Hurricane Simulation Research Articles

On the Limits of Estimating the Maximum Wind Speeds in Hurricanes

AbstractThis study uses an observing system simulation experiment (OSSE) approach to test the limitations of even nearly ideal observing systems to capture the peak wind speed occurring within a tropical storm or hurricane. The dataset is provided by a 1-km resolution simulation of an Atlantic hurricane with surface wind speeds saved every 10 s. An optimal observing system consisting of a dense field of anemometers provides perfect measurements of the peak 1-min wind speed as well as the average peak wind speed. Suboptimal observing systems consisting of a small number of anemometers are sampled and compared to the truth provided by the optimal observing system. Results show that a single, perfect anemometer experiencing a direct hit by the right side of the eyewall will underestimate the actual peak intensity by 10%–20%. Even an unusually large number of anemometers (e.g., 3–5) experiencing direct hits by the storm together will underestimate the peak wind speeds by 5%–10%. However, the peak winds of just one or two anemometers will provide on average a good estimate of the average peak intensity over several hours. Enhancing the variability of the simulated winds to better match observed winds does not change the results. Adding observational errors generally increases the reported peak winds, thus reducing the underestimates. If the average underestimate (negative bias) were known perfectly for each case, it could be used to correct the wind speeds, leaving only mean absolute errors of 3%–5%.

Objective convective updraft identification and tracking: Part 1. Structure and thermodynamics of convection in the rainband regions of two hurricane simulations

AbstractTo better understand the life cycles of convective elements in numerical model simulations, a convective updraft tracking algorithm, dubbed the Statistical and Programmable Objective Updraft Tracker (SPOUT), is developed. SPOUT identifies updraft cores in horizontal slices through a given model data set, links these cores in three dimensions to form vertically coherent updrafts, and then tracks updrafts in time via estimates of the mean local advection. SPOUT is evaluated in idealized hurricane simulations from both the Regional Atmospheric Modeling System (RAMS) and Weather Research and Forecasting (WRF). In both simulations, SPOUT indicates most updrafts live less than 60 min, but some can persist for over 2 h. Updrafts are typically manifested as shallow cumulus, cumulus congestus, and deep cumulonimbus. Overall, there is reasonable agreement between the vigor of convection and the amount of localized conditional convective instability. Also, the majority of updrafts attain a tilted structure that is in accordance with the localized vertical wind shear. Nonetheless, there are striking differences between models, with the higher‐resolution WRF simulation producing around twice as many updrafts as RAMS. Moreover, the WRF updrafts are significantly weaker. This paper highlights the utility of SPOUT for studying convective life cycles in high‐resolution numerical simulations and provides some basic features of convection in two hurricane simulations.

Mapping joint hurricane wind and surge hazards for Charleston, South Carolina

Combined effects of hurricane wind and surge can pose significant threats to coastal cities. Although current design codes consider the joint occurrence of wind and surge, information on site-specific joint distributions of hurricane wind and surge along the US Coast is still sparse and limited. In this study, joint hazard maps for combined hurricane wind and surge for Charleston, South Carolina (SC), were developed. A stochastic Markov chain hurricane simulation program was utilized to generate 50,000 years of full-track hurricane events. The surface wind speeds and surge heights from individual hurricanes were computed using the Georgiou's wind field model and the Sea, Lake and Overland Surges from Hurricanes (SLOSH) model, respectively. To validate the accuracy of the SLOSH model, the simulated surge levels were compared to the surge levels calculated by another state-of-the-art storm surge model, ADCIRC (Advanced Circulation), and the actual observed water elevations from historical hurricane events. Good agreements were found between the simulated and observed water elevations. The model surface wind speeds were also compared with the design wind speeds in ASCE 7-10 and were found to agree well with the design values. Using the peak wind speeds and maximum surge heights, the joint hazard surfaces and the joint hazard maps for Charleston, SC, were developed. As part of this study, an interactive computer program, which can be used to obtain the joint wind speed and surge height distributions for any location in terms of latitude and longitude in Charleston area, was created. These joint hazard surfaces and hazard maps can be used in a multi-hazard design or risk assessment framework to consider the combined effects of hurricane wind and surge. Language: en

