Wind shear

Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal wind shear. Vertical wind shear is a change in wind speed or direction with change in altitude. Horizontal wind shear is a change in wind speed with change in lateral position for a given altitude.[1]

Wind shear is a microscale meteorological phenomenon occurring over a very small distance, but it can be associated with mesoscale or synoptic scale weather features such as squall lines and cold fronts. It is commonly observed near microbursts and downbursts caused by thunderstorms, fronts, areas of locally higher low-level winds referred to as low level jets, near mountains, radiation inversions that occur due to clear skies and calm winds, buildings, wind turbines, and sailboats. Wind shear has significant effects on control of an aircraft, and it has been a sole or contributing cause of many aircraft accidents.

Wind shear is sometimes experienced by pedestrians at ground level when walking across a plaza towards a tower block and suddenly encountering a strong wind stream that is flowing around the base of the tower.

Sound movement through the atmosphere is affected by wind shear, which can bend the wave front, causing sounds to be heard where they normally would not, or vice versa. Strong vertical wind shear within the troposphere also inhibits tropical cyclone development, but helps to organize individual thunderstorms into longer life cycles which can then produce severe weather. The thermal wind concept explains how differences in wind speed at different heights are dependent on horizontal temperature differences, and explains the existence of the jet stream.[2]

Downdraft Wind shear-2
Down draft winds with associated virga allow these clouds in the eastern sky at civil twilight to mimic aurora borealis in the Mojave desert
Cirrus clouds2
Cirrus uncinus ice crystal plumes showing high level wind shear, with changes in wind speed and direction.

Definition

Wind shear refers to the variation of wind over either horizontal or vertical distances. Airplane pilots generally regard significant wind shear to be a horizontal change in airspeed of 30 knots (15 m/s) for light aircraft, and near 45 knots (23 m/s) for airliners at flight altitude.[3] Vertical speed changes greater than 4.9 knots (2.5 m/s) also qualify as significant wind shear for aircraft. Low level wind shear can affect aircraft airspeed during take off and landing in disastrous ways, and airliner pilots are trained to avoid all microburst wind shear (headwind loss in excess of 30 knots [15 m/s]).[4] The rationale for this additional caution includes:

  • microburst intensity can double in a minute or less,
  • the winds can shift to excessive cross wind,
  • 40–50 knots (21–26 m/s) is the threshold for survivability at some stages of low-altitude operations, and
  • several of the historical wind shear accidents involved 35–45 knots (18–23 m/s) microbursts.

Wind shear is also a key factor in the creation of severe thunderstorms. The additional hazard of turbulence is often associated with wind shear.

Where and when it is strongly observed

Microburstnasa
Microburst schematic from NASA. 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.

Weather situations where shear is observed include:

  • Weather fronts. Significant shear is observed when the temperature difference across the front is 5 °C (9 °F) or more, and the front moves at 30 knots (15 m/s) or faster. Because fronts are three-dimensional phenomena, frontal shear can be observed at any altitude between surface and tropopause, and therefore be seen both horizontally and vertically. Vertical wind shear above warm fronts is more of an aviation concern than near and behind cold fronts due to their greater duration.[2]
  • Upper-level jet streams. Associated with upper level jet streams is a phenomenon known as clear air turbulence (CAT), caused by vertical and horizontal wind shear connected to the wind gradient at the edge of the jet streams.[5] The CAT is strongest on the anticyclonic shear side of the jet,[6] usually next to or just below the axis of the jet.[7]
  • Low-level jet streams. When a nocturnal low-level jet forms overnight above the Earth's surface ahead of a cold front, significant low level vertical wind shear can develop near the lower portion of the low level jet. This is also known as nonconvective wind shear since it is not due to nearby thunderstorms.[2]
  • Mountains. When winds blow over a mountain, vertical shear is observed on the lee side. If the flow is strong enough, turbulent eddies known as "rotors" associated with lee waves may form, which are dangerous to ascending and descending aircraft.[8]
  • Inversions. When on a clear and calm night, a radiation inversion is formed near the ground, the friction does not affect wind above the top of the inversion layer. The change in wind can be 90 degrees in direction and 40 knots (21 m/s) in speed. Even a nocturnal (overnight) low level jet can sometimes be observed. It tends to be strongest towards sunrise. Density differences cause additional problems to aviation.[2]
  • Downbursts. When an outflow boundary forms due to a shallow layer of rain-cooled air spreading out near ground level from the parent thunderstorm, both speed and directional wind shear can result at the leading edge of the three dimensional boundary. The stronger the outflow boundary is, the stronger the resultant vertical wind shear will become.[9]

Horizontal component

Weather fronts

Weather fronts are boundaries between two masses of air of different densities, or different temperature and moisture properties, which normally are convergence zones in the wind field and are the principal cause of significant weather. Within surface weather analyses, they are depicted using various colored lines and symbols. The air masses usually differ in temperature and may also differ in humidity. Wind shear in the horizontal occurs near these boundaries. Cold fronts feature narrow bands of thunderstorms and severe weather, and may be preceded by squall lines and dry lines. Cold fronts are sharper surface boundaries with more significant horizontal wind shear than warm fronts. When a front becomes stationary, it can degenerate into a line which separates regions of differing wind speed, known as a shear line, though the wind direction across the front normally remains constant. In the tropics, tropical waves move from east to west across the Atlantic and eastern Pacific basins. Directional and speed shear can occur across the axis of stronger tropical waves, as northerly winds precede the wave axis and southeast winds are seen behind the wave axis. Horizontal wind shear can also occur along local land breeze and sea breeze boundaries.[10]

