Microburst

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Illustration of a microburst. Note the downward motion of the air until it hits ground level, then spreads outward in all directions. The wind regime in a microburst is completely opposite to a tornado.
Illustration of a microburst. Note the downward motion of the air until it hits ground level, then spreads outward in all directions. The wind regime in a microburst is completely opposite to a tornado.

A microburst is a very localized column of sinking air, producing damaging divergent and straight-line winds at the surface that are similar to but distinguishable from tornadoes which generally have convergent damage.

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[edit] History of term

The term was defined by severe weather expert Tetsuya Theodore 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. Dr. Fujita also coined the term macroburst for downbursts larger than 4 km (2.5 mi).

A distinction can be made between a wet microburst which consists of precipitation and a dry microburst which consists of virga. They generally are formed by precipitation-cooled air rushing to the surface, but they perhaps also could be powered from the high speed winds of the jet stream deflected to the surface in a thunderstorm (see downburst).

Microbursts are recognized as capable of generating wind speeds higher than 75 m/s (168 mph; 270 km/h).

Dry microburst schematic from NWS.
Dry microburst schematic from NWS.

[edit] Dry microbursts

When rain falls below 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 and this divergence of the wind is the signature of the microburst.

Dry microbursts, produced by high based thunderstorms that generate little 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 of three important variables: mid-level moisture, a deep and dry adiabatic lapse rate in the sub-cloud layer, and low surface relative humidity.

Wet microburst schematic from NWS.
Wet microburst schematic from NWS.

[edit] Wet microbursts

Wet microbursts are downbursts accompanied by significant precipitation at the surface (Fujita, 1985) which are warmer than their environment (Wakimoto, 1998). These downbursts rely more on the drag of precipitation for downward acceleration of parcels than 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 one kilometer above ground level (Proctor, 1989). These factors, among others, make forecasting wet microbursts a difficult task.

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 Usually below 850 mb
Features below Cloud Base Virga Shafts of strong precipitation reaching the ground
Primary Catalyst Evaporative cooling Downward transport of higher momentum
Environment below Cloud Base Deep dry layer/low relative humidity/dry adiabatic lapse rate Shallow dry layer/high relative humidity/moist adiabatic lapse rate
Surface Outflow Pattern Omni-directional Gusts of the direction of the mid-level wind

[edit] Development stages of microbursts

The University of Illinois breaks the evolution of downbursts into three stages, the contact stage, the outburst stage and the cushion stage.

A downburst initially develops as the downdraft begins its descent from cloud base. The downdraft accelerates and within minutes, reaches the ground (contact stage). It is during the contact stage that the highest winds are observed.

During the outburst stage, the wind "curls" as the cold air of the downburst moves away from the point of impact with the ground.

During the cushion stage, winds about the curl continue to accelerate, while the winds at the surface slow due to friction.

[edit] Physical processes of dry and wet microbursts

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[edit] Simple explanation

In the case of a wet microburst, the atmosphere is warm and humid in the lower levels and dry aloft. As a result, when thunderstorms develop, heavy rain is produced but some of the rain evaporates in the drier air aloft. As a result the air aloft is cooled thereby causing it to sink and spread out rapidly as it hits the ground. The result can be both strong damaging winds and heavy rainfall occurring in the same area. Wet downbursts can be identified visually by such features as a shelf cloud, while on radar they sometimes produce bow echoes.

In the case of a dry microburst, the atmosphere is warm but dry in the lower levels and moist aloft. Thus when showers and thunderstorms develop, most of the rain evaporates before reaching the ground.

[edit] Basic physical processes using simplified buoyancy equations

Start by using the vertical momentum equation

{dw\over dt} = -{1\over\rho} {\partial p\over\partial z}-g

By decomposing the variables into a basic state and a perturbation, defining the basic states, and using the Ideal Gas Law (p = ρRTv), then the equation can be written in the form

B \equiv -{\rho^\prime\over\bar\rho}g = g{T^\prime_v - \bar T_v \over \bar T_v}

where B is used to denote buoyancy. Note that 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 parameterized by including a term that decreases buoyancy as the liquid water mixing ratio (\ell) increases, leading to the final form of the parcel's momentum equation:

{dw^\prime\over dt} = {1\over\bar\rho}{\partial p^\prime\over\partial z} + B - g\ell

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. Cleary, 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 takes 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 buoyance, if a parcel has a liguid water mixing ration of 1.0 gkg-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.

[edit] Negative vertical motion associated only with buoyancy

Using pure "parcel theory" results in a prediction of the maximum downdraft of

-w_{\rm max} = \sqrt{2\times\hbox{NAPE}}

where NAPE is the Negative Available Potential Energy,

\hbox{NAPE} = -\int_{\rm SFC}^{\rm LFS} B\,dz

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 results from the relatively modest NAPE value of 312.5 m²s-2. To a first approximation, the maximum gust is roughly equal to the maximum downdraft speed.

See the following reference and link for more information on derivation of buoyancy equations:

A photograph of the surface curl soon after a microburst impacted the surface
A photograph of the surface curl soon after a microburst impacted the surface

[edit] Danger to aircraft

Further information: Downburst

The scale and suddenness of a microburst makes it a great danger to aircraft, particularly those at low altitude which are taking off and 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 Pan Am flight is a notable exception). 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 sudden loss of air moving across the wings causes the aircraft to literally drop out of the air. The best way to deal with a microburst in an aircraft would be to increase speed as soon as the spike in airspeed is noticed. This will allow the aircraft to remain in the air when traveling through the tailwind portion of the microburst and also pass through the microburst with less difficulty, although it is possible that for light aircraft, the descent rate induced by the microburst will exceed their maximum climb rate, leading to an unavoidable crash.

