Thursday, November 15, 2018

Trapped and Vertically Propagating Wave


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.

May 5th Flight - Colored lines show the peak of each climb.

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.


October 9th Flight - Yellow line shows the peak of the climb over the Mt. Washington Auto Road parking lot.

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 flightsRick 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.

October 10, 2011 - Red arrows are the top of the primary wave.  Green arrows show the top of the secondary wave.


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/

4 comments:

Tim said...

Here is what Rick Roelke said about October 10, 2011:

Well, today was Stellar. Not the typical classic wave profile where it was blowing 60kts at altitude, but all the right things came together for a delightful day. First off, Diamond Jim David, topped out at 29,000 ft. and this was on a day where I saw 10kts of wind at 12,000 ft.



It was really like no other wave day that I can remember. In the range from 12- 15k, the wind was so light that you had to circle like you were thermal flying. Very unlike “normal” wave flying. The lift in that region was not really strong, sometimes perhaps 5kts, but then, climb above that and you were back in classic wave conditions, with 9 kts of lift. The strongest lift reported today was 14kts. After some early climbs, many pilots headed of cross country. I expect there will be many interesting flights to be seen on the olc. Not all made it back, with pilots landing at dean, mount Washington regional, and even Springfield VT.



Yours truly pulled off a flight that I have been trying for years, and that is all 3 diamonds in one flight. After telling Rick Sheppe, he pointed out that it was really all the badge legs there are, in one flight. The task was Gorham to Sugarloaf, to Mousalake, to Saddleback, to Gorham. A diamond climb, 500k distance and 300k goal. Took about 6hrs, could have been done faster, but I had plenty of daylight, and that is all I was racing…



I am quite sure this if the first time it has been done in New England, and perhaps the first time east of the Mississippi…



Great Big Fun…

Moshe Braner said...

Thanks Tim. Fascinating. On a "vertical wave" day, which mountains would produce wave? When people flew XC on that day in 2011, did they find wave in other places besides right by Mt. Washington? I looked now at RR and T8 and they only climbed significantly in the Mt. Washington area.

T8 said...

On 10/10/11 we had to drop back to the secondary to climb to 25K plus and I recall the distinct "forward" lean of the zone of best lift in both primary and secondary. This makes sense in light of Fig 7.7. Like RR, I noted essentially vertical streamlines in some places that day -- circling without drift in very strong wave lift.

Great article, thanks Tim!

Evan

T8 said...

Moshe -- look again. The strong climbs on 10/10/11 were all at the Mt Wash primary, but there was lift all along the Mahoosic range, so I cruised all the way to Sugarloaf Mtn (75 miles or there about) essentially level. There was big sink out there too! Towards the end of my flight I got squashed hard in the vicinity of Old Speck because we'd had a frontal passage with dramatically increasing wind speed, longer wavelength. I was slow figuring this out and lost most of a mile of altitude before I had the thought to shift downwind to reacquire the lift. That was one of the spookier/scarier moments I've had in any sort of flying machine.

T8