Mesoscale Structure ofMesoscale Structure ofPrecipitation Regions inPrecipitation Regions inNortheast Winter StormsNortheast Winter Storms

Matthew D. Greenstein, Lance F. Bosart, and Daniel KeyserMatthew D. Greenstein, Lance F. Bosart, and Daniel KeyserDepartment of Earth and Atmospheric SciencesDepartment of Earth and Atmospheric Sciences

University at Albany, Albany, NY 12222University at Albany, Albany, NY 12222David J. NicosiaDavid J. Nicosia

National Weather ServiceNational Weather ServiceBinghamton Weather Forecast Office, Johnson City, NY Binghamton Weather Forecast Office, Johnson City, NY

1379013790

7 April 20067 April 2006CSTAR-II support provided by NOAA Grant NA04NWS4680005CSTAR-II support provided by NOAA Grant NA04NWS4680005

OutlineOutline

• IntroductionIntroduction• Case selectionCase selection• Radar classificationRadar classification• Cross section analysisCross section analysis• Summary of results Summary of results • Future workFuture work

IntroductionIntroduction• Forecasters Forecasters cancan predict likely areas of precipitation predict likely areas of precipitation• Forecasters Forecasters cannotcannot always skillfully predict always skillfully predict mesoscale features mesoscale features• Forecasting mesoscale details adds value to a Forecasting mesoscale details adds value to a forecast:forecast:

• Prediction of snowfall amount and variabilityPrediction of snowfall amount and variability• Differentiating between high-impact andDifferentiating between high-impact and low-impact snows low-impact snows

IntroductionIntroduction• Precipitation regions have multiple modes Precipitation regions have multiple modes (patterns)(patterns)• Goal is to examine ingredients …Goal is to examine ingredients … * Lift* Lift * Instability * Instability * Moisture * Moisture * Microphysics * Microphysics … … to find ways of distinguishing the to find ways of distinguishing the modesmodes

Introduction: Introduction: Previous banded Previous banded studiesstudies• Matejka, Houze, and Hobbs (1980)Matejka, Houze, and Hobbs (1980)

Warm frontalWarm frontal

SurgSurgee

PostfrontaPostfrontall

Cold frontalCold frontal

Warm Warm sectorsector

Introduction: Introduction: Previous banded Previous banded studiesstudies• Nicosia and Grumm (1999)Nicosia and Grumm (1999)

Introduction: Introduction: Previous banded Previous banded studiesstudies• Novak et al. (2004)Novak et al. (2004)

Banded Nonbanded

Introduction: Introduction: Previous banded Previous banded studiesstudies• Novak et al. (2004)Novak et al. (2004)

Banded Nonbanded

Case SelectionCase Selection• Cases occur in area bounded by Cases occur in area bounded by 36.5°N, 50°N, 65°W, and 85°W36.5°N, 50°N, 65°W, and 85°W• Within U.S. radar coverage Within U.S. radar coverage • 1 October – 30 April 1 October – 30 April • No warm sector precipitationNo warm sector precipitation• P–type predominantly snow P–type predominantly snow • “ “Heavy snow” = 15+ cm in 12 h over area the Heavy snow” = 15+ cm in 12 h over area the size of CTsize of CT• No lake effect snows and enhancementsNo lake effect snows and enhancements• Past three winters (2002–3, 2003–4, 2004–5)Past three winters (2002–3, 2003–4, 2004–5)

Case SelectionCase Selection• Data usedData used

• NCDC national hourly mosaic reflectivity NCDC national hourly mosaic reflectivity imagesimages• Public Information Statements (PNS)Public Information Statements (PNS)• Northeast River Forecast Center snowfall mapsNortheast River Forecast Center snowfall maps• NCDC’s U.S. Storm Events DatabaseNCDC’s U.S. Storm Events Database• ASOS reportsASOS reports

20 Cases20 Cases• 26–27 Nov 26–27 Nov 20022002• 4–6 Dec 20024–6 Dec 2002• 25–26 Dec 25–26 Dec 20022002• 2–5 Jan 20032–5 Jan 2003• 6–7 Feb 20036–7 Feb 2003• 15–18 Feb 15–18 Feb 20032003• 6 Mar 20036 Mar 2003

• 5–8 Dec 20035–8 Dec 2003• 13–15 Dec 13–15 Dec 20032003• 14–15 Jan 14–15 Jan 20042004• 27–28 Jan 27–28 Jan 20042004• 16–17 Mar 16–17 Mar 20042004• 18–19 Mar 18–19 Mar 20042004

• 19–20 Jan 200519–20 Jan 2005• 22–23 Jan 200522–23 Jan 2005• 24–25 Feb 200524–25 Feb 2005• 28 Feb–2 Mar 28 Feb–2 Mar 20052005• 8–9 Mar 20058–9 Mar 2005• 11–13 Mar 200511–13 Mar 2005• 23–24 Mar 200523–24 Mar 2005

Radar ClassificationRadar Classification• 2km WSI NOW2km WSI NOWradrad mosaics mosaics

* 15-min resolution* 15-min resolution* 3 levels of quality control* 3 levels of quality control* Composite reflectivity* Composite reflectivity

1.1. UniformUniform2.2. Classic BandClassic Band3.3. Transient BandTransient Band4.4. BandletsBandlets5.5. FracturedFractured6.6. UnclassifiableUnclassifiable

Multiple modes may Multiple modes may exist in a storm’s exist in a storm’s

lifecycle and at one lifecycle and at one timetime

Radar Classification: Radar Classification: UniformUniform

1200 UTC 27 Nov 1200 UTC 27 Nov 20022002

Radar Classification: Radar Classification: Classic Classic BandBand

1900 UTC 7 Feb 1900 UTC 7 Feb 20032003

Radar Classification: Radar Classification: Transient BandTransient Band

12001200––2100 UTC 16 Feb 2100 UTC 16 Feb 20032003

Evolving Band

Radar Classification: Radar Classification: Transient BandTransient Band

1600 UTC 6 Dec 1600 UTC 6 Dec 20032003

Broken Band

Radar Classification: Radar Classification: Transient BandTransient Band

2115 UTC 14 Dec 2115 UTC 14 Dec 20032003

Messy Band

Radar Classification: Radar Classification: BandletsBandlets

1500 UTC 17 Feb 1500 UTC 17 Feb 20032003

Radar Classification: Radar Classification: FracturedFractured

1500 UTC 16 Mar 1500 UTC 16 Mar 20042004

Cross Section AnalysisCross Section Analysis• Previous research:Previous research: frontogenesis in the presence of weak frontogenesis in the presence of weak moist symmetric stability yields bands moist symmetric stability yields bands

• Negative saturation equivalent potential Negative saturation equivalent potential vorticity vorticity (EPV*) indicates conditional slantwise instability (EPV*) indicates conditional slantwise instability (CSI) (CSI) and/or conditional upright instability (CI) and/or conditional upright instability (CI)

• CI dominates CSICI dominates CSI

EPV* = – g (EPV* = – g (ζζ · · θθee), where ), where ζζ is the absolute vorticity vector is the absolute vorticity vector**

Cross Section AnalysisCross Section Analysis• 32–32–km North American Regional Reanalysis (NARR)km North American Regional Reanalysis (NARR)• Cross sections contain …Cross sections contain …

• Saturation equivalent potential temperature – θSaturation equivalent potential temperature – θee (K)(K)• Relative humidity (%)Relative humidity (%)• 2D Petterssen Frontogenesis (ºC 100 km2D Petterssen Frontogenesis (ºC 100 km-1-1 3 h 3 h-1-1))• Saturation equivalent potential vorticity - EPV* Saturation equivalent potential vorticity - EPV* (PVU)(PVU) (calculated with the full wind) (calculated with the full wind)• Vertical motion (μb sVertical motion (μb s-1-1))• Dendritic growth zone,Dendritic growth zone, i.e., −12ºC and − i.e., −12ºC and −18ºC isotherms8ºC isotherms

**

Cross Section Analysis: Cross Section Analysis: Classic BandClassic Band

Strong, Strong, steep, steep,

surface-surface-based based

frontogenesifrontogenesiss

Strong, tilted Strong, tilted ascent ascent

rooted in the rooted in the

boundary boundary layerlayer

Weakly Weakly positive EPV*positive EPV*

CI CI unimportantunimportant

2100 UTC 7 Feb 2100 UTC 7 Feb 20032003

Cross Section Analysis: Cross Section Analysis: UniformUniform

Weak, flat Weak, flat frontogenesifrontogenesi

ss

Upright Upright ascentascent

Ascent Ascent strength not strength not

a factora factor

Weakly Weakly positive & positive & negative negative

EPV*EPV*has no effecthas no effect

No CINo CI

2100 UTC 22 Jan 2100 UTC 22 Jan 20052005

Cross Section Analysis: Cross Section Analysis: Transient BandTransient Band

Weak, Weak, decoupled decoupled

frontogenesifrontogenesiss

Inhibits Inhibits continuous continuous boundary boundary

layer layer moisture moisture

feedfeed

Weakly Weakly positive EPV* positive EPV*

seen in all seen in all modesmodes

1500 UTC 16 Feb 1500 UTC 16 Feb 20032003

Cross Section Analysis: Cross Section Analysis: BandletsBandlets

FrontogenesiFrontogenesis lifts air s lifts air

parcels to CI parcels to CI regionregion

Escalator-Escalator-elevatorelevator

0000 UTC 1 Mar 0000 UTC 1 Mar 20052005

Cross Section Analysis: Cross Section Analysis: FracturedFractured

Weak, Weak, decoupled, decoupled, fragmented fragmented frontogenesifrontogenesi

ss

SeparateSeparateEPV mins EPV mins

and ascent and ascent maxesmaxes

Lower RHLower RH

1500 UTC 16 Mar 1500 UTC 16 Mar 20042004

Summary of Results: Summary of Results: Distinguishing Distinguishing featuresfeatures Frontogenesis Other

Uniform weak and/or flator none

no CI

ClassicBand

strong and steep; surface-based

ascent indicates frontal circulation dominates

Transient

Band

decoupled ω not well rooted in B.L.;CI alters precip pattern

Bandlets thin, weak, or none; surface-based

CI / escalator–elevator;deep CI = messier pattern

Fractured

fragmented;decoupled

CI or frontogenesis enhances precip; lower RH ‡ ‡ = some look like a band= some look like a band CI enhances updrafts & CI enhances updrafts &

downdraftsdowndrafts

‡‡

‡‡

Summary of Results: Summary of Results: Nondistinguishing featuresNondistinguishing features• Ascent strengthAscent strength

* * Uniform: −4 to −24 μb s-1

** Classic band: ≤ −20 μb s-1

• Intersection of max ascent with DGZ• Depth of DGZ (~50–100 hPa in most cases)–100 hPa in most cases)• Intersection of max ascent with CI regionIntersection of max ascent with CI region• RH patternsRH patterns• Reduced EPV*Reduced EPV*

* All cases contain EPV* 0–0.25 PVU (WMSS) * All cases contain EPV* 0–0.25 PVU (WMSS) and CSIand CSI

* Shape and location of reduced EPV* regions* Shape and location of reduced EPV* regions

Summary of Results: Summary of Results: Nondistinguishing featuresNondistinguishing features• From plan-view analyses…From plan-view analyses…

• QG–forcing ratio: DCVA / (DCVA + WAA)• Depths of reduced EPV* satisfying various criteria

• EPV* ≤ 0, ≤ 0.25, 0–0.25, or –0.25, or ≤ −0.25 + RH ≥ 70% + Ascent

• Max vertical speed shearMax vertical speed shear• 850–500 hPa vertical speed shear850–500 hPa vertical speed shear

Summary of Results: Summary of Results: EPV* vs. EPVEPV* vs. EPV

• Reasons for EPVReasons for EPV• Symmetric instability theory: thermal wind Symmetric instability theory: thermal wind balancebalance• MMgg more accurately captures growing instability more accurately captures growing instability

• Reasons for EPV*• Better representation of curved flow• Assumption that time scale of convection << time scale for large-scale environmental changes not valid? Potential for slantwise convection better found by using an evolving and unbalanced environment?

**gg**gg

EPVEPV

EPV*EPV*

**gg

0600 UTC 23 Jan 0600 UTC 23 Jan 20052005

Summary of Results: Summary of Results: EPV* vs. EPV EPV* vs. EPV

• EPV produces a messier pattern with more EPV produces a messier pattern with more negative negative values, especially in dry areas values, especially in dry areas• EPV “bull’s-eyes” line up with band positions EPV “bull’s-eyes” line up with band positions

**gg**gg

**gg

• Because… Because… 1) Value does not seem to matter1) Value does not seem to matter2) WMSS is a necessary but not distinguishing 2) WMSS is a necessary but not distinguishing

factorfactor3) CI plays an important role3) CI plays an important role

• Use EPV* because it produces a cleaner imageUse EPV* because it produces a cleaner image• If classic band is indicated, use EPV for positionIf classic band is indicated, use EPV for position

**g g

Summary of Results: Summary of Results: Conceptual Conceptual ModelsModels

Summary of Results: Summary of Results: Conceptual Conceptual ModelsModels

Summary of Results: Summary of Results: Conceptual Conceptual ModelsModels

Summary of Results: Summary of Results: Conceptual Conceptual ModelsModels

Summary of Results: Summary of Results: Conceptual Conceptual ModelsModels

Summary of Results: Summary of Results: FlowchartFlowchart

Frontogenesis?

YES NO

CI?

YES NO

Decoupled frontogenesis?

NO YES

Strong, steep frontogenesis?

Fragmented frontogenesis?

YES NO

Classic Band

Bandlets

Transient Band

YES NO

CI?

YES NO

Bandlets

Uniform

Fractured

Uniform

Future WorkFuture Work• Is the “fractured” mode really just a hybrid of Is the “fractured” mode really just a hybrid of “bandlets” & “transient bands” but with drier “bandlets” & “transient bands” but with drier spaces?spaces?

• Prove decoupled frontogenesis hypothesisProve decoupled frontogenesis hypothesis

• Investigate band lagInvestigate band lag

• Examine the Examine the EPV “bull’s-eyes”EPV “bull’s-eyes” **gg

Special ThanksSpecial Thanks• Lance and DanLance and Dan

• David Ahijevych (NCAR)David Ahijevych (NCAR)• Kevin TyleKevin Tyle• Alan SrockAlan Srock• Anantha AiyyerAnantha Aiyyer• Keith WagnerKeith Wagner• Celeste, Sharon, and LynnCeleste, Sharon, and Lynn• My parentsMy parents

Questions? Questions? Comments?Comments?

e-mail: greenstein@atmos.albany.edue-mail: greenstein@atmos.albany.edu