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Peter J. Vickery

Abstract

Modeling the increase in the central pressure of tropical cyclones following landfall plays a critical role in the estimation of the hurricane wind hazard at locations removed from the coastline. This paper describes the development of simple empirical models for estimating the rate at which tropical cyclones decay after making landfall. For storms making landfall along the Gulf of Mexico Coast and the coast of the Florida Peninsula, it is shown that the rate of storm filling is proportional to the central pressure difference and translation speed at the time of landfall and is inversely proportional to the radius to maximum winds. Along the Atlantic Coast the effect of radius to maximum winds does not play as significant a role in the rate of storm decay as compared with that seen in Florida and along the Gulf Coast. The models developed here can readily be included in any hurricane simulation model designed for estimating wind speeds in the United States.

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Peter J. Vickery and Dhiraj Wadhera

Abstract

In many hurricane risk models the inclusion of the Holland B parameter plays an important role in the risk prediction methodology. This paper presents an analysis of the relationship between B and a nondimensional intensity parameter. The nondimensional parameter includes the strong negative correlation of B with increasing hurricane size [as defined by the radius to maximum winds (RMW)] and latitude as well as a positive correlation with sea surface temperature. A weak positive correlation between central pressure deficit and B is also included in the single parameter term. Alternate statistical models relating B to RMW and latitude are also developed. Estimates of B are derived using pressure data collected during hurricane reconnaissance flights, coupled with additional information derived from the Hurricane Research Division’s H*Wind snapshots of hurricane wind fields. The reconnaissance data incorporate flights encompassing the time period 1977 through 2001, but the analysis was limited to include only those data collected at the 700-hPa-or-higher level. Statistical models relating RMW to latitude and central pressure derived from the dataset are compared to those derived for U.S. landfalling storms during the period 1900–2005. The authors find that for the Gulf of Mexico, using only the landfall hurricanes, the data suggest that there is no inverse relationship between RMW and the central pressure deficit. The RMW data also demonstrate that Gulf of Mexico hurricanes are, on average, smaller than Atlantic Ocean hurricanes. A qualitative examination of the variation of B, central pressure, and radius to maximum winds as a function of time suggests that along the Gulf of Mexico coastline (excluding southwest Florida), during the final 6–24 h before landfall, the hurricanes weaken as characterized by both an increase in central pressure and the radius to maximum winds and a decrease in B. This weakening characteristic of landfalling storms is not evident for hurricanes making landfall elsewhere along the U.S. coastline.

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Peter J. Vickery, Dhiraj Wadhera, Mark D. Powell, and Yingzhao Chen

Abstract

This article examines the radial dependence of the height of the maximum wind speed in a hurricane, which is found to lower with increasing inertial stability (which in turn depends on increasing wind speed and decreasing radius) near the eyewall. The leveling off, or limiting value, of the marine drag coefficient in high winds is also examined. The drag coefficient, given similar wind speeds, is smaller for smaller-radii storms; enhanced sea spray by short or breaking waves is speculated as a cause. A fitting technique of dropsonde wind profiles is used to model the shape of the vertical profile of mean horizontal wind speeds in the hurricane boundary layer, using only the magnitude and radius of the “gradient” wind. The method slightly underestimates the surface winds in small but intense storms, but errors are less than 5% near the surface. The fit is then applied to a slab layer hurricane wind field model, and combined with a boundary layer transition model to estimate surface winds over both marine and land surfaces.

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Forrest J. Masters, Peter J. Vickery, Phuong Bacon, and Edward N. Rappaport

Extreme wind climatology and event-specific intensity assessments rely heavily on surface wind field observations. The most widely used platforms sited at airports are the Automated Surface Observing System (ASOS) and its predecessor, the Automated Weather Observing System (AWOS). The terrain immediately surrounding most of these stations may be nominally characterized as aero-dynamically very smooth because of the runways and flat expanses of grass that define most airport layouts. Outside of most airports, a wide spectrum of marine, open, suburban, and heavily built-up terrain conditions are present. The results of this research indicate that the wind speeds recorded by AWOS/ASOS are deeply sensitive to this terrain. Prior research has shown that direct usage of the raw surface data can introduce surface-layer wind speed errors on the order of 30%–40% due to terrain effects. Similar values are observed for gust speeds in this paper, when averaging technique and anemometer response characteristics are considered. A solution is developed to automatically compute the directional effective roughness length (z 0) values using simple averages of peak-to-mean wind speed ratios (gust factors). Adjustments are also made to cup anemometer data to correct for gust attenuation caused by frequency response and block averaging. A new effective surface roughness database is offered that can be used to convert a raw wind speed measurement (sustained or gust) to any predefined aerodynamic metadata (height, terrain, and average period) to serve the needs of operational and research users.

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Bruce A. Harper, John D. Holmes, Jeffrey D. Kepert, Luciano B. Mason, and Peter J. Vickery

Abstract

Cook and Nicholls recently argued in this journal that the city of Darwin (Northern Territory), Australia, should be located in wind region D rather than in the current region C in the Australian/New Zealand Standard AS/NZS 1170.2 wind actions standard, in which region D has significantly higher risk. These comments critically examine the methods used by Cook and Nicholls and find serious flaws in them, sufficient to invalidate their conclusions. Specific flaws include 1) invalid assumptions in their analysis method, including that cyclones are assumed to be at the maximum intensity along their entire path across the sampling circle even after they have crossed extensive land areas; 2) a lack of verification that the simulated cyclone tracks are consistent with the known climatological data and in particular that the annual rate of simulated cyclones at each station greatly exceeds the numbers recorded for the entire Australian region; and 3) the apparent omission of key cyclones when comparing the risk at Darwin with two other locations. It is shown here that the number of cyclones that have affected Port Hedland (Western Australia), a site in Australia’s region D, greatly exceeds the number that have influenced Darwin over the same period for any chosen threshold of intensity. Analysis of the recorded gusts from anemometers at Port Hedland and Darwin that is presented here further supports this result. On the basis of this evidence, the authors conclude that Darwin’s tropical cyclone wind risk is adequately described by its current location in region C.

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Ian M. Giammanco, John L. Schroeder, Forrest J. Masters, Peter J. Vickery, Richard J. Krupar III, and Juan-Antonio Balderrama

Abstract

The deployment of ruggedized surface observing platforms by university research programs in the path of landfalling tropical cyclones has yielded a wealth of information regarding the near-surface wind flow characteristics. Data records collected by Texas Tech University’s Wind Engineering Mobile Instrument Tower Experiment and StickNet probes and by the Florida Coastal Monitoring Program along the Gulf Coast of the United States from 2004 to 2008 were compiled to examine influences on near-surface gust factors. Archived composite reflectivity data from coastal WSR-88D instruments were also merged with the tower records to investigate the influence of precipitation structure. Wind records were partitioned into 10-min segments, and the ratio of the peak moving-average 3-s-gust wind speed to the segment mean was used to define a gust factor. Observations were objectively stratified into terrain exposure categories to determine if factors beyond those associated with surface frictional effects can be extracted from the observations. Wind flow characteristics within exposure classes were weakly influenced by storm-relative position and precipitation structure. Eyewall observations showed little difference in mean gust factors when compared with other regions. In convective precipitation, only peak gust factors were slightly larger than those found in stratiform conditions, with little differences in the mean. Gust factors decreased slightly with decreasing radial distance in rougher terrain exposures and did not respond to radar-observed changes in precipitation structure. In two limited comparisons, near-surface gusts did not exceed the magnitude of the wind maximum aloft detected through wind profiles that were derived from WSR-88D velocity–azimuth displays.

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