1.2.4.2. Interesting things a downdraft does: GLEJ TOLE

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Downdrafts in thunderstorms are often linked with strong winds at the surface. This phenomenon is called a downburst.

A downburst is a strong ground-level wind phenomenon often producing severe straight-line winds, up to and sometimes over 150 km/h. A downburst forms as a mass of rain-cooled air rapidly descends from a thunderstorm and impacts the ground, spreading outwards in all directions from the impact point. Downbursts can produce severe damage to buildings, equivalent to strong tornado damage. They are also much more common than tornadoes.

Downbursts are produced by rapidly descending air in a thunderstorm, impacting the ground and producing severe straight line winds. There are two types of downbursts, which differ somewhat in how they form and storm environments they are found in: dry and wet downbursts. A typical downburst goes through three stages: Contact, Outburst and Cushion stage. While downbursts differ in their appearance based on their size, cloud base height and the amount of precipitation they contain, they all share these three stages.

  • Contact stage: is the first stage, before and during contact of the downburst airmass with the ground. In this stage the downburst plummets from the cloud base towards the ground, assuming a shape similar to a water baloon.
  • Outburst stage: is the mid stage, where the downburst airmass hits the ground and spreads outwards, creating severe straight line winds.
  • Cushion stage: is the final stage of a downburst, as the downburst airmass spreads out further and wind speeds diminish. The cold airmass forms a cold pool on the ground.


Size, type and effects of downbursts

Depending on the area they affect, downbursts are divided into two categories:

  • Microburst: is a downburst affecting an area 4 km in diameter or less. Microbursts typically last 5 to 15 minutes. The strongest winds can exceed 180 km/h.
  • Macroburst: is a downburst affecting an area more than 4 km in diameter.

Opis vetra. // Vlažen zrak je manj gost od suhega -> večji negative buoyancy wet downburstov v BL. Below freezing temperatures in the cloud, ice sublimation draws more latent heat, produces more cooling.

There are two types of downbursts, depending on the environment they form in, the driving mechanism (the process that produces rapid descent of air) and visual cues: dry downbursts and wet downbursts.

Dry downbursts

Dry downbursts are tricky. They form in thunderstorm environments that do not readily appear dangerous. They are also very difficult to spot visually prior to them reaching the surface, and even then they may be difficult to see, particularly from a distance. Let us see why.

A dry downburst is produced by evaporative cooling of the air through evaporation of rain in very dry air. As the rain falls through the dry air, rain drops evaporate. Evaporation absorbs heat from the surrounding air, cooling it. The cooled air is denser (negatively buoyant) than the surrounding air and rapidly sinks towards the surface. This process is called dry air entrainment. Dry downbursts occur in environments with very dry air in the lowest several kilometers (lower troposphere). Thunderstorms that produce dry downbursts are high-based, due to low moisture near the ground, and produce little or no rainfall. Virtually all from the thunderstorm rainfall evaporates in the dry air, forming virga. As there is little or no rainfall associated, a dry downburst is difficult to spot prior to ground contact. Severe winds kick up dust on the surface and can produce a distinctive swirl along the gust front, providing visual cues of the downburst. Thunderstorms that have the potential to produce a dry downburst will have high bases and little or no rainfall. They often produce intense lightning – as there is little rainfall they are sometimes called dry thunderstorms.

// precipitation at the surface is either very light or does not occur at all, but virga may be evident. Dry microbursts can develop in the absence of lightning and thunder. High-based Cumulus and Altoculumulus have been observed to produce damaging dry microbursts.

Characteristics of thunderstorms and environments that produce dry downbursts:

  • Little or even no rain reaches the surface. Dry downbursts are often associated with virga shafts.
  • Negative buoyancy of air is produced by evaporation of precipitation below the cloud base.
  • Downdraft entrainment minimal, due to near or complete absence of precipitation. TA LAHKO LETI VEN
  • High cloud base (CCL or LCL). Very dry air below cloud base.
  • Convective updraft can be weak. A dry downburst is not necessarily produced by a strong thunderstorm.
  • Function of solar heating, most frequent around mid-afternoon local time.
  • Small drop in surface temperature during the downburst.

VISUAL CUES OF DRY DOWNBURSTS:

  • High cloud bases.
  • Presence of virga.
  • Little or no rainfall reaches the ground.
  • May be spotted by dust being kicked up from the surface.

Wet downburst

Wet downbursts form in environments with very dry air aloft (mid troposphere), but very moist air in the lowest several kilometers (lower troposphere). In wet downbursts evaporative cooling is still important, but a significant part of the downward air motion is produced by the weight of the rain dragging the air downward (this is called water loading). A wet downburst is accompanied by heavy precipitation at the surface. As seen from a distance a wet downburst in the contact page looks like a water balloon hanging from below the cloud base. In outburst stage it may resemble an upside down mushroom, with a visually distinct rain shaft and a more or less distinct rain foot.

