Cold Front and Mountains A Doppler Radar View March , 2009 Case Mark Pilon

Observation Sites VOD... located to the north and a little above the Cross Country Stadium. It has a reasonably good exposure with a gradual rise in terrain to the north and east which steepens significantly beyond 1/2 km. VOY... located immediately west of the Biathlon shooting range in a narrow valley gap that is bounded by a steep tree covered ridge to the north and a much smaller forested mountain to the south. For this reason, winds at this site are from two primarily directions, east and west. VOW... located just north of the "lift-off" portion of the Ski Jump, it is around the mid level of the westward facing slope of a steep ridge. It was recently relocated to the ski jump coaches stand which may have improved its exposure, however trees, the ridge, and the ski jump ramp itself, likely reduce the full impact of the wind at that level. VOX... located at the bottom of the Ski Jump, this is the newest of the instruments with a limited amount of historical data. The best exposure for this instrument is from about degrees with the steep ski jump ridge to the east and considerable tree cover in the remaining sectors, however there is a ski path cut immediately to the north of the instrument that exposes the site to northerly winds.

Mar. 15, VVO 11.0 Doppler 0430 UTC0400 UTC0330 UTC Surface winds increase sharply between 04 and 05 UTC at Callaghan VOD. Note at this time upper level winds are actually decreasing in strength. However winds are increasing in the low levels – this is evident near the origin of the radar where the beam is close to the ground. See next slide for low angle beam.

Mountain Weather Scale ~90 min Similar to Summer Convective Scale!

Mar. 15 VVO 2.0 Doppler This animation shows the low level winds increasing from about 6 m/s (darker blues) to over 12 m/s (purples) in the sea to sky valley

Threshold for Biathlon Thresholds for Jump

WSK-Whistler profiles Yellow lines mark purple vs blue and red vs pink contours. Note that: 1.winds are towards the radar from both 187 and 73 degrees in low levels. 2.Layering of winds is evident indicating fairly stable stratification

Red arrow marks approach of a front moving up the valley Area of heavier precipitation associated with front

Stronger winds are beginning to descend in the Callaghan to Squamish segment ahead of frontal feature.

Reaches Radar Approx 0410 UTC. Main area of precipitation gets “lost” as it goes over the radar

24 km in 30 mins ~ 50 km/h

Total lead time – assuming forecaster notices feature right away: ~ 20 mins? At this time layer appears to be well mixed with fairly uniform winds – vertical stratification has disappeared.

Squamish-Whistler

Callaghan OAN Winds Feature reaches Whistler around No noticeable effect on the winds

Whistler Alpine stations show effects after 0500 UTC with more variability in speed and direction. In some of the data there seems to be an abrupt discontinuity in the trends shortly after 0500 UTC.

Why the hour difference between Callaghan and Whistler winds? Passage of front marked beginning of a series of changes Change in Whistler winds may have been affected by other factors associated with front Front is not quite a sharp discontinuity in weather Took over an hour for all of these changes to occur. This is evident from the Doppler seen below

1.Changes in wind field take 40 mins to stabilize from 0400 UTC to 0440 UTC 2.lessening of precipitation coverage takes over one hour

Callaghan Valley

Time front reached VVO radar (0410)

10 mins later – stronger winds have surfaced

Threshold for Biathlon Thresholds for Jump

March 20, 2009 Case

Is there an enhanced flow or LLJ up the Valley? One has to be careful in interpreting these RHIs – a reduction in wind “strength” may actually be simply a shift in wind direction!

These high ridges parallel the wind and seem to block it and give a “V” notch pattern

Does the enhanced flow / jet impact winds / precipitation? Was the wind event at the Callaghan due to a descending jet or enhanced flow? In this case the airmass was not well mixed after the onset of winds. Some of the strong wind events may be due to descending jets rather than mixing down of stronger winds.

Light winds, smooth flow, max winds above 1 km Winds pick up at VOD, smooth flow, appearance of LLJ / lowering of winds aloft. Winds stregthen at VOD, smooth flow, LLJ below 1 km Winds Diminish at VOD, smooth flow. LLJ very weak or moved away. Max winds above 2 km Strongest winds at VOD, note the turbulent flow. No vertical layering of winds evident. Max winds mixed down to surface. Note on the Whistler cross section at this time a couple of slides down that there must still be a stable layer of air trapped in the valley as winds remain stratified.

There is a big change in the appearance of the wind field after 22/23 UTC in the wake of the cold front – it is much more turbulent

Whistler Winds

Passage of front not clear from Doppler winds Perhaps winds were so strong signal was lost. Passage of front more evident in reflectivity. Assumed to pass WSK 2300 and Whistler 2330 UTC.

In next set of slides effect of front is visible by reduction in intensity of precipitation.

Circle marks possible location of front

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March 17, 2009

March 17/06z – VOY 04G08 m/s March 17/07z – VOY 05G10 m/s Beiges here not evident on next slide. Jet is below 1 km

March 17/06z – VOY 04G08 m/s March 17/07z – VOY 05G10 m/s Note the presence of the v notch and the jet south of the jump. Though the winds appear lighter at the jump they are not. The cross section in the previous slide shows that the jet is tilting towards the bottom left as one goes lower in elevation.