Maximum sustained wind

Definition

The maximum sustained wind normally occurs at a distance from the center known as the radius of maximum wind, within a mature tropical cyclone's eyewall, before winds decrease at farther distances away from a tropical cyclone's center. Most weather agencies use the definition for sustained winds recommended by the World Meteorological Organization (WMO), which specifies measuring winds at a height of 10 m for 10 minutes, and then taking the average. However, the United States National Weather Service defines sustained winds within tropical cyclones by averaging winds over a period of one minute, measured at the same 10 m height. This is an important distinction, as the value of the highest one-minute sustained wind is about 14% greater than a ten-minute sustained wind over the same period.

Estimation and measurement

Main article: Tropical cyclone observation

In most tropical cyclone basins, use of the satellite-based Dvorak technique is the primary method used to estimate a tropical cyclone's maximum sustained winds. The extent of spiral banding and difference in temperature between the eye and eyewall is used within the technique to assign a maximum sustained wind and pressure. Central pressure values for their centers of low pressure are approximate. The tracking of individual clouds on minutely satellite imagery could be used in the future in estimating surface winds speeds for tropical cyclones.

Ship and land observations are also used, when available. In the Atlantic as well as the Central and Eastern Pacific basins, reconnaissance aircraft are still utilized to fly through tropical cyclones to determine flight level winds, which can then be adjusted to provide a fairly reliable estimate of maximum sustained winds. A reduction of 10 percent of the winds sampled at flight level is used to estimate the maximum sustained winds near the surface, which has been determined during the past decade through the use of GPS dropwindsondes. Doppler weather radar can be used in the same manner to determine surface winds with tropical cyclones near land.

Variation

Friction between the atmosphere and the Earth's surface causes a 20% reduction in the wind at the surface of the Earth. Surface roughness also leads to significant variation of wind speeds. Over land, winds maximize at hill or mountain crests, while sheltering leads to lower wind speeds in valleys and lee slopes. Compared to over water, maximum sustained winds over land average 8% lower. More especially, over a city or rough terrain, the wind gradient effect could cause a reduction of 40% to 50% of the geostrophic wind speed aloft; while over open water or ice, the reduction is between 10% and 30%.

Relationship to tropical cyclone strength scales

Main article: Tropical cyclone scales

In most basins, maximum sustained winds are used to define the category of a tropical cyclone on each basin's tropical cyclone scale. In the Atlantic and northeast Pacific oceans, the Saffir–Simpson scale is used. This scale can be used to determine possible storm surge and damage impact on land. In most basins, the category of the tropical cyclone (for example, tropical depression, tropical storm, hurricane/typhoon, super typhoon, depression, deep depression, intense tropical cyclone) is determined from the cyclone's maximum sustained wind over one minute.

Notes

References

References

1. (July 2007). "Relation Between Saffir-Simpson Hurricane Scale Wind Speeds and Peak 3-s Gust Speeds Over Open Terrain". [[Journal of Structural Engineering]].
2. Brian W. Blanchard and S. A. Hsu. [http://www.nwas.org/ej/pdf/2006-EJ4.pdf ON THE RADIAL VARIATION OF THE TANGENTIAL WIND SPEED OUTSIDE THE RADIUS OF MAXIMUM WIND DURING HURRICANE WILMA (2005).] {{webarchive. link. (2012-09-05 Retrieved on 2008-07-04.)
3. Tropical Cyclone Weather Services Program. (June 1, 2006). "Tropical cyclone definitions". [[National Weather Service]].
4. "SECTION 2. INTENSITY OBSERVATION AND FORECAST ERRORS".
5. "Objective Dvorak Technique". [[University of Wisconsin–Madison]].
6. [[Chris Landsea]] (June 8, 2010). [http://www.aoml.noaa.gov/hrd/tcfaq/H1.html Subject: H1) What is the Dvorak technique and how is it used?] [[Atlantic Oceanographic and Meteorological Laboratory]]. Retrieved on 2011-01-14.
7. A. F. Hasler, K. Palaniappan, C. Kambhammetu, P. Black, E. Uhlhorn, and D. Chesters. [http://ams.allenpress.com/archive/1520-0477/79/11/pdf/i1520-0477-79-11-2483.pdf High-Resolution Wind Fields within the Inner Core and Eye of a Mature Tropical Cyclone from GOES 1-min Images.] Retrieved on 2008-07-04.
8. Franklin, James L., Michael L. Black, and Krystal Valde. [http://cat.inist.fr/?aModele=afficheN&cpsidt=15034652 GPS dropwindsonde wind profiles in hurricanes and their operational implications.] Retrieved on 2008-07-04.
9. J. TUTTLE and R. GALL. [http://cat.inist.fr/?aModele=afficheN&cpsidt=1801644 A single-radar technique for estimating the winds in tropical cyclones.] Retrieved on 2008-06-12.
10. Haby, Jeff. "The Importance of Friction".
11. [http://www.nhc.noaa.gov/jht/05-07reports/JHT07_Miller_midyear.pdf Mapping of Topographic Effects on Maximum Sustained Surface Wind Speeds in Landfalling Hurricanes.] Retrieved on 2008-07-04.
12. Peter Black. [http://www.aoml.noaa.gov/hrd/hurdat/august01/Franklin_response(2).txt Subject: Re: Offshore vs nearshore sonde composite.] Retrieved on 2008-07-04.
13. Harrison, Roy. (1999). "Understanding Our Environment". Royal Society of Chemistry.
14. Thompson, Russell. (1998). "Atmospheric Processes and Systems". Routledge.