Michael J Muccilli NOAANWS Burlington VT NROW XV 2014 1 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 ID: 512731
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Slide1
Using The Froude Number to Improve Orographic Snow Forecasts in the Green Mountains of Vermont
Michael J Muccilli
NOAA/NWS Burlington, VT
NROW XV 2014
1Slide2
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
2Slide3
Outline
Motivation
Synoptic Overview
Define the Froude NumberGreen Mountain Study
Operational Use at WFO BTV
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Motivation
4
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January 2
nd
- 3
rd
, 2012
Source: Weather Prediction Center
5
NROW XV 2014 – Michael J MuccilliSlide6
December 28
th
- 29
th
, 2011
6
NROW XV 2014 – Michael J Muccilli
Source: Weather Prediction CenterSlide7
December 28
th
- 29th, 2011January 2nd – 3
rd
, 2012
Froude = 4.4
Froude = 0.91
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Synoptic Overview
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Terrain of WFO Burlington County Warning Area
4395 feet
100 feet
15 Miles9
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Synoptic Overview
Upper Level Trough or Closed Low Progressing through the Region
Source: NOAA ESRL
Composite of 25 Upslope Cases10
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Synoptic Overview
Surface & Low Level Pressure System exiting the Region
Source: NOAA ESRLComposite of 25 Upslope Cases
11
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Synoptic Overview
Increasing west to northerly flow in low levels along with lingering low level moisture
Source: NOAA ESRLComposite of 25 Upslope Cases
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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 componentEquivalent potential temperature decreasing with height in the low levelsEvent duration of at least 12 hours13
NROW XV 2014 – Michael J MuccilliSlide14
Objectives
Make it easier to identify significant orographic (upslope) snow events and placement/orientation of heavy snow using:
The Froude Number
Low/Mid Level Humidity ProfilesVertical Profile of Wind Speed & DirectionLow level Stability14
NROW XV 2014 – Michael J MuccilliSlide15
The Froude Number
Potential Temperature (Surface & Mountain Top)
Mountain Height
Speed of Wind Perpendicular to the BarrierFroude 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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The Froude Number
Subcritical (Blocked),
Froude < 1Precipitation likely to fall
upwind of barrierCritical, Froude =~1Precipitation likely to fall
along barrier
Supercritical (Unblocked),
Froude > 1
Precipitation likely to fall on
lee side of barrier
16
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Data & Methods - Green Mountain Study
25 cases (2007 – 2012)
12 “Blocked” (Froude < 1)
13 “Unblocked” (Froude >1)11 Stations Used2 Champ Valley5 West Slopes3 East Slopes
1 East Valley
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Data & Methods - Green Mountain Study
Froude Number calculated for each event, in the mid-point of each event
Used archived NAM/RUC Soundings at KBTVAfter calculation, grouped by calculated Froude Number into 8 bins
0.25, 0.25 - 0.49, 0.50 - 0.84, 0.84 - 0.991.0 - 1.33, 1.34 - 1.75, 1.76 - 2.0, >2.018
NROW XV 2014 – Michael J MuccilliSlide19
Froude > 2
Fr 1.76-2
Fr 1.34-1.75
Fr 1.0-1.33
Fr 0.85-0.99
Fr 0.50-0.84
Fr 0.25-0.49
Fr <0.25
Results
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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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Results – 3 Types of Events
Fr >1
Fr 0.5-1
Fr <0.521
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Results – 3 Types of Events
Unblocked
Froude #: 4.4
Low level RH 80-90%WNW winds throughout low levelsWell Mixed to Mountain Height22
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Results – 3 Types of Events
Unblocked
Froude #: 4.4Slide24
Results – 3 Types of Events
Classic Western Slopes
Blocked
Froude #: 0.98Low level RH ~90%West winds veering to Northwest throughout low levelsIsothermal layer below mountain height24
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Results – 3 Types of Events
Blocked
Froude #: 0.98Slide26
Results – 3 Types of Events
“Champlain Valley Powder”
Very Blocked
Froude #: 0.07Low 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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NROW XV 2014 – Michael J Muccilli
Results – 3 Types of Events
Very Blocked
Froude #: 0.07Slide28
Results – Blocked vs. Unblocked
Blocked
Unblocked
Source: NOAA ESRLComposite of 12 Blocked/13 Unblocked Upslope Cases28
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Results –
Snow Ratios29
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 71:1Slide30
Results –
Snow Ratios30
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, 25-35:1Slide31
Results –
Snow Ratios31
NROW XV 2014 – Michael J MuccilliSlide32
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, & GFS32
NROW XV 2014 – Michael J MuccilliSlide33
R2O- Use at WFO BTV
Also have a real-time calculation based off vertical temperature profile along mountain chain
33
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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 amountsImproved lead time and verification scores of winter weather products 34
NROW XV 2014 – Michael J MuccilliSlide35
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 35
NROW XV 2014 – Michael J MuccilliSlide36
References
Baxter, 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
, 11
th
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,
20
th
Conference on Weather Analysis and Forecasting
. Seattle, WA, Amer. Meteor. Soc., 18.4.
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Questions?
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