Downburst

Wind shear Ted Fujita Tornado
Advertisement - You can get this game from STEAM

Illustration of a microburst. The air moves in a downward motion until it hits ground level. It then spreads outward in all directions. The wind regime in a microburst is opposite to that of a tornado.
Downburst seen from the ARMOR Doppler Weather Radar in Huntsville, Alabama in 2012. Note the winds in green going towards the radar, and the winds in red going away from the radar.

A downburst is a strong ground-level wind system that emanates from a point source above and blows radially, that is, in straight lines in all directions from the point of contact at ground level. Often producing damaging winds, it may be confused with a tornado, where high-velocity winds circle a central area, and air moves inward and upward; by contrast, in a downburst, winds are directed downward and then outward from the surface landing point.

Downbursts are created by an area of significantly rain-cooled air that, after reaching ground level, spreads out in all directions producing strong winds. Dry downbursts are associated with thunderstorms with very little rain, while wet downbursts are created by thunderstorms with high amounts of rainfall. Microbursts and macrobursts are downbursts at very small and larger scales, respectively. Another variety, the heat burst, is created by vertical currents on the backside of old outflow boundaries and squall lines where rainfall is lacking. Heat bursts generate significantly higher temperatures due to the lack of rain-cooled air in their formation. Downbursts create vertical wind shear or microbursts, which is dangerous to aviation, 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 recognize and recover from a microburst/wind shear event. They usually last for seconds to minutes.

They go through three stages in their cycle: the downburst, outburst, and cushion stages.[1]

Definition

Downburst damages in a straight line. (Source NOAA)

A downburst is created by a column of sinking air that after hitting ground level, spreads out in all directions and is capable of producing damaging straight-line winds of over 240 km/h (150 mph), often producing damage similar to, but distinguishable from, that caused by tornadoes. This is because the physical properties of a downburst are completely different from those of a tornado. Downburst damage will radiate from a central point as the descending column spreads out when hitting the surface, whereas tornado damage tends towards convergent damage consistent with rotating winds. To differentiate between tornado damage and damage from a downburst, the term straight-line winds is applied to damage from microbursts.

Downbursts are particularly strong downdrafts from thunderstorms. Downbursts in air that is precipitation free or contains virga are known as dry downbursts;[2] those accompanied with precipitation are known as wet downbursts. Most downbursts are less than 4 km (2.5 mi) in extent: these are called microbursts.[3] Downbursts larger than 4 km (2.5 mi) in extent are sometimes called macrobursts.[3] Downbursts can occur over large areas. In the extreme case, a derecho can cover a huge area more than 320 km (200 mi) wide and over 1,600 km (1,000 mi) long, lasting up to 12 hours or more, and is associated with some of the most intense straight-line winds,[4] but the generative process is somewhat different from that of most downbursts.

The term microburst was defined by mesoscale meteorology expert Ted Fujita as affecting an area 4 km (2.5 mi) in diameter or less, distinguishing them as a type of downburst and apart from common wind shear which can encompass greater areas.[5] Fujita also coined the term macroburst for downbursts larger than 4 km (2.5 mi).[6]

A distinction can be made between a wet microburst which consists of precipitation and a dry microburst which typically consists of virga.[2] 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 microbursts

Dry microburst schematic

When 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.[7]

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, cloudbase in the mid atmosphere, and low surface relative humidity. These conditions evaporate the moisture from the air as it falls, cooling the air and making it fall faster because it is more dense.

Wet microbursts

Wet microbursts are downbursts accompanied by significant precipitation at the surface.[8] 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 1 km (0.62 mi) above ground level (Proctor, 1989). These factors, among others, make forecasting wet microbursts difficult.

