Using The Froude Number to Improve Orographic Snow Forecast - PowerPoint Presentation

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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

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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 NumberGreen Mountain Study

Operational Use at WFO BTV

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Motivation

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January 2

nd

- 3

rd

, 2012

Source: Weather Prediction Center

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December 28

th

- 29

th

, 2011

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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

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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

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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

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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

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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

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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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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

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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

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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

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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

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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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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

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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

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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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