heavy precipitation indicated by the contours in the figure. Downstream of the strong convergence is where the second wind wave of Wilma occurs. This second wind wave is strong enough to cause significant inland wind damage. Thus from a hurricane mitigation perspective, understanding the effect of land surface roughness on localized damaging winds is as important as, if not more than, its effect on the overall storm intensity. To understand why increasing land surface roughness may lead to strong surface wind gust despite a (a) 6 5 D 4 D (b) (EXP+3) (CNTL) (EXP+25) (CNTL) (c) (EXP+25) (CNTL) Time (UTC) Day: (EXP+3) (CNTL) Figure 3. (a) Maximum 1 m wind speed over land in domains D-3 (light line) and D-4 (dark line) from CNTL. (b) Difference of maximum 1 m winds in D-4 between EXPs and CNTL. (c) Difference of maximum 1 m winds over land in D-3 between EXPs and CNTL. The dark line is (EXP+3) minus (CNTL); the light line is (EXP+25) minus (CNTL). Local standard time is UTC 4. Copyright 28 Royal Meteorological Society Q. J. R. Meteorol. Soc. 134: (28) DOI: 1.12/qj

5 LAND-SURFACE ROUGHNESS EFFECT ON HURRICANE SURFACE WINDS 155 Figure 4. (a) 1 m winds (m s 1, shading and arrows) at 1428 UTC (128 LST) simulated by CNTL. Simulated winds from domain D-4 (indicated by the black box) have been overlapped on those of D-3. The black contours (.5, 1. mm min 1 ) indicate the precipitation rate. The red box indicates the area used for the comparison between CNTL and EXP+25 in (b) and (c). (b) 1 m winds (shading) and surface convergence (contours) from CNTL (left) and EXP+25 (right). Convergence (positive) is indicated by white contours (.1,.2,.3, and.4 s 1 ) and divergence (negative) by black contours (.2 and.1 s 1 ). (c) 5 m vertical velocity from CNTL (left) and EXP+25 (right). The horizontal line indicates the cross-section used for the vertical structure analysis in (d). (d) Cross-section of horizontal wind speed (shading) and the secondary circulations (vectors) in EXP+25. To better illustrate the secondary circulations, the vertical velocities have been multiplied by 4. This figure is available in colour online at Copyright 28 Royal Meteorological Society Q. J. R. Meteorol. Soc. 134: (28) DOI: 1.12/qj

6 156 P. ZHU weak hurricane intensity, Figures 4(b) and (c) show, for CNTL and EXP+25, the detailed wind structures of localized damaging winds (simulated by domain D-4) in the area bounded by the red box in Figure 4(a). Compared with CNTL, EXP+25 simulated stronger localized surface winds at the scale of a few kilometres. The localized damaging winds are associated with pairs of surface divergence and convergence or downdraught and updraught. Generally the stronger damaging winds, the stronger is the divergence convergence or downdraught updraught pair, suggesting that the localized damaging winds are only loosely linked to the overall strength of the storm. Rather, they are directly produced by local secondary circulations. To illustrate the relationship between surface damaging winds and vertical secondary circulation, Figure 4(d) shows the vertical structure of local secondary circulations along the cross-section line shown in Figure 4(c). It shows that the downward leg of the circulation tends to transport momentum aloft to the surface resulting in a surface wind maximum, while the upward leg of the circulation is the momentum sink since the air is slowed down by the surface friction. As shown in Figure 4(d), there is a slight phase shift between the downdraught of a vertical circulation and the strong horizontal momentum aloft. This phase shift may be caused by the shear and horizontal inhomogeneity. Note that the simulated circulations are similar to the observed kilometre and sub-kilometre well-organized vortical structures by Doppler radar (e.g. Wurman and Winslow, 1998; Morrison et al., 25) and Synthetic Aperture Radar (e.g. Katsaros et al., 2). The stronger damaging winds simulated by EXP+25 may be attributed to its stronger local secondary circulation. It can be argued that large land-surface roughness may enhance local convergence, which in turn fosters stronger local convection and induced downdraughts. As a result, the stronger damaging winds may be produced despite a weaker storm intensity once the strong downdraughts of secondary circulations and high hurricane momentum aloft go hand-in-hand. In addition, secondary circulations may also be sensitive to the horizontal gradients in land-surface roughness. For the experiments performed in this study, scaling up landsurface roughness will cause a scaling up of the gradient, which may also lead to stronger secondary circulations. In short, we may argue that localized damaging winds are controlled by a combination of storm intensity, local secondary circulation, its phase relation to high hurricane momentum aloft, and land-surface conditions. A further analysis of the simulation results also shows that the stronger local damaging winds are also accompanied by stronger local turbulence reflected by the large surface frictional velocity and turbulent kinetic energy (not shown here). The strong local turbulence may further enhance the secondary circulation and strengthen local damaging winds through large turbulent transport. From a wind engineering perspective, strong turbulence can further cause structural fatigue, and thus lead to more serious damage. 4. Summary and discussion In this study, the sensitivity of surface winds of landfalling hurricane Wilma to land surface roughness was investigated based on the idealized numerical experiments performed by the WRF model. It shows that the overall storm intensity decreases with the increase in land-surface roughness during landfall as reflected by the weak averaged surface winds, rising in storm centre pressure, and weakened surface convergence. The wet Everglades do not appear to have an substantial effect on delaying the decay of Wilma as the surface latent heat fluxes also decrease with the increase of land-surface roughness. A likely explanation is that the reduced surface winds cause a land-surface evaporation which is weaker than in a condition with stronger surface winds. However, there is no apparent trend in reducing the maximum gusting winds during Wilma s landfall when the land-surface roughness was increased. Localized damaging winds, which are intimately involved with the hurricane boundary-layer secondary circulations, can retain their strength despite the overall weakened storm intensity. This is true even for the experiment when land-surface roughness was increased by a factor of 3. The WRF simulations revealed the dynamic structure of the vertical circulation and its relation to damaging surface winds. The secondary circulation consists of axis-asymmetric updraught and downdraught. The downdraught may not be in phase with the strong hurricane momentum aloft. But once they go hand-in-hand, the downdraught can effectively transport momentum aloft downward to result in localized damaging winds. Increasing land-surface roughness can strengthen secondary circulation by creating large local convergence and divergence, which explains why the maximum winds do not have an apparent reduction and even increase a little in certain periods during landfall when surface friction was increased. This result suggests that hurricane wind damage may not be assessed properly solely based on the overall storm intensity. The secondary circulation that produces localized damaging winds generally has a small scale at or less than a few kilometres. They are also associated with strong local turbulence. However, the model resolution used in this study is not sufficiently high to explicitly resolve turbulent eddies. Thus, this study cannot provide the detailed information of the interaction between secondary circulations and local turbulence. Lastly, this study only focuses on the down-scale impact of secondary circulations on surface damaging winds. In fact, these organized structures may also have an important up-scale bearing on landfalling hurricane decay. Investigation of the interaction between hurricane boundary-layer organized structures and local turbulence, Copyright 28 Royal Meteorological Society Q. J. R. Meteorol. Soc. 134: (28) DOI: 1.12/qj

7 LAND-SURFACE ROUGHNESS EFFECT ON HURRICANE SURFACE WINDS 157 and its up-scale impact on landfalling hurricane decay and down-scale impact on hurricane wind damage, will be the focus of future research. Acknowledgements The author wishes to acknowledge the support for this work from the NOAA Florida Hurricane Alliance. I am also very grateful to the two anonymous reviewers for their constructive comments. Their helpful suggestions led to improvements of this paper. References Deng A, Stauffer DR, Dudhia J, Otte TL, Hunter GK. 27. Update on analysis nudging FDDA in WRF-ARW. The 8th WRF Users Workshop, June, Boulder, CO Deng.pdf. Dudhia J Numerical study of convection observed during the winter monsoon experiment using a mesoscale two-dimensional model. J. Atmos. Sci. 46: Fujita TT Damage survey of Hurricane Andrew in south Florida. NCDC. Storm Data 34: Janjić ZI. 22. Nonsingular implementation of the Mellor Yamada level 2.5 scheme in the NCEP meso model. NCEP Office Note, No pdf. Kain JS, Fritsch JM Convective parameterization for mesoscale models: The Kain Fritsch scheme. Pp in The Representation of Cumulus Convection in Numerical Models. Meteorol. Monogr. No. 46, Amer. Meteorol. Soc: Boston, USA. Katsaros KB, Vachon PW, Black PG, Dodge PP, Uhlhorn EW. 2. Wind fields from SAR: Could they improve our understanding of storm dynamics? Johns Hopkins APL Tech. Digest 21(1): Kurihara Y, Bender MA, Ross RJ An initialization scheme of hurricane models by vortex specification. Mon. Weather Rev. 121: Leslie LM, Holland GJ On the bogussing of tropical cyclones in numerical models: A comparison of vortex profiles. Meteorol. Atmos. Phys. 56: Mlawer EJ, Taubman SJ, Brown PD, Iacono MJ, Clough SA Radiative transfer for inhomogeneous atmospheres: RRTM, a validated k-correlated model for the longwave. J. Geophys. Res. 12: Morrison I, Businger S, Marks F, Dodge P, Businger JA. 25. An observational case for the prevalence of roll vortices in the hurricane boundary layer. J. Atmos. Sci. 62: Shen W, Ginis I, Tuleya RE. 22. A numerical investigation of land surface water on landfalling hurricanes. J. Atmos. Sci. 59: Singh R, Pal PK, Kishtawal CM, Joshi PC. 25. Impact of bogus vortex for track and intensity prediction of tropical cyclone. J. Earth System Sci. 114: Skamarock WC, Klemp JB, Dudhia J, Gill DO, Barker DM, Wang W, Powers JG. 25. A description of the advanced research WRF, version 2. Technical Note TN-468+STR, NCAR: Boulder. v2.pdf. Thompson G, Rasmussen RM, Manning K. 24. Explicit forecasts of winter precipitation using an improved bulk microphysics scheme. PartI: Description and sensitivity analysis. Mon. Weather Rev. 132: Tuleya RE, Kurihara Y A numerical simulation of the landfall of tropical cyclones. J. Atmos. Sci. 35: Tuleya RE, Bender MA, Kurihara Y A simulation study of the landfall of tropical cyclones using a movable nested-mesh model. Mon. Weather Rev. 112: Wakimoto RM, Black PG Damage survey of Hurricane Andrew and its relationship to the eyewall. Bull. Am. Meteorol. Soc. 75: Wurman J, Winslow J Intense sub-kilometer-scale boundary layer rolls observed in Hurricane Fran. Science 28: Copyright 28 Royal Meteorological Society Q. J. R. Meteorol. Soc. 134: (28) DOI: 1.12/qj