UNSTABLE Science Question 1: ABL Processes Water Vapour and Convergence Boundaries Neil Taylor Hydrometeorology and Arctic Lab, Environment Canada Dave Sills Cloud Physics and Severe Weather Research Section, Environment Canada

1st UNSTABLE Science Workshop Outline Taylor Factors important for CI Science Question 1 What do we need to do to resolve processes? Required Instrumentation and deployment Sills Further instrumentation and deployment strategies (ATMOS, AMMOS, Aircraft) February 22, 2019 1st UNSTABLE Science Workshop 18-19 April 2007

Factors Important for CI: Water Vapour CI potential can depend critically on small variations in temperature (1˚C) and mixing ratio (1 gkg-1) Variations of this magnitude are common over short distances but not resolved by synoptic-scale observation networks Fabry (2006) February 22, 2019 1st UNSTABLE Science Workshop 18-19 April 2007

Factors Important for CI: Water Vapour Availability / Depth Availability (e.g., Td, mixing ratio) Conceptually: need lifted parcel to attain level of free convection (reduces CIN and increases CAPE) Impacts on stability (CAPE / CIN) CAPE likely overestimated using SFC-based parcels 1 gkg-1 change in mixing ratio 2.5x impact on CAPE as 1 ˚C scales < 20 km CIN more sensitive to changes in mixing ratio scales > 20 km CIN more sensitive to changes in temperature Depth (depth of mixed moist layer) Mixed-layer parcels better represent convective cloud-base heights than SFC-based parcels CI less likely in shallow moisture conditions Deeper moisture more conducive to severe storms February 22, 2019 1st UNSTABLE Science Workshop 18-19 April 2007

Factors Important for CI: ABL Convergence Strength / Depth of lift Important for CI / maintenance of existing storms More intense storms triggered by ABL convergence lines Local deepening of moisture associated with convergence lines Modify environment within ~10 km February 22, 2019 1st UNSTABLE Science Workshop 18-19 April 2007

Factors Important for CI: Meso. Circulations and Wind Shear Mesoscale circulations associated with ABL convergence lines and surface heterogeneities, e.g., land-breeze (see question 2) Divergence below the LFC can counter low-level convergence Erect updrafts extend higher into ABL – favoured with equal shear on both sides of convergence boundary Ambient shear tilts parcel updrafts allowing more entrainment along a longer trajectory below cloud base Shear acts to remove parcel from influence of low-level convergence February 22, 2019 1st UNSTABLE Science Workshop 18-19 April 2007

Mesoscale Circulations and Convergence Boundaries Ziegler and Rasmussen (1998) Crook and Klemp (2000) February 22, 2019 1st UNSTABLE Science Workshop 18-19 April 2007 Weiss et al. (2004)

Factors Important for CI: The Dryline Mesoscale boundary forms in AB foothills Associated with low-level convergence and strong gradient in moisture – focus for CI Well documented over U.S. Plains ‘Recently’ shown to be important for CI over the foothills though formation mechanism may differ from that in the Great Plains region One of the boundaries of primary interest for UNSTABLE February 22, 2019 1st UNSTABLE Science Workshop 18-19 April 2007

Factors Important for CI: The Dryline Dryline observed with and without mobile observations (Hill 2006) Drylines and severe storm tracks from summer 2000 Dryline transect (Strong) February 22, 2019 1st UNSTABLE Science Workshop 18-19 April 2007 Taylor (2004)

1st UNSTABLE Science Workshop Science Question 1 What are the contributions of ABL processes to the initiation of deep moist convection and the development of severe thunderstorms in the Alberta Foothills? What is ABL evolution especially wrt water vapour prior to and during CI? What is role and importance of mesoscale convergence boundaries and circulations associated with CI? How are they influenced by terrain and synoptic-scale processes? How do they affect storms (motion, intensity, morphology)? What is 4D characterization of the dryline and importance for CI? Which storms become severe and why? How related to boundaries associated with CI? Are conceptual models adequate? How improve observational network to aid forecasters? February 22, 2019 1st UNSTABLE Science Workshop 18-19 April 2007

What is Needed to Resolve ABL and Other Processes Related to CI? Mobile SFC Aircraft Soundings Profilers Tethersonde Fixed Mesonet N February 22, 2019 1st UNSTABLE Science Workshop 18-19 April 2007

Supplemental Instrumentation 19 Station Configuration 15 Station Configuration Fixed Mesonet stations (10-20) 2 radiosondes Tethersonde 2 WV radiometers Profiling radiometer (H2O profile) GPS PW sensors Eddy Correlation Flux Tower(s)? Additional Profiling Radiometer (T, RH)? Mobile AMMOS / Strong Mobile (T, P, RH) MARS (PW, SFC wx, profile – wind, T, RH) 3 radiosondes Aircraft Photography Locations of fixed radiometers, GPS sensors, tethersonde to be determined February 22, 2019 1st UNSTABLE Science Workshop 18-19 April 2007

Instrumentation Deployment* Design fixed mesonet to take advantage of existing stations and climatologically favoured regions for CI and severe storms (includes high-resolution lines of mesonet stations) Two fixed sounding locations upstream (at low-levels) of foothills – catch moist advection and pre-storm ABL Fixed tethersonde, WV radiometers, GPS PW sensors, profiling radiometer(?) in primary study area near expected CI regions Mobile surface platforms to be deployed on intensive observation days to obtain four-dimensional characteristics of ABL and upper troposphere Target mesoscale boundaries and favoured CI regions within study area(s) ‘Bookend’ AMMOS (and Strong’s) mobile mesonet with mobile radiosondes Attempt to place MARS near to, and east of, observed boundaries (thermal, moisture, wind profiles) Supplement ground-based observations with aircraft stepped traverses * Details of deployment will appear in UNSTABLE field plan February 22, 2019 1st UNSTABLE Science Workshop 18-19 April 2007

1st UNSTABLE Science Workshop Summary CI depends on thermodynamic and kinematic processes both at the surface and aloft Water vapour availability / depth, ABL convergence, mesoscale circulations, ambient wind shear, the dryline all considered important for CI in the AB foothills These features are not resolved by existing observation networks – uncertainty remains with respect to their role and significance for CI in the region on varying spatial and temporal scales Have posed science questions to address these uncertainties in the context of the UNSTABLE field experiment Targeted, high-resolution measurements are required to resolve processes associated with CI and severe thunderstorm development UNSTABLE will include fixed and mobile surface, upper-air and profiling measurements – central to the success of UNSTABLE will be the ATMOS / AMMOS mesonet stations and aircraft measurements… February 22, 2019 1st UNSTABLE Science Workshop 18-19 April 2007