Hurricane-force Winds Research Articles

An Axisymmetric View of Concentric Eyewall Evolution in Hurricane Rita (2005)

Abstract Multiplatform observations of Hurricane Rita (2005) were collected as part of the Hurricane Rainband and Intensity Change Experiment (RAINEX) field campaign during a concentric eyewall stage of the storm’s life cycle that occurred during 21–22 September. Satellite, aircraft, dropwindsonde, and Doppler radar data are used here to examine the symmetric evolution of the hurricane as it underwent eyewall replacement. During the approximately 1-day observation period, developing convection associated with the secondary eyewall became more symmetric and contracted inward. Latent heating in the emergent secondary eyewall led to the development of a distinct toroidal (overturning) circulation with inertially constrained radial inflow above the boundary layer and compensating subsidence in the moat region, properties that are consistent broadly with the balanced vortex response to an imposed ring of diabatic heating outside the primary eyewall. The primary eyewall’s convection became more asymmetric during the observation period, but the primary eyewall was still the dominant swirling wind and vorticity structure throughout the period. The observed structure and evolution of Rita’s secondary eyewall suggest that spinup of the tangential winds occurred both within and above the boundary layer, and that both balanced and unbalanced dynamical processes played an important role. Although Rita’s core intensity decreased during the observation period, the observations indicate a 125% increase in areal extent of hurricane-force winds and a 19% increase in integrated kinetic energy resulting from the eyewall replacement.

Data Assimilation within the Advanced Circulation (ADCIRC) Modeling Framework for Hurricane Storm Surge Forecasting

AbstractAccurate, real-time forecasting of coastal inundation due to hurricanes and tropical storms is a challenging computational problem requiring high-fidelity forward models of currents and water levels driven by hurricane-force winds. Despite best efforts in computational modeling there will always be uncertainty in storm surge forecasts. In recent years, there has been significant instrumentation located along the coastal United States for the purpose of collecting data—specifically wind, water levels, and wave heights—during these extreme events. This type of data, if available in real time, could be used in a data assimilation framework to improve hurricane storm surge forecasts. In this paper a data assimilation methodology for storm surge forecasting based on the use of ensemble Kalman filters and the advanced circulation (ADCIRC) storm surge model is described. The singular evolutive interpolated Kalman (SEIK) filter has been shown to be effective at producing accurate results for ocean models using small ensemble sizes initialized by an empirical orthogonal function analysis. The SEIK filter is applied to the ADCIRC model to improve storm surge forecasting, particularly in capturing maximum water levels (high water marks) and the timing of the surge. Two test cases of data obtained from hindcast studies of Hurricanes Ike and Katrina are presented. It is shown that a modified SEIK filter with an inflation factor improves the accuracy of coarse-resolution forecasts of storm surge resulting from hurricanes. Furthermore, the SEIK filter requires only modest computational resources to obtain more accurate forecasts of storm surge in a constrained time window where forecasters must interact with emergency responders.

Aerosol Effects on Microstructure and Intensity of Tropical Cyclones

Improving the forecasts of the intensity of tropical cyclones (TCs) remains a major challenge. One possibility for improvement is consideration of the effects that aerosols have on tropical clouds and cyclones. The authors have been pursuing this under the Hurricane Aerosol and Microphysics Program, supported by the U.S. Department of Homeland Security. This was done through observations of aerosols and resulting cloud microphysical structure within tropical cyclones and simulating their effects using high-resolution TC models that treat cloud internal processes explicitly. In addition to atmospheric aerosols, special attention was given also to the impact of the intense sea-spray-generated aerosols and convective rolls in the hurricane boundary layer (BL) under hurricane- force winds. The results of simulations and observations show that TC ingestion of aerosols that serve as cloud condensation nuclei can lead to significant reductions in their intensities. This is caused by redistribution of the precipitation and latent heating to more vigorous convection in the storm periphery that cools the low levels and interferes with the inflow of energy to the eyewall, hence making the eye larger and the maximum winds weaker. The microphysical effects of the pollution and dust aerosols occur mainly at the peripheral clouds. Closer to the circulation center, the hurricane-force winds raise intense sea spray that is lifted efficiently in the roll vortices that form in the BL and coalesce into rain of mostly seawater already at cloud base, which dominates the microstructure and affects the dynamics of the inner convective cloud bands.

