Pre-Frontal and Partial Lee: The makings of a bipolar variometer

The forecast ...

Winds:
0m: light and variable
1000m: 240 @ 8 kts becoming 260 @ 12+ kts post noon

Lapse Rate:
0m to 700m: -3.0 C/200m (unstable).
700m to 1200m: -1.5 C/200m (inverted)

Cloud Cover:
None expected over terrain under 1200m due to strong inversion.

Flying picture:
Approaching warm front expected to shutdown strong thermic activity with lowering cloud base but providing good to strong ridge lift. Use of Harvest Market bailout LZ a few kilometres behind launch expected.

The reality ...

We arrive on launch  @ 11AM to find the cycles blowing straight in from south west and increasing in strength and duration. Occasionally, the last remnants of the nightly catabatic outflow from Harrison Lake would make its presence known with a north cross at the bottom of launch, while smoke in the valley indicated that the increasing meteorologic wind was not yet a concern, likely held in check for the time being by the inversion.  No cloud development over launch rounded out the reassuring synoptic picture.

Knowing that winds were only going to increase as we passed noon, I quickly kit up in the wind shadow behind launch, rosette up the wing, and proceed a third of the way down the slope. The cycles were making laying out the wing unwise, requiring a progressive A and C riser inflation straight from the rosette.

Those last remnants of catabatic outflow? Not so last remnant as one might think. The wind sock at the base of launch was still showing cross cycles while the sock at the top was showing straight in.

Timing would be key to a successful launch. Launch during a cross cycle and I would end up kicking tree tops.

And so I start the waiting game, watching the upper and lower socks and anticipating the moment they would reach consensus.

A few minutes pass, the lower sock begins to show straight in while the upper is dead still. The cycle makes its presence felt while my fingers wrap tighter around the A riser mallions. The cycle grows and I bring the wing overhead, keeping it in check by softening tension on the Cs.

Turn.

One step.

A gust.

Pause.

Kite the wing, allow it to settle following the sudden change in airspeed.

Two more steps, airborne, into the pod ...

... and right into the claws of the sink monster.

I immediately begin extracting feet from the pod, thinking I might have snagged the speed bar (more like speed rope). No dice, but the wing planes out and I no longer need to pick between tree LZs A, B or C.

And proceed directly into the arms of the lift monster followed quickly by another visit from the sink monster.

Two more times, I rapidly yo-yo between +-4.0 m/s, my variometer concurrently sounding lift while showing sink (and vice versa). A Hand and Hammer of God type of flying experience.

Some might call managing a wing under these conditions active piloting, me, personally, would call it spastic puppet chair dancing. In either case, the wing remained overhead with little oscillation and nary a collapse.

Fleeing what is obviously a partial lee situation coupled with sharp edged spring thermals, I make my way to the LZ and land.

A short flight but an educational one. +1 for the judgement bucket.

The picture below identifies the unusually strong catabatic outflow from Harrison Lake rotoring over the ridge to the north of launch (in blue) versus the thermic activity making its way up the southwest face of the mountain (in red).

This is a classic partial lee situation and can be very dangerous, a trap lying in wait for unsuspecting pilots. One enters it expecting to find a consistent thermal generator only to be caught in a washing machine of particularly violent rotor.

Learning More

Kelly Farina @ Austrian Arena examines dangers of partial lee flying in one episode of his video series 'Understanding the Alps'. It would benefit every pilot to watch this video and become familiar with both the risks of partial lee and the techniques required to identify it in advance.