Near coastlines

Wind shear along the coast with low level clouds moving towards the east and higher level clouds moving towards the south-west

The magnitude of winds offshore are nearly double the wind speed observed onshore. This is attributed to the differences in friction between land masses and offshore waters. Sometimes, there are even directional differences, particularly if local sea breezes change the wind on shore during daylight hours.[11]

Vertical component

Thermal wind

Thermal wind is a meteorological term not referring to an actual wind, but a difference in the geostrophic wind between two pressure levels p1 and p0, with p1 < p0; in essence, wind shear. It is only present in an atmosphere with horizontal changes in temperature (or in an ocean with horizontal gradients of density), i.e. baroclinicity. In a barotropic atmosphere, where temperature is uniform, the geostrophic wind is independent of height. The name stems from the fact that this wind flows around areas of low (and high) temperature in the same manner as the geostrophic wind flows around areas of low (and high) pressure.[12]

The thermal wind equation is

where the φ are geopotential height fields with φ1 > φ0, f is the Coriolis parameter, and k is the upward-pointing unit vector in the vertical direction. The thermal wind equation does not determine the wind in the tropics. Since f is small or zero, such as near the equator, the equation reduces to stating that ∇(φ1φ0) is small.[12]

This equation basically describes the existence of the jet stream, a westerly current of air with maximum wind speeds close to the tropopause which is (even though other factors are also important) the result of the temperature contrast between equator and pole.

Effects on tropical cyclones

Thunderhead.anvil
Strong wind shear in the high troposphere forms the anvil-shaped top of this mature cumulonimbus cloud, or thunderstorm.[13]

Tropical cyclones are, in essence, heat engines that are fueled by the temperature gradient between the warm tropical ocean surface and the colder upper atmosphere. Tropical cyclone development requires relatively low values of vertical wind shear so that their warm core can remain above their surface circulation center, thereby promoting intensification. Vertical wind shear tears up the "machinery" of the heat engine causing it to break down. Strongly sheared tropical cyclones weaken as the upper circulation is blown away from the low level center.

The vertical wind shear in a tropical cyclone's environment is very important. When the wind shear is weak, the storms that are part of the cyclone grow vertically, and the latent heat from condensation is released into the air directly above the storm, aiding in development. When there is stronger wind shear, this means that the storms become more slanted and the latent heat release is dispersed over a much larger area.[14][15]

Effects on thunderstorms and severe weather

Severe thunderstorms, which can spawn tornadoes and hailstorms, require wind shear to organize the storm in such a way as to maintain the thunderstorm for a longer period of time. This occurs as the storm's inflow becomes separated from its rain-cooled outflow. An increasing nocturnal, or overnight, low level jet can increase the severe weather potential by increasing the vertical wind shear through the troposphere. Thunderstorms in an atmosphere with virtually no vertical wind shear weaken as soon as they send out an outflow boundary in all directions, which then quickly cuts off its inflow of relatively warm, moist air and kills the thunderstorm.[16]

Planetary boundary layer

PBLimage
Depiction of where the planetary boundary layer lies on a sunny day

The atmospheric effect of surface friction with winds aloft force surface winds to slow and back counterclockwise near the surface of the Earth blowing inward across isobars (lines of equal pressure), when compared to the winds in frictionless flow well above the Earth's surface.[17] This layer where friction slows and changes the wind is known as the planetary boundary layer, sometimes the Ekman layer, and it is thickest during the day and thinnest at night. Daytime heating thickens the boundary layer as winds at the surface become increasingly mixed with winds aloft due to insolation, or solar heating. Radiative cooling overnight further enhances wind decoupling between the winds at the surface and the winds above the boundary layer by calming the surface wind which increases wind shear. These wind changes force wind shear between the boundary layer and the wind aloft, and is most emphasized at night.

Effects on flight

FAA-8083-13 Fig 7-20
Glider ground launch affected by wind shear.

In gliding, wind gradients just above the surface affect the takeoff and landing phases of flight of a glider. Wind gradient can have a noticeable effect on ground launches, also known as winch launches or wire launches. If the wind gradient is significant or sudden, or both, and the pilot maintains the same pitch attitude, the indicated airspeed will increase, possibly exceeding the maximum ground launch tow speed. The pilot must adjust the airspeed to deal with the effect of the gradient.[18]

When landing, wind shear is also a hazard, particularly when the winds are strong. As the glider descends through the wind gradient on final approach to landing, airspeed decreases while sink rate increases, and there is insufficient time to accelerate prior to ground contact. The pilot must anticipate the wind gradient and use a higher approach speed to compensate for it.[19]

Wind shear is also a hazard for aircraft making steep turns near the ground. It is a particular problem for gliders which have a relatively long wingspan, which exposes them to a greater wind speed difference for a given bank angle. The different airspeed experienced by each wing tip can result in an aerodynamic stall on one wing, causing a loss of control accident.[19][20]

Wind shear or wind gradients are a threat to parachutists, particularly to BASE jumping and wingsuit flying. Skydivers have been pushed off of their course by sudden shifts in wind direction and speed, and have collided with bridges, cliffsides, trees, other skydivers, the ground, and other obstacles. Skydivers routinely make adjustments to the position of their open canopies to compensate for changes in direction while making landings to prevent accidents such as canopy collisions and canopy inversion.