A 300-year history of Pacific Northwest windstorms inferred from tree rings

Hurricane-force winds are frequently allied with mid-latitude cyclones yet little is known about their historical timing and geographic extent over multiple centuries. This research addresses these issues by extending the historical record of major mid-latitude windstorms along North America's Pacific Northwest (PNW) coast using tree-ring data collected from old-growth (>350years), wind-snapped trees sampled at seven coastal sites in Oregon, USA. Our objectives were to: 1) characterize historical windstorm regimes; 2) determine the relationship between high-wind events (HWEs) and phases of the PDO, ENSO and NPI; and 3) test the hypothesis that PNW HWEs have migrated northward. We based our study on the identification of tree-growth anomalies resulting from windstorm-induced canopy changes corresponding to documented (1880–2003) and projected HWEs (1701–1880). Our methods identified all major windstorm events recorded since the late 1800s and confirmed that variations in coastal tree-growth are weakly related to temperature, precipitation, and drought, but are significantly related to peak wind speeds. These results suggest wind-induced changes in canopy conditions control tree growth at all sites. Comparisons between the tree-ring record and the PDO, NPI, and ENSO revealed a significant positive correlation between HWEs and neutral to warm PDO conditions and a slightly weaker correlation with the NPI. ENSO events were not significantly related to the occurrence of HWEs. Latitudinal groupings of our sites revealed a gradual and non-significant northerly shift of HWEs until the late 19th century followed by a significant northward shift during the past 120years. These results mark the application of dendroanemology as a method for characterizing windstorm regimes for multiple centuries.

In the beginning

In the beginning

Hurricane Igor Impacts at Northern Latitudes: Factors Influencing Tree Fall in an Urban Setting

Hurricane Igor was a Category 1 hurricane when it passed the island of Newfoundland, Canada, causing extensive damage. Hurricanes are uncommon at northern latitudes, and boreal species are not adapted to hurricane-force winds. Moreover, much of the storm damage was in the urban area of the City of St. John’s, where there are also numerous non-native trees. This research tested whether there were attributes of trees (e.g., height, diameter at breast height, slenderness, species, age, or distance to nearest tree) that may have influenced whether a tree fell or was left standing. The study authors sampled 70 trees and found that DBH was a significant predictor of tree fall (snapping or uprooting). Conifers were no more or less likely to fall in the storm than deciduous trees, nor were native trees more or less susceptible to wind damage than non-natives. These results suggest that for a boreal, urban ecosystem, there are no target species available that could be planted strategically to minimize risk of tree fall in a major wind event. Thus, to minimize storm damage to human-built infrastructure in regions where hurricanes are rare, the best strategy would be to avoid having large trees located in close proximity to infrastructure.

Circular conditional autoregressive modeling of vector fields

As hurricanes approach landfall, there are several hazards for which coastal populations must be prepared. Damaging winds, torrential rains, and tornadoes play havoc with both the coast and inland areas; but, the biggest seaside menace to life and property is the storm surge. Wind fields are used as the primary forcing for the numerical forecasts of the coastal ocean response to hurricane force winds, such as the height of the storm surge and the degree of coastal flooding. Unfortunately, developments in deterministic modeling of these forcings have been hindered by computational expenses. In this paper, we present a multivariate spatial model for vector fields, that we apply to hurricane winds. We parameterize the wind vector at each site in polar coordinates and specify a circular conditional autoregressive (CCAR) model for the vector direction, and a spatial CAR model for speed. We apply our framework for vector fields to hurricane surface wind fields for Hurricane Floyd of 1999 and compare our CCAR model to prior methods that decompose wind speed and direction into its N-S and W-E cardinal components.

Estimating Gale to Hurricane Force Winds Using the Satellite Altimeter

Abstract A new model is provided for estimating maritime near-surface wind speeds (U10) from satellite altimeter backscatter data during high wind conditions. The model is built using coincident satellite scatterometer and altimeter observations obtained from QuikSCAT and Jason satellite orbit crossovers in 2008 and 2009. The new wind measurements are linear with inverse radar backscatter levels, a result close to the earlier altimeter high wind speed model of Young (1993). By design, the model only applies for wind speeds above 18 m s−1. Above this level, standard altimeter wind speed algorithms are not reliable and typically underestimate the true value. Simple rules for applying the new model to the present-day suite of satellite altimeters (Jason-1, Jason-2, and Envisat RA-2) are provided, with a key objective being provision of enhanced data for near-real-time forecast and warning applications surrounding gale to hurricane force wind events. Model limitations and strengths are discussed and highlight the valuable 5-km spatial resolution sea state and wind speed altimeter information that can complement other data sources included in forecast guidance and air–sea interaction studies.

Does Increased Horizontal Resolution Improve Hurricane Wind Forecasts?

