UNSTABLE

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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

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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)

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Factors Important for CI Water Vapour

• CI potential can depend critically on small
  variations in temperature (1C) 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)
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Moisture Varies in Vertical Too
Weckwerth et al. (1996)
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Factors Important for CIWater 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 lt 20 km CIN more sensitive to changes in
  mixing ratio
• scales gt 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

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SFC-Based vs. Mean Layer Parcel
SFC-based LCL underestimated
Craven et al. (2002) comparisons of SFC and
mean-layer parcels to observed cloud bases
SBCAPE consistently higher than MLCAPE
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Factors Important for CIABL 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

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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

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Mesoscale Circulations and Convergence Boundaries
Ziegler and Rasmussen (1998)
Crook and Klemp (2000)
Weiss et al. (2004)
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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

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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)
Taylor (2004)
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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?

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What is Needed to Resolve ABL and Other Processes
Related to CI?
N
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Supplemental Instrumentation
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
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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 Strongs) 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
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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