Soaring related to wind shear, also called dynamic soaring, is a technique used by soaring birds like albatrosses, who can maintain flight without wing flapping. If the wind shear is of sufficient magnitude, a bird can climb into the wind gradient, trading ground speed for height, while maintaining airspeed.[21] By then turning downwind, and diving through the wind gradient, they can also gain energy.[22] It has also been used by glider pilots on rare occasions.

Wind shear can also create wave. This occurs when an atmospheric inversion separates two layers with a marked difference in wind direction. If the wind encounters distortions in the inversion layer caused by thermals coming up from below, it will create significant shear waves that can be used for soaring.[23]

Windshearaircraftnasa
Effect of wind shear on aircraft trajectory. Note how merely correcting for the initial gust front can have dire consequences.

Strong outflow from thunderstorms causes rapid changes in the three-dimensional wind velocity just above ground level. Initially, this outflow causes a headwind that increases airspeed, which normally causes a pilot to reduce engine power if they are unaware of the wind shear. As the aircraft passes into the region of the downdraft, the localized headwind diminishes, reducing the aircraft's airspeed and increasing its sink rate. Then, when the aircraft passes through the other side of the downdraft, the headwind becomes a tailwind, reducing lift generated by the wings, and leaving the aircraft in a low-power, low-speed descent. This can lead to an accident if the aircraft is too low to effect a recovery before ground contact.

Delta 191 wreckage
Wreckage of Delta Air Lines Flight 191 tail section after a microburst slammed the aircraft into the ground. Another aircraft can be seen flying in the background past the crash scene.

As the result of the accidents in the 1970s and 1980s, most notably following the 1985 crash of Delta Air Lines Flight 191, in 1988 the U.S. Federal Aviation Administration mandated that all commercial aircraft have on-board wind shear detection systems by 1993. Between 1964 and 1985, wind shear directly caused or contributed to 26 major civil transport aircraft accidents in the U.S. that led to 620 deaths and 200 injuries.[24] Since 1995, the number of major civil aircraft accidents caused by wind shear has dropped to approximately one every ten years, due to the mandated on-board detection as well as the addition of Doppler weather radar units on the ground (NEXRAD). The installation of high-resolution Terminal Doppler Weather Radar stations at many U.S. airports that are commonly affected by wind shear has further aided the ability of pilots and ground controllers to avoid wind shear conditions.[25]

Sailing

Wind shear affects sailboats in motion by presenting a different wind speed and direction at different heights along the mast. The effect of low level wind shear can be factored into the selection of sail twist in the sail design, but this can be difficult to predict since wind shear may vary widely in different weather conditions. Sailors may also adjust the trim of the sail to account for low level wind shear, for example using a boom vang.[26]

Sound propagation

Wind shear can have a pronounced effect upon sound propagation in the lower atmosphere, where waves can be "bent" by refraction phenomenon. The audibility of sounds from distant sources, such as thunder or gunshots, is very dependent on the amount of shear. The result of these differing sound levels is key in noise pollution considerations, for example from roadway noise and aircraft noise, and must be considered in the design of noise barriers.[27] This phenomenon was first applied to the field of noise pollution study in the 1960s, contributing to the design of urban highways as well as noise barriers.[28]

Hodographe NOAA
Hodograph plot of wind vectors at various heights in the troposphere. Meteorologists can use this plot to evaluate vertical wind shear in weather forecasting. (Source: NOAA)

The speed of sound varies with temperature. Since temperature and sound velocity normally decrease with increasing altitude, sound is refracted upward, away from listeners on the ground, creating an acoustic shadow at some distance from the source.[29] In the 1862, during the American Civil War Battle of Iuka, an acoustic shadow, believed to have been enhanced by a northeast wind, kept two divisions of Union soldiers out of the battle,[30] because they could not hear the sounds of battle only six miles downwind.[31]

Effects on architecture

Wind engineering is a field of engineering devoted to the analysis of wind effects on the natural and built environment. It includes strong winds which may cause discomfort as well as extreme winds such as tornadoes, hurricanes and storms which may cause widespread destruction. Wind engineering draws upon meteorology, aerodynamics and a number of specialist engineering disciplines. The tools used include climate models, atmospheric boundary layer wind tunnels and numerical models. It involves, among other topics, how wind impacting buildings must be accounted for in engineering.[32]

Wind turbines are affected by wind shear. Vertical wind-speed profiles result in different wind speeds at the blades nearest to the ground level compared to those at the top of blade travel, and this in turn affects the turbine operation.[33] This low level wind shear can create a large bending moment in the shaft of a two bladed turbine when the blades are vertical.[34] The reduced wind shear over water means shorter and less expensive wind turbine towers can be used in shallow seas.[35]