Essential Dynamics of Secondary Eyewall Formation

Abstract The authors conduct an analysis of the dynamics of secondary eyewall formation in two modeling frameworks to obtain a more complete understanding of the phenomenon. The first is a full-physics, three-dimensional mesoscale model in which the authors examine an idealized hurricane simulation that undergoes a canonical eyewall replacement cycle. Analysis of the mesoscale simulation shows that secondary eyewall formation occurs in a conditionally unstable environment, questioning the applicability of moist-neutral viewpoints and related mathematical formulations thereto for studying this process of tropical cyclone intensity change. The analysis offers also new evidence in support of a recent hypothesis that secondary eyewalls form via a progressive boundary layer control of the vortex dynamics in response to a radial broadening of the tangential wind field. The second analysis framework is an axisymmetric, nonlinear, time-dependent, slab boundary layer model with radial diffusion. When this boundary layer model is forced with the aforementioned mesoscale model's radial profile of pressure at the top of the boundary layer, it generates a secondary tangential wind maximum consistent with that from the full-physics, mesoscale simulation. These findings demonstrate that the boundary layer dynamics alone are capable of developing secondary wind maxima without prescribed secondary heat sources and/or invocation of special inertial stability properties of the swirling flow either within or above the boundary layer. Finally, the time-dependent slab model reveals that the simulated secondary wind maximum contracts inward, as secondary eyewalls do in mesoscale models and in nature, pointing to a hitherto unrecognized role of unbalanced dynamics in the eyewall replacement cycle.

Building envelope failure assessment framework for residential communities subjected to hurricanes

This paper presents a modular framework utilized to assess the building envelope failures of light-frame wood construction subjected to hurricane wind hazards. A component-based approach is taken to develop an integrated building envelope model that is based on previous research of individual component capacities. Key modules of the proposed framework include a wind-borne Debris Generation Module and an impact-tracking module that interact with a Hurricane Simulation Module capable of simulating suites of synthetic or historical hurricanes. A significant difference between the proposed framework and other assessment models is that the proposed framework is extremely flexible in allowing the user to define the building stock within the area of interest, which will provide the user with the ability to investigate an unlimited number of “what if” scenarios. Another distinction of the proposed framework is that it is driven by a mechanics-based three-dimensional probabilistic debris trajectory model developed by the authors as opposed to incorporating empirical fragility curves developed from observed post-hurricane assessments, insurance claim data, or basic probabilistic analyses of wind-borne debris. As an illustrative example of results provided by the proposed framework, a debris impact vulnerability envelope presented in a polar coordinate system are developed for a representative South Carolina coastal subdivision subjected to Hurricane Hugo. The probabilities obtained from the debris impact vulnerability envelope generated by the framework can be utilized by officials, developers, architects, and designers either pre- or post-construction to mitigate damage to the building envelope of homes within a residential community located within hurricane-prone regions.

Survivors, not invaders, control forest development following simulated hurricane

Wind disturbance profoundly shapes temperate forests but few studies have evaluated patterns and mechanisms of long-term forest dynamics following major windthrows. In 1990, we initiated a large hurricane simulation experiment in a 0.8-ha manipulation (pulldown) and 0.6-ha control area of a maturing Quercus rubra--Acer rubrum forest in New England. We toppled 276 trees in the pulldown, using a winch and cable, in the northwesterly direction of natural treefall from major hurricanes. Eighty percent of canopy trees and two-thirds of all trees > or = 5 cm dbh (diameter at breast height) suffered direct and indirect damage. We used 20 years of measurements to evaluate the trajectory and mechanisms of forest response after intense disturbance. Based on the patch size and disturbance magnitude, we expected pioneer tree and understory species to drive succession. The first decade of analyses emphasized tree seedling establishment and sprouting by damaged trees as the dominant mechanisms of forest recovery in this extensive damaged area. However, despite 80% canopy damage and 8000-m2 patch size, surviving overstory and advance regeneration controlled longer-term forest development. Residual oaks make up 42% of stand basal area after 20 years. The new cohort of trees, dominated by black birch advance regeneration, contributes 30% of stand basal area. There were shifts in understory vegetation composition and cover, but few species were gained or lost after 20 years. Stand productivity rebounded quickly (litterfall recovered to pre-disturbance levels in six years), but we predict that basal area in the pulldown will lag behind the control (which gained 6 m2/ha over 20 years) for decades to come. This controlled experiment showed that although the scale and intensity of damage were great, abundant advance regeneration, understory vegetation, and damaged trees remained, allowing the forest to resist changes in ecosystem processes and invasion by new species.