Abstract The representation of tropical cyclone track, intensity, and structure in a set of 69 parallel forecasts performed at each of two horizontal grid increments with the Advanced Research Hurricane (AHW) component of the Weather and Research and Forecasting Model (WRF) is evaluated. These forecasts covered 10 Atlantic tropical cyclones: 6 from the 2005 season and 4 from 2007. The forecasts were integrated from identical initial conditions produced by a cycling ensemble Kalman filter. The high-resolution forecasts used moving, storm-centered nests of 4- and 1.33-km grid spacing. The coarse-resolution forecasts consisted of a single 12-km domain (which was identical to the outer domain in the forecasts with nests). Forecasts were evaluated out to 120 h. Novel verification techniques were developed to evaluate forecasts of wind radii and the degree of storm asymmetry. Intensity (maximum wind) and rapid intensification, as well as wind radii, were all predicted more accurately with increased horizontal resolution. These results were deemed to be statistically significant based on the application of bootstrap confidence intervals. No statistically significant differences emerged regarding storm position errors between the two forecasts. Coarse-resolution forecasts tended to overpredict the extent of winds compared to high-resolution forecasts. The asymmetry of gale-force winds was better predicted in the coarser-resolution simulation, but asymmetry of hurricane-force winds was predicted better at high resolution. The skill of the wind radii forecasts decayed gradually over 120 h, suggesting a synoptic-scale control of the predictability of outer winds.

Calculations of Asteroid Impacts into Deep and Shallow Water

Contrary to received opinion, ocean impacts of small (<500 m) asteroids do not produce tsunamis that lead to world-wide devastation. In fact the most dangerous features of ocean impacts, just as for land impacts, are the atmospheric effects. We present illustrative hydrodynamic calculations of impacts into both deep and shallow seas, and draw conclusions from a parameter study in which the size of the impactor and the depth of the sea are varied independently. For vertical impacts at 20 km/s, craters in the seafloor are produced when the water depth is less than about 5–7 times the asteroid diameter. Both the depth and the diameter of the transient crater scale with the asteroid diameter, so the volume of water excavated scales with the asteroid volume. About a third of the crater volume is vaporised, because the kinetic energy per unit mass of the asteroid is much larger than the latent heat of vaporisation of water. The vaporised water carries away a considerable fraction of the impact energy in an explosively expanding blast wave which is responsible for devastating local effects and may affect worldwide climate. Of the remaining energy, a substantial portion is used in the crown splash and the rebound jet that forms as the transient crater collapses. The collapse and rebound cycle leads to a propagating wave with a wavelength considerably shorter than classical tsunamis, being only about twice the diameter of the transient crater. Propagation of this wave is hindered somewhat because its amplitude is so large that it breaks in deep water and is strongly affected by the blast wave’s perturbation of the atmosphere. Even if propagation were perfect, however, the volume of water delivered per metre of shoreline is less than was delivered by the Boxing Day 2004 tsunami for any impactor smaller than 500 m diameter in an ocean of 5 km depth or less. Near-field effects are dangerous for impactors of diameter 200 m or greater; hurricane-force winds can extend tens of kilometers from the impact point, and fallout from the initial splash can be extremely violent. There is some indication that near-field effects are more severe if the impact occurs in shallow water.

Wind-Uplift Capacity of Residential Wood Roof-Sheathing Panels Retrofitted with Insulating Foam Adhesive

This research evaluated and compared the wind-uplift capacity of wood roof-sheathing panels fabricated by using nails with retrofitted roof panels made with nails and closed-cell sprayed polyurethane foam (ccSPF) adhesive. In hurricane-prone areas, structural retrofits of light-framed wood roof structures are needed to mitigate wind damage to existing residential roof structures because the majority of these have inadequate design strength to resist hurricane-force winds. A steel pressure chamber connected to a pressure loading actuator was used to conduct uplift pressure tests on 186 roof panels. The panels were fabricated using 11.1 mm (7/16 in.) thick oriented strand board sheathing fastened to nominal 51 by 102 mm (2 by 4 in.) southern yellow pine framing members spaced 610 mm (24 in.) apart. There were 123 panels tested in typical as-built conditions, and 63 panels tested after they were retrofitted with ccSPF. The parametric study determined the effect of several factors on wind-uplift failure pressure: (1) three nail types, (2) two nail spacings, and (3) three retrofit methods using ccSPF. The hypothesis tested was that ccSPF (traditionally used as wall and roof insulation in houses) could also act as a structural adhesive to increase the wind resistance of existing roofs. Standardized tests do not currently exist for wood roof panels, and so a uniform, static pressure-test protocol was developed on the basis of the ASTM E330, Method B Test Procedure. The results showed that ccSPF retrofits increase the wind-uplift capacity of the pre-1994 code-minimum wood roof panels by as much as 250–300%. This finding is important because it could provide a means to improve the wind resistance of these older roof designs, which may still account for more than 60% of the existing residential inventory. The distributions of roof-failure capacities of as-built and retrofitted roof panels are presented, and statistical parameters are presented for use in developing performance-based design criteria. The documentation and approach is presented as a model test protocol as the basis of a standardized wind-uplift test method for wood roof-sheathing panels.