See also

References

  1. ^ "Vertical wind shear. Retrieved on 2015-10-24".
  2. ^ a b c d "Low-Level Wind Shear". Integrated Publishing. Retrieved 2007-11-25.
  3. ^ FAA FAA Advisory Circular Pilot Wind Shear Guide. Retrieved on 2007-12-15.
  4. ^ "Wind Shear". NASA. Archived from the original on 2007-10-09. Retrieved 2007-10-09.
  5. ^ "Jet Streams in the UK". BBC. Archived from the original on January 18, 2008. Retrieved 2008-05-08.
  6. ^ Knox, John A. (1997). Possible Mechanisms of Clear-Air Turbulence in Strongly Anticyclonic Flows. Retrieved on 2015-01-13.
  7. ^ CLARK T. L., HALL W. D., KERR R. M., MIDDLETON D., RADKE L., RALPH F. M., NEIMAN P. J., LEVINSON D. Origins of aircraft-damaging clear-air turbulence during the 9 December 1992 Colorado downslope windstorm : Numerical simulations and comparison with observations. Retrieved on 2008-05-08.
  8. ^ National Center for Atmospheric Research. T-REX: Catching the Sierra’s waves and rotors Archived 2006-11-21 at the Wayback Machine Retrieved on 2006-10-21.
  9. ^ Fujita, T.T. (1985). "The Downburst, microburst and macroburst". SMRP Research Paper 210, 122 pp.
  10. ^ David M. Roth. Hydrometeorological Prediction Center. Unified Surface Analysis Manual. Retrieved on 2006-10-22.
  11. ^ Franklin B. Schwing and Jackson O. Blanton. The Use of Land and Sea Based Wind Data in a Simple Circulation Model. Retrieved on 2007-10-03.
  12. ^ a b James R. Holton (2004). An Introduction to Dynamic Meteorology. ISBN 0-12-354015-1
  13. ^ McIlveen, J. (1992). Fundamentals of Weather and Climate. London: Chapman & Hall. p. 339. ISBN 0-412-41160-1.
  14. ^ University of Illinois. Hurricanes. Retrieved 2006-10-21.
  15. ^ "Hurricanes: a tropical cyclone with winds > 64 knots". University of Illinois.
  16. ^ University of Illinois. Vertical Wind Shear Retrieved on 2006-10-21.
  17. ^ "AMS Glossary of Meteorology, Ekman layer". American Meteorological Association. Retrieved 2015-02-15.
  18. ^ Glider Flying Handbook. U.S. Government Printing Office, Washington D.C.: U.S. Federal Aviation Administration. 2003. pp. 7–16. FAA-8083-13_GFH.
  19. ^ a b Piggott, Derek (1997). Gliding: a Handbook on Soaring Flight. Knauff & Grove. pp. 85–86, 130–132. ISBN 978-0-9605676-4-5.
  20. ^ Knauff, Thomas (1984). Glider Basics from First Flight to Solo. Thomas Knauff. ISBN 0-9605676-3-1.
  21. ^ Alexander, R. (2002). Principles of Animal Locomotion. Princeton: Princeton University Press. p. 206. ISBN 0-691-08678-8.
  22. ^ Alerstam, Thomas (1990). Bird Migration. Cambridge: Cambridge University Press. p. 275. ISBN 0-521-44822-0.
  23. ^ Eckey, Bernard (2007). Advanced Soaring Made Easy. Eqip Verbung & Verlag GmbH. ISBN 3-9808838-2-5.
  24. ^ National Aeronautics and Space Administration, Langley Research Center (June 1992). "Making the Skies Safer From Windshear". Archived from the original on March 29, 2010. Retrieved 2012-11-16.
  25. ^ "Terminal Doppler Weather Radar Information". National Weather Service. Retrieved 4 August 2009.
  26. ^ Garrett, Ross (1996). The Symmetry of Sailing. Dobbs Ferry: Sheridan House. pp. 97–99. ISBN 1-57409-000-3.
  27. ^ Foss, Rene N. (June 1978). "Ground Plane Wind Shear Interaction on Acoustic Transmission". WA-RD 033.1. Washington State Department of Transportation. Retrieved 2007-05-30.
  28. ^ "C. Michael Hogan, Analysis of highway noise, Journal of Water, Air, & Soil Pollution, Volume 2, Number 3, Biomedical and Life Sciences and Earth and Environmental Science Issue, Pages 387-392, September, 1973, Springer Verlag, Netherlands". ISSN 0049-6979.
  29. ^ Everest, F. (2001). The Master Handbook of Acoustics. New York: McGraw-Hill. pp. 262–263. ISBN 0-07-136097-2.
  30. ^ Cornwall, Sir (1996). Grant as Military Commander. Barnes & Noble Inc. p. 92. ISBN 1-56619-913-1.
  31. ^ Cozzens, Peter (2006). The Darkest Days of the War: the Battles of Iuka and Corinth. Chapel Hill: The University of North Carolina Press. ISBN 0-8078-5783-1.
  32. ^ Professor John Twidell. Wind Engineering. Retrieved on 2007-11-25.
  33. ^ Heier, Siegfried (2005). Grid Integration of Wind Energy Conversion Systems. Chichester: John Wiley & Sons. p. 45. ISBN 0-470-86899-6.
  34. ^ Harrison, Robert (2001). Large Wind Turbines. Chichester: John Wiley & Sons. p. 30. ISBN 0-471-49456-9.
  35. ^ Lubosny, Zbigniew (2003). Wind Turbine Operation in Electric Power Systems: Advanced Modeling. Berlin: Springer. p. 17. ISBN 3-540-40340-X.

