词条 | Microburst | ||||||||||||||||||||||||
释义 |
A microburst is an intense small-scale downdraft produced by a thunderstorm or rain shower. There are two types of microbursts: wet microbursts and dry microbursts. They go through three stages in their cycle, the downburst, outburst, and cushion stages also called "Suriano's Stroke".[1] A microburst can be particularly dangerous to aircraft, especially during landing, due to the wind shear caused by its gust front. Several fatal and historic crashes have been attributed to the phenomenon over the past several decades, and flight crew training goes to great lengths on how to properly recover from a microburst/wind shear event. A microburst often has high winds that can knock over fully grown trees. They usually last for seconds to minutes. History of termThe term was defined by mesoscale meteorology expert Ted Fujita as affecting an area {{convert|4|km|abbr=on}} in diameter or less, distinguishing them as a type of downburst and apart from common wind shear which can encompass greater areas.[2] Fujita also coined the term macroburst for downbursts larger than {{convert|4|km|abbr=on}}.[3] A distinction can be made between a wet microburst which consists of precipitation and a dry microburst which typically consists of virga.[4] They generally are formed by precipitation-cooled air rushing to the surface, but they perhaps also could be powered by strong winds aloft being deflected toward the surface by dynamical processes in a thunderstorm (see rear flank downdraft). Dry microburstsWhen rain falls below the cloud base or is mixed with dry air, it begins to evaporate and this evaporation process cools the air. The cool air descends and accelerates as it approaches the ground. When the cool air approaches the ground, it spreads out in all directions. High winds spread out in this type of pattern showing little or no curvature are known as straight-line winds.[5] Dry microbursts produced by high based thunderstorms that generate little to no surface rainfall, occur in environments characterized by a thermodynamic profile exhibiting an inverted-V at thermal and moisture profile, as viewed on a Skew-T log-P thermodynamic diagram. Wakimoto (1985) developed a conceptual model (over the High Plains of the United States) of a dry microburst environment that comprised three important variables: mid-level moisture, a deep and dry adiabatic lapse rate in the sub-cloud layer, and low surface relative humidity. Wet microburstsWet microbursts are downbursts accompanied by significant precipitation at the surface.[6] These downbursts rely more on the drag of precipitation for downward acceleration of parcels as well as the negative buoyancy which tend to drive "dry" microbursts. As a result, higher mixing ratios are necessary for these downbursts to form (hence the name "wet" microbursts). Melting of ice, particularly hail, appears to play an important role in downburst formation (Wakimoto and Bringi, 1988), especially in the lowest {{convert|1|km|abbr=on}} above ground level (Proctor, 1989). These factors, among others, make forecasting wet microbursts difficult.
Development stages of microburstsThe evolution of microbursts is broken down into three stages: the contact stage, the outburst stage, and the cushion stage. Physical processes of dry and wet microbursts{{Expert-subject|meteorology|date=February 2009}}Basic physical processes using simplified buoyancy equationsStart by using the vertical momentum equation: By decomposing the variables into a basic state and a perturbation, defining the basic states, and using the ideal gas law (), then the equation can be written in the form where B is buoyancy. The virtual temperature correction usually is rather small and to a good approximation; it can be ignored when computing buoyancy. Finally, the effects of precipitation loading on the vertical motion are parametrized by including a term that decreases buoyancy as the liquid water mixing ratio () increases, leading to the final form of the parcel's momentum equation: The first term is the effect of perturbation pressure gradients on vertical motion. In some storms this term has a large effect on updrafts (Rotunno and Klemp, 1982) but there is not much reason to believe it has much of an impact on downdrafts (at least to a first approximation) and therefore will be ignored. The second term is the effect of buoyancy on vertical