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Mountain Waves

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Article Information
Category: Weather Weather
Content source: SKYbrary About SKYbrary
Content control: SKYbrary About SKYbrary
WX
Tag(s) Weather Phenomena,
Turbulence

Lee Waves or Standing Waves

Definition

Mountain Waves is defined as oscillations to the lee side (downwind) of high ground resulting from the disturbance in the horizontal air flow caused by the high ground.

Description

The wavelength and amplitude of the oscillations depends on many factors including the height of the high ground relative to surrounding terrain, the wind speed and the instability of the atmosphere.

Formation of mountain waves can occur in the following conditions:

  • Wind direction within 30 degrees of the perpendicular to the ridge of high ground and no change in direction over a significant height band.
  • Wind speeds at the crest of the ridge in excess of 15 kts27.78 km/h
    7.71 m/s
    , increasing with height.
  • Stable air above the crest of the ridge with less stable air above and a stable layer below the ridge.
Leewaves
Mountain waves

Vertical currents within the oscillations can reach 2,000 ft/min10.16 m/s. The combination of these strong vertical currents and surface friction may cause rotors[1] to form beneath the mountain waves causing severe turbulence.

Effects

Mountain Waves are associated with severe turbulence, strong vertical currents, and icing.

  • Loss of Control and / or Level Bust. The vertical currents in the waves can make it difficult for an aircraft to maintain en route altitude leading to level busts and can cause significant fluctuations in airspeed potentially leading, in extremis, to loss of control. Loss of Control can also occur near to the ground prior to landing or after take off with a risk of terrain contact or a hard landing if crew corrective response to a downdraft is not prompt.
  • Turbulence. Aircraft can suffer structural damage as a result of encountering severe clear air turbulence. In extreme cases this can lead to the break up of the aircraft. In even moderate turbulence, damage can occur to fittings within the aircraft especially as a result of collision with unrestrained items of cargo or passenger luggage. If caught unaware, passengers and crew walking around the aircraft cabin can be injured.
  • Icing. Severe icing can be experienced within the clouds associated with the wave peaks.

Defences

Lenticular Clouds
Lenticular Clouds (lens shaped clouds)
Lenticular clouds over Luino, Italy, phoptographed on 17 March 2008. Source: Jacob Kollegger, RMetS.
  • Awareness.
    • When approaching a mountain ridge, it is advantageous, if heading upwind towards it, to cross at an angle of around 30 - 45 degrees in order to allow an escape should downdrafts prove excessive.
    • In the Alps regions, particularly in the Zurich – Milano regions, a general rule of thumb that a QNH difference of more than 5 – 8 mb between LSZH and LIMC, for example, or between north and south of the Alps, will provide for significant mountain wave activity over the Alps. A higher QNH in Zurich will result in mountain waves south of the Alps, for example.
    • If significant mountain wave activity is expected, as a rule of thumb and if possible plan a flight at least 5000 – 8000 feet above the highest elevation along your route.
  • Forecasting. Local knowledge of the conditions which tend to cause the formation of mountain waves enables forecasting of potential wave propagation.
  • Cloud Formation. Lenticular Clouds (lens shaped clouds) can form in the crest of the mountain waves if the air is moist. Roll Clouds can also occur in the rotors below the waves if the air is moist. These clouds are a good indication of the presence of mountain waves but, if the air is dry, there may not be any cloud to see. Windward of the mountains IMC conditions may likely be present, whereas due to the “Foehn Effect” VMC conditions are generally expected to the leeward.
  • Restraint Systems. Passengers and flight crew should routinely wear their seat belts / harnesses when seated to provide protection against unexpected turbulence.

Scenarios

An aircraft tracking perpendicularly across, or downwind of, a mountain range or a significant mountain ridge experiences a sudden loss of altitude followed by a significant and sudden reduction in airspeed during severe turbulence. Regaining the desired flght path may be difficult, for a relatively short period, until the wave is exited.

Solutions

  • Airspeed. Reducing the aircraft speed reduces the risk of structural damage and reduces vibration making instruments easier to read in turbulence BUT beware the effect of vertical currents on airspeed and the risk of stalling the aircraft.
  • Strap in. Aircraft commanders should ensure that if conditions in which there is a possibility of encountering mountain wave turbulence are envisaged en route that not only are seat belt signs selected on but that all cabin crew are instructed to cease service and secure their equipment and themselves in good time.
  • Inform ATC. Notify ATC of mountain wave activity.

Related Articles

Accidents & Incidents

The following reports include reference to Mountain Wave activity:

  • AT75, vicinity Cork Ireland, 2014 (On 2 January 2014, the crew of an ATR 72-212A lost forward visibility due to the accumulation of a thick layer of salt deposits on the windshield whilst the aircraft was being radar positioned to an approach at Cork on a track which took it close to and at times over the sea in the presence of strong onshore winds. The Investigation concluded that the prevailing strong winds over and near to the sea in relatively dry air with little visible moisture present had been conducive to high concentrations of salt particles at low levels.)
  • A332, en-route, near Dar es Salaam Tanzania, 2012 (On 27 February 2012, the crew of an Airbus A330 en route at night and crossing the East African coast at FL360 encountered sudden violent turbulence as they flew into a convective cell not seen on their weather radar and briefly lost control as their aircraft climbed 2000 feet with resultant minor injuries to two occupants. The Investigation concluded that the isolated and rapidly developing cell had not been detected because of crew failure to make proper use of their weather radar, but noted that activation of flight envelope protection and subsequent crew action to recover control had been appropriate.)
  • AS32, en-route, North Sea UK, 2002 (On 28th February 2002, an Aerospatiale AS332L Super Puma helicopter en route approximately 70 nm northeast of Scatsa, Shetland Islands was in the vicinity of a storm cell when a waterspout was observed about a mile abeam. Soon afterwards, violent pitch, roll and yaw with significant negative and positive ‘g’ occurred. Recovery to normal flight was achieved after 15 seconds and after a control check, the flight was completed. After flight, all five tail rotor blades and tail pylon damage were discovered. It was established that this serious damage was the result of contact between the blades and the pylon.)
  • C404, Kulusuk Greenland, 2002 (On 1 August 2002, a Cessna 404, en-route at FL130 over Greenland, experienced sudden power loss on both engines, probably as a result of ice in the induction systems, leading to loss of control. The crew regained control at 3000 feet.)
  • B732, vicinity Washington National DC USA, 1982 (On 13 January 1982, an Air Florida Boeing 737-200 took off in daylight from runway 36 at Washington National in moderate snow but then stalled before hitting a bridge and vehicles and continuing into the river below after just one minute of flight killing most of the occupants and some people on the ground. The accident was attributed entirely to a combination of the actions and inactions of the crew in relation to the prevailing adverse weather conditions and, crucially, to the failure to select engine anti ice on which led to over reading of actual engine thrust.)
  • C501, vicinity Trier-Fohren Germany, 2014 (On 12 January 2014, the crew of a Cessna 501 on a private business flight with a two-pilot crew attempted to make an unofficial GPS-based VNAV approach in IMC to the fog-bound VFR-only uncontrolled aerodrome at Trier-Fohren. However, after apparently mis-programming the 'descend-to' altitude and deviating from the extended centre, the aircraft emerged from the fog very close to the ground and after pulling up collided with obstructions, caught fire and crashed killing all occupants. The Investigation noted an apparent absence of pre-flight weather awareness beyond the intended destination and that there was a suitable fog-free diversion.)
  • B738, Delhi India, 2014 (On 5 January 2014, a Boeing 737-800 operating a domestic flight into Dehli diverted to Jaipur due to destination visibility being below approach minima but had to break off the approach there when the aircraft ahead was "substantially damaged" during landing, blocking the only runway. There was just enough fuel to return to Dehli as a MAYDAY flight and successfully land below applicable minima and with minimal fuel remaining. The Investigation found that a different alternate with better weather conditions would have been more appropriate and that the aircraft operator had failed to provide sufficient ground-based support to the flight.)
  • A321, en-route, Gimpo South Korea, 2006 (On 9 June 2006, an Airbus 321-100, operated by Asiana Airlines, encountered a thunderstorm accompanied by Hail around 20 miles southeast of Anyang VOR at an altitude of 11,500 ft, while descending for an approach to Gimpo Airport. The radome was detached and the cockpit windshield was cracked due to impact with Hail.)
  • GLF3, Biggin Hill UK, 2014 (On 24 November 2014, the crew of a privately-operated Gulfstream III carrying five passengers inadvertently commenced take off at night in poor visibility when aligned with the runway edge instead of the runway centreline. When the aircraft partially exited the paved surface, the take-off was rejected but not before the aircraft had sustained substantial damage which put it beyond economic repair. The Investigation found that chart and AIP information on the taxiway/runway transition made when lining up was conducive to error and that environmental cues, indicating the aircraft was in the wrong place to begin take-off, were weak.)
  • MD81, Kiruna Sweden, 1997 (A scheduled passenger flight from Stockholm Arlanda to Kiruna left the runway during the night landing at destination performed in a strong crosswind with normal visibility.)
  • B732, vicinity Islamabad Pakistan, 2012 (On 20 April 2012, the crew of a Boeing 737-200 encountered negative wind shear during an ILS final approach at night in lMC and failed to respond with the appropriate recovery actions. The aircraft impacted the ground approximately 4 nm from the threshold of the intended landing runway. The Investigation attributed the accident to the decision to continue to destination in the presence of adverse convective weather and generally ineffective flight deck management and noted that neither pilot had received training specific to the semi-automated variant of the 200 series 737 being flown and had no comparable prior experience.)
  • SF34, en-route, Santa Maria CA USA, 2006 (On 2 January 2006, an American Eagle Saab 340 crew failed to notice a progressive loss of climb performance in icing conditions and control of the aircraft was lost when it stalled at 11,700 feet and was only recovered after a 5200 feet height loss. The Investigation noted that the aircraft had stalled prior to the activation of the Stall Protection System and that the climb had been conducted with the AP engaged and, contrary to SOP, with VS mode selected. It was concluded that SLD icing conditions had prevailed. Four Safety Recommendations were made and two previous ones reiterated.)

"Mountain Waves" is not in the list of possible values (In Flight Airframe Icing, In Flight Icing - Piston Engine, In Flight Icing - Turbine Engine, CAT encounter, En route In-cloud air turbulence, Hail damage, Volcanic Ash Effects, Fog, In Cloud on Visual Clearance, Precipitation-limited IFV, Strong Surface Winds, Lightning Damage, Low Level Windshear, In Flight Icing - Turboprop Engine, In Flight Icing - Jet Engine, Sand/Dust limited Forward Visibility, Mountain Wave/Rotor Conditions, Triggered Lightning Strike, Layer Cloud Airframe Icing) for this property.

Notes

  1. ^ Rotor: a turbulent horizontal vortex generated around the "troughs" of mountain wave activity (see diagram above).