Lee waves

Lee waves – a danger for aviation?

Tim Takeoff
27.04.2020
4 minutes

Especially on stormy days the enormous energy of the wind causes numerous weather phenomena. One of these are so-called lee waves on the lee side of a mountain range. They reach high altitudes and consequently also affect commercial aviation.

Even with moderate winds from 20 knots and, given certain basic conditions, vibrations are generated in the atmosphere. These can be caused either by different air stratifications or corresponding terrain.

Of water and air

The air that we breathe basically behaves exactly like the medium “water”. It has a certain density and consists of smallest particles. If we stick to the idea of water, it encounters obstacles when flowing down the course of a river. Branches and stones create waves behind them. Also, different currents in the ocean promise recurring wave movements.

Upwind (luv) and downwind (lee)

These waves can be larger or smaller, wider or narrower. Exactly the same thing happens in our atmosphere. Large air masses flow over a mountain range. The air is forced to rise on the windward side. This uplift often causes dense clouds (accumulation of clouds). However, the “other” side, the side facing away from the wind, the “lee”, is much more interesting.

Lee waves
Very clearly visible cloud formation of lee waves / © Wikimedia Commons Torsten Paul

A hydraulic process

In the lee of the mountain, the air now has the tendency to surrender to the force of gravity. It falls, bounces off near the ground and develops dynamics of its own. The air mass begins to oscillate. This phenomenon is called “lee waves”, or “gravity waves”. It is a purely hydraulic process, which can be easily explained in physical terms. Depending on the stability of the air mass, the size of the mountains, wind direction and force, this oscillation can be very variable.

Gliders have been taking advantage of this phenomenon for a long time. They literally “surf” in the upwind range of the wave up to enormous heights. In the Andes of Argentina or the Rocky Mountains, high altitude flights of up to 8 kilometres are not uncommon. By comparison, a commercial airliner cruises at an altitude of about 10 kilometres.

In this video you can see some impressive pictures of a wave gliding flight. Again and again very easy to recognize: the wavelike cloud formations:

Origin

If you take a closer look at such a lee wave, it can be divided into three main areas. On the windward side (luv), energy is supplied to the wave, creating an upwind area, which is opposed to the downwind area. The upwind area has an absolutely calm, so-called “laminar” flow, and little or no turbulence is to be expected. It is more dangerous in the area below the sinusoidal curve of the wave. Here the air is strongly swirled and one speaks of “rotors”.

If the air mass is moist enough, you can see lee waves very clearly from the cloud image. Dust cloud on the windward side of the mountains, cloud resolution on the leeward side. Often you can see this on the north side of the Alps when the “Südföhn” (Southern Foehn) is blowing. Humid air masses from the south are pressed over the main ridge of the Alps through a Genoa low. On the south side there are densest clouds, from which heavy rain often falls. In the process of overflow and fall on the north side, the air mass dries out more and more. In the northern area a warm, dry wind from southern directions is thus created.

Rotors as a danger to aviation

In the lower lee area you can discover smaller cumulus clouds. They appear again and again at the same place, then turn around their own axis and literally “roll out” with the wind to the back to dissolve. It is clearly a “rotor cloud” and thus the turbulent area below the wave.

UFOs?

If you look at the higher atmosphere, you can often see the “tip” of the wave. Flat, lenticular clouds form there. Many a discoverer of alleged “UFOs” has certainly seen such a cloud before. In technical terminology these clouds are called “lenticularis”. They consist mainly of ice crystals and are primarily harmless for flying.

Depending on the wind stratification of the atmosphere, such waves can even overlap each other and create real wave systems. Multi-storey lenticularis draw imposing cloud formations into the sky, below which, however, extreme turbulence can be expected even without signs of rotor cloudiness.

Lenticularis - cloud - windward - lee waves
A lenticularis cloud above the Essener and Rostocker Hütte – Hohe Tauern, Austria / © Wikimedia Commons Rosebud 23

New prediction models to increase safety

In order to minimise this threat to commercial aviation, meteorologists are successfully developing ever newer forecasting models. Vibration predictions are made to highlight areas of turbulence in the pilots’ weather charts. Since a lee wave is very dynamic and never the same, these predictions are not easy to make.

There have already been several incidents of extreme turbulence that have caused serious injuries to people on board commercial aircraft. During these incidents, the aircraft was flown into a highly turbulent area of the lee waves. This can happen in all phases of the flight near mountainous terrain.

PIREPS and Perlan

It shows once again how energy-rich our atmosphere is. As a pilot you should always be alert and rely on so-called “Pilot Reports” (short: PIREPS). On the weather radar, air traffic controllers inform themselves about possible turbulence zones, which they can pass on to all aircraft at the affected altitude.

We reported on the “Perlan Project” some time ago. A team of scientific glider pilots is trying to investigate lee waves more closely and take advantage of them. Commercial aviation is expected to benefit from this in the near future. In 2018, a new world record in gliding was set: 23,200 meters.

Lenticular cloud
Lenticular cloud above Madeira / © Wikimedia Commons KarlHeinz Essl

Cover picture © Wikimedia Commons Torsten Paul

by Tim Takeoff

Related Posts