Characteristic Dry Microburst Wet Microburst
Location of highest probability within the United States Midwest / West Southeast
Precipitation Little or none Moderate or heavy
Cloud bases As high as 500 mb (hPa) As high as 850 mb (hPa)
Features below cloud base Virga Precipitation shaft
Primary catalyst Evaporative cooling Precipitation loading and evaporative cooling
Environment below cloud base Deep dry layer/low relative humidity/dry adiabatic lapse rate Shallow dry layer/high relative humidity/moist adiabatic lapse rate

Straight-line winds

Straight-line winds (also known as plough winds, thundergusts and hurricanes of the prairie) are very strong winds that can produce damage, demonstrating a lack of the rotational damage pattern associated with tornadoes.[9] Straight-line winds are common with the gust front of a thunderstorm or originate with a downburst from a thunderstorm. These events can cause considerable damage, even in the absence of a tornado. The winds can gust to 58 m/s (130 mph)[10] and winds of 26 m/s (58 mph) or more can last for more than twenty minutes.[11] In the United States, such straight-line wind events are most common during the spring when instability is highest and weather fronts routinely cross the country.[citation needed] Straight-line wind events in the form of derechos can take place throughout the eastern half of the U.S.[12]

Straight-line winds may be damaging to marine interests. Small ships, cutters and sailboats are at risk from this meteorological phenomenon.[citation needed]

Formation

The formation of a downburst starts with hail or large raindrops falling through drier air. Hailstones melt and raindrops evaporate, pulling latent heat from surrounding air and cooling it considerably. Cooler air has a higher density than the warmer air around it, so it sinks to the ground. As the cold air hits the ground it spreads out and a mesoscale front can be observed as a gust front. Areas under and immediately adjacent to the downburst are the areas which receive the highest winds and rainfall, if any is present. Also, because the rain-cooled air is descending from the middle troposphere, a significant drop in temperatures is noticed. Due to interaction with the ground, the downburst quickly loses strength as it fans out and forms the distinctive "curl shape" that is commonly seen at the periphery of the microburst (see image). Downbursts usually last only a few minutes and then dissipate, except in the case of squall lines and derecho events. However, despite their short lifespan, microbursts are a serious hazard to aviation and property and can result in substantial damage to the area.

Heat bursts

A special, and much rarer, kind of downburst is a heat burst, which results from precipitation-evaporated air compressionally heating as it descends from very high altitude, usually on the backside of a dying squall line or outflow boundary.[13] Heat bursts are chiefly a nocturnal occurrence, can produce winds over 160 km/h (100 mph), are characterized by exceptionally dry air, can suddenly raise the surface temperature to 38 °C (100 °F) or more, and sometimes persist for several hours.

Development stages of microbursts

The 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

Microburst crosssection (vectored).svg

Basic physical processes using simplified buoyancy equations

Start 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.[15]

Negative vertical motion associated only with buoyancy

Using 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 25 m/s (56 mph; 90 km/h) 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.[15]

Danger to aviation

A series of photographs of the surface curl soon after a microburst impacted the surface

Downbursts, particularly microbursts, are exceedingly dangerous to aircraft which are taking off or landing due to the strong vertical wind shear caused by these events. A number of fatal crashes have been attributed to downbursts.[16]

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.[16] 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).[18]

Danger to buildings

Strong microburst winds flip a several-ton shipping container up the side of a hill, Vaughan, Ontario, Canada

