1. Genesis and Morphology of the Alberta Dryline
Neil M. Taylor
Hydrometeorology and Arctic Lab
Meteorological Service of Canada

2. MOIP Simulator: Drylines
Outline
brief review of dryline ‘theory’
mechanism responsible for dryline genesis in Alberta
dryline morphology
operational forecast issues
future work
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MOIP Simulator: Drylines

3. MOIP Simulator: Drylines
The Dryline
strong moisture gradient in the BL (obs at SFC)
dry air: hot -- deep, well mixed boundary layer
moist air: cooler & capped in BL -- dry and well mixed aloft => loaded gun sounding
literature: frontogenetic (wrt moisture) circulation driven mainly by differential heating (e.g., land breeze effect)
convergence at dryline => convective initiation
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MOIP Simulator: Drylines

4. Dryline Interpretation
circulation near dryline important for low-level mesocyclogenesis (tornadoes)
forms capping lid + convergence line a focus for convective initiation
simplest interpretation: storm - no storm boundary (e.g., too dry => no storms)
IMPLICATIONS
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MOIP Simulator: Drylines

5. Severe Convection in Alberta
“breakdown of upper ridge” associated with upper trough or low advancing from BC
12Z 250mb and 500mb charts showing advancing upper trough / low
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MOIP Simulator: Drylines

6. Southwest Flow Over Mountains
subsidence in lee of Rockies mb flow
warmer & drier air mixes down to SFC
30kt Jet
25kt Jet
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MOIP Simulator: Drylines

7. MOIP Simulator: Drylines
Genesis of the Dryline
Dryline
Moist, capped
Dry, Well Mixed
dry air mixes to SFC and converges with moist air over plains
SW’ly 700mb flow
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MOIP Simulator: Drylines

8. Schematic Dryline Cross-section2 4 3
capped 1
Dry Air
10-20km
Moist Air
1. SFC: dry air convergent with moist air over / near foothills
2. Aloft: dry air overruns moist air
3. Convergence at dryline breaks capping lid => deep convection
4. Secondary circulation generates horizontal vorticity
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MOIP Simulator: Drylines

9. Severe Weather Days from 2000
Days from summer 2000 (AB and SK only) with...
3 or more reports of ≥ 20mm hail or
1 or more reports of ≥ 30mm hail or
at least 1 report of a tornado
33 days met criteria
Wx Reports
Alberta
Saskatchewan
Total
Tornado 33 24 57
Severe Hail 91 51
142
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MOIP Simulator: Drylines

10. MOIP Simulator: Drylines
Dryline Days
Defined as a day where severe weather reports met 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 ‘dryline day’ criteria
Wx Reports
Tornado
≥20mm Hail
All Reports
Dryline Days 21 78 99
Severe Days 57
142
199
% Dryline
37%
56%
50%
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11. MOIP Simulator: Drylines
12Z 500mb 570dam on Dryline Days
500mb TROF/Low
Upper trof in BC leads to SW flow over Rocky mountains => subsidence necessary to form dryline
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12. Dryline Day Severe Weather Events
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13. Dryline Locations Summer 2000
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14. Dryline Associated Storm Tracks
Considered storms initiated near dryline that produced severe weather (or led to severe storm via outflow boundary cell generation / storm splitting)
Remote sensing used to define a storm track via radar (1.5km cappi – first echo to <40dBZ), VIS satellite + CLDN lightning flash data
26 severe thunderstorms initiated near dryline (or led to severe storm development via outflow boundary / splitting)
All 26 storms initiated within 75km of dryline (61% within 50km and mean distance of 45km)
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MOIP Simulator: Drylines

15. Dryline Associated Storm Tracks
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16. Dryline Locations and Storm Tracks
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17. MOIP Simulator: Drylines
Dryline Morphology
develops rapidly (<6 hours) parallel to foothills
evolution strongly linked to flow over mountains (~700mb)
Two Types: (depending on 700mb flow)
I: general 15-20kt SW flow over Rockies => QS
II: 20-30kt 700mb jet => bulges eastward
direction / extent of bulge function of location, angle, and speed of jet over mountain barrier
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MOIP Simulator: Drylines

18. MOIP Simulator: Drylines
QS Dryline
little “bulging”
Cu/TCu over foothills due differential heating
capping lid “breaks” at dryline -- obs convergence and moisture gradient
15-20kt
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19. 00Z 1 July 2000 35mm hail / tornado south of YQF 12Z 20kt YEG YQF YYC
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20. Bulging Dryline eastward bulge in south -- jet momentum mixed to SFC
20-30kt
30kt
eastward bulge in south -- jet momentum mixed to SFC
RM storms may interact with bulging portion -- LL MESO
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MOIP Simulator: Drylines

21. MOIP Simulator: Drylines
00Z 15 July 2000
Pine Lake Tornado
YEG
YQF
12Z 25-30kt
YYC
00Z 30kt
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22. Storm Interaction with Dryline
streamwise vorticity near dryline boundary
cyclonic vorticity
within storm --
LL mesocyclone E
storm propagates eastward along dryline feeding on moist air
streamwise vorticity tilted into vertical and ingested into storm updraft
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MOIP Simulator: Drylines

23. MOIP Simulator: Drylines
Forecast Issues
SW flow aloft (upper trough / low)
moisture gradient over foothills -- SFC isodrosotherms, mixing ratio, theta e/w
at least 15-20kt SW flow over mountains (700mb)
monitor SFC winds / moisture SW Alberta -- often develops by 18Z
mid-level jet over mtns -- monitor dryline for bulge ( bulging can trigger storms in SW/W SK)
beware RM storms near E-W portion of bulge
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MOIP Simulator: Drylines

24. MOIP Simulator: Drylines
Summary
a significant feature for summer severe weather in Alberta (13 of 22 days from 2000)
develops mainly in response to subsidence in lee of mountains
synoptic scale => mesoscale => microscale
anticipate dryline development from 12Z analysis (previous day?) -- i.e., forecastable
two types depending on 700mb flow
RM storms can interact with bulging dryline -- tornado
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MOIP Simulator: Drylines

25. MOIP Simulator: Drylines
Future Work
establish dryline climatology (threshold wind speeds, dewpoint gradients, time of convection, strength of cap…)
high-resolution mesoscale modeling of Alberta dryline
where’s the dryline? 100km SFC obs.
obtain in-situ measurements of BL moisture and winds (mesonet, instrumented vehicle, soundings, etc.) through UNSTABLE
2/18/2019
MOIP Simulator: Drylines