External links

2015 Pacific hurricane season

The 2015 Pacific hurricane season was the second-most active Pacific hurricane season on record, with 26 named storms, only behind the 1992 season. A record-tying 16 of those storms became hurricanes, and a record 11 storms further intensified into major hurricanes throughout the season. The Central Pacific, the portion of the Northeast Pacific Ocean between the International Date Line and the 140th meridian west, had its most active year on record, with 16 tropical cyclones forming in or entering the basin. Moreover, the season was the third-most active season in terms of accumulated cyclone energy, amassing a total of 287 units. The season officially started on May 15 in the Eastern Pacific and on June 1 in the Central Pacific; they both ended on November 30. These dates conventionally delimit the period of each year when most tropical cyclones form in the Northeast Pacific basin. However, the formation of tropical cyclones is possible at any time of the year. This was shown when a tropical depression formed on December 31. The above-average activity during the season was attributed in part to the very strong 2014–16 El Niño event.

The season featured several long-tracking and powerful storms, although land impacts were often minimal. In June, Hurricane Blanca, an early season Category 4 hurricane, killed four people due to rough seas. Hurricane Carlos caused minor damage while passing a short distance off the coast of Mexico. In July, the remnants of Hurricane Dolores brought record rainfall to Southern California, killing one and causing losses worth over $50 million. On August 29, three Category 4 hurricanes (Kilo, Ignacio, Jimena) were all active simultaneously in the Pacific east of the International Date Line for the first time in recorded history. In September, moisture from Hurricane Linda contributed to storms that killed 21 people in Utah. Later that month, Hurricane Marty inflicted $30 million in damage to the southwestern coast of Mexico. In October, Hurricane Patricia became the most intense hurricane ever recorded in the Western Hemisphere, with a central pressure of 872 mbar (hPa; 25.75 inHg) and 1-minute sustained winds of 215 mph (345 km/h). After also becoming the strongest landfalling Pacific hurricane on record, Patricia claimed 13 lives and was responsible for $463 million in damage. The season's activity continued into November when Hurricane Sandra became the strongest Pacific hurricane ever recorded in that month.

Air-mass thunderstorm

An air-mass thunderstorm, also called an "ordinary", "single cell", or "garden variety" thunderstorm, is a thunderstorm that is generally weak and usually not severe. These storms form in environments where at least some amount of Convective Available Potential Energy (CAPE) is present, but very low levels of wind shear and helicity. The lifting source, which is a crucial factor in thunderstorm development, is usually the result of uneven heating of the surface, though they can be induced by weather fronts and other low-level boundaries associated with wind convergence. The energy needed for these storms to form comes in the form of insolation, or solar radiation. Air-mass thunderstorms do not move quickly, last no longer than an hour, and have the threats of lightning, as well as showery light, moderate, or heavy rainfall. Heavy rainfall can interfere with microwave transmissions within the atmosphere.

Lightning characteristics are related to characteristics of the parent thunderstorm, and could induce wildfires near thunderstorms with minimal rainfall. On unusual occasions there could be a weak downburst and small hail. They are common in temperate zones during a summer afternoon. Like all thunderstorms, the mean-layered wind field the storms form within determine motion. When the deep-layered wind flow is light, outflow boundary progression will determine storm movement. Since thunderstorms can be a hazard to aviation, pilots are advised to fly above any haze layers within regions of better visibility and to avoid flying under the anvil of these thunderstorms, which can be regions where hail falls from the parent thunderstorm. Vertical wind shear is also a hazard near the base of thunderstorms which have generated outflow boundaries.

Airborne wind shear detection and alert system

The Airborne wind shear detection and alert system, fitted in an aircraft, detects and alerts the pilot both visually and aurally of a wind shear condition. In case of reactive wind shear detection system, the detection takes place when the aircraft penetrates a wind shear condition of sufficient force, which can pose a hazard to the aircraft. In case of predictive wind shear detection system, the detection takes place, if such wind shear condition is ahead of the aircraft. In 1988, the U.S. Federal Aviation Administration (FAA) mandated that all turbine-powered commercial aircraft must have on-board wind shear detection systems by 1993. Airlines successfully lobbied to have commercial turbo-prop aircraft exempted from this requirement.

In the predictive wind shear detection mode, the weather radar processor of the aircraft detects the presence of the microburst, a type of vertical wind shear condition by detecting the Doppler frequency shift of the microwave pulses caused by the microburst ahead of the aircraft and displays the area where it is present in the Navigation Display Unit (of the Electronic Flight Instrument System) along with an aural warning.

Clear-air turbulence

Clear-air turbulence (CAT) is the turbulent movement of air masses in the absence of any visual clues, such as clouds, and is caused when bodies of air moving at widely different speeds meet.The atmospheric region most susceptible to CAT is the high troposphere at altitudes of around 7,000–12,000 metres (23,000–39,000 ft) as it meets the tropopause. Here CAT is most frequently encountered in the regions of jet streams. At lower altitudes it may also occur near mountain ranges. Thin cirrus clouds can also indicate high probability of CAT.

CAT can be hazardous to the comfort, but rarely the safety, of air travelers.

CAT in the jet stream is expected to become stronger and more frequent because of climate change, with transatlantic wintertime CAT increasing by 59% (light), 94% (moderate), and 149% (severe) by the time of CO2 doubling.

Cyclone Cliff

Tropical Cyclone Cliff was first noted as a weak tropical disturbance on April 1, 2007, within a trough of low pressure about 210 km (130 mi) to the southwest of Rotuma. Over the next couple of days the system drifted towards the southeast and Fiji, in an area of strong wind shear. During April 3, the system slightly accelerated, as it moved towards the south-southeast before the westerly wind shear around the system relaxed sufficiently to allow the depression to consolidate while it was located near Vanua Levu.

