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Using The Froude Number to Improve Orographic Snow
Forecasts in the Green Mountains of Vermont
Michael J MuccilliNOAA/NWS Burlington, VT
NROW XV 2014
NROW XV 2014 – Michael J Muccilli
Top 3 Take-Aways
The Froude Number is a useful tool for determining the characteristics of Orographic Snow Events
How and Why the Froude Number works for orographic snow forecasts
How to use in Operations
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Outline
Motivation
Synoptic Overview
Define the Froude Number
Green Mountain Study
Operational Use at WFO BTV
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NROW XV 2014 – Michael J Muccilli
Motivation
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NROW XV 2014 – Michael J Muccilli
January 2nd - 3rd, 2012
Source: Weather Prediction Center
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NROW XV 2014 – Michael J Muccilli
December 28th - 29th, 2011 6
NROW XV 2014 – Michael J Muccilli
Source: Weather Prediction Center
December 28th - 29th, 2011
January 2nd – 3rd, 2012
Froude = 4.4
Froude = 0.91
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NROW XV 2014 – Michael J Muccilli
Synoptic Overview
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NROW XV 2014 – Michael J Muccilli
Terrain of WFO Burlington County Warning Area
4395 feet100 feet
15 Miles
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NROW XV 2014 – Michael J Muccilli
Synoptic Overview
Upper Level Trough or Closed Low Progressing through the Region
Source: NOAA ESRLComposite of 25 Upslope Cases
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NROW XV 2014 – Michael J Muccilli
Synoptic Overview
Surface & Low Level Pressure System exiting the Region
Source: NOAA ESRLComposite of 25 Upslope Cases
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NROW XV 2014 – Michael J Muccilli
Synoptic Overview
Increasing west to northerly flow in low levels along with lingering low level moistureSource: NOAA ESRL
Composite of 25 Upslope Cases
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NROW XV 2014 – Michael J Muccilli
Synoptic Overview
St. Jean et al. (2004) found specific important factors to the development of significant upslope snow events:
Near-Saturated Conditions from surface to ridge-top level
Strong low level winds (>10 m/s) with significant cross-barrier component
Equivalent potential temperature decreasing with height in the low levels
Event duration of at least 12 hours
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NROW XV 2014 – Michael J Muccilli
Objectives
Make it easier to identify significant orographic (upslope) snow events and placement/orientation of heavy snow using:
I. The Froude NumberII. Low/Mid Level Humidity ProfilesIII. Vertical Profile of Wind Speed & DirectionIV. Low level Stability
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NROW XV 2014 – Michael J Muccilli
The Froude Number
Potential Temperature (Surface & Mountain Top) Mountain Height Speed of Wind Perpendicular to the Barrier
Froude Number Equation Brunt-Vaisala Equation
***RESULT: A Unit-less expression that represents the flow of air when it comes in contact with a barrier (Green Mountains)
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NROW XV 2014 – Michael J Muccilli
The Froude Number
Subcritical (Blocked), Froude < 1 Precipitation likely to fall
upwind of barrier
Critical, Froude =~1 Precipitation likely to fall
along barrier
Supercritical (Unblocked), Froude > 1 Precipitation likely to fall on
lee side of barrier
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NROW XV 2014 – Michael J Muccilli
Data & Methods - Green Mountain Study 25 cases (2007 – 2012)
12 “Blocked” (Froude < 1) 13 “Unblocked” (Froude >1)
11 Stations Used 2 Champ Valley 5 West Slopes 3 East Slopes 1 East Valley
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NROW XV 2014 – Michael J Muccilli
Data & Methods - Green Mountain Study
Froude Number calculated for each event, in the mid-point of each event
Used archived NAM/RUC Soundings at KBTV After calculation, grouped by calculated Froude
Number into 8 bins 0.25, 0.25 - 0.49, 0.50 - 0.84, 0.84 - 0.99 1.0 - 1.33, 1.34 - 1.75, 1.76 - 2.0, >2.0
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NROW XV 2014 – Michael J Muccilli
Froude > 2Fr 1.76-2Fr 1.34-1.75Fr 1.0-1.33Fr 0.85-0.99Fr 0.50-0.84Fr 0.25-0.49Fr <0.25
Results 19
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Results – 3 Types of Events
Consistent with theory behind the Froude number Can then be separated into 3 Types of Events
Froude > 1: Spine and eastward (Unblocked) Froude 0.5 – 1: Heaviest Western Slopes (Classic
Blocked) Froude < 0.5: Champlain Valley (Very Blocked)
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NROW XV 2014 – Michael J Muccilli
Results – 3 Types of Events
Fr >1Fr 0.5-1Fr <0.5
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NROW XV 2014 – Michael J Muccilli
Results – 3 Types of Events
Unblocked Froude #: 4.4 Low level RH 80-90% WNW winds
throughout low levels Well Mixed to
Mountain Height
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NROW XV 2014 – Michael J Muccilli
Results – 3 Types of Events