motion. Clearly, in the case of microbursts, one expects to find that B is negative meaning the parcel is cooler than its environment. This cooling typically takes place as a result of phase changes (evaporation, melting, and sublimation). Precipitation particles that are small, but are in great quantity, promote a maximum contribution to cooling and, hence, to creation of negative buoyancy. The major contribution to this process is from evaporation. The last term is the effect of water loading. Whereas evaporation is promoted by large numbers of small droplets, it only requires a few large drops to contribute substantially to the downward acceleration of air parcels. This term is associated with storms having high precipitation rates. Comparing the effects of water loading to those associated with buoyancy, if a parcel has a liquid water mixing ratio of 1.0 g kg−1, this is roughly equivalent to about 0.3 K of negative buoyancy; the latter is a large (but not extreme) value. Therefore, in general terms, negative buoyancy is typically the major contributor to downdrafts.[8] Negative vertical motion associated only with buoyancyUsing pure "parcel theory" results in a prediction of the maximum downdraft of where NAPE is the negative available potential energy, and where LFS denotes the level of free sink for a descending parcel and SFC denotes the surface. This means that the maximum downward motion is associated with the integrated negative buoyancy. Even a relatively modest negative buoyancy can result in a substantial downdraft if it is maintained over a relatively large depth. A downward speed of {{convert|25|m/s|mi/h km/h|abbr=on}} results from the relatively modest NAPE value of 312.5 m2 s−2. To a first approximation, the maximum gust is roughly equal to the maximum downdraft speed.[8] Danger to aircraft{{further|Downburst|Wind shear|Cumulonimbus and aviation}}The scale and suddenness of a microburst makes it a notorious danger to aircraft, particularly those at low altitude which are taking off or landing. The following are some fatal crashes and/or aircraft incidents that have been attributed to microbursts in the vicinity of airports:
A microburst often causes aircraft to crash when they are attempting to land (the above-mentioned BOAC and Pan Am flights are notable exceptions). The microburst is an extremely powerful gust of air that, once hitting the ground, spreads in all directions. As the aircraft is coming in to land, the pilots try to slow the plane to an appropriate speed. When the microburst hits, the pilots will see a large spike in their airspeed, caused by the force of the headwind created by the microburst. A pilot inexperienced with microbursts would try to decrease the speed. The plane would then travel through the microburst, and fly into the tailwind, causing a sudden decrease in the amount of air flowing across the wings. The decrease in airflow over the wings of the aircraft causes a drop in the amount of lift produced. This decrease in lift combined with a strong downward flow of air can cause the thrust required to remain at altitude to exceed what is available, thus causing the aircraft to stall.[9] If the plane is at a low altitude shortly after takeoff or during landing, it will not have sufficient altitude to recover. The strongest microburst recorded thus far occurred at Andrews Field, Maryland on August 1st 1983, with wind speeds reaching 240.5 km/h (149.5 mi/h).[11] Danger to buildings
See also
ReferencesNotes1. ^{{cite web|url=https://www.weather.gov/bmx/outreach_microbursts|title=What is a Microburst?|date=n.d.|website=National Weather Service|access-date=March 10, 2018}} Bibliography2. ^Glossary of Meteorology. Microburst. {{webarchive|url=https://web.archive.org/web/20081212043551/http://amsglossary.allenpress.com/glossary/search?p=1&query=microburst&submit=Search |date=2008-12-12 }} Retrieved on 2008-07-30. 3. ^Glossary of Meteorology. Macroburst. Retrieved on 2008-07-30. 4. ^Fernando Caracena, Ronald L. Holle, and Charles A. Doswell III. Microbursts: A Handbook for Visual Identification. Retrieved on 2008-07-09. 5. ^Glossary of Meteorology. Straight-line wind. {{webarchive|url=https://web.archive.org/web/20080415101545/http://amsglossary.allenpress.com/glossary/search?p=1&query=straight&submit=Search |date=2008-04-15 }} Retrieved on 2008-08-01. 