Peak Gust Wind Research Articles

An Examination of Tropical and Extratropical Gust Factors and the Associated Wind Speed Histograms

Abstract A gust factor, defined as the ratio between a peak wind gust and mean wind speed over a period of time, can be used along with other statistics to examine the structure of the wind. Gust factors are heavily dependent on upstream terrain conditions (roughness), but are also affected by transitional flow regimes (specifically, changes in terrain and the distance from the upstream terrain change to the measuring device), anemometer height, stability of the boundary layer, and, potentially, the presence of deep convection. Previous studies have yielded conflicting results regarding differences in gust factors that might exist between winds generated by tropical cyclones and those generated by extratropical systems. Using high-resolution wind speed data collected from both landfalling tropical cyclones and extratropical systems, two databases of wind characteristics were developed. Gust factors from tropical cyclone and extratropical winds were examined, summarized, and compared. Further analysis was conducted to examine and compare the characteristics of the associated tropical and extratropical wind speed histograms. As expected, the mean gust factor was found to increase with increasing upstream surface roughness. Some differences were observed between data from the tropical environment and the extratropical environment. Mean gust factors from the tropical regime were found to be higher than mean gust factors from the extratropical environment within each roughness regime and the wind speed histograms generated from data from the two environments indicated some differences.

The 30 May 1998 Spencer, South Dakota, Storm. Part II: Comparison of Observed Damage and Radar-Derived Winds in the Tornadoes

Abstract A violent supercell tornado passed through the town of Spencer, South Dakota, on the evening of 30 May 1998 producing large gradients in damage severity. The tornado was rated at F4 intensity by damage survey teams. A Doppler On Wheels (DOW) mobile radar followed this tornado and observed the tornado at ranges between 1.7 and 8.0 km during various stages of the tornado's life. The DOW was deployed less than 4.0 km from the town of Spencer between 0134 and 0145 UTC, and during this time period, the tornado passed through Spencer, and peak Doppler velocity measurements exceeded 100 m s−1. Data gathered from the DOW during this time period contained high spatial resolution sample volumes of approximately 34 m × 34 m × 37 m along with frequent volume updates every 45–50 s. The high-resolution Doppler velocity data gathered from low-level elevation scans, when sample volumes are between 20 and 40 m AGL, are compared to extensive ground and aerial damage surveys performed by the National Weather Service (NWS) and the National Institute of Standards and Technology (NIST). Idealized radial profiles of tangential velocity are computed by fitting a model of an axisymmetric translating vortex to the Doppler radar observations, which compensates for velocity components perpendicular to the radar beam as well as the translational motion of the tornado vortex. Both the original single-Doppler velocity data and the interpolated velocity fields are compared with damage survey Fujita scale (F-scale) estimates throughout the town of Spencer. This comparison on a structure-by-structure basis revealed that radar-based estimates of the F-scale intensity usually exceeded the damage-survey-based F-scale both inside and outside the town of Spencer. In the town of Spencer, the radar-based wind field revealed two distinct velocity time series inside and outside the passage of the core-flow region. The center of the core-flow region tracked about 50 m farther north than the damage survey indicated because of the asymmetry induced by the 15 m s−1 translational motion of the tornado. The radar consistently measured the strongest winds in the lowest 200 m AGL with the most extreme Doppler velocities residing within 50 m AGL. Alternate measures of tornado wind field intensity that incorporated the effects of the duration of the extreme winds and debris were explored. It is suggested that damage may not be a simple function of peak wind gust and structural integrity, but that the duration of intense winds, directional changes, accelerations, and upwind debris loading may be critical factors.

ENSO Impacts on Peak Wind Gusts in the United States

Abstract Changes in the peak wind gust magnitude in association with the warm and cold phases of the El Nino–Southern Oscillation (ENSO) are identified over the contiguous United States. All calculations of the peak wind gust are differences in the extreme phases of ENSO (warm and cold) relative to neutral for all stations in the study that pass the completeness criteria. Monthly composites were created for all years in the study (1 January 1948 through 31 August 1998). The differences in the mean peak wind gust are calculated for each month. A nonparametric statistical test was invoked to determine significant shifts in the extreme phase distributions. Differences in the frequency of gale-force wind gusts were also calculated. The results show a dominant, ENSO cold phase wintertime signal. Regions most greatly affected are the Pacific Northwest, Southwest, the Great Plains, and the region extending from the Great Lakes through the Ohio River valley, and southwest into Texas. During the cold phase months ...

A once in 50-year wind speed map for Europe derived from mean sea level pressure measurements

Once in 50-year geostrophic wind speeds have been calculated for Europe using mean sea level pressure maps. The resulting wind speeds have then been converted to the equivalent 10-min mean surface wind speeds using the geostrophic drag law. Surface wind speeds are found to be in good agreement with those for the United Kingdom and Denmark calculated using observed hourly or 10-min mean wind speeds, but are less than those for France and Ireland, which have been calculated using observed peak gust wind speeds. The worst discrepancy occurs for northern France where wind speeds are 8–10 m/s less than those determined from observed peak gust wind speeds.

