SUMMER SEVERE WEATHER

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SUMMER SEVERE WEATHER

• Transport Canada
• Aviation Safety Seminar
• March 15, 2006
• Nick Czernkovich

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Outline

• Scales of Motion
• Thunderstorms
• Mesoscale Convective Systems
• Downbursts
• Flight Planning

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Weather SystemsWhy does weather occur?

• NATURE LOVES AN EQUILIBRIUM !!
• Net energy imbalance
• More incoming energy in the tropics
• Less incoming energy at the poles

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Scales Of MotionGeneral

• Can be divided based on
• Observations
• Energy contained within the scales

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Scales Of MotionPlanetary Scale
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Scales Of MotionSynoptic Scale
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Scales Of MotionMesoscale
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Scales Of MotionTime- vs. Length-Scales
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Scales Of MotionLimits of Predictability

• Theoretical Limit 2 weeks
• In General
• Synoptic Scales 2-5 days
• Mesoscale 1-6 hours (or less!)

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Mesoscale Features

• Thunderstorms
• Garden Variety/Airmass
• Multicell
• Supercell
• Mesoscale Convective Systems
• Squall Lines
• Bow Echoes
• Downbursts

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A Note on Humidity

• Relative Humidity saturation
• Temperature Dew Point spread is a measure of RH
• Smaller T-Td spread Higher RH
• Ok so far?
• Airmass 1 has T20C and Td5C
• Airmass 2 has T8C and Td5C
• Which has a higher RH?
• Which contains more water vapour?

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A Note on Humidity

• Airmass 2 has a higher RH because the T-Td
  spread is smaller
• They both hold the same amount of water vapour
• Temperature puts a cap on dew point because T gt
  Td, ALWAYS
• Td is a measure of water vapour available, not T

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Thunderstorms
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ThunderstormsParcel Theory

• Parcel Theory
• Considers a lifted parcel to be a closed system
• No mass is exchanged with the environment
• As parcel rises, it expands to equalize pressure
  with surroundings

700 mb
850 mb
Parcel
900 mb
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ThunderstormsCloud Formation
Latent Heat Release
Rising air expands and cools
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Thunderstorms An Idealized Example
Altitude
Temperature
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ThunderstormsCapping Inversion
Capping Inversion
Parcel
Add Moisture
Warming
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ThunderstormsTemperature and Moisture

• Increases in TEMPERATURE and DEW POINT can
  destabilize a parcel
• An increase in dew point will destabilize a
  parcel MORE than an equivalent increase in
  temperature.

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ThunderstormsTriggering Mechanisms

• Heating
• Moisture
• Lifting
• Terrain
• Mechanical (Turbulence)
• Frontal
• Convergence (Southwestern Ontario!)
• Upper-level divergence

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ThunderstormsSingle Cell

• Cumulus
• Air is lifted to LFC
• Updrafts only
• Mature
• Parcel reaches maximum altitude (Tropopause)
• Precipitations forms
• Updrafts and downdrafts co-exist
• Dissipating
• Precipitation falls
• Downdrafts only

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ThunderstormsSingle Cell

• Often referred to as popcorn convection or
  pulse storms
• Lifetimes 30 min to 1 hr
• Difficult to predict location of formation
• Usually disorganized
• Form in low shear environments

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ThunderstormsSingle Cell
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ThunderstormsMulticell

• Organized group of single cells
• Self sustaining
• Each cell goes through the typical single cell
  lifecycle
• Outflow from old cells generates new cells
• Usually on the southern flank

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ThunderstormsMulticell

• Lifetimes 1 to 3 hr
• Form in moderate shear environments
• Heaviest precipitation on downwind side
• Weather
• Locally high winds due to outflow
• Heavy rainfall
• Hail/Tornado possible

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ThunderstormsMulticell
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ThunderstormsSupercell

• MOST SEVERE Hail/Wind/Tornadoes
• Form in Strong Shear Environments
• Typically, wind direction rotates with height
• Organized and long-lived 1 to 3 hr
• Weather
• Locally high winds due to outflow
• Heavy rainfall
• Hail/Tornado possible

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ThunderstormsSupercell
T
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ThunderstormsSupercell
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ThunderstormsSupercell
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ThunderstormsSupercell
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ThunderstormsSupercell

• Sunday March 12, 2006
• Kansas Missouri
• 2.25 Hail
• Damaging Wind
• 5 Tornado Reports

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ThunderstormsSupercell
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ThunderstormsSupercell
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ThunderstormsHail

• Form in severe thunderstorms
• Strong updrafts / displaced downdrafts
• Hail tends to fall DOWNWIND of storm
• Can fall as far out beneath the anvil
• Golf ball and Baseball sized hail possible!

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ThunderstormsHail

• Storms
• Fast moving (not necessary)
• Long-lived (supercell or multicell)
• Radar
• Dry hail DOES NOT show up well on radar
• Look for BWER Hail down shear
• High radar reflectivities (strong rain rates)

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ThunderstormsHail
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ThunderstormsMotion

• Synoptic systems tend to move with the 500 mb
  wind (18 000 ft)
• Individual thunderstorms (single cells) move with
  the mean wind in cloud layer
• At 45 deg latitude, 700 mb (9000 ft) wind
• Organized thunderstorms (multi- and supercells)
  move due to advection propagation

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ThunderstormsMotion

• Single Cells
• Move with the mean wind in the cloud layer
• At 45 deg latitude, this is 700 mb (9000 ft)
• Multicells
• Advection Propagation
• Embedded cells move with mean wind
• Storm system usually moves right of mean wind
• Supercells
• Storm system usually moves right of mean wind

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ThunderstormsMotion

• Angle between Mean Wind and Storm Motion varies
• Larger angle more organized
• Often more severe
• Fast moving storms often more severe

Propagation
Mean Wind
Storm Motion
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ThunderstormsFlying Considerations

• Counter-Clockwise isnt always best!
• Storm systems tend to move SOUTHWEST to
  NORTHEAST
• Organized storms tend to move to the right of
  the mean wind
• Mean wind 700 mb (9000 ft)

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ThunderstormsFlying Considerations

• Hail
• Typically falls downwind of storm
• Caution under thunderstorm anvil
• Turbulence
• Updrafts can reach 3000 ft/min in the core
• Under anvil
• Stay above cloud base Gust front VERY
  turbulent
• Gravity waves Above thunderