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Northwest Flow Snowfall (NWFS) Events: GSP Froude Number Study. NWFS Discussion Group Conference Call Harry Gerapetritis and Blair Holloway 11/19/10. Froude Review. Fr = U/(N*H) U: wind speed orthogonal to the barrier N: the Brunt- Vaisala frequency (static stability)

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Northwest flow snowfall nwfs events gsp froude number study

Northwest Flow Snowfall (NWFS) Events: GSP Froude Number Study

NWFS Discussion Group Conference Call

Harry Gerapetritis and Blair Holloway

11/19/10


Froude review
Froude Review Study

  • Fr = U/(N*H)

    • U: wind speed orthogonal to the barrier

    • N: the Brunt-Vaisala frequency (static stability)

    • H: height of the barrier

  • Unitless parameter, Fr increases when U increases or N decreases

  • Therefore, stronger flow and/or decreased stability will result in larger Fr values

  • The thinking for NWFS is that when Fr is high, more flow over the mountains, more precipitation generated, more snow


Study background
Study Background Study

  • Fr archived each hour in the edit area to the right

  • 5 outputs include:

    • GFS (FroudeGFS)

    • NAM (FroudeNAM)

    • RUC (FroudeRUC)

    • Local WRF (FroudeWRF)

    • Average of the 4 (Froude)

  • For our purposes these are the inputs to the calculation

    • Barrier height = 3500 ft.

    • Orthogonal wind direction = 320 degrees

    • Averaged within the 850-950 hPa layer


Study background1
Study Background Study

  • Data collected for the 2009-2010 winter season

  • Events identified using Baker Perry’s catalog of events, synoptic classification (Perry et al. 2010)

  • 15 total events

    • 6 pure upslope NWFS

    • 9 transitioned to NWFS


Study background2
Study Background Study

  • Relied heavily on Baker’s data from Flat Springs

  • Most consistent and reliable dataset available

  • Used Baker’s start/end times to denote events and analyze Fr

  • Might result in some bias towards our northern mountains and elevations above 3500 feet



10 11 february 2010 nwfs event
10-11 February 2010 NWFS Event Study

  • Highest impact event of the season

  • Occurred with NW flow on the backside of a departing surface low

  • Event that GSP issued some Blizzard Warnings


10 11 february 2010 nwfs event1
10-11 February 2010 NWFS Event Study

  • Warning level accumulations primarily limited to the northern mountains and the Smoky Mountains

  • Perhaps because low level flow was more like 270-290 degrees, not a particularly amplified upper pattern

  • Also, maybe upstream snow cover played a role?


Season summary guide
Season Summary Guide Study

  • Snow/12 Hours – Flat Springs snow per 12 hours

  • SLR – Snow-to-Liquid Ratio from Flat Springs, NC

  • Fr Before – Average hourly Fr in the 24 hour period before the event began

  • Fr During – Average hourly Fr during the event

  • Difference – Change between Fr Before and Fr During

  • Fr_SD– Average hourly Fr standard deviation within the edit area


Summary
Summary Study

  • Season long Fr trace reveals periods of time when Fr is high but there is little or no upslope snow

  • Can have NWFS with low or high Fr but it appears that higher end events have relatively higher Fr values

  • Overall, very small sample size, especially higher end events

  • Flat Springs only had 2 periods that would have met GSP’s new winter storm warning criteria (4”/12 hours)

    • 1/2/10 07-18 UTC – 5.1” (average Fr=1.06), but no other warning level obs

    • 2/10/10 12-00 UTC – 6.4” (average Fr=2.01)

  • Maybe 12 hour Fr average ~2.00 is more indicative of heavy snow (4”/12 hours)?

  • For operational use, consider in conjunction with traditional NWFS parameters to determine event magnitude?


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