Hurricane Ekeka

Hurricane Ekeka was the most intense off-season tropical cyclone on record in the north-eastern pacific basin. The first storm of the 1992 Pacific hurricane season, Ekeka developed on January 28 well to the south of Hawaii. It gradually intensified to reach major hurricane status on February 2, although it subsequently began to weaken due to unfavorable wind shear. It crossed the International Date Line as a weakened tropical storm, and shortly thereafter degraded to tropical depression status. Ekeka continued westward, passing through the Marshall Islands and later over Chuuk State, before dissipating on February 9 about 310 miles (500 km) off the north coast of Papua New Guinea. The storm did not cause any significant damage or deaths.

Hurricane Epsilon

Hurricane Epsilon was the final of fifteen hurricanes within the record-breaking 2005 Atlantic hurricane season. Originating from a cold front beneath an upper-level low, Epsilon formed on November 29 about 915 mi (1470 km) east of Bermuda. Initially, the National Hurricane Center (NHC) forecast the storm to transition into an extratropical cyclone within five days, due to conditions unfavorable for significant intensification. Epsilon continually defied forecasts, at first due to an unexpected loop to the southwest, and later due to retaining its strength despite cold waters and strong wind shear.

On December 1, Epsilon began a northeast motion due to an approaching trough, and the next day it attained hurricane status. After turning to the east, it developed characteristics of an annular hurricane, meaning it had a circular eye, a ring of convection, and had few fluctuations in its intensity. On December 5 Epsilon attained peak winds of 85 mph (140 km/h), and the next day it turned to the south and southwest. Late on December 7, the winds dropped below hurricane status for the first time in five days, making Epsilon the longest-lasting December hurricane on record. Stronger wind shear caused rapid weakening, and the storm could no longer be classified as a tropical cyclone late on December 8. The next day the remnant circulation of Epsilon dissipated.

Hurricane Genevieve (2014)

Hurricane Genevieve, also referred to as Typhoon Genevieve, was the fourth-most intense tropical cyclone of the North Pacific Ocean in 2014. A long-lasting system, Genevieve was the first one to track across all three northern Pacific basins since Hurricane Dora in 1999. Genevieve developed from a tropical wave into the eighth tropical storm of the 2014 Pacific hurricane season well east-southeast of Hawaii on July 25. However, increased vertical wind shear caused it to weaken into a tropical depression by the following day and degenerate into a remnant low on July 28. Late on July 29, the system regenerated into a tropical depression, but it weakened into a remnant low again on July 31, owing to vertical wind shear and dry air.

The remnants redeveloped into a tropical depression and briefly became a tropical storm south of Hawaii on August 2, yet it weakened back into a tropical depression soon afterwards. Late on August 5, Genevieve re-intensified into a tropical storm, and intensified into a Category 1 hurricane on the next day when undergoing rapid deepening because of favorable conditions. Early on August 7, Genevieve strengthened into a Category 4 hurricane, shortly before it crossed the International Date Line and was reclassified as a typhoon, also becoming the thirteenth named storm of the 2014 Pacific typhoon season. Late on the same day, Genevieve reached peak intensity while it was located west-southwest of Wake Island.

Typhoon Genevieve started to gradually weaken at noon on August 8, and stronger vertical wind shear provided by a TUTT cell began to weaken the system further on August 9. The typhoon crossed 30° north at noon on August 10 and weakened to a severe tropical storm soon afterwards, because of unfavorable sea surface temperature and expanding subsidence. Genevieve weakened into a tropical storm on August 11 and a tropical depression the following day, as its deep convection diminished.

Hurricane Hector (2018)

Hurricane Hector was a powerful and long-lived tropical cyclone that was the first to traverse all three North Pacific basins since Genevieve in 2014. The eighth named storm, fourth hurricane, and third major hurricane of the 2018 Pacific hurricane season, Hector originated from an area of low pressure that formed a couple hundred miles west-southwest of Mexico on July 28. Amid favorable weather conditions, a tropical depression formed a few days later on July 31. The depression continued strengthening and became Tropical Storm Hector on the next day. Hector became a hurricane on August 2, and rapidly intensified into a strong Category 2 hurricane later in the day. After weakening while undergoing an eyewall replacement cycle, Hector quickly strengthened into a Category 4 hurricane late on August 5. Over the next week, Hector fluctuated in intensity multiple times due to eyewall replacement cycles and shifting wind shear. Hector achieved its peak intensity on August 6, as a high-end Category 4 hurricane with winds of 155 mph (250 km/h). On the following day, the hurricane bypassed Hawaii approximately 200 mi (320 km) to the south. Increasing wind shear resulted in steady weakening of the storm, beginning on August 11. At that time, Hector accumulated the longest continuous stretch of time as a major hurricane in the northeastern Pacific since reliable records began. Eroding convection and dissipation of its eye marked its degradation to a tropical storm on August 13. The storm subsequently traversed the International Dateline that day. Hector later weakened into a tropical depression on August 15, before dissipating late on August 16.

Hector prompted several islands in the Papahānaumokuākea Marine National Monument to issue tropical storm watches after the close pass by in Hawaii that warranted the issuance of a tropical storm warning for Hawaii County. Despite Hector having passed a couple hundred miles to the south of Hawaii, it still brought numerous adverse weather effects to Hawaii County and the surrounding islands.

