There are two divisions of mountain wave: Trapped and vertically propagating. Trapped wave assume the form of a series of waves running parallel to the ridge and can extend many miles downwind of the ridge. They occur when the wind increases {and stability deceases} with altitude (figure 7.9). The increasing wind traps the wave against the earth's surface.
With vertically propagating waves (figure 7.7) the energy is directed upwards instead downstream. They occur when the wind speed is constant (does not increase) with altitude and the wavelength is shorter than the base of the mountain. Wavelength is proportional to wind speed so, expect vertically propagated waves with low or moderate wind speeds (higher stability also gives shorter wavelengths).
Trapped Wave
I had a couple interesting wave flights this season. The first was on
May 5th in a trapped wave. I was able to climb to 15,000' right over Post
Mills Airport and then travel up wind 4 wavelengths to the Sugarbush
airport. At Sugarbush I climbed to 18,000'. From
there I was able to cross the spine of the Green Mountains and climb
back up to 18,000' over the flat Champlain Valley. Pushing ahead 2 more
wave crests put me over the middle of Lake Champlain. From there I
could see small cumulus clouds making wave crests
all the way to Lake Placid. The day had the ideal amount of clouds.
They marked the wave but did not interfere with navigation. This was a
trapped mountain wave. The wave generated by Sugarbush (and Mt. Marcy
near Lake Placid) was tapped close to the
earths surface as it traveled downwind many cycles to Post Mills. The
characteristic wind profile for trapped wave is high winds and winds
increasing with attitude.
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| May 5th Flight - Colored lines show the peak of each climb. |
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| October 5th Radiosonde data from Albany at 8:00 AM - Note the increasing winds from 5,000' to 30,000'. |
Vertically Propagating Wave
The second interesting flight occurred on October 9 during the Mt. Washington Wave Camp in a vertically propagating wave. The vertical wave allowed a climb to over 32,000'. The wind profile showed relatively lighter winds (about 25 kts at 6,000') and the wind speed did not significantly increase from 5,000' to 50,000' (always below 50 kts). Without the increasing
wind (with altitude) forcing the wave along the surface it is free to
oscillate up into the upper atmosphere. The wave length was very short with the primary lift over the Auto Road parking lot.
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| October 9th Flight - Yellow line shows the peak of the climb over the Mt. Washington Auto Road parking lot. |
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| October 9th Radiosonde data from Albany at 8:00 AM - Note the light consistent winds from 5,000' to 50,000'. |
The only other time I have experienced a vertically propagating wave was on October 10, 2011. This was probably the best day in modern wave camp history. There were
many spectacular flights.
Rick Roelke had a triple diamond flight (the 3 legs of a diamond badge in one flight).
Evan went to 26,000' and then flew 440 km in 8 hours.
Jim David went over 29,000' (he flew all the way down to Mt. Ascutney but didn't find wave along the way because it was vertical wave day). I think there were 5 new Lennies that day. Below is my flight in PM on that day. It shows the pattern of lift found in the lee of Mt. Washington that can be seen in all the flights from that day. The red arrow shows the location of the top of climb in the primary. It is right over the Auto Road parking lot (very short wavelength). The primary only went to about 13,000'. To go higher you had to drop back to the secondary over the top of Wildcat (green arrow). In a vertically propagating wave the secondary can go much higher than the primary (see figure 7.7). The wind profile from that day was very similar to October 9 of this year.
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| October 10, 2011 - Red arrows are the top of the primary wave. Green arrows show the top of the secondary wave. |
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| October 10, 2011 - Radiosonde data from Albany at 8:00 AM - Note the light, relatively consistent winds from 5,000' to 50,000'. |
In conclusion we need a trapped wave to fly out of Post Mills. We are many wavelengths downwind of the Green Mountains. We want high winds and we want the wind to increase with altitude. The best days at Mt. Washington (long XC and high altitudes) are going to be those with rare vertically propagating wave. We want lighter winds and we don't want the winds to increase with altitude. The best lift may be in the secondary.
References:
Figure 7.7 and 7.9 are from
Dieter
Etling: Atmospheric Gravity Waves and Soaring Flight - Physical principles and practical applications
available for free at:
https://www.schwerewelle.de/
