Cold Front and MountainsA Doppler Radar View

March 13- 15, 2009 Case

Mark Pilon

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

Callaghan Winds (20090315)

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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 Winds (20090315)

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Callaghan OAN WindsFeature reaches Whistler around 0430. No noticeable effect on the winds

Whistler Alpine Wind Speed (20090315)

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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.

Whistler Alpine Wind Gusts (20090315)

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Whistler Alpine Wind Direction (20090315)

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• 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

Callaghan Winds (20090315)

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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.

Peak Wind & Hrly Pcpn

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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.

Peak Wind & Hrly Pcpn

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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

Circle marks possible location of front

Circle marks possible location of front

Circle marks possible location of front

?

March 17, 2009

March 17/06z – VOY 04G08 m/sMarch 17/07z – VOY 05G10 m/s

Beiges here not evident on next slide. Jet is below 1 km

March 17/06z – VOY 04G08 m/sMarch 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.