On 8/28/2012 7:23 PM, BobW wrote:
On 8/28/2012 11:08 AM, John Cochrane wrote:

One point not reiterated yet here -- the atmosphere down low is very
different from what you're used to at 2000 feet and above.
Snip...

A short list of what's different down low: The atmosphere is much more
turbulent. Thermals, such as they are are much smaller. In this layer,
many small punchy thermals will start. Many will die. The ones we use
up higher consist of many little parcels of hot air that have
coalesced. Most thermals are either short lived, or basically
unworkable to a modern glider. You're in the boundary layer where wind
is being affected by the ground, so there is wind-induced turbulence.
Punches of strong lift/gust followed by sink when you make a half turn
will be the norm.

The ground picture will be totally different to the pilot. If you turn
downwind at altitude, you don't notice that much. If you turn downwind
at 300 feet, all of a sudden the ground will rush by and, this being a
high stress moment, you may pull back. Just as the gust you turned in
fades, or the thermal turns to sink. And when the canopy fills with
trees going by at 70 mph, the urge to pull back will be really strong.
You may push forward to recover at altitude, but it's really really
hard to do with the ground coming up fast.

So, just because you've never unintentionally spun at altitude does
not mean your chances at 300 feet are the same.
Snip...

John Cochrane

Science now can create movies & pictures of the accuracy/reality of what John
asserts above. Roughly 10 years ago I attended a presentation that included
LIDAR movies and pictures of thermals from ground to ~1500' agl. I expect
atmospheric imaging technology has significantly advanced since then.

Apologizing for "replying" to my own post, below is an announcement for a
recent presentation - using the latest in real-time imaging technology - that
woulda likely been of interest to any soaring pilot interested in visualizing
and flying in wave conditions. Wish I coulda attended. Undoubtedly the dynamic
imagery woulda been compelling stuff...

Though written somewhat in "scientific-ese language," a not terribly
inaccurate synopsis (for you Twitter fans) might be: Dynamic wave-n-rotor
radar movies show why the wind blows hard in different directions in Wyoming.

Sorry, no links available...

Begin cut-n-insert...
For those science inclined members here is an interesting talk coming up:

Announcement will run from Wed, 08/15/2012 to Tue, 08/28/2012
Stefano Serafin and Lukas Strauss
Department of Meteorology and Geophysics, University of Vienna

On January 26th 2006, the University of Wyoming King Aircraft (UWKA)
documented the occurrence of a wave-induced boundary-layer separation (BLS)
event in the lee of the Medicine Bow Range (Wyoming). Remote sensing
measurements with the dual-Doppler Wyoming Cloud Radar (WCR) aboard UWKA
indicate strong wave activity, downslope winds in excess of 30 m/s within 200
m above the ground and near-surface flow reversal in the lee of the mountain
range. Owing to its fine resolution, the radar is also able to capture
small-scale coherent vortical structures (subrotors) embedded within the main
rotor zone.

A distinctive feature of the observed phenomenon is its unsteadiness, as
demonstrated by the BLS line moving upstream for about 8 km in approximately
half an hour. Mesoscale simulations with the WRF model at a maximum horizontal
grid spacing of 400 m reveal the dynamic forcing leading to this rapid
evolution. The upstream motion of the BLS line and of the related rotor appear
to depend on the decreasing nonlinearity of the impinging flow, which causes
the transition from a flow regime characterized by low-level wave breaking, to
another one where trapped lee waves form as a consequence of wave reflection
at an elevated neutral level. The observed upstream drift of the rotor is
shown to be dynamically consistent with the cessation of wave breaking. The
overall evolution of the phenomenon displays striking analogies with
documented unsteady Bora events, observed in the Northern Adriatic Sea.

Model simulations are verified against airborne measurements along a number of
cross-mountain flight legs, as well as against surface data. Also, a
quantification of turbulence intensity in this BLS event, using both
high-frequency in situ and radar measurements, is attempted. Given the complex
topography and the limited period of time of the observations, measuring
turbulence proves to be a challenging task. Preliminary estimates of turbulent
kinetic energy and eddy-dissipation rate along the flight trajectory will be
presented.

Tuesday August 28, 2012, 2:00 PM – 3:00PM
End cut-n-insert...