An Improved C-Band Ocean Surface Emissivity Model at Hurricane-Force Wind Speeds Over a Wide Range of Earth Incidence Angles

An improved microwave radiometric ocean surface emissivity model has been developed to support forward radiative transfer modeling of brightness temperature and geophysical retrieval algorithms for the next-generation airborne Hurricane Imaging Radiometer instrument. This physically based C-band emissivity model extends current model capabilities to hurricane-force wind speeds over a wide range of incidence angles. It was primarily developed using brightness temperature observations during hurricanes with coincident high-quality surface-truth wind speeds, which were obtained using the airborne Stepped-Frequency Microwave Radiometer. Also, other ocean emissivity models available through the published literature and the spaceborne WindSat radiometer measurements were used.

Effect of Two-Way Air–Sea Coupling in High and Low Wind Speed Regimes

AbstractA recent advance in the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) is described and used to study two-way air–sea coupling and its impact on two different weather scenarios. The first case examines the impact of a hurricane-induced cold ocean wake on simulated changes in the structure of Hurricane Katrina. The second case investigates the effect of wind- and current-induced island wakes and their impact on the local electromagnetic (EM) and acoustic propagation characteristics in the Southern California Bight region. In the Katrina case, both the atmosphere and ocean show a strong response from air–sea interaction. The model results show that wind-induced turbulent mixing, vertical advection, and horizontal advection are the three primary causes of the development of the trailing cold ocean wake. A distinct spatial separation is seen in these three primary forcing terms that are generating the bulk of the cooling in the ocean mixed layer. An asymmetric tropical cyclone structure change has been documented in detail from a more realistic, full physics, and tightly coupled model. These changes include a broadening of the eye, a reduced radius of hurricane-force wind, and a pronounced inner-core dry slot on the west side of the storm. In the island wake experiment, many finescale variations in the wind, current, and static stability structure resulting from the two-way interaction are described. These variations take the form of narrow vorticity and temperature anomalies that are found to reside in the ocean and atmosphere well downwind from the Channel Islands. Upwind differences in the lower-atmospheric wind and thermal structure also arise and are found to have a small impact on the lee-flow structure and EM characteristics of the southernmost Channel Islands.

Multi-Frequency Synthetic Thinned Array Antenna for the Hurricane Imaging Radiometer

A C-band four-frequency resonant stacked-patch array antenna is developed for synthetic thinned aperture radiometer measurements of hurricane force wind speeds. This antenna is being integrated into an aircraft instrument referred to as the Hurricane Imaging Radiometer (HIRAD). Details of the antenna design are presented along with antenna performance tests and laboratory measurements using a full-scale prototype array with a subset model of the HIRAD instrument.

Obtaining Accurate Ocean Surface Winds in Hurricane Conditions: A Dual-Frequency Scatterometry Approach

We describe a method for retrieving winds from colocated Ku- and C-band ocean wind scatterometers. The method utilizes an artificial neural network technique to optimize the weighting of the information from the two frequencies and to use the extra degrees of freedom to account for rain contamination in the measurements. A high-fidelity scatterometer simulation is used to evaluate the efficacy of the technique for retrieving hurricane force winds in the presence of heavy precipitation. Realistic hurricane wind and precipitation fields were simulated for three Atlantic hurricanes, Katrina and Rita in 2005 and Helene in 2006, using the Weather Research and Forecasting model. These fields were then input into a radar simulation previously used to evaluate the Extreme Ocean Vector Wind Mission dual-frequency scatterometer mission concept. The simulation produced high-resolution dual-frequency normalized radar cross-section (NRCS) measurements. The simulated NRCS measurements were binned into 5 x 5 km wind cells. Wind speeds in each cell were estimated using an artificial neural network technique. The method was shown to retrieve accurate winds up to 50 m/s even in intense rain.

