Saturday, November 01, 2008

First Mountain Wave - well, hill wave anyway

Very interesting weather at Hemet today. A low pressure system to the east was to bring rain in by evening, and there were lots of thin cumulus clouds hanging around. The thermal forecast looked good, though no CU were forecast. The temperature profile looked good - no inversion at all. Driving in, I could see occasional little lenticular clouds, indicating wave activity. Huh? There were big "lennies" on the west side of Mt. San Jacinto and smaller ones to the north and west of the valley, but only CU and clear areas over the valley. The tricky part for me has been to figure out from the lennies which direction the wind is actually blowing, and therefore where the upwind side should be. The sounding map showed the wind at about 240, and driving in I was able to observe the trailing edge of a lennie dissipating, and that was the northeast side, which confirmed for me that the wind aloft was from the southwest.

The thermals were only supposed to go up to 7300' MSL, not high enough to reach the big lennies... but maybe the little ones?

The temperature was about 90F, higher than forecast, and way over trigger temperature. The wind at ground level was 13 to 16 knots, and was forecast to be about 20 at 5,000'. By the time I took off in the PW5 about 2:00, the sky was 90% obscured, and I was afraid thermal activity would shut down. But I let off tow in lift at 4500, and worked up to about 5500, and headed southwest toward the closest cloud that looked lenticular. To get there I had to head directly into the wind (approaching what I thought would be the lift area by flying under the cloud - watch out for rotor!). That's usually a killer for altitude, but I was in zero sink much of the way. Groundspeed was way slow! (I didn't bring my PDA, so I didn't have any true wind or groundspeed info.)

The cloud was further away than I thought, beyond the hills beyond the little town of Winchester. Thinking there might be rotor directly under the cloud, I skirted the edge between it and the next cloud. Wrong idea: there was no lift, even a little sink between the clouds, and the zero-sink or 1-knot lift had been under the cloud. I turned around and headed back toward the airport. I was fairly low, but I knew I'd be flying downwind to get back, so I would not lose too much altitude.

Close to the airport, I found lift again and worked it up to 5800'. My drift in the thermal confirmed the wind direction. I headed off toward the lennie again. This time, I was starting higher and closer, so I had a better chance of making it all the way. The minor lift was there again, all the way over to Winchester and beyond. I stayed under the cloud and drifted up, though I never reached cloudbase (which AWOS reported as 7,500'). But the cloud was still further southwest of the airport than I was comfortable going, so I did not go all the way to the leading edge. I eventually turned back and headed home, planning for about a one-hour flight duration.

Under a cloud, it's hard to see the actual shape, but earlier I had been able to see it was a combination of lenticular and CU: smooth and curved at the upwind side, then broken and puffy and going upward on the downwind side. And the next cloud over was definitely smooth and lens-shaped, clearly a wave cloud. So I conclude that I was flying in wave lift, not "cloud suck". It was not as smooth as most wave is described, but then I was not at the leading edge. And these clouds did not quite fit the classic lennies caused by wind blowing across a ridge. The hills upwind that created these waves are small and isolated, so the waves were not wide areas, more like small points. Since I was only up at between 5 and 6 thousand feet, I also conclude that the wind was probably bouncing off the ground, not off a stable air layer. So when I was flying under the cloud, I was essentially flying upwind in a huge updraft between the ground and the condensing moisture. Really a strange flight: I flew about 7 miles directly into a 20+ knot headwind, and actually went UP!

Coming back to the airport, I hit some fairly strong turbulence and some 8- to 10-knot sink. It was either rotor, or all that air that went UP coming back DOWN.

The thermal forecast certainly got the condensation level wrong - it forecast 18,000' and the cloudbase was actually 7,500.

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