6. ^* Fujita, T.T. (1985). "The Downburst, microburst and macroburst". SMRP Research Paper 210, 122 pp. 7. ^University of Illinois – Urbana Champaign. Microbursts. Retrieved on 2008-08-04. 8. ^1 Charles A. Doswell III. Extreme Convective Windstorms: Current Understanding and Research. Retrieved on 2008-08-04. 9. ^1 2 3 NASA Langley Air Force Base. Making the Skies Safer From Windshear. {{webarchive|url=https://web.archive.org/web/20100329221032/http://oea.larc.nasa.gov/PAIS/Windshear.html |date=2010-03-29 }} Retrieved on 2006-10-22. 10. ^Aviation Safety Network. Damage Report. Retrieved on 2008-08-01. 11. ^{{Cite book|title=Guinness Book of World Records 2014|last=Glenday|first=Craig|publisher=The Jim Pattinson Group|year=2013|isbn=978-1-908843-15-9|location=|pages=20|quote=|via=}} 12. ^{{cite news|last=Roberts|first=Samantha|title=What happened in Cleveland Heights Tuesday night?|work=KLTV|date=August 10, 2016|accessdate=August 15, 2016|url=http://www.kltv.com/story/32729599/what-happened-in-cleveland-heights-tuesday-night}} 13. ^1 {{cite news|last1=Steer|first1=Jen|last2=Wright|first2=Matt|title=Damage in Cleveland Heights caused by microburst|work=Fox8.com|date=August 10, 2016|accessdate=August 15, 2016|url=http://fox8.com/2016/08/10/damage-in-cleveland-heights-caused-by-microburst/}} 14. ^1 {{cite news|last=Reardon|first=Kelly|title=Wind gusts reached 58 mph, lightning struck 10 times a minute in Tuesday's storms|work=The Plain Dealer|date=August 10, 2016|accessdate=August 15, 2016|url=http://www.cleveland.com/weather/blog/index.ssf/2016/08/update_on_northeast_ohios_stor.html}} 15. ^1 {{cite news|last=Higgs|first=Robert|title=About 4,000 customers, mostly in Cleveland Heights, still without power from Tuesday's storms|work=The Plain Dealer|date=August 11, 2016|accessdate=August 15, 2016|url=http://www.cleveland.com/metro/index.ssf/2016/08/about_4000_customers_mostly_in.html}} 16. ^{{cite news| last=Evbouma, Andrei | title=Storm Knocks Out Power to 206,000 in Chicago Area| newspaper=Chicago Sun-Times|date=July 12, 2012}} 17. ^{{cite web|last=Gorman |first=Tom |url=http://www.lasvegassun.com/news/2011/sep/08/8-injured-nellis-afb-when-aircraft-shelters-collap/ |title=8 injured at Nellis AFB when aircraft shelters collapse in windstorm – Thursday, Sept. 8, 2011 | 9 p.m. |publisher=Las Vegas Sun |date= |accessdate=2011-11-30}} 18. ^{{cite web|author=|url=http://www.chicagobreakingnews.com/2010/09/storm-front-leaves-damage-in-its-wake.html |title=Microbursts reported in Hegewisch, Wheeling |publisher=Chicago Breaking News |date=2010-09-22 |accessdate=2011-11-30}} 19. ^{{cite news| url=http://www.nydailynews.com/ny_local/2010/09/17/2010-09-17_national_weather_service_confirms_that_two_tornadoes_touched_down_in_new_york_ci.html | location=New York | work=Daily News | title=New York News, Local Video, Traffic, Weather, NY City Schools and Photos – Homepage – NY Daily News}} 20. ^{{cite web|url=http://www.nbcnewyork.com/news/local-beat/Days-After-Tornadoes-All-Power-Restored-to-Storm-Battered-103271949.html |title=Power Restored to Tornado Slammed Residents: Officials |publisher=NBC New York |date=2010-09-20 |accessdate=2011-11-30}} 21. ^{{cite web |url=http://www.newsplex.com/news/headlines/97104629.html |title=Archived copy |accessdate=2010-06-26 |deadurl=yes |archiveurl=https://archive.is/20120903212356/http://www.newsplex.com/news/headlines/97104629.html |archivedate=2012-09-03 |df= }} and http://www.nbc29.com/Global/story.asp?S=12705577 22. ^{{cite web|author=Brian Kushida |url=http://www.keloland.com/news/news/NewsDetail7807.cfm?ID=101172 |title=Strong Winds Rip Through SF Neighborhood – News for Sioux Falls, South Dakota, Minnesota and Iowa |publisher=Keloland.com |date=2010-06-11 |accessdate=2011-11-30 |deadurl=yes |archiveurl=https://web.archive.org/web/20110927224401/http://www.keloland.com/news/news/NewsDetail7807.cfm?ID=101172 |archivedate=2011-09-27 |df= }} 23. ^{{cite news | title = Their view on matter: Patriots checking practice facility | publisher = The Boston Globe | date = May 6, 2009 | url = http://www.boston.com/sports/football/patriots/articles/2009/05/06/their_view_on_matter/ | accessdate = 2009-05-12| first=Christopher L.| last=Gasper}} 24. ^"One year after microburst, recovery progresses" KU.edu. Retrieved 21 July 2009.
External links
6 : Severe weather and convection|Storm|Weather hazards|Weather hazards to aircraft|Wind|Microscale meteorology |
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