See also

References

  1. ^ "What is a Microburst?". National Weather Service. n.d. Retrieved 10 March 2018.
  2. ^ a b Fernando Caracena, Ronald L. Holle, and Charles A. Doswell III. Microbursts: A Handbook for Visual Identification. Retrieved on 9 July 2008.
  3. ^ a b Glossary of Meteorology. Macroburst. Retrieved on 30 July 2008.
  4. ^ Peter S. Parke and Norvan J. Larson.Boundary Waters Windstorm. Retrieved on 30 July 2008.
  5. ^ Glossary of Meteorology. Microburst. Archived 2008-12-12 at the Wayback Machine Retrieved on 2008-07-30.
  6. ^ Glossary of Meteorology. Macroburst. Retrieved on 2008-07-30.
  7. ^ Glossary of Meteorology. Straight-line wind. Archived 2008-04-15 at the Wayback Machine Retrieved on 2008-08-01.
  8. ^ * Fujita, T.T. (1985). "The Downburst, microburst and macroburst". SMRP Research Paper 210, 122 pp.
  9. ^ Glossary of Meteorology. Straight-line wind. Archived 15 April 2008 at the Wayback Machine Retrieved on 1 August 2008.
  10. ^ http://www.spc.noaa.gov/misc/AbtDerechos/derechofacts.htm#strength
  11. ^ http://www.spc.noaa.gov/misc/AbtDerechos/casepages/jun291998page.htm
  12. ^ http://www.spc.noaa.gov/misc/AbtDerechos/derechofacts.htm#climatology
  13. ^ "Oklahoma "heat burst" sends temperatures soaring". USA Today|1999-07-08. 8 July 1999. Archived from the original on 25 December 1996. Retrieved 9 May 2007.
  14. ^ University of Illinois – Urbana Champaign. Microbursts. Retrieved on 2008-08-04.
  15. ^ a b Charles A. Doswell III. Extreme Convective Windstorms: Current Understanding and Research. Retrieved on 2008-08-04.
  16. ^ a b c d e NASA Langley Air Force Base. Making the Skies Safer From Windshear. Archived 2010-03-29 at the Wayback Machine Retrieved on 2006-10-22.
  17. ^ Aviation Safety Network. Damage Report. Retrieved on 2008-08-01.
  18. ^ Glenday, Craig (2013). Guinness Book of World Records 2014. The Jim Pattinson Group. pp. 20. ISBN 978-1-908843-15-9.
  19. ^ Roberts, Samantha (10 August 2016). "What happened in Cleveland Heights Tuesday night?". KLTV. Retrieved 15 August 2016.
  20. ^ a b Steer, Jen; Wright, Matt (10 August 2016). "Damage in Cleveland Heights caused by microburst". Fox8.com. Retrieved 15 August 2016.
  21. ^ a b Reardon, Kelly (10 August 2016). "Wind gusts reached 58 mph, lightning struck 10 times a minute in Tuesday's storms". The Plain Dealer. Retrieved 15 August 2016.
  22. ^ a b Higgs, Robert (11 August 2016). "About 4,000 customers, mostly in Cleveland Heights, still without power from Tuesday's storms". The Plain Dealer. Retrieved 15 August 2016.
  23. ^ Evbouma, Andrei (12 July 2012). "Storm Knocks Out Power to 206,000 in Chicago Area". Chicago Sun-Times.
  24. ^ Gorman, Tom. "8 injured at Nellis AFB when aircraft shelters collapse in windstorm – Thursday, Sept. 8, 2011 | 9 p.m." Las Vegas Sun. Retrieved 30 November 2011.
  25. ^ "Microbursts reported in Hegewisch, Wheeling". Chicago Breaking News. 22 September 2010. Retrieved 30 November 2011.
  26. ^ "New York News, Local Video, Traffic, Weather, NY City Schools and Photos – Homepage – NY Daily News". Daily News. New York.
  27. ^ "Power Restored to Tornado Slammed Residents: Officials". NBC New York. 20 September 2010. Retrieved 30 November 2011.
  28. ^ "Archived copy". Archived from the original on 3 September 2012. Retrieved 26 June 2010.CS1 maint: archived copy as title (link) and http://www.nbc29.com/Global/story.asp?S=12705577
  29. ^ Brian Kushida (11 June 2010). "Strong Winds Rip Through SF Neighborhood – News for Sioux Falls, South Dakota, Minnesota and Iowa". Keloland.com. Archived from the original on 27 September 2011. Retrieved 30 November 2011.
  30. ^ Gasper, Christopher L. (6 May 2009). "Their view on matter: Patriots checking practice facility". The Boston Globe. Retrieved 12 May 2009.
  31. ^ "One year after microburst, recovery progresses" KU.edu. Retrieved 21 July 2009.
  32. ^ "Storm Wrecks New Copters". The New York Times. 20 May 1989. Retrieved 2 June 2020.