Saturday, December 22, 2012

Mind the gap!

Some places, that means "don't fall in the gap between the platform and the train." In soaring, that means "don't get trapped when the Foehn gap closes under you." I've learned about it in training and read about it, but never experienced it until today.

The RASPtable forecast showed possible wave from a southwest wind. The NWS forecast showed increasing cloudiness after noon in the high desert. Both were spot on! When I arrived at Crystalaire there were lennies far to the west, but it was clear overhead. By the time I was ready to fly, wispy wave clouds were starting to form over the local mountains. I had to wait quite a while for the tow, and we watched rotor clouds forming over the foothills, and increasing wave clouds over the mountains.

I launched at about 12:45 and there was only moderate rotor turbulence on tow. The tow pilot did a great job of dragging me right into the wave lift. When things got smooth, I let off at 7,800 MSL, which at 4,400 AGL was quite a high tow for me. But it worked! I worked the wave back and forth a bit, trying to find the strongest part. It did not form a very long line, and I had the best luck staying pretty stationary over the Devil's Punchbowl and facing into the wind. I bet my groundspeed was only 5 knots.

For quite a while, the only clouds were over the mountains. By the shape of the bottom sections, they were clearly driven by wind, but the tops were not smooth like classic lenticular clouds. There was some wind shear, with Hemlholtz waves visible occasionally. We had rain earlier this week and the mountains were dusted with snow, though not as much as I expected. There were a couple other gliders exploring the wave, but not very close to me. I eventually got to 12,200' MSL, much better than I expected today!

Secondary wave clouds started forming behind me over the desert. Initially they were pretty rough but eventually turned into classic lenticular clouds. Look how smooth the top of this one is. They look like static formations, but if you ever get a close look at the upwind edge you'll see it is constantly forming as the wind moves moist air up into it, and on the downwind edge it is constantly dissolving as the wind pushes the moist air down and it re-evaporates. The smooth top indicates the boundary of fast-flowing, condensed moisture.  You can see a gap called the "Foehn gap" between the trailing edge of the clouds over the mountains, and the secondary clouds over the desert.

The "standing waves" appear to keep the clouds in pretty much the same places for quite a while, but in  fact they are constantly evolving. The gap in the previous picture disappeared at its western end, as the two clouds merged. Over about a 5 to 10 minute period, that gap got smaller and gradually closed to the east - picture a zipper closing, with me as the pull-tab! I spotted it happening pretty early on, and headed east at a pretty good speed. I was actually higher than the clouds, probably by about 1,500 to 2,000 feet. I suppose I could have hopped over the northern (secondary) wave, but what if it developed even more to the north? So I continued east (in moderate lift, not losing altitude) until I was clear of it. This picture is looking back from where I came. See the narrow gap in the middle of the picture? That was much bigger 5 minutes earlier! I had heard about this phenomenon, so I was prepared for it, but I had never seen it in action.

Along the way, I saw a "cloud bow", a bull's-eye-shaped rainbow in thin clouds below me, with the sun behind me in the opposite direction. I tried to take pictures of it, but I only had a few seconds - and I was concentrating on getting out of the gap - so they didn't come out very well.

You can see the huge difference in cloud cover between picture 1 and picture 4. That cloud growth occurred in about 45 minutes! And hmm... now the cloud was covering the airport! If it grew much bigger, I'd be stuck above the clouds far from the field. I decided to duck under the cloud so I could be sure of getting back, even if that meant cutting my flight short. At this point I was still close to 12,000' MSL, and the cloud base was probably about 8,000' MSL, so I had a lot of altitude to lose. By a combination of speeding up to 80+ knots, and using spoilers, I got down under the clouds. But I had to fight quite a headwind to get back to the airport, which cost even more altitude. I didn't get very low, but low enough that I did not have much altitude left for exploring.

A couple of other gliders and I explored the area to the west of the airport, trying to reconnect with the lower part of the wave. It was there, but not wide enough or strong enough to let me go southwest far enough to really reach the leading edge of the cloud. So I eventually landed after just over an hour. By this time the clouds were covering 70% to 80% of the sky - just like NWS had said.

All in all, a very satisfying, interesting, and challenging flight.