Assessing the Impacts of Different WRF Precipitation Physics in Hurricane Simulations

AbstractNumerical weather prediction models play a major role in weather forecasting, especially in cases of extreme events. The Weather Research and Forecasting Model (WRF), among others, is extensively used for both research and practical applications. Previous studies have highlighted the sensitivity of this model to microphysics and cumulus schemes. This study investigated the performance of the WRF in forecasting precipitation, hurricane track, and landfall time using various microphysics and cumulus schemes. A total of 20 combinations of microphysics and cumulus schemes were used, and the model outputs were validated against ground-based observations. While the choice of microphysics and cumulus schemes can significantly impact model output, it is not the case that any single combination can be considered “ideal” for modeling all characteristics of a hurricane, including precipitation amount, areal extent, hurricane track, and the time of landfall. For example, the model’s ability to simulate precipitation (with the least total bias) is best achieved using Betts–Miller–Janjić (BMJ) cumulus parameterization in combination with the WRF single-moment five-class microphysics scheme (WSM5). It was determined that the WSM5–BMJ, WSM3 (the three-class version of the WSM scheme)–BMJ, and Ferrier microphysics in combination with the Grell–Devenyi cumulus scheme were the best combinations for simulation of the landfall time. However, the hurricane track was best estimated using the Lin et al. and Kessler microphysics options with BMJ cumulus parameterization. Contrary to previous studies, these results indicated that the use of cumulus schemes improves model outputs when the grid size is smaller than 10 km. However, it was found that many of the differences between parameterization schemes may be well within the uncertainty of the measurements.

Effects of Parameterized Diffusion on Simulated Hurricanes

Abstract In this study the authors analyze and interpret the effects of parameterized diffusion on the nearly steady axisymmetric numerical simulations of hurricanes presented in a recent study. In that study it was concluded that horizontal diffusion was the most important control factor for the maximum simulated hurricane intensity. Through budget analysis it is shown here that horizontal diffusion is a major contributor to the angular momentum budget in the boundary layer of the numerically simulated storms. Moreover, a new scale analysis recognizing the anisotropic nature of the parameterized model diffusion shows why the horizontal diffusion plays such a dominant role. A simple analytical model is developed that captures the essence of the effect. The role of vertical diffusion in the boundary layer in the aforementioned numerical simulations is more closely examined here. It is shown that the boundary layer in these simulations is consistent with known analytical solutions in that boundary layer depth increases and the amount of “overshoot” (maximum wind in excess of the gradient wind) decreases with increasing vertical diffusion. However, the maximum wind itself depends mainly on horizontal diffusion and is relatively insensitive to vertical diffusion; the overshoot variation with vertical viscosity mainly comes from changes in the gradient wind with vertical viscosity. The present considerations of parameterized diffusion allow a new contribution to the dialog in the literature on the meaning and interpretation of the Emanuel potential intensity theory.

Geostationary Doppler Radar and Tropical Cyclone Surveillance

Abstract The potential usefulness of spaceborne Doppler radar as a tropical cyclone observing tool is assessed by conducting a high-resolution simulation of an intense hurricane and generating synthetic observations of reflectivity and radial velocity. The ground-based velocity track display (GBVTD) technique is used to process the radial velocity observations and generate retrievals of meteorologically relevant metrics such as the maximum wind (MW), radius of maximum wind (RMW), and radius of 64-kt wind (R64). Results indicate that the performance of the retrieved metrics compares favorably with the current state-of-the-art satellite methods for intensity estimation and somewhat better than current methods for structure (i.e., wind radii).

Secondary eyewall formation in WRF simulations of Hurricanes Rita and Katrina (2005)

[1] An analysis is presented of two high-resolution hurricane simulations of Katrina and Rita (2005) that exhibited secondary eyewall formation (SEF). The results support the notion of vortex Rossby waves (VRWs) having an important role in SEF and suggest that VRW activity is a defining aspect of the moat. SEF occurs at a radius of ∼65 (80) km in Katrina (Rita), close to the hypothesized stagnation radius of VRWs. VRW activity appears to be the result of eye-eyewall mixing events, themselves a product of the release of barotropic instability. The convection in the radial region that becomes the moat is mainly in the form of VRWs propagating radially outward from the primary eyewall until the negative radial gradient of potential vorticity is no longer conducive for their propagation. These convectively coupled waves, originating and being expelled from the eyewall, are rotation dominated and have the coherency necessary to survive their passage through the strain-dominated region outside the eyewall.