  • Heavy rainfall and precipitation core is mainly in the form of ice (e.g. melting hail).
  • Surface winds caused by precipitation loading and negative buoyancy. Downwind momentum transfer and/or dynamically induced pressure gradient may also contribute, particularly in strong events (copy).
  • Downdraft entrainment of environmental air important.
  • Moist air below cloud base, dry at mid level.
  • Strong convection / updraft.
  • Low cloud base.
  • Function of solar heating, most frequent around mid-afternoon local time.
  • Relatively large lowering of surface temperature during the downburst.

//The wet microburst is accompanied by significant precipitation at the surface. Wet microbursts occur with a range of thunderstorm types.

The wet microburst develops in environments characterised by weak vertical wind shear and deep moisture capped by a dry layer as depicted in Figure 8.6. Water drops evaporating in the dry layer cool the air that then falls through the cloud. The falling air, combined with the drag of falling raindrops, produces strong downburst winds. When the downburst hits the ground it spreads out and away from the rain shaft as depicted in Figures 8.6 and 8.7.

In the United States during the 1970s and early 1980s a number of large passenger jets crashed just after take-off or just before landing. The microburst has been established as the major factor in those accidents.

Wet downbursts can produce intense rainfall, with rates >200 mm/h over several minutes. Downbursts are frequently asymmetric: stronger and more spread out in certain directions than others.

VISUAL CUES OF WET DOWNBURSTS

There are several distinct visual cues by which wet downburst can be identified [if seen from a distance]:

  • Distinct rain shafts.
  • Developing or developed rain foot in later stages.

SOME INTERESTING DOWNBURSTS

  • The strongest microburst ever recorded occurred on August 1, 1983 at Andrews Air Force Base (Maryland, USA). Peak recorded wind speed reached 240 km/h. Interestingly, Air Force One with president Reagan on board, landed at Andrews AFB only 6 minutes prior to the onset of the microburst.
  • A downburst can also capsize vessels (for example the Sea Wing on Lake Peppin between Wisconsin and Minnesota on July 13, 1890).

Characteristic dust swirl on the gust front of a dry downburst. Photo: OAR/ERL/National Severe Storms Laboratory (NSSL) (NOAA Photo Library, NOAA Central Library).

EXAMPLES

EXAMPLE 1/4

Textbook massive rain foot on a new developed storm while following the storms in SW Texas, June 1st, 2016. Notice the curved rain foot.

With annotations.

Detail on the rainfoot and gust front. Several precipitation ‘curls’ have developed on the leading part of the rain foot, curving backward on themselves. A depression has formed in the upper edge of the rain foot, next to the rain shaft, indicative of a vortex rollup / formation.

Annotated photo of the rain foot and gust front. Dotted cyan lines indicate individual precipitation curls just behind the gust front on the leading edge of the rain foot. The gust front is denoted by the dark blue line.

EXAMPLE 2/4

An isolated pulse storm with a massive rainfoot over northern Istra, Slovenia on July 25th, 2016.

EXAMPLE 3/4

Night time wet downburst with a well-developed rain foot off the coast of Istra (NW Croatia) over the Adriatic sea on September 24, 2017. Photo: Marko Korošec.

EXAMPLE 4/4

Night time wet microburst near Eraclea (NE Veneto, NE Italy) on June 24, 2014. The image sequence spans a little under two minutes. The first image (upper left) shows the microburst expanding with a distinct rain foot developed on the left. The left edge of the rain shaft is very sharply defined and slightly convex, indicaing a new pulse descending towards the ground. 24 seconds later (upper right) the rain foot had expanded significantly. By 90 second later (lower left), a very distinct and sharply defined backwards curved rain foot had developed on the right, rapidly expanding outwards by 108 seconds later (lower right).

See annotated version below for easier identification of features.

Heat burst

A heat burst is a special, very rare type of downburst characterized by a rapid rise in temperature, in addition to strong to severe straight line winds. Heat bursts typically occur in evening or night hours and only in a very particular set of conditions. Precipitation must start from a very high altitude and fall through very dry airmass, evaporating completely as it falls through it. As the rain evaporates the air evaporatively cools, accelerating towards the ground and building up momentum. As the rain evaporates completely, the cooling stops and the descending cooled air begins rapidly warming adiabatically by dry adabatic lapse rate. As it descends and warms it passes the altitude where the surrounding air temperature is the same and the descending air is neutrally buoyant – the equilbirium level. If the descending air has sufficient momentum it reaches all the way to the ground, while warming above the temperature of surrounding air – producing a heat burst.

The air in a heat burst is not only very warm, but also exceedingly dry, desiccating the ground and plants. It can persist over an area for several hours before cooling.