Hurricane Karen (2007)

Hurricane Karen was the eleventh named storm and fourth hurricane of the 2007 Atlantic hurricane season. Karen was a Cape Verde-type hurricane that developed in the eastern tropical Atlantic out of a large tropical wave. The storm briefly reached Category 1 hurricane intensity before slowly weakening due to increased wind shear. As the storm remained away from land, no damages or fatalities were reported in association with Karen.

Meteorological history of Hurricane Wilma

Hurricane Wilma was the most intense tropical cyclone in the Atlantic basin on record, with an atmospheric pressure of 882 hPa (mbar, 26.05 inHg). Wilma's destructive journey began in the second week of October 2005. A large area of disturbed weather developed across much of the Caribbean Sea and gradually organized to the southeast of Jamaica. By late on October 15, the system was sufficiently organized for the National Hurricane Center to designate it as Tropical Depression Twenty-Four.

The depression drifted southwestward, and under favorable conditions, it strengthened into Tropical Storm Wilma on October 17. Initially, development was slow due to its large size, though convection steadily organized. From October 18, and through the following day, Wilma underwent explosive deepening over the open waters of the Caribbean; in a 30-hour period, the system's central atmospheric pressure dropped from 982 mbar (29.00 inHg) to the record-low value of 882 mbar (26.05 inHg), while the winds increased to 185 mph (298 km/h). At its peak intensity, the eye of Wilma was about 2.3 miles (3.7 km) in diameter, the smallest known eye in an Atlantic hurricane. After the inner eye dissipated due to an eyewall replacement cycle, Hurricane Wilma weakened to Category 4 status, and on October 21, it made landfall on Cozumel and on the Mexican mainland with winds of about 150 mph (240 km/h).

Wilma weakened over the Yucatán Peninsula, and reached the southern Gulf of Mexico before accelerating northeastward. Despite increasing amounts of vertical wind shear, the hurricane re-strengthened to hit Cape Romano, Florida, as a major hurricane. Wilma weakened as it quickly crossed the state, and entered the Atlantic Ocean near Jupiter, Florida. The hurricane again re-intensified before cold air and wind shear penetrated the inner core of convection. By October 26, it transitioned into an extratropical cyclone, and the next day, the remnants of Wilma were absorbed by another extratropical storm over Atlantic Canada.

Squall

A squall is a sudden, sharp increase in wind speed lasting minutes, contrary to a wind gust lasting seconds. They are usually associated with active weather, such as rain showers, thunderstorms, or heavy snow. Squalls refer to the increase to the sustained winds over that time interval, as there may be higher gusts during a squall event. They usually occur in a region of strong sinking air or cooling in the mid-atmosphere. These force strong localized upward motions at the leading edge of the region of cooling, which then enhances local downward motions just in its wake.

Thunderstorm

A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are sometimes called thundershowers. Thunderstorms occur in a type of cloud known as a cumulonimbus. They are usually accompanied by strong winds, and often produce heavy rain and sometimes snow, sleet, or hail, but some thunderstorms produce little precipitation or no precipitation at all. Thunderstorms may line up in a series or become a rainband, known as a squall line. Strong or severe thunderstorms include some of the most dangerous weather phenomena, including large hail, strong winds, and tornadoes. Some of the most persistent severe thunderstorms, known as supercells, rotate as do cyclones. While most thunderstorms move with the mean wind flow through the layer of the troposphere that they occupy, vertical wind shear sometimes causes a deviation in their course at a right angle to the wind shear direction.

Thunderstorms result from the rapid upward movement of warm, moist air, sometimes along a front. As the warm, moist air moves upward, it cools, condenses, and forms a cumulonimbus cloud that can reach heights of over 20 kilometres (12 mi). As the rising air reaches its dew point temperature, water vapor condenses into water droplets or ice, reducing pressure locally within the thunderstorm cell. Any precipitation falls the long distance through the clouds towards the Earth's surface. As the droplets fall, they collide with other droplets and become larger. The falling droplets create a downdraft as it pulls cold air with it, and this cold air spreads out at the Earth's surface, occasionally causing strong winds that are commonly associated with thunderstorms.

Thunderstorms can form and develop in any geographic location but most frequently within the mid-latitude, where warm, moist air from tropical latitudes collides with cooler air from polar latitudes. Thunderstorms are responsible for the development and formation of many severe weather phenomena. Thunderstorms, and the phenomena that occur along with them, pose great hazards. Damage that results from thunderstorms is mainly inflicted by downburst winds, large hailstones, and flash flooding caused by heavy precipitation. Stronger thunderstorm cells are capable of producing tornadoes and waterspouts.

There are four types of thunderstorms: single-cell, multi-cell cluster, multi-cell lines and supercells. Supercell thunderstorms are the strongest and most severe. Mesoscale convective systems formed by favorable vertical wind shear within the tropics and subtropics can be responsible for the development of hurricanes. Dry thunderstorms, with no precipitation, can cause the outbreak of wildfires from the heat generated from the cloud-to-ground lightning that accompanies them. Several means are used to study thunderstorms: weather radar, weather stations, and video photography. Past civilizations held various myths concerning thunderstorms and their development as late as the 18th century. Beyond the Earth's atmosphere, thunderstorms have also been observed on the planets of Jupiter, Saturn, Neptune, and, probably, Venus.