[edit] Effects of microbursts

Strong Microburst winds flip a several ton shipping container up the side of a hill, Vaughan Ontario, Canada
Strong Microburst winds flip a several ton shipping container up the side of a hill, Vaughan Ontario, Canada
Tree damage from a downburst
Tree damage from a downburst

A microburst often has high winds that can knock over full grown trees. They usually last for a couple of seconds.

[edit] List of notable microbursts

  • July 15, 1995 - Watertown (city), New York, and surrounding areas, suffered severe wind and water damage in the early AM hours. This storm was bizarre in that it threw large objects several feet while small items were left untouched.
  • August 14, 1996 - Runyan calls this the costliest storm in Arizona history. A severe thunderstorm and its accompanying dry microburst hit the northwest portion of the Phoenix metro area – ripping off tile roofs and causing $160 million in damage. An Arizona record wind gust of 115 miles per hour is recorded at the Deer Valley Airport. A few locations had to go without power for several days.
  • A microburst squall with windspeeds of 80 miles per hour is responsible for capsizing and sinking the Pride of Baltimore in May 1986 in the Caribbean, about 250 miles north of Puerto Rico. The ship took the lives of her captain and three of her other 11 crew members.
  • A particularly violent microburst is a possible alternative explanation to the 1961 sinking of the American school brigantine Albatross. The ship's captain Dr. Christopher Sheldon claimed that the ship was hit by a white squall on the voyage from Progreso, Yucatán, to Nassau in the Bahamas.
  • A microburst cost the New Jersey suburban towns of Bloomfield, Cedar Grove Montclair, and Verona a combined total of a little less than $1 million in damages when a storm passed through the area on July 18, 2006[1][2]
  • A microburst moved through northern and western Utah during the late afternoon and evening hours on June 5, 1995. Some of the higher reported wind gusts were: Tremonton 95 mph, Highland and American Fork 90 mph, Pleasant Grove 88 mph, and north Orem 86 mph. According to data received from the Western Insurance Information Service, damage estimates totaled $15 million.
  • In September 1998, a microburst hit the city of Syracuse, New York. Syracuse University was closed for this first time in over a decade because of the destruction.
  • In 2003, a microburst hit the city of Rockford, Illinois. Over 10,000 trees were destroyed and the city suffered over 1 million dollars in damages.
  • On May 31, 2002, a microburst struck the suburban town of West Mifflin, Pennsylvania and surrounding areas, also hitting Kennywood Park. The high winds caused the pavillion of one of one of the park's oldest ride, "The Whip," to collapse, killing one woman and injuring at least 54 others, many of which were children. Damage consisted of flooding, trees and wires down, mudslides, and several damaged buildings.
  • On July 22, 2003 at approximately 8:30 PM, a microburst caused damaged to Bernards Township (specifically the Basking Ridge section of the township) and extended into neighboring Long Hill Township, NJ. The damage estimates were between $1 million and $2 million. The damage extended 3 miles and damaged 50 homes, 35 vehicles, and took down hundreds of trees in the area. Trees as large as 10 feet in diameter were ripped from the ground. More than dozens of roads in the area were closed including Exit 36 of Interstate 78. Power was not restored to some area homes until two days later. Some residents in the center of the wind burst described the sounds coming from the outdoors as if "the sky was falling."
  • On March 12, 2006 at approximately 8:10 AM, a severe microburst with winds varying from 70 to 90 mph damaged large portions of Lawrence, Kansas. Reported damage included downed power lines, stop lights and trees, overturned semi-trailers, collapsed farm silos and damage to roofs. Seventy buildings on the University of Kansas campus reported damage. In total, over $8 million in damages was estimated.[3]
  • On August 2, 2006 a microburst damaged Wah-Tut-Ca Scout Reservation in Northwood, New Hampshire. The microburst crushed one building (the camp's trading post) to the ground, damaged roofs of two other buildings, destroyed nine vehicles and knocked down 30 to 40 trees, some of which were 25-30 inches in diameter. Two Boy Scouts were injured during the event, although their injuries were not serious.
  • On August 9, 2007 a microburst with winds of 85 mph affected the city of Salem, Ohio. The roofs of two gas stations were damaged. One of the gas stations, which had only been open for 5 weeks, had portions of its roof torn off, which subsequently damaged cars as it was blown down the street. Windows in cars and at the Giant Eagle grocery store were broken. Many other businesses reported damage.[4]
  • On September 9, 2007 a severe microburst hit the Massachusetts College of Liberal Arts (MCLA) in North Adams, Massachusetts. Many trees came down and the roofs of several town-house style dorms were damaged. The National Weather Service in Albany, New York estimated that there were straight-line winds with gusts of 110 miles per hour. There were no injuries reported.[5]
  • On October 29, 2007 a severe microburst hit the Australian town of Gayndah in Queensland. Roofs were lifted off at least 5 houses and many trees were blown down. Recorded gusts at the Gayndah weather station reached 180kmh (110mph) and the town received 70mm of rain within 30 minutes.[6]
  • On January 9, 2008 A Microburst hit the town of Webster, New York causing severe damage and winds reaching 80mph. Over 100 hundred trees were knocked down or snapped in half within a time period less than 30 seconds in just a small part of the town. Repair costs could reach $500,000.

[edit] Printed media

[edit] References

  1. ^ Montclair mayhem: Massive winds sweep through town, damaging homes, destroying trees
  2. ^ Towns pick up after storm
  3. ^ National Weather Service 2006 Report, National Weather Service Institute.
  4. ^ Salem hit by 85 mph microburst, Salem News.
  5. ^ Winds batter MCLA, The Berkshire Eagle.
  6. ^ Severe storm cell hits Gayndah - Queensland - BrisbaneTimes

[edit] See also

[edit] External links