Genesis and Morphology of Genesis and Morphology of the Alberta Drylinethe Alberta Dryline

Neil M. TaylorNeil M. Taylor

Hydrometeorology and Arctic LabHydrometeorology and Arctic Lab

Meteorological Service of CanadaMeteorological Service of Canada

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OutlineOutline

• brief review of dryline ‘theory’brief review of dryline ‘theory’

• mechanism responsible for dryline mechanism responsible for dryline genesis in Albertagenesis in Alberta

• dryline morphologydryline morphology

• operational forecast issuesoperational forecast issues

• future workfuture work

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The DrylineThe Dryline• strong moisture gradient in the BL (obs at SFC)strong moisture gradient in the BL (obs at SFC)• dry air: hot -- deep, well mixed boundary layer dry air: hot -- deep, well mixed boundary layer • moist air: cooler & capped in BL -- dry and well moist air: cooler & capped in BL -- dry and well

mixed aloft => loaded gun soundingmixed aloft => loaded gun sounding• literature: frontogenetic (wrt moisture) literature: frontogenetic (wrt moisture)

circulation driven mainly by differential heating circulation driven mainly by differential heating (e.g., land breeze effect)(e.g., land breeze effect)

• convergence at dryline => convective initiationconvergence at dryline => convective initiation

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Dryline InterpretationDryline Interpretation

• circulation near dryline important for circulation near dryline important for low-level mesocyclogenesis low-level mesocyclogenesis (tornadoes)(tornadoes)

• forms capping lid + convergence line forms capping lid + convergence line a focus for convective initiationa focus for convective initiation

• simplest interpretation: storm - no simplest interpretation: storm - no storm boundary (e.g., too dry => no storm boundary (e.g., too dry => no storms)storms)

IMP

LIC

AT

ION

SIM

PLI

CA

TIO

NS

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Severe Convection in AlbertaSevere Convection in Alberta• ““breakdown of upper ridge” associated with breakdown of upper ridge” associated with

upper trough or low advancing from BCupper trough or low advancing from BC

12Z 250mb and 500mb charts showing advancing upper trough / low12Z 250mb and 500mb charts showing advancing upper trough / low

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Southwest Flow Over Southwest Flow Over MountainsMountains

• subsidence in lee of Rockies -- 700mb flowsubsidence in lee of Rockies -- 700mb flow

• warmer & drier air mixes down to SFCwarmer & drier air mixes down to SFC

25kt Jet25kt Jet30kt Jet30kt Jet

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Genesis of the Dryline Genesis of the Dryline

Moist, cappedMoist, capped

SW’ly 700mb flowSW’ly 700mb flow

DrylineDryline

• dry air mixes to SFC and converges with moist air over plains

Dry, Well MixedDry, Well Mixed

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3-5km

Schematic Dryline Cross-sectionSchematic Dryline Cross-section

Dry Air Moist Air10-20km10-20km

cappedcapped

1. SFC: dry air convergent with moist air over / near foothills1. SFC: dry air convergent with moist air over / near foothills2. Aloft: dry air overruns moist air2. Aloft: dry air overruns moist air3. Convergence at dryline breaks capping lid => deep convection3. Convergence at dryline breaks capping lid => deep convection4. Secondary circulation generates horizontal vorticity 4. Secondary circulation generates horizontal vorticity

33

11

22

44

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Severe Weather Days from 2000Severe Weather Days from 2000• Days from summer 2000 (AB and SK only) with...Days from summer 2000 (AB and SK only) with...

3 or more reports of 3 or more reports of ≥≥ 20mm hail 20mm hail or or

1 or more reports of 1 or more reports of ≥≥ 30mm hail 30mm hail or or

at least at least 1 report of a tornado1 report of a tornado• 33 days met criteria33 days met criteria

Wx ReportsWx Reports AlbertaAlberta SaskatchewanSaskatchewan TotalTotal

TornadoTornado 3333 2424 5757

Severe HailSevere Hail 9191 5151 142142

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Dryline DaysDryline Days• Defined as a day where severe weather reports Defined as a day where severe weather reports

met criteria and eithermet criteria and either– The severe storm that produced the event was initiated

in close proximity to the dryline, or– The severe storm developed from outflow boundary or

storm split of a storm that initiated in close proximity to the dryline

• 12 of 33 (36%) severe weather days met the 12 of 33 (36%) severe weather days met the ‘dryline day’ criteria‘dryline day’ criteria

Wx ReportsWx Reports TornadoTornado ≥≥20mm Hail20mm Hail All ReportsAll Reports

Dryline DaysDryline Days 2121 7878 9999

Severe DaysSevere Days 5757 142142 199199

% Dryline% Dryline 37%37% 56%56% 50%50%

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12Z 500mb 570dam on Dryline Days12Z 500mb 570dam on Dryline Days

500mb T

ROF/Low

500mb T

ROF/Low

Upper trof in BC leads to SW Upper trof in BC leads to SW flow over Rocky mountains => flow over Rocky mountains => subsidence necessary to form subsidence necessary to form drylinedryline

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Dryline Day Severe Weather EventsDryline Day Severe Weather Events

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Dryline Locations Summer 2000Dryline Locations Summer 2000

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Dryline Associated Storm TracksDryline Associated Storm Tracks• Considered storms initiated near dryline that Considered storms initiated near dryline that

produced severe weather (or led to severe produced severe weather (or led to severe storm via outflow boundary cell generation / storm via outflow boundary cell generation / storm splitting)storm splitting)

• Remote sensing used to define a storm track via Remote sensing used to define a storm track via radar (1.5km cappi – first echo to <40dBZ), VIS radar (1.5km cappi – first echo to <40dBZ), VIS satellite + CLDN lightning flash datasatellite + CLDN lightning flash data

• 26 severe thunderstorms initiated near 26 severe thunderstorms initiated near drylinedryline (or led to severe storm development via (or led to severe storm development via outflow boundary / splitting)outflow boundary / splitting)

• All 26 storms initiated within 75km of dryline All 26 storms initiated within 75km of dryline (61% within 50km and mean distance of 45km)(61% within 50km and mean distance of 45km)

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Dryline Associated Storm TracksDryline Associated Storm Tracks

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Dryline Locations and Storm TracksDryline Locations and Storm Tracks

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Dryline MorphologyDryline Morphology• develops rapidly (<6 hours) parallel to foothillsdevelops rapidly (<6 hours) parallel to foothills

• evolution strongly linked to flow over mountains (~700mb)evolution strongly linked to flow over mountains (~700mb)

Two Types: (Two Types: (depending on 700mb flowdepending on 700mb flow))

I: general 15-20kt SW flow over Rockies => QSI: general 15-20kt SW flow over Rockies => QS

II: 20-30kt 700mb jet => bulges eastwardII: 20-30kt 700mb jet => bulges eastward

• direction / extent of bulge function of location, angle, and direction / extent of bulge function of location, angle, and speed of jet over mountain barrierspeed of jet over mountain barrier

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QS DrylineQS Dryline

• little “bulging”little “bulging”• Cu/TCu over foothills Cu/TCu over foothills

due differential due differential heating heating

• capping lid “breaks” capping lid “breaks” at dryline -- obs at dryline -- obs convergence and convergence and moisture gradientmoisture gradient

15-20kt

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00Z 1 July 200000Z 1 July 2000

12Z 20kt12Z 20kt

00Z 15-20kt00Z 15-20kt

35mm hail / tornado south of YQF35mm hail / tornado south of YQF

YQFYQF

YEGYEG

YYCYYC

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Bulging DrylineBulging Dryline

• eastward bulge in south -- jet momentum mixed to SFCeastward bulge in south -- jet momentum mixed to SFC

RM storms may interact with bulging portion -- LL MESORM storms may interact with bulging portion -- LL MESO

20-30kt 30kt

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00Z 15 July 200000Z 15 July 2000Pine Lake TornadoPine Lake Tornado

12Z 25-30kt

00Z 30kt

YQFYQF

YYCYYC

YEG

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Storm Interaction with DrylineStorm Interaction with Dryline

streamwise vorticity tilted into streamwise vorticity tilted into vertical and ingested into storm vertical and ingested into storm updraftupdraft

cyclonic vorticity cyclonic vorticity within storm --within storm --LL mesocycloneLL mesocyclone

streamwise vorticity streamwise vorticity near dryline boundarynear dryline boundary

storm propagates storm propagates eastward along dryline eastward along dryline feeding on moist airfeeding on moist air

EE

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Forecast IssuesForecast Issues• SW flow aloft (upper trough / low)SW flow aloft (upper trough / low)• moisture gradient over foothills -- SFC moisture gradient over foothills -- SFC

isodrosotherms, mixing ratio, theta e/w isodrosotherms, mixing ratio, theta e/w • at least 15-20kt SW flow over mountains (700mb)at least 15-20kt SW flow over mountains (700mb)• monitor SFC winds / moisture SW Alberta -- often monitor SFC winds / moisture SW Alberta -- often

develops by 18Zdevelops by 18Z• mid-level jet over mtns -- monitor dryline for bulge mid-level jet over mtns -- monitor dryline for bulge

( bulging can trigger storms in SW/W SK)( bulging can trigger storms in SW/W SK)• beware RM storms near E-W portion of bulgebeware RM storms near E-W portion of bulge

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SummarySummary• a significant feature for summer severe weather in a significant feature for summer severe weather in

Alberta (13 of 22 days from 2000)Alberta (13 of 22 days from 2000)• develops mainly in response to subsidence in lee of develops mainly in response to subsidence in lee of

mountains mountains • synoptic scale => mesoscale => microscale synoptic scale => mesoscale => microscale • anticipate dryline development from 12Z analysis anticipate dryline development from 12Z analysis

(previous day?) -- i.e., forecastable(previous day?) -- i.e., forecastable• two types depending on 700mb flowtwo types depending on 700mb flow• RM storms can interact with bulging dryline -- tornadoRM storms can interact with bulging dryline -- tornado

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Future WorkFuture Work• establish dryline climatology (threshold wind establish dryline climatology (threshold wind

speeds, dewpoint gradients, time of convection, speeds, dewpoint gradients, time of convection, strength of cap…)strength of cap…)

• high-resolution mesoscale modeling of Alberta high-resolution mesoscale modeling of Alberta dryline dryline

• where’s the dryline? 100km SFC obs.where’s the dryline? 100km SFC obs.

• obtain in-situ measurements of BL moisture and obtain in-situ measurements of BL moisture and winds (mesonet, instrumented vehicle, winds (mesonet, instrumented vehicle, soundings, etc.) through UNSTABLEsoundings, etc.) through UNSTABLE