Risk assessment of hurricane winds for Eglin air force base in northwestern Florida, USA

Hurricane winds present a significant hazard for coastal infrastructure. An estimate of the local risk of extreme wind speeds is made using a new method that combines historical hurricane records with a deterministic wind field model. The method is applied to Santa Rosa Island located in the northwestern panhandle region of Florida, USA. Firstly, a hurricane track is created for a landfall location on the island that represents the worst-case scenario for Eglin Air Force Base (EAFB). The track is based on averaging the paths of historical hurricanes in the vicinity of the landfall location. Secondly, an extreme-value statistical model is used to estimate 100-year wind speeds at locations along the average track based again on historical hurricanes in the vicinity of the track locations. Thirdly, the 100-year wind speeds together with information about hurricane size and forward speed are used as input to the HAZUS hurricane wind field model to produce a wind swath across EAFB. Results show a 100-year hurricane wind gust on Santa Rosa Island of 58 (±5) m s−1 (90% CI). A 100-year wind gust at the same location based on a 105-year simulation of hurricanes is lower at 55 m s−1, but within the 90% confidence limits. Based on structural damage functions and building stock data for the region, the 100-year hurricane wind swath results in $574 million total loss to residential and commercial buildings, not including military infrastructure, with 25% of all buildings receiving at least some damage. This methodology may be applied to other coastal areas and adapted to predict extreme winds and their impacts under climate variability and change.

Probabilistic life cycle analysis model for evaluating electric power infrastructure risk mitigation investments

One effect of climate change may be increased hurricane frequency or intensity due to changes in atmospheric and geoclimatic factors. It has been hypothesized that wetland restoration and infrastructure hardening measures may improve infrastructure resilience to increased hurricane frequency and intensity. This paper describes a parametric decision model used to assess the tradeoffs between wetland restoration and infrastructure hardening for electric power networks. We employ a hybrid economic input–output life-cycle analysis (EIO-LCA) model to capture: construction costs and life-cycle emissions for transitioning from the current electric power network configuration to a hardened network configuration; construction costs and life-cycle emissions associated with wetland restoration; and the intrinsic value of wetland restoration. Uncertainty is accounted for probabilistically through a Monte Carlo hurricane simulation model and parametric sensitivity analysis for the number of hurricanes expected to impact the project area during the project cycle and the rate of wetland storm surge attenuation. Our analysis robustly indicates that wetland restoration and undergrounding of electric power network infrastructure is not preferred to the “do-nothing” option of keeping all power lines overhead without wetland protection. However, we suggest a few items for future investigation. For example, our results suggest that, for the small case study developed, synergistic benefits of simultaneously hardening infrastructure and restoring wetlands may be limited, although research using a larger test bed while integrating additional costs may find an enhanced value of wetland restoration for disaster loss mitigation.

Effect of Tree Size, Root Pruning, and Production Method on Root Growth and Lateral Stability of Quercus virginiana

This research aimed to evaluate impact of slicing the outer edge of container root balls, initial tree size at planting, and root ball composition on post-planting tree stability in a simulated wind storm. One-hundred twenty Cathedral Oak® live oak were planted in March 2005. Thirty field-grown trees were transplanted, and 60 trees of similar size were planted from 170 L containers. Root ball sides on 30 containers were sliced prior to planting. Thirty smaller trees from 57 L containers were planted without slicing. Trees were pulled with an electric winch, and blown with a hurricane simulator in 2007. Slicing the root ball had no impact on root growth, bending moment, or bending stress. More bending stress was required to pull field-grown trees than trees planted from containers of either size. Growing trees in containers for three years prior to landscape planting changed root morphology compared to field-grown trees, which corresponded to reduced stability. Trees planted from small containers were as stable as those from larger containers. Root cross-sectional area windward correlated the most with bending stress required to tilt trees with a winch and cable. Bending moment scaled to the 3.4 power of trunk diameter.

