A Method to Reliably A Method to Reliably Predict Convective Predict Convective

Modes in Late Season Modes in Late Season Lake-Effect Snow Lake-Effect Snow

EventsEvents

Michael L. Jurewicz, Sr.Michael L. Jurewicz, Sr.NOAA/NWS Binghamton, NYNOAA/NWS Binghamton, NY

November 1, 2006November 1, 2006NROW 8NROW 8

Albany, NYAlbany, NY

MotivationMotivation

Lake-effect snow forecasting can be Lake-effect snow forecasting can be quite challenging quite challenging – Particularly late in the season (February Particularly late in the season (February

through April)through April) Due to the increased sun angle, changes in Due to the increased sun angle, changes in

mode / organization tend to follow the diurnal mode / organization tend to follow the diurnal heating cycleheating cycle

However, this doesn’t always workHowever, this doesn’t always work– For example, well defined bands during peak For example, well defined bands during peak

heating time (afternoon); or disorganized open-heating time (afternoon); or disorganized open-cellular snow showers late at night or in the cellular snow showers late at night or in the morningmorning

GoalsGoals

To identify the atmospheric To identify the atmospheric parameters primarily responsible for parameters primarily responsible for governing the organization / different governing the organization / different modes of Lake-effect snow modes of Lake-effect snow

Utilize this information to formulate a Utilize this information to formulate a technique for predicting convective technique for predicting convective mode in Lake-effect snow situationsmode in Lake-effect snow situations– In order for this method to have value, skill In order for this method to have value, skill

must be demonstrated over and above must be demonstrated over and above simply following diurnal trends simply following diurnal trends

MethodologyMethodology

Central NY Lake-effect snow events have Central NY Lake-effect snow events have been archived since the winter of 2002-03been archived since the winter of 2002-03– Only looked at Feb., March, and April cases for this Only looked at Feb., March, and April cases for this

project project Utilized radar and sounding information from Utilized radar and sounding information from

this database this database – Radar imagery was the basis for categorizing Radar imagery was the basis for categorizing

individual events (banded structures vs. open-individual events (banded structures vs. open-cellular convection)cellular convection)

– NAM soundings used to determine shear and NAM soundings used to determine shear and stability parameters at 6-hourly time steps (0000, stability parameters at 6-hourly time steps (0000, 0600, 1200, and 1800 UTC)0600, 1200, and 1800 UTC)

111 different time periods evaluated for this study111 different time periods evaluated for this study Specific site was chosen based on proximity to Specific site was chosen based on proximity to

greatest radar coveragegreatest radar coverage

ExamplesExamples

Example of Data Example of Data CatalogingCataloging

OutlineOutline

Review of earlier research on the Review of earlier research on the morphology of Lake-effect precipitationmorphology of Lake-effect precipitation– Horizontal Roll concepts (Multiple Bands)Horizontal Roll concepts (Multiple Bands)

Similar transverse circulations / convergence on Similar transverse circulations / convergence on the edges of intense single bandsthe edges of intense single bands

Overview of how technique was Overview of how technique was developed for predicting convective developed for predicting convective mode mode

Demonstration of potential utility in an Demonstration of potential utility in an operational forecast settingoperational forecast setting

Horizontal Convective Horizontal Convective RollsRolls

• Counter-rotating horizontal vortices in CBL• Aligned along mean wind direction• Due to combination of surface heat flux and wind• Clouds often above updraft branches

The COMET Program

Formation of BandsFormation of Bands

Clouds are suppressed in between bandsClouds are suppressed in between bands

Lake-ICE ExperimentLake-ICE Experiment

Project conducted in January of Project conducted in January of 19981998

Used mobile soundings, observation Used mobile soundings, observation sites, and airborne Doppler radar to sites, and airborne Doppler radar to look at structure / behavior of Lake-look at structure / behavior of Lake-effect bands / cells over Lake effect bands / cells over Lake MichiganMichigan– Kristovich, Laird, and Hjemfeldt (2003) Kristovich, Laird, and Hjemfeldt (2003)

Satellite View of Satellite View of Project AreaProject Area

Sounding Data Sounding Data

Airborne Radar Airborne Radar DepictionDepiction

Brief Summary of Brief Summary of FindingsFindings

In 100 kmIn 100 km22 box over Lake Michigan:– Snow showers displayed mainly

disorganized / open cellular appearance Further south over Lake Michigan:

– Snow showers displayed a banded look to them; more consistent with organized horizontal rolls

Stronger low-level shear and more boundary layer stability (lower air-lake temperature differentials) across this region

Consistency with Other Consistency with Other ResearchResearch

It has been shown that roll-type It has been shown that roll-type convection tends to prevail when:convection tends to prevail when:– Low-level environment (1-2 km AGL) has Low-level environment (1-2 km AGL) has

moderate to strong speed shear; although moderate to strong speed shear; although little directional shear little directional shear

– Some low-level heat flux / instability is Some low-level heat flux / instability is presentpresent

However, seems to be an upper-limitHowever, seems to be an upper-limit– If too unstable, can detract from overall organization If too unstable, can detract from overall organization

– Weckwerth, et al. (1997); Stull (1988); and Weckwerth, et al. (1997); Stull (1988); and Miura (1986)Miura (1986)

Warm Season Warm Season ComparisonsComparisons

Can make an interesting analogy with Pulse vs. Can make an interesting analogy with Pulse vs. Organized thunderstormsOrganized thunderstorms– Need sufficient vertical shear to balance CAPENeed sufficient vertical shear to balance CAPE

Will typically result in more organized multi-cell systems, Will typically result in more organized multi-cell systems, squall lines, etc.squall lines, etc.

– Weakly sheared environments Weakly sheared environments Will typically see shorter-lived and disorganized storms Will typically see shorter-lived and disorganized storms

One important difference is with directional shearOne important difference is with directional shear– Favorable ingredient to strengthen updrafts and cold Favorable ingredient to strengthen updrafts and cold

pool dynamics with upright convection / thunderstormspool dynamics with upright convection / thunderstorms– Unfavorable for maintaining narrow updraft branches / Unfavorable for maintaining narrow updraft branches /

corridors with horizontal roll type convectioncorridors with horizontal roll type convection

A Plan Coming A Plan Coming TogetherTogether

Given that we’ve established the Given that we’ve established the importance of importance of both vertical speed both vertical speed shear and at least some CBL shear and at least some CBL instabilityinstability to the existence of to the existence of horizontal rolls / Lake-effect bands; horizontal rolls / Lake-effect bands; these questions logically follow:these questions logically follow:– Is there a preferred amount of either one; Is there a preferred amount of either one;

or an optimal balance between them?or an optimal balance between them?– How would one best quantify and then How would one best quantify and then

illustrate these parameters?illustrate these parameters?

BUFKITBUFKIT

Several advantages of using BUFKIT Several advantages of using BUFKIT soundings as part of the study:soundings as part of the study:– Good data availability (archived back to Good data availability (archived back to

2002)2002)– Many shear / stability parameters Many shear / stability parameters

already quantified within the programalready quantified within the program– Widely used forecast tool in Lake-effect Widely used forecast tool in Lake-effect

situations situations

Trial and Error Trial and Error

Initially unsure of what specific quantities Initially unsure of what specific quantities to look at, we decided to try the following:to look at, we decided to try the following:– For instability – Lapse Rates, CAPE, and For instability – Lapse Rates, CAPE, and

depths of the Mixed Layer (for any depths of the Mixed Layer (for any normalization)normalization)

LR and CAPE values from the surface to inversion LR and CAPE values from the surface to inversion basebase

– For shear – Bulk Speed Shear and the Mean For shear – Bulk Speed Shear and the Mean Flow near the top of the CBL Flow near the top of the CBL

Bulk Speed Shear values also from the surface to Bulk Speed Shear values also from the surface to inversion baseinversion base

Spreadsheet SampleSpreadsheet Sample

CorrelationsCorrelations

For statistical purposes, we assigned For statistical purposes, we assigned Banded Banded events a value of 0 and Disorganized / events a value of 0 and Disorganized / Cellular events a value of 1Cellular events a value of 1

After tabulating results for the entire After tabulating results for the entire database, here’s how some of the numbers database, here’s how some of the numbers fell out:fell out:– Bulk Speed Shear (-0.66)Bulk Speed Shear (-0.66)– CAPE (0.58)CAPE (0.58)– Normalized Bulk Shear (-0.57)Normalized Bulk Shear (-0.57)– Lapse Rate (0.43)Lapse Rate (0.43)– Wind Speed just below Inversion (-0.23)Wind Speed just below Inversion (-0.23)– CBL Depth (-0.05)CBL Depth (-0.05)

Scatter Plot DiagramScatter Plot DiagramCAPE vs. Bulk Shear

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Purple Markers = Open Cellular Events

Dark Blue Markers = Banded Events

Scatter Plot DiagramScatter Plot DiagramCAPE vs. Bulk Shear

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Line of Best Fit

Is This Worth It?Is This Worth It?

As mentioned earlier, the value of As mentioned earlier, the value of this technique will be measured this technique will be measured by how much skill it can show by how much skill it can show over normal diurnal trends over normal diurnal trends

To that end, let’s look at some To that end, let’s look at some statistics, then case study statistics, then case study examplesexamples

Statistical Statistical ComparisonsComparisons

If one were to simply follow diurnal If one were to simply follow diurnal trends to forecast convective mode trends to forecast convective mode (in other words, (in other words, 06z or 12z = 06z or 12z = Banded Banded ; and ; and 18z or 00z = 18z or 00z = CellularCellular), here’s how the numbers ), here’s how the numbers added up:added up:– For Banded Events : For Banded Events : POD = 0.74 and POD = 0.74 and

FAR = 0.16FAR = 0.16– For Cellular Events : For Cellular Events : POD = 0.82 and POD = 0.82 and