Hurricane Wind Field Model for Use in Hurricane Simulations

A critical component in the simulation of hurricanes is a good representation of the hurricane wind field when given information of the storm intensity, size, and translation speed. In the investigation described here, the full nonlinear solution to the equations of motion of a hurricane are solved and then parameterized for use in fast-running simulations. The hurricane model described here takes into account the effects of changing sea surface roughness and the air-sea temperature difference on the estimated surface-level wind speeds. Comparisons between modeled and observed hurricane wind speed records are performed where one compares both the 10-min mean wind speeds and the peak gust wind speeds. The resulting wind field model represents the most physically based and validated model used in the estimate of hurricane wind speed exceedance probabilities.

Analyses of mesoscale events and local climate using The Automated Weather Source school weather network

A cold front moved through the greater Baltimore, MD, and Washington, DC, metropolitan areas during the late evening on 1 December, 1996 and early morning on 2 December 1996 (local time). This paper describes an analysis of the near-surface meteorological conditions associated with the frontal passage using observations from an automated school weather network. Data acquired from the school network were used to determine the location of the cold front as it passed through the area. Observations from the school network are compared with those from a network of more widely separated airport stations. In several instances, peak wind gusts, temperature fall rates, and precipitation totals from the network exceeded those reported at the airports. The development of time-averaged wind statistics with and without rainfall is also discussed. Wind roses from one network station were shown to differ when the co-occurrence of wind and rain was considered compared with findings using all wind data.

Analysis of peak gust versus fastest-mile wind statistics

The relationship between peak gust wind speed and fastest 1 min (or fastest-mile) wind speed occurring within the same hour has been studied for five stations in the United States. For each station at least ten years of observations were used to investigate the relationship and its variation seasonally, diurnally and with different concomitant weather events. As a result it appears that peak gust wind data could be used to estimate the fastest 1 min wind for stations where the latter is not available.

Hurricane Impacts to Tropical and Temperate Forest Landscapes

Hurricanes represent an important natural disturbance process to tropical and temperate forests in many coastal areas of the world. The complex patterns of damage created in forests by hurricane winds result from the interaction of meteorological, physiographic, and biotic factors on a range of spatial scales. To improve our understanding of these factors and of the role of catastrophic hurricane wind as a disturbance process, we take an integrative approach. A simple meteorological model (HURRECON) utilizes meteorological data to reconstruct wind conditions at specific sites and regional gradients in wind speed and direction during a hurricane. A simple topographic exposure model (EXPOS) utilizes wind direction predicted by HURRECON and a digital elevation map to estimate landscape—level exposure to the strongest winds. Actual damage to forest stands is assessed through analysis of remotely sensed, historical, and field data. These techniques were used to evaluate the characteristics and impacts of two important hurricanes: Hurricane Hugo (1989) in Puerto Rico and the 1938 New England Hurricane, storms of comparable magnitude in regions that differ greatly in climate, vegetation, physiography, and disturbance regimes. In both cases patterns of damage on a regional scale were found to agree with the predicted distribution of peak wind gust velocities. On a landscape there was also good agreement between patterns of forest damage and predicted exposure in the Luquillo Experimental Forest in Puerto Rico and the town of Petersham, Massachusetts. At the Harvard and Pisgah Forests in central New England the average orientation of wind—thrown trees was very close to the predicted peak wind direction, while at Luquillo there was also good agreement, with some apparent modification of wind direction by the mountainous terrain. At Harvard Forest there was evidence that trees more susceptible to windthrow were felled earlier in the storm. This approach may be used to study the effects of topography on wind direction and the relation of forest damage to wind speed and duration; to establish broad—scale gradients of hurricane frequency, intensity, and wind direction for particular regions; and to determine landscape—level exposure to long—term hurricane disturbance at particular sites.

Criteria for environmental wind conditions

Since 1971 a number of authors have published criteria for the acceptability of environmental wind conditions for human comfort for a range of activities. This paper notes that it is the forces caused by peak gust wind speeds and associated gradients which people feel most and discusses the relation between peak gust and mean wind speeds. Melbourne's criteria, which have been stated in terms of maximum gust speeds per annum, are shown to define a range of wind-speed probabilities, in particular, the frequency of occurrence of mean wind speeds, which then facilitates comparison between the various published criteria. It is shown that, in spite of the apparent numerical differences in published wind speed criteria and the various subjective assumptions used in their development, there is remarkably good agreement when they are compared on a proper probabilistic basis.

Wind environment studies in Australia

The assessment of prospective environmental wind conditions about proposed building developments in Australia has been discussed. Assessment techniques, making use of wind tunnel studies, have been illustrated with examples from a study of two possible building configurations for a very exposed site on the north side of the City of Melbourne. A method of predicting the probability of occurrence of a given wind speed at a particular location has been detailed, and examples have been given of the integration of model measurements of local velocities with the wind speed probability distribution for the geographic area. The comparisons of these probabilistic estimates with environmental wind speed criteria have been discussed and illustrated. A method of measuring peak gust wind speeds at model scale in situations of high turbulence intensity has been given and a comparison is given with a full scale situation.

A Basis for Forecasting Peak Wind Gusts in Non-Frontal Thunderstorms

From sixty-two cases of non-frontal thunderstorms passing over reporting stations, a relation is found between peak wind gusts and the temperature difference between the surface air underneath and in advance of the thunderstorm. The temperature under the thunderstorm is forecast by lowering the wet-bulb temperature at the freezing level to the surface along a saturation adiabat on a pseudoadiabatic chart, permitting use of the relation as an aid to thunderstorm gust forecasting.