Downburst
In meteorology, a downburst is a strong downward and outward gushing wind system that emanates from a point source above and blows radially, that is, in straight lines in all directions from the area of impact at surface level. It originates under deep, moist convective conditions like cumulus congestus or cumulonimbus. Capable of producing damaging winds, it may sometimes be confused with a tornado, where high-velocity winds circle a central area, and air moves inward and upward. These usually last for seconds to minutes. Downbursts are particularly strong downdrafts within thunderstorms. Downbursts are most often created by an area of significantly precipitation-cooled air that, after reaching the surface, spreads out in all directions producing strong winds.
Dry downbursts are associated with thunderstorms that exhibit very little rain, while wet downbursts are created by thunderstorms with significant amounts of precipitation. Microbursts and macrobursts are downbursts at very small and larger scales, respectively. A rare variety of dry downburst, 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 and compressional heating during descent.
Downbursts are a topic of notable discussion in aviation, since they create vertical wind shear, which has the potential to be dangerous to aviation, especially during landing, where airspeed performance windows are the most narrow. Several fatal and historic crashes in past decades are attributed to the phenomenon and flight crew training goes to great lengths on how to properly recognize and recover from a downburst/wind shear event; wind shear recovery, among other adverse weather events, are standard topics across the world in flight simulator training that flight crews receive and must successfully complete. Detection and nowcasting technology was also implemented in much of the world and particularly around major airports, which in many cases actually have wind shear detection equipment on the field. This detection equipment helps air traffic controllers and pilots make decisions on the safety and feasibility of operating on or in the vicinity of the airport during storms.
Definition
A downburst is created by a column of sinking air that after hitting the surface spreads out in all directions and is capable of producing straight-line winds of over, often producing damage similar to, but distinguishable from, that caused by tornadoes. Downburst damage radiates 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 in air that is precipitation free or contains virga are known as dry downbursts; those accompanied with precipitation are known as wet downbursts. These 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. Most downbursts are less than in extent: these are called microbursts. Downbursts larger than in extent are sometimes called macrobursts. Downbursts can occur over large areas. In the extreme case, a series of continuing downbursts results in a derecho, which covers huge areas of more than wide and over long, persisting for 12 hours or more, and which is associated with some of the most intense straight-line winds.
The term microburst was defined by mesoscale meteorology expert Ted Fujita as affecting an area in diameter or less, distinguishing them as a type of downburst and apart from common wind shear which can encompass greater areas. Fujita also coined the term macroburst for downbursts larger than.
Dry downbursts
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 denser cool air descends and accelerates as it approaches the surface. When the cool air approaches the surface, 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.Dry downbursts are typically produced by high based thunderstorms that contain little to no surface rainfall. They 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 developed a conceptual model of a dry downburst environment that comprised three important variables: mid-level moisture, cloud base in the mid troposphere, 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 downbursts
Wet downbursts are accompanied by significant precipitation at the surface. 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" downbursts. As a result, higher mixing ratios are necessary for these downbursts to form. Melting of ice, particularly hail, appears to play an important role in downburst formation, especially in the lowest above surface level. These factors, among others, make forecasting wet microbursts difficult.| Characteristic | Dry Downburst | Wet Downburst |
| Location of highest probability within the United States | Midwest / West | Southeast |
| Precipitation | Little or none | Moderate or heavy |
| Cloud bases | As high as 500 hPa | As high as 850 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 are very strong winds that can produce damage, demonstrating a lack of the rotational damage pattern associated with tornadoes. 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 and winds of or more can last for more than twenty minutes. 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. Straight-line wind events in the form of derechos can take place throughout the eastern half of the U.S.Straight-line winds may be damaging to marine interests. Small ships, cutters and sailboats are at risk from this meteorological phenomenon.
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 surface. As the cold air hits the ground or water it spreads out and a mesoscale front can be observed as a gust front. Areas under and immediately adjacent to the downburst 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 surface, 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. 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.Downbursts go through three stages in their cycle: the downburst, outburst, and cushion stages.
Development stages of downbursts
The evolution of downbursts is broken down into three stages: the contact stage, the outburst stage, and the cushion stage:- A downburst initially develops as the downdraft begins its descent from the cloud base. The downdraft accelerates, and within minutes reaches the surface.
- During the outburst stage, the wind "curls" as the cold air of the downburst moves away from the point of impact with the surface.
- During the cushion stage, winds about the curl continue to accelerate, while the winds at the surface slow due to friction.
Physical processes of dry and wet downbursts
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 but there is not much reason to believe it has much of an impact on downdrafts and therefore will be ignored.
The second term is the effect of buoyancy on vertical motion. Clearly, in the case of downbursts, 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. 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 value. Therefore, in general terms, negative buoyancy is typically the major contributor to downdrafts.