FAR = 0.29FAR = 0.29

Stats for New MethodStats for New MethodCAPE vs. Bulk Shear

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Line of Best Fit

For Banded: POD=0.84 and FAR = 0.05

For Cellular: POD = 0.94 and FAR = 0.19

Graphical ComparisonGraphical Comparison

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Banded Cellular Banded Cellular

POD POD FAR FAR

Diurnal Method vs. New Technique

Diurnal

New

March 13, 2004March 13, 2004

Appeared to be a situation where Appeared to be a situation where consolidated LES bands typically consolidated LES bands typically develop / evolve in Central NY:develop / evolve in Central NY:– Steady-state and moist 290 to 300 degree Steady-state and moist 290 to 300 degree

flow in the CBLflow in the CBL– Little directional shear Little directional shear – Late night / early morning time frameLate night / early morning time frame

Despite these factors, LES remained Despite these factors, LES remained disorganized / cellular in naturedisorganized / cellular in nature– Not enough vertical shear to balance Not enough vertical shear to balance

lingering instability? lingering instability?

Radar Images at 0600 Radar Images at 0600 UTC, 03/13/04UTC, 03/13/04

Sounding from Ithaca, Sounding from Ithaca, NY at 0600 UTC, NY at 0600 UTC,

03/13/0403/13/04

Snowfall TotalsSnowfall Totals

Specific PlotsSpecific PlotsCAPE vs. Bulk Shear

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Line of Best Fit

March 13, 2004 at 06z and 12z

February 12, 2003February 12, 2003

It was approaching the right time of It was approaching the right time of year for LES bands to break up near year for LES bands to break up near peak heating in the afternoon peak heating in the afternoon

Yet, in this case, a single LES band Yet, in this case, a single LES band stayed well in tactstayed well in tact– Not enough instability to disrupt “roll Not enough instability to disrupt “roll

circulation,” that was kept well in tact circulation,” that was kept well in tact by strong vertical shear by strong vertical shear

Radar Images at 1800 Radar Images at 1800 UTC, 02/12/03UTC, 02/12/03

Sounding from Syracuse, Sounding from Syracuse, NY at 1800 UTC, 02/12/03NY at 1800 UTC, 02/12/03

Snowfall InformationSnowfall Information

Specific PlotSpecific PlotCAPE vs. Bulk Shear

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Line of Best FitFebruary 12, 2003 at 1800Z

SummarySummary

The following are preliminary results The following are preliminary results (from a database of 4 different winter (from a database of 4 different winter seasons and 100+ time periods):seasons and 100+ time periods):– How well LES bands were able to organize How well LES bands were able to organize

into banded structures, or remain into banded structures, or remain consolidated, seemed to hinge on a preferred consolidated, seemed to hinge on a preferred balance of CBL CAPE and Bulk Speed Shearbalance of CBL CAPE and Bulk Speed Shear

Better vertical shear and some instability were Better vertical shear and some instability were most conducive; while too much instability and/or most conducive; while too much instability and/or too little shear were the primary detractorstoo little shear were the primary detractors

Fits conceptual model of Horizontal Rolls well and Fits conceptual model of Horizontal Rolls well and supports previous LES researchsupports previous LES research

Analogous to some aspects of warm-season Analogous to some aspects of warm-season convectionconvection

Summary (continued)Summary (continued)

““Best fit” line was drawn on scatter Best fit” line was drawn on scatter plot of CAPE vs. Bulk Speed Shearplot of CAPE vs. Bulk Speed Shear– Discriminated fairly well between Discriminated fairly well between

Banded and Cellular LES events Banded and Cellular LES events – New technique showed improvement New technique showed improvement

over simply using diurnal trendsover simply using diurnal trends ““Odd ball” cases provided the best Odd ball” cases provided the best

support (well developed LES bands near support (well developed LES bands near peak heating or disorganized cellular peak heating or disorganized cellular convection late at night / early in the convection late at night / early in the morning) morning)

Future WorkFuture Work

Continue to evaluate this technique over coming Continue to evaluate this technique over coming winter seasons to see if success continues with winter seasons to see if success continues with a larger databasea larger database– If success continues, possibly include as a diagnostic tool If success continues, possibly include as a diagnostic tool

within BUFKITwithin BUFKIT Also look at different parts of the Great Lakes Also look at different parts of the Great Lakes

region / Other times of yearregion / Other times of year– Varying terrain, lake / land interfaces, upwind influences, etc.Varying terrain, lake / land interfaces, upwind influences, etc.– Examine November to January casesExamine November to January cases

Strong single bands less sensitive to downstream changes in Strong single bands less sensitive to downstream changes in stability / shear ?stability / shear ?

How much vertical shear is too much ?How much vertical shear is too much ?– Especially with shorter-fetch bands Especially with shorter-fetch bands

Stability Differences between Land / Lake Stability Differences between Land / Lake SurfacesSurfaces

A Model for Future A Model for Future Application?Application?

Thank You !!Thank You !!

Questions ??Questions ??