Observations of Antarctic Polynya With Unmanned Aircraft Systems

Working in the polar environment is always challenging, particularly during the winter, when environmental conditions are harshest. With hurricane force winds, frigid temperatures, and the potential to alter the global thermohaline circulation, the Terra Nova Bay region of Antarctica in the western Ross Sea is an environment where acquiring observations of local atmospheric and oceanic interactions is critical and also extremely challenging.An important feature of Terra Nova Bay is a recurring polynya—an area of nearly ice‐free water surrounded by sea ice and land. Strong katabatic winds (cold, negatively buoyant air that flows downslope under the influence of gravity) drain from the interior of the continent and blow over the open water of the polynya, resulting in large upward fluxes of heat and moisture. Sea ice production occurs as a result of the large transfer of heat from sea to air, with the newly formed sea ice blown offshore, effectively removing freshwater from the coastal ocean. The high‐salinity water created through this process becomes part of the global thermohaline circulation as Antarctic bottom water. Coastal polynyas, such as the one in Terra Nova Bay, are of interest to atmospheric scientists and oceanographers due to the intense air‐sea coupling and the impact of these fluxes on the state of the atmosphere and ocean.

A Climatology of Inland Winds from Tropical Cyclones for the Eastern United States

Abstract Tropical cyclones pose a significant threat to life and property along coastal regions of the United States. As these systems move inland and dissipate, they can also pose a threat to life and property, through heavy rains, high winds, and other severe weather such as tornadoes. While many studies have focused on the impacts from tropical cyclones on coastal counties of the United States, this study goes beyond the coast and examines the impacts caused by tropical cyclones on inland locations. Using geographical information system software, historical track data are used in conjunction with the radial maximum extent of the maximum sustained winds at 34-, 50-, and 64-kt (1 kt ≈ 0.5 m s−1) thresholds for all intensities of tropical cyclones and overlaid on a 30-km equal-area grid that covers the eastern half of the United States. The result is a series of maps with frequency distributions and an estimation of return intervals for inland tropical storm– and hurricane-force winds. Knowing where the climatologically favored areas are for tropical cyclones, combined with a climatological expectation of the inland penetration frequency of these storms, can be of tremendous value to forecasters, emergency managers, and the public.

Major Extratropical Cyclones of the Northwest United States: Historical Review, Climatology, and Synoptic Environment

Abstract The northwest United States is visited frequently by strong midlatitude cyclones that can produce hurricane-force winds and extensive damage. This article reviews these storms, beginning with a survey of the major events of the past century. A climatology of strong windstorms is presented for the area from southern Oregon to northern Washington State and is used to create synoptic composites that show the large-scale evolution associated with such storms. A recent event, the Hanukkah Eve Storm of December 2006, is described in detail, with particular attention given to the impact of the bent-back front/trough and temporal changes in vertical stability and structure. The discussion section examines the general role of the bent-back trough, the interactions of such storms with terrain, and the applicability of the “sting jet” conceptual model. A conceptual model of the evolution of Northwest windstorm events is presented.

Effects of Bubbles and Sea Spray on Air–Sea Exchange in Hurricane Conditions

The lower limit on the drag coefficient under hurricane force winds is determined by the break-up of the air–sea interface due to Kelvin–Helmholtz instability and formation of the two-phase transition layer consisting of sea spray and air bubbles. As a consequence, a regime of marginal stability develops. In this regime, the air–sea drag coefficient is determined by the turbulence characteristics of the two-phase transition layer. The upper limit on the drag coefficient is determined by the Charnock-type wave resistance. Most of the observational estimates of the drag coefficient obtained in hurricane conditions and in laboratory experiments appear to lie between the two extreme regimes: wave resistance and marginal stability.

Pre-landfall evacuee perception of the meteorological hazards associated with Hurricane Gustav

In this study, evacuees from the path of Hurricane Gustav were surveyed to determine which meteorological hazards most influenced their decision to leave. Surveys were conducted along two major evacuation routes on August 30 and 31, 2008, to collect time-sensitive data on individual evacuation decisions related to the meteorological hazards from Hurricane Gustav. The regions of New Orleans, Houma, and Lafayette were represented most frequently, as determined by zip code data collected from the surveys. Responses were evaluated first by meteorological hazard for the entire dataset and then by three-digit zip code region. Overall, storm surge was the most important meteorological variable, followed by the size of the storm, wind, rain, and tornadoes. When separated into three-digit zip code regions, analyses revealed evacuees from in and around New Orleans were driven to evacuate as a result of the perceived threat from storm surge and storm size; residents in the Houma, Louisiana region were motivated to leave due to the threat from storm surge; and Lafayette and the surrounding areas were most-concerned with the threats posed by hurricane-force winds. Given the forecast track and intensity, survey respondents understood the meteorological hazards from Gustav and were motivated to leave based on personal evaluations of risk associated with the storm.