Wind and the Built Environment U.S. Needs in Wind Engineering and Hazard Mitigation (1993) / Chapter Skim
Currently Skimming:
2. The Nature of Wind
2. The Nature of Wind
Pages 19-54
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From page 19... ...
A knowledge of probable, extreme wind speeds is required so that buildings may be designed to resist these wind effects. Wind direction information is used to select runways for aircraft takeoffs and landings.
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From page 20... ...
However, adjustment procedures usually entail assumptions about the variation of wind with time and height; these can lead to biases in the climatological values. The Wind Climatology of the United States The mean annual wind speed for the contiguous 48 states is ~ to 12 mph (4 to 6 m/s)
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From page 22... ...
Although the highest, near-surface wind speeds likely occur beneath thunderstorms and in hurncanes, very high wind speeds can also be associated with deep extratropical cyclones developed from winter low-pressure systems. In these weather systems, strong winds may be found both swirling around the center of lowest pressure and in association with the steep gradients of temperature and pressure that characterize the attendant cold frontal zones.
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An observer on the surface experiences rapid changes in wind speed and Erection as a system of thunderstorm winds moves past. As implied in Figure 2-3, gusts exceeding 50 knots are frequently reported in nontornadic, severe thunderstorms (Kelly et al., 1985~.
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- -~ of\\ ~ '.
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26 Wnd and the Built Environment l ~ Dissipating Thunderstorm ~ atlv0 sta~0 Radar refletibvely detects ~e advancing rain at lowlevels .3 it_ Early - ~e ~~e Precipiftabon Foci late Burt Stem No Thunderstorm Dissipating STAB FIGURE 2-4 A conceptual mode] for the evolution of the flow in a thunderstorm gust Font and the following current.
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From page 27... ...
or less in horizontal extent. They can contain very high winds; the record observed wind speed in a m~croburst is 150 mph (67 m/s)
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Very little detail is actually known about the airflow in tornadoes. Tornado wind speeds are categorized by using the Fujita Tornado Scale (commonly called the F-scale)
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0 ~ ~ o to to ~ · · o · ~ ~ · ~ a ~ N O V N O (V)
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For an L4 wind speed of ZW mph, the duration of the damaging wind at a point Would thus be about 4 seconds. Source: Fujita, 1971.
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Topography can compress and funnel the strong air flows accompanying extratropical cyclones to produce very high wind speeds locally. Examples of this effect are provided by the extreme winds cited in Table 2-1 for Mt.
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. As examples of extreme winds that can be produced by these systems, consider that during the storm of November 24-26, 1950, a fastest-m~le wind speed of 82 mph (37 m/s)
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z to ~ 1 ~ '7 )
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shown in Table 2-3 has been developed to express the severity of these storms in terms of their maximum wind speed (usually estimated from ship and aircraft data) and depth of the storm surge.
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As illustrated in Figure 2-15, the actual hour-by-hour track of Hurricane Hugo's eye, when observed by radar, is much more complex and illustrates the problems of predicting landfall time and location. Although satellite observations have proved invaluable in continuously monitoring the overall development and motion of a storm system, most of our detailed knowledge of the inner workings of hurricanes has come from aircraft reconnaissance flights by the National Oceanic and Atmospheric Administration (NOAA)
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Most frequently cited extreme wind speeds in hurricanes were measured at aircraft altitude. Using aircraft data, Riehl (1979)
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an, \ \ \ i ~ , _ _ \ FIGURE 2-15 A detailed mapping of the track of the center of the eye of Hurricane Hugo as it swept past the Virgin Islands and Puerto Rico September 18, 1989. This track was determined by continuous monitoring of the storm system by the NWS radar at San Juan, Puerto Rico.
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Although wings have been measured at the ocean shore during hurricane landfalIs, most observations have been made some distance inland from the coast. Because of poor mast design or inadequate maintenance, anemometers positioned at the shore have usually failed before peak wind speeds are suspected to have occurred, particularly in the extreme storms of most interest.
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SPEED (kts)
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r F1AMPTOK ~ 5 / ~~ - \~ O 3 \ \~ E f F ~ NGHA:~' ~ B E ~ U F O R T NI '' SAVER ;, '~ Ah \3 999A ~5 \ a? ,.Q~\~ at\ it'\ FIGURE 2-17 Estimated distribution of peak gust speed along the track of Hurricane Hugo as this rapidly moving storm came indand.
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From page 48... ...
Currently, near-surface wind data are measured at approximately 750 stations across the Uruted States. Approximately 225 of these are NOAA National Weather Service facilities; others are operated by the Federal Aviation Administration, the military services, and other cooperating organizations, both public and private.
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The Federal Aviation Administration wall deploy a complementary system of shorter-range Terminal Doppler Weather Radars to watch for the occurrence of downbursts and wind shears near airports. The ASOS wall record both 10-m mean wind speed and direction, and peak gust speed at 1-m~nute intervals.
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Some states contain only one station that provides data usable for the prediction of wind speed. To predict wind speeds for the lifespan of a building or longer, it is necessary to have long-term, w~nd-speed records at a closely spaced network of stations.
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RECOMMENDATIONS Measurements of Extreme Wind Speeds There is a need to confirm the maximum wind speeds that can occur in tornadoes, downbursts, hurricanes, and other high-speed winds. Technology in the form of portable Doppler radar (Bluestein and Unruh, 1989)
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Fastest-mile wind speed/ direction 5. Standard deviation of 20 min Pollutant dispersion wind fluctuations 2 s 1 min Continuity with previous data Low-level wind data (at levels of approximately 100, 300, and 500 m above ground)
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From page 53... ...
There is also a need to install Doppler radars in the Caribbean (presently there are only three conventional radars to cover this region: one each at the National Weather Service ounces in Key West, Flonda, and San Juan, Puerto Rico, and the third in the Bahamas)
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From page 54... ...
Dunng an extreme wind event, commercial service often fails before the extreme wind speeds occur, with consequent loss of data. Particularly in hurricane-prone coastal regions, NEXRAD and Terminal Doppler Weather Radar systems should be provided with full backup power capability to ensure that full sets of data become available.
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