Tutorial: Mountain Wave for Airplane Pilots

Strong mountain wave can present several distinct hazards to airplanes, but airplane pilots typically aren’t well-informed about wave phenomena.  This tutorial briefly summarizes these hazards and provides guidance for minimizing them.

A strong mountain wave system potentially presents four types of hazards:

1. Turbulence

2. Sink

3. Lift

4. Icing

 

Turbulence

The turbulence can be found in any of four distinct regions:

  1. At or near the surface, as low-level turbulence
  2. In the rotor, if present
  3. At the tropopause
  4. In the form of mid-level turbulence

 

The low-level turbulence due to the interaction of locally strong winds with the surface have little to do with the wave itself, but merely accompanies it.  However, when waves are present there’s a good chance that downslope winds, that in effect bring high-level winds aloft to the surface, will enhance the local turbulence.  Use your standard windshear countermeasures, but be alert to the possibility that a diversion to another airport may be the best option.

Even in strong waves, rotor isn’t always present.  One important parameter is the character of the terrain underlying the wave crest; if the valley is narrow, so that the slope of the terrain broadly parallels the wave flow, rotor is much less likely.

Wave flow is from left to right in this photo taken near the Air Sailing gliderport.  Because of the slope toward Tule Peak (right, in this photo) rotor is entirely absent in this wave.

Rotors, if present, come in two varieties:  Type I rotor, the classic eddy formation beneath each wave crest with axes parallel to the mountain range, and Type II, which is very similar to a hydraulic jump in a water channel.

Type I rotors rarely reach more than 2000 feet above the ridge line, whereas Type II rotor, which is far more likely to generate severe or extreme turbulence, can on occasion extend as high as FL300.

Type II rotor tends to accompany waves that don’t form trains of successive wave crests.  The synoptic discriminant appears to be the character of the winds aloft profile:  a steeper wind gradient “traps” the wave energy into a channel with a distinct upper boundary (this is also why these waves form successive downstream crests) whereas a shallow or nonexistent wind gradient favors the single-wave/hydraulic jump form.

A Type II wave and rotor system generally is set further back from the mountain range, often picks up dust (especially in the Great Basin) from the valley floor, and the rotor clouds or dust tend to be straight rather than following any bends in the mountain range.  Often the only visual clue is a sharp line of dust rising vertically from the surface:

Recommended Type II rotor penetration altitude is FL250-300.  The original Sierra Wave Project recommendation was FL250, but nowadays there’s a lot more traffic at FL250, so being slightly higher would be prudent.

The turbulence at the tropopause at the top of the wave system is simply the shear zone such as is often encountered near any inversion.  (The lower portion of the stratosphere is generally a deep isothermal layer, and thus acts like an inversion to inhibit mixing.)  The hazard it presents is mainly to cabin crew.  Turn the seatbelt sign ON and consider avoiding the altitudes within 2-4000 feet of the tropopause.

Mid-level turbulence in wave is associated with wave “breaking” which is just what it sounds like:  rather than continuing as a smooth sine-wave undulation, the wave breaks in much the same way as waves become breakers at the beach.  This sort of turbulence, like rotor, is characterized by very rapid onset:  as the flight progresses, it cuts through the very sharp boundary between smooth and turbulent air with very little warning.  The best way to minimize the hazard is to check the winds aloft forecast for an abrupt reduction in wind speed, or even a reversal, at altitude.  Any breaking waves will form at the altitudes with these abrupt reversals or reductions.

Sink

Strong sink is a hazard primarily to piston aircraft and is most hazardous to flights attempting to fly upstream.  Coupled with the strong headwinds, it’s easy to lose a surprising amount of altitude over a rather short distance.  Under these conditions it may be useful to accept the fact that your airplane is effectively a glider, and to adopt glider pilots’ tactics.  Rather than lifting the nose and flying slowly upwind in the sink, attempting to maintain altitude, consider flying faster and accepting a greater sink rate.  You’ll lose more altitude per minute, but spend less time in the sinking air, so your overall altitude loss is likely to be reduced.

Strong sink can also be a hazard to turbine-powered aircraft, as explained in the next section.

Lift

Strong lift is a hazard primarily to turbine equipment.  At high altitudes the “coffin corner” (the indicated airspeed band between low speed buffet and never-exceed speed) can be quite narrow, and there just isn’t much room for an airspeed increase.  In strong lift it’s possible that if the aircraft is flown at a constant altitude, even at idle thrust the never-exceed speed may be exceeded.  Conversely, on some aircraft types extending the speed brakes may result in an inadvertent excursion into the low speed buffet and a probable altitude loss.

Note the narrow interval between high- and low-speed limits:  in this photo, a mere 27 knots!

You can avoid these hazards by either requesting a block altitude, so as to allow room for the aircraft to drift up or down in lift and sink, or by requesting clearance to a lower altitude where your buffet margins will be more generous.

Icing

Icing in wave clouds can be hazardous for two reasons.  One is due to the stable and laminar nature of the atmosphere in mountain waves, water droplets carried through the freezing level can exist in a supercooled state, permitting very rapid ice build-up.  The other is the fact that when transiting a wave system ice may be deposited unexpectedly because of the rapidly varying potential temperature of the air at any given constant altitude.  An ice-free altitude outside the wave system may well harbor icing conditions within the wave, so complacency isn’t an option.  Keep a close eye on your SATs and follow your flight manual guidance accordingly.

In this photo, the lenticular clouds have long “tails” because the ice crystals take much longer to sublimate than liquid water takes to evaporate, and so they’re carried a distance downstream.