UnblockedFroude #: 4.4
Results – 3 Types of Events
Classic Western Slopes Blocked Froude #: 0.98 Low level RH ~90% West winds veering to
Northwest throughout low levels
Isothermal layer below mountain height
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NROW XV 2014 – Michael J Muccilli
Results – 3 Types of Events
BlockedFroude #: 0.98
Results – 3 Types of Events “Champlain Valley Powder” Very Blocked Froude #: 0.07 Low level RH 95-100% Northwest winds veering to
North throughout low levels (WAA)/(Less Perpendicular Flow)
Strong Inversion below mountain height (Very Stable)
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Results – 3 Types of Events
Very BlockedFroude #: 0.07
Results – Blocked vs. Unblocked
Blocked
Unblocked
Source: NOAA ESRLComposite of 12 Blocked/13 Unblocked Upslope Cases
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Results – Snow Ratios 29
NROW XV 2014 – Michael J Muccilli
Snow Ratios were found to be much greater than average synoptic snowfall cases in CWA Long Term Average (Baxter et al 2005) – 13:1 Upslope cases averaged 28:1 Ranged from 7:1 to 71:1
Results – Snow Ratios 30
NROW XV 2014 – Michael J Muccilli
Snow Ratios were found to have some association with 850mb Temperatures Average 850mb temperature during events was -13C Highest ratios occurred when 850mb temperatures were
between -11C and -15C, 25-35:1
Results – Snow Ratios 31
NROW XV 2014 – Michael J Muccilli
-8 -9 -10 -11 -12 -13 -14 -15 -16 -17 -18 -190
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Average Snow-Liquid Ratios vs. Average 850 mb Temps
850 mb Temperatures (C)
Snow
-Liq
uid
Ratio
s
R2O- Use at WFO BTV
Worked with SOO to develop into GFE Smart Tool
Calculates Froude Number, wind speed/direction, relative humidity, and QPF
Run off local WRF model, NAM, & GFS
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R2O- Use at WFO BTV
Also have a real-time calculation based off vertical temperature profile along mountain chain
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NROW XV 2014 – Michael J Muccilli
R2O- Use at WFO BTV
These two products have led to: An increased situational awareness of upslope
events Improved forecasts of location of heavy snow
band, snow intensity, and snow amounts Improved lead time and verification scores of
winter weather products
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NROW XV 2014 – Michael J Muccilli
In Summary
The Froude Number is a useful tool in determining characteristics of orographic snow bands
Events can be unblocked (Fr > 1), Blocked (Fr < 1), or Very Blocked (Fr << 1)
Can be transitioned into operations and used to improve orographic snowfall forecasts
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ReferencesBaxter, M.A., C.E. Graves, and J.T. Moore, 2005: A Climatology of Snow-to-Liquid Ratio for the Contiguous United States. Wea. Forecasting, 20, 729-744.
Bell, G.D. and L.F. Bosart, 1988: Appalachian Cold Air Damming. Mon. Wea. Rev., 116, 137-161.
Chen, S-H., Y.L Lin, and Z. Zhao, 2008: Effects of Unsaturated Moist Froude Number and Orographic Aspect Ratio on a Conditionally Unstable Flow over a Mesoscale Mountain. J. of the Meteor. Soc. Japan, 86, 353-367.
Chu, C-M. and Y.L. Lin, 2000: Effects of Orography on the Generation and Propagation of Mesoscale Convective Systems in a two-dimensional conditionally unstable flow. J. of Atmos. Sci., 57, 3817-3837.
Lee, L.G. and H. Gerapetritis, 2012: The Northwest Flow Snow Event of 11 February 2012.
NOAA Earth System Research Laboratory (ESRL), Physical Sciences Division (PSD), 2012: Interactive Plotting and Analysis Pages. [Available online at http://www.esrl.noaa.gov/psd/]
NOAA Air Resources Laboratory (ARL), 2012: Gridded Meteorological Data Archives. [Available online at http://ready.arl.noaa.gov/archives.php]
National Weather Service Burlington, VT, 2012: Daily Climate Maps. [Available online at http://www.weather.gov/btv/climatemaps]
Keighton, S., L. Lee, B. Holloway, D. Hotz, S. Zubrick, J. Hovis, G. Votaw, L.B. Perry, G. Lackmann, S.E. Yuter, C. Konrad, D. Miller, and B. Etherton, 2009: A Collaborative Approach to Study Northwest Flow Snow in the Southern Appalachians. Bull. Amer. Meteor. Soc., 90, 980-999.
Rutledge, S.A. and P. Hobbs, 1983: The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. VIII: A Model for the “Seeder-Feeder” Process in Warm-Frontal Rainbands. J. Atmos. Sci., 40, 1185-1206.
Sisson, P.A., D. St. Jean, E. Evenson, W.E. Murray, S.F. Hogan, L. Bosart, D. Keyser, and B. Smith, 2004: Applying local research to National Weather Service operations: Forecasting heavy mountain snowfalls in Vermont and Northern New York. Preprints, 11th Conference on Mountain Meteorology and the Annual Mesoscale Alpine Program, Bartlett, NH, Amer. Meteor Soc., 17.5. St. Jean, D.P., and P.A. Sisson , 2004: Characteristics of upslope snowfall events in northern new York state and northern Vermont: Diagnostics and model simulations of several northwest flow cases. Preprints, 20th Conference on Weather Analysis and Forecasting. Seattle, WA, Amer. Meteor. Soc., 18.4.
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Questions?
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