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

MOIP Simulator: Drylines Outline brief review of dryline ‘theory’ mechanism responsible for dryline genesis in Alberta dryline morphology operational forecast issues future work 2/18/2019 MOIP Simulator: Drylines

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 2/18/2019 MOIP Simulator: Drylines

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 2/18/2019 MOIP Simulator: Drylines

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 2/18/2019 MOIP Simulator: Drylines

Southwest Flow Over Mountains subsidence in lee of Rockies -- 700mb flow warmer & drier air mixes down to SFC 30kt Jet 25kt Jet 2/18/2019 MOIP Simulator: Drylines

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 2/18/2019 MOIP Simulator: Drylines

Schematic Dryline Cross-section 2 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 2/18/2019 MOIP Simulator: Drylines

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 2/18/2019 MOIP Simulator: Drylines

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% 2/18/2019 MOIP Simulator: Drylines

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 2/18/2019 MOIP Simulator: Drylines

Dryline Day Severe Weather Events 2/18/2019 MOIP Simulator: Drylines

Dryline Locations Summer 2000 2/18/2019 MOIP Simulator: Drylines

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) 2/18/2019 MOIP Simulator: Drylines

Dryline Associated Storm Tracks 2/18/2019 MOIP Simulator: Drylines

Dryline Locations and Storm Tracks 2/18/2019 MOIP Simulator: Drylines

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 2/18/2019 MOIP Simulator: Drylines

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 2/18/2019 MOIP Simulator: Drylines

00Z 1 July 2000 35mm hail / tornado south of YQF 12Z 20kt YEG YQF YYC 2/18/2019 MOIP Simulator: Drylines

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 2/18/2019 MOIP Simulator: Drylines

MOIP Simulator: Drylines 00Z 15 July 2000 Pine Lake Tornado YEG YQF 12Z 25-30kt YYC 00Z 30kt 2/18/2019 MOIP Simulator: Drylines

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 2/18/2019 MOIP Simulator: Drylines

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 2/18/2019 MOIP Simulator: Drylines

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 2/18/2019 MOIP Simulator: Drylines

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