Some remarkable heat burst events:

  • Torcy, Seine-et-Marne, France – April 29, 2012:
  • Wichita, Kansas – June 9, 2011: between 12:22 and 12:24 am temperatures rose from 29 °C to 39 °C in winds up to 80 km/h.
  • Sheppard Air Force Base, Wichita Falls, Texas – June 11, 2004: National Weather Service in Norman, Oklahoma reported a heat burst at Sheppard Air Force Base, Wichita Falls, Texas just before local midnight. Temperature abruptly rose from 28.3 °C to 34.4 °C in winds gusting up to 116 km/h. Dewpoint fell from 21.1 °C to 3.9 °C.
  • Midland, Texas – June 16, 2008: wind gusts up to 100 km/h recorded, temperature rose from 21.7 °C to 36.1 °C in minutes.
  • Emporia, Kansas – May 25, 2008: temperature increased from 21.7 °C to 32.8 °C betwen 4:44 and 5:11 am.
  • Cherokee, Oklahoma – July 11, 1909: at 3 am in the morning temperature rose to 57.8 °C, reportedly desiccating crops.

Downbursts and aircraft safety

Downbursts present a particular hazard to air traffic and have been responsible for a number of major aircraft accidents. Aircraft caught within the rapidly descending air close to the ground on take off or landing experience could rapid and unexpected descent rates as well as strong turbulence which may result in loss of control of the aircraft and crash. Major crashes involving downbursts in the 1980s have resulted in aircraft now being equipped with on-board sensors to successfully and reliably detect and avoid downbursts.

NASA artist’s rendering of a microburst.

How a downburst affects an aircraft at low altitude

An aircraft entering a downburst will initially encounter strong headwinds from the expanding gust front close to the ground. The indicated airspeed will increase; a pilot on approach, wanting to to fly at a set airspeed may reduce power. As the airraft passes through the downburst, the headwind becomes a tailwind the indicated airspeed and lift drops. Aircraft performance is decreased and descent rate increased. Also, the rapidly downward moving airmass may exert a significant downward force on the aircraft, increasing its descent rate. Very rapid descent rate combined with poor visibility may result in a crash. Also, very strong tailwind may result in the aircraft stalling and crashing.

Over the past several decades, a number of major aircraft accidents have been attributed to downbursts/microbursts:

  • Malév Ilyushin Il-18 crash at Copenhagen Airport – August 28, 1971
  • Eastern Air Lines Flight 66, Boeing 727 at John F. Kennedy International Airport, New York – June 24, 1975 [111 fatalities, 13 survivors; crashed on approach to JFK airport over 700 m short of the runway]
  • Pan Am Flight 759, Boeing 727, New Orleans International Airport, Louisiana – July 9, 1982 [153 fatalities – 145 on the aircraft, 4 on the ground; aircraft crashed on takeoff, reaching 30-45 m altitude and then crashing into trees and houses just 1400 m from the end of the runway. Winds reported as “gusty and swirling”.]
  • Delta Air Lines Flight 191, Lockheed L-1011 TriStar, Dallas/Fort Worth International Airport, Texas – August 2, 1985 [137 fatalities, 27 survivors; crashed on approach at Dallas/Fort Worth International Airport; aircraft encountered a downburst, descending at up to 15 m/sec, missing the runway and crashing into Texas State Highway 114. Crash investigation resulted in NASA testing a Doppler weather radar on board an airplane, ultimately resulting in the airborne wind shear detection and alert system. FAA now mandates that all commercial aircraft must have on board windshear detection capability]
  • Martinair Flight 495, McDonnell Douglas DC-10, Faro Airport, Portugal – December 21, 1992 [56 fatalities, 284 survivors]
  • USAir Flight 1016, Douglas DC-9, Charlotte/Douglas International Airport, North Carolina – July 2, 1994 [37 fatalities, 20 survivors; aircraft entered rapid descent, crashed 800 m from the runway]
  • Bhoja Air Flight 213, Boeing 737-200, Islamabad International Airport, Pakistan – April 20, 2012 [127 fatalities; aircraft entered successive strong downfrafts, on exit from the final downdraft the crew maintained high pitch angle, stalling the aircraft and crashing]

Several other aircraft accidents have also been attributed to downbursts.

Downbursts and aircraft safety / nova stran

Low-level Wind Shear Alert Systems (LLWSAS)

//July 8, 1989 – Denver Stapleton International Airport. After being alerted by LLWAS of an imminent encounter with a 95 kn (109 mph; 176 km/h) microburst, Captain Craig Levine initiated a missed approach, taking his Boeing 737 to full takeoff power, climbing 400 ft (120 m) and adding 40 kn (46 mph; 74 km/h) of airspeed. Encountering the microburst at full takeoff power, they lost 400 ft (120 m) of altitude and lost 50 kn (58 mph; 93 km/h) of airspeed in about one-half of a minute. This event is a documented “save” of an airplane by a windshear alert system.[7][8][9][10]

Airborne wind shear detection and alert system and Predictive wind shear (PWS) systems

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