Tropical Depression Ten (2005)

Tropical Depression Ten was a short-lived and weak tropical cyclone that was the tenth system of the record-breaking 2005 Atlantic hurricane season. It formed on August 13 from a tropical wave that emerged from the west coast of Africa on August 8. As a result of strong wind shear, the depression remained weak and did not strengthen beyond tropical depression status. It degenerated on August 14, but its remnants partially contributed to the formation of Hurricane Katrina. Thus, it is often referred to as "the precursor to Katrina". The cyclone had no effect on land and did not directly result in any fatalities or damage.

Tropical Storm Josephine (2008)

Tropical Storm Josephine was the tenth tropical storm of the 2008 Atlantic hurricane season. Josephine developed out of a strong tropical wave which moved off the African coast on August 31. The wave quickly became organized and was declared Tropical Depression Ten while located 170 mi (270 km) to the south-southeast of the Cape Verde Islands on September 2. The depression was quickly upgraded to Tropical Storm Josephine around noon the same day. Over the next several days, Josephine moved in a general west-northwest direction and reached its peak intensity early on September 3. Strong wind shear, some due to the outflow of Hurricane Ike, and dry air caused the storm to weaken. On September 6, the combination of wind shear, dry air, and cooling waters caused Josephine to weaken into a tropical depression. Josephine deteriorated into a remnant low shortly after as convection continued to dissipate around the storm. The low ultimately dissipated while located 520 mi (835 km) east of Guadeloupe on September 10. However, the remnant moisture led to minor flooding on the island of St. Croix.

Tropical cyclogenesis

Tropical cyclogenesis is the development and strengthening of a tropical cyclone in the atmosphere. The mechanisms through which tropical cyclogenesis occurs are distinctly different from those through which temperate cyclogenesis occurs. Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment.Tropical cyclogenesis requires six main factors: sufficiently warm sea surface temperatures (at least 26.5 °C (79.7 °F)), atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to develop a low-pressure center, a pre-existing low-level focus or disturbance, and low vertical wind shear.Tropical cyclones tend to develop during the summer, but have been noted in nearly every month in most basins. Climate cycles such as ENSO and the Madden–Julian oscillation modulate the timing and frequency of tropical cyclone development. There is a limit on tropical cyclone intensity which is strongly related to the water temperatures along its path.An average of 86 tropical cyclones of tropical storm intensity form annually worldwide. Of those, 47 reach hurricane/typhoon strength, and 20 become intense tropical cyclones (at least Category 3 intensity on the Saffir–Simpson Hurricane Scale).

Tropical cyclone rainfall climatology

A tropical cyclone rainfall climatology is developed to determine rainfall characteristics of past tropical cyclones. A tropical cyclone rainfall climatology can be used to help forecast current or upcoming tropical cyclone impacts. The degree of a tropical cyclone rainfall impact depends upon speed of movement, storm size, and degree of vertical wind shear. One of the most significant threats from tropical cyclones is heavy rainfall. Large, slow moving, and non-sheared tropical cyclones produce the heaviest rains. The intensity of a tropical cyclone appears to have little bearing on its potential for rainfall over land, but satellite measurements over the last several years show that more intense tropical cyclones produce noticeably more rainfall over water. Flooding from tropical cyclones remains a significant cause of fatalities, particularly in low-lying areas.

Tropical cyclone rainfall forecasting

Tropical cyclone rainfall forecasting involves using scientific models and other tools to predict the precipitation expected in tropical cyclones such as hurricanes and typhoons. Knowledge of tropical cyclone rainfall climatology is helpful in the determination of a tropical cyclone rainfall forecast. More rainfall falls in advance of the center of the cyclone than in its wake. The heaviest rainfall falls within its central dense overcast and eyewall. Slow moving tropical cyclones, like Hurricane Danny and Hurricane Wilma, can lead to the highest rainfall amounts due to prolonged heavy rains over a specific location. However, vertical wind shear leads to decreased rainfall amounts, as rainfall is favored downshear and slightly left of the center and the upshear side is left devoid of rainfall. The presence of hills or mountains near the coast, as is the case across much of Mexico, Haiti, the Dominican Republic, much of Central America, Madagascar, Réunion, China, and Japan act to magnify amounts on their windward side due to forced ascent causing heavy rainfall in the mountains. A strong system moving through the mid latitudes, such as a cold front, can lead to high amounts from tropical systems, occurring well in advance of its center. Movement of a tropical cyclone over cool water will also limit its rainfall potential. A combination of factors can lead to exceptionally high rainfall amounts, as was seen during Hurricane Mitch in Central America.Use of forecast models can help determine the magnitude and pattern of the rainfall expected. Climatology and persistence models, such as r-CLIPER, can create a baseline for tropical cyclone rainfall forecast skill. Simplified forecast models, such as the Kraft technique and the eight and sixteen-inch rules, can create quick and simple rainfall forecasts, but come with a variety of assumptions which may not be true, such as assuming average forward motion, average storm size, and a knowledge of the rainfall observing network the tropical cyclone is moving towards. The forecast method of TRaP assumes that the rainfall structure the tropical cyclone currently has changes little over the next 24 hours. The global forecast model which shows the most skill in forecasting tropical cyclone-related rainfall in the United States is the ECMWF IFS (Integrated Forecasting System) .

Vertical draft

An updraft is a small‐scale current of rising air, often within a cloud.

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