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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. 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

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  1. Northwest Flow Snowfall (NWFS) Events: GSP Froude Number Study NWFS Discussion Group Conference Call Harry Gerapetritis and Blair Holloway 11/19/10

  2. Froude Review • 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

  3. Study Background • 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

  4. Study Background • 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

  5. Study Background • 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

  6. 10-11 February 2010 Event

  7. 10-11 February 2010 NWFS Event • 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

  8. 10-11 February 2010 NWFS Event • 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?

  9. Season Summary Guide • 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

  10. Summary • 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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