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Summary: RUC has shown a remarkable ability to predict the development
and movement of two different convergence lines, one line being
associated with a mountain ridgeline and the other one not. An
implication is that Tstorms generated by terrain-generated
convergences can also be predicted. A single comparision of predicted
convergence vs observed clouds can be viewed at:
http://www.drjack.net/BLIPMAP/CASES/LABORDAY-CONVERGENCE/prediction_vs_satellite.1z.html
I supply daily prediction maps of BL parameters to soaring pilots
(www.drjack.net/BLIPMAP), so I am interested in the ability of RUC to
predict convergence lines generated by terrain since soaring pilots
have consistently found and utilized such features. Convergence lines
appear in many forecasts, particularly over mt peaks/ridgelines, but I
am especially interested in those which move since they would seem to
be a tougher test of model prediction capability. Large convergence
line movements have been anecdotally reported by pilots and
occasionally found in the predictions, but such large movement is
infrequent - more generally both the predicted and reported
convergences remain relatively close to the mt top/ridgeline. So I
asked for pilot reports of large movement cases and on Labor Day got
such a report that I could compare to RUC predictions.
Summarized, the pilot reported launching at 1930Z into a convergence
line just west of Lake Tahoe and flying down the Sierras to Mt
Whitney, then moving east to the White Mts (which parallel the
southern end of the Sierras) and returning north along the convergence
line (which had moved eastward), finding that the northern end of the
convergence line had moved about 90 km eastward during the 6 hour
flight (the flight distance totaled over 900 km).
For that day RUC does predict such a convergence line, with excellent
agreement with the reported timing and amount of movement. The model
shows an initial early morning development of a convergence line at
the southern and western sides of the Sierras, which then extends
itself further to the north and moves eastward. The eastward movement
is more rapid to the north since in the south it remains over
Mt. Whitney until after 0Z, when it moves east to+beyond the White
Mts.
A loop showing the predicted convergence line development and movement
(the parameter being the maximum vertical velocity within the BL) can
be viewed at:
http://www.drjack.net/BLIPMAP/SPECIAL/jplayer-website.html
Satellite cloud images also confirm the model predictions (and verify
the pilot report). Moreover, the satellite indicates that early AM
convection was strong over the southern Sierras but later in the day
was stronger over the White Mts, an eastward shift in agreement with
model predictions.
A loop of satellite images at 30 min intervals can be viewe at (2.4MB
so high bandwidth required):
http://www.drjack.net/BLIPMAP/CASES/LABORDAY-CONVERGENCE/jviewer3.html
Further, examination of the satellite images reveals _another_
convergence line - at the a right angle to the Tahoe-Sierra line, at
the latter's northern end, cutting across the NW corner of Nevada> -
which also develops and moves during the day. And this second line
and its movement also agrees in location and timing with the RUC
predictions!
This second line is not associated with a ridgeline and thus differs
significantly from the Tahoe-Sierra line. I think the second line is
a downstream wake of Mt. Lassen, which lies at the NE end of CA's
Central Valley - though the line's 300 km extension from Mt Lassen is
remarkable. I expect the Tahoe-Sierra line to be a non-infrequent
event at similar mt ridgelines so likely someone has explained it
somewhere and I am just unaware - if you know of any published
explanation please let me know. I do note that the movement does
not appear to result from entrainment of westerly flow with BL
growth.
The actual convergence lines were, of course, significantly smaller in
scale than the model's 20 km resolution, so the fact that such
convergence is predicted implies that the signal is fairly strong.
The bottom line is that for this one case the RUC model has shown a
remarkable ability to predict terrain-forced convergence lines. While
the present case did not initiate any Tstorms (CAPE values were zero
in the region), the development of Tstorms at or downstream of
specific terrain features has been documented. The present case study
suggests that RUC should be capable of predicting such occurrences for
terrain it can resolve. And that with further increases in terrain
resolution increasingly more of such phenomena will be predicted.
Some details:
(1) The maximum vertical velocity occurs near mid-BL
(2) The BL top is defined using the RUC definition and is above 22,000 ft over the Sierras
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