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Using The Froude Number to Improve Orographic Snow Forecast
Using The Froude Number to Improve Orographic Snow Forecast

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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 Download Presentation

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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

3

NROW XV 2014 – Michael J MuccilliSlide4

Motivation

4

NROW XV 2014 – Michael J MuccilliSlide5

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

7

NROW XV 2014 – Michael J MuccilliSlide8

Synoptic Overview

8

NROW XV 2014 – Michael J MuccilliSlide9

Terrain of WFO Burlington County Warning Area

4395 feet

100 feet

15 Miles9

NROW XV 2014 – Michael J MuccilliSlide10

Synoptic Overview

Upper Level Trough or Closed Low Progressing through the Region

Source: NOAA ESRL

Composite of 25 Upslope Cases10

NROW XV 2014 – Michael J MuccilliSlide11

Synoptic Overview

Surface & Low Level Pressure System exiting the Region

Source: NOAA ESRLComposite of 25 Upslope Cases

11

NROW XV 2014 – Michael J MuccilliSlide12

Synoptic Overview

Increasing west to northerly flow in low levels along with lingering low level moisture

Source: NOAA ESRLComposite of 25 Upslope Cases

12

NROW XV 2014 – Michael J MuccilliSlide13

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)

15

NROW XV 2014 – Michael J MuccilliSlide16

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

NROW XV 2014 – Michael J MuccilliSlide17

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

17

NROW XV 2014 – Michael J MuccilliSlide18

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

19

NROW XV 2014 – Michael J MuccilliSlide20

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)

20

NROW XV 2014 – Michael J MuccilliSlide21

Results – 3 Types of Events

Fr >1

Fr 0.5-1

Fr <0.521

NROW XV 2014 – Michael J MuccilliSlide22

Results – 3 Types of Events

Unblocked

Froude #: 4.4

Low level RH 80-90%WNW winds throughout low levelsWell Mixed to Mountain Height22

NROW XV 2014 – Michael J MuccilliSlide23

23

NROW XV 2014 – Michael J Muccilli

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

NROW XV 2014 – Michael J MuccilliSlide25

25

NROW XV 2014 – Michael J Muccilli

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

)

26

NROW XV 2014 – Michael J MuccilliSlide27

27

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

NROW XV 2014 – Michael J MuccilliSlide29

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

NROW XV 2014 – Michael J MuccilliSlide34

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.

36

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Questions?

37

NROW XV 2014 – Michael J Muccilli

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