Lagrangian mixing in an axisymmetric hurricane model

Abstract. This paper discusses the extension of established Lagrangian mixing measures to make them applicable to data extracted from a 2-D axisymmetric hurricane simulation. Because of the non-steady and unbounded characteristics of the simulation, the previous measures are extended to a moving frame approach to create time-dependent mixing rates that are dependent upon the initial time of particle integration, and are computed for nonlocal regions. The global measures of mixing derived from finite-time Lyapunov exponents, relative dispersion, and a measured mixing rate are applied to distinct regions representing different characteristic feautures within the model. It is shown that these time-dependent mixing rates exhibit correlations with maximal tangential winds during a quasi-steady state, establishing a connection between mixing and hurricane intensity.

Adiabatic rearrangement of hollow PV Towers

Diabatic heating from deep moist convection in the hurricane eyewall produces a towering annular structure of elevated potential vorticity (PV). This structure has been referred to as a hollow PV tower. The sign reversal of the radial gradient of PV satisfies the Charney‐Stern necessary condition for combined barotropic‐baroclinic instability. For thin enough annular structures, small perturbations grow exponentially, extract energy from the mean flow, and lead to hollow tower breakdown, with significant vortex structural and intensity change. The three‐dimensional adiabatic rearrangements of two prototypical hurricane‐like hollow PV towers (one thick and one thin) are examined in an idealized framework. For both hollow towers, dynamic instability causes air parcels with high PV to be mixed into the eye preferentially at lower levels, where unstable PV wave growth rates are the largest. Little or no mixing is found to occur at upper levels. The mixing at lower and middle levels is most rapid for the breakdown of the thin hollow tower, consistent with previous barotropic results. For both hollow towers, this advective rearrangement of PV affects the tropical cyclone structure and intensity in a number of ways. First, the minimum central pressure and maximum azimuthal mean velocity simultaneously decrease, consistent with previous barotropic results. Secondly, isosurfaces of absolute angular momentum preferentially shift inward at low levels, implying an adiabatic mechanism by which hurricane eyewall tilt can form. Thirdly, a PV bridge, similar to that previously found in full‐physics hurricane simulations, develops as a result of mixing at the isentropic levels where unstable PV waves grow most rapidly. Finally, the balanced mass field resulting from the PV rearrangement is warmer in the eye between 900 and 700 hPa. The location of this warming is consistent with observed warm anomalies in the eye, indicating that in certain instances the hurricane eye inversion is enhanced through PV mixing on isentropic surfaces and is thus partially dynamically controlled.

Evaluation of Planetary Boundary Layer Parameterizations in Tropical Cyclones by Comparison of In Situ Observations and High-Resolution Simulations of Hurricane Isabel (2003). Part I: Initialization, Maximum Winds, and the Outer-Core Boundary Layer

Abstract In this study, the first of two parts, the planetary boundary layer (PBL) depicted in high-resolution Weather Research and Forecast Model (WRF) simulations of Hurricane Isabel (2003) is studied and evaluated by direct comparisons with in situ data obtained during the Coupled Boundary Layer and Air–Sea Transfer Experiment (CBLAST). In particular, two boundary layer schemes are evaluated: the Yonsei University (YSU) parameterization and the Mellor–Yamada–Janjić (MYJ) parameterization. Investigation of these schemes is useful since they are available for use with WRF, are both widely used, and are based on entirely different methods for simulating the PBL. In this first part, the model domains and initialization are described. For additional realism of the low-level thermodynamic environment, a simple mixed layer ocean model is used to simulate ocean cooling. The YSU and MYJ schemes are discussed, along with some modifications. Standard measures of the accuracy of the hurricane simulations, such as track, maximum surface wind speed, and minimum surface pressure are described for a variety of parameter choices and for the two parameterizations. The effects on track and intensity of increased horizontal and vertical resolutions are also shown. A modification of the original YSU and MYJ schemes to have ocean roughness lengths more in agreement with recent studies considerably improves the results of both schemes. Instantaneous wind maxima on the innermost grid with 1.33-km resolution are shown to be an accurate representation of the simulated 1-min sustained winds. The simulated boundary layers are evaluated by direct comparison of the PBL as simulated and as observed by in situ data from the CBLAST experiment in the “outer core” region of the storm. The two PBL schemes and their modified counterparts reproduce the observed PBL remarkably well. Comparisons are also made to the observed vertical fluxes of momentum, heat, and moisture. In Part II, the detailed comparisons of the intensities and structures of the simulated and observed inner-core boundary layers are presented, and the reasons for the differences are discussed.

Optimizing the selection of hazard-consistent probabilistic scenarios for long-term regional hurricane loss estimation

Long-term regional hurricane loss estimation is typically conducted by simulating the occurrence of many hurricanes in a region using a historical or synthetic set of hurricanes; estimating losses caused by each hurricane; and using the resulting loss database to calculate loss-exceedence curves for each location. This paper introduces a new way to identify a set of hurricanes to be simulated, so as to substantially reduce the number of events required and thus the computation, while maintaining the spatial coherence of individual scenarios. Using a mixed-integer linear programming formulation, a relatively small subset of all possible hurricanes is identified (tens to a couple hundred), and the annual occurrence probability of each is adjusted so that the regional hazard estimated from the reduced set matches the regional hazard estimated from the full synthetic hurricane simulation. The user can explicitly tradeoff number of hurricanes (i.e., computation) and error based on the intended use of the analysis. The method also allows the user to force errors to be smaller at geographic locations or return periods that are particularly important for a given analysis. In a case study application for North Carolina, the method provides unbiased results with errors that are small enough for practical use.

Investigation of Hurricane Katrina characteristics for future, warmer climates

Simulations of Hurricane Katrina using the Weather Research and Forecasting (WRF) model on a 9 km grid over the Gulf of Mexico and the southeast United States are analyzed. Global Forecast System (GFS) analyses provided the initial fields and lateral boundary conditions (LBC) 4 times per day to drive a control simulation during the period 27 to 30 August 2005. The control cap- tured many of the observed characteristics of Katrina. A new approach, mean signal nesting, was devised to make climate change projections of the storm for each decade in the 21st century. Mean climate change signals were extracted from A2 scenario projections of the future climate by an atmosphere-ocean global climate model (AOGCM). These signals were combined with the GFS data used in the control to create the initial fields and LBC for WRF climate change simulations. This inno- vative method allows the LBC to retain realistic sub-daily variability present in GFS data, but still include the climate change signal. Hurricane simulations representing the earlier decades tracked east of the 2005 trajectory, and west of that track for 4 of the 5 later decades. Sensitivity experiments suggest that anticipated atmospheric warming versus expected positive sea surface temperature trends have opposing influences on developing storms. Warming trends during the 21st century are associated with ever-increasing vertical thermal stability, inhibiting initial hurricane intensification and limiting their diameters. Eye wall wind speeds in excess of 60 m s -1 at the time of landfall could be sustained for a somewhat longer duration in storms toward the end of the 21st century.

U.S. Hurricane Wind Speed Risk and Uncertainty

This paper presents a summary of updates made to the hurricane simulation model that has formed the basis of the hurricane wind speeds given in ASCE 7 for the period 1998 through 2010. Since the development of the model used for the ASCE 7-98 wind speeds, significantly more hurricane data have become available to improve the modeling process. These new data have resulted in an improved representation of the hurricane wind field, including the modeling of the sea-land transition and the hurricane boundary layer height, a new model for hurricane weakening after landfall, and an improved statistical model for the Holland B parameter. The new hurricane hazard model yields hurricane wind speeds that are lower than those given in ASCE 7-05 even though the overall rate of intense storms (as defined by central pressure) produced by the new model is increased compared to those produced by the hurricane simulation model used to develop the ASCE 7-98 through ASCE 7-05 wind speeds. Estimates of uncertainties in predi...

Analysis of Wind-Induced Economic Losses Resulting from Roof Damage to a Metal Building

Metal roofs are highly susceptible to hurricane wind damage. Roof damage is a particularly important element in the estimation of economic losses as even minor roof damage can augment the total loss due to entrance of rain and subsequent interior and content loss. Dabral (2007) developed a probabilistic damage model to predict the physical damage in a metal roof due to wind. This study estimates economic losses associated with high-wind, hurricane events that cause roof damage to metal buildings using Hurricane Katrina data and a hurricane simulation data set. A cost benefit study is also performed to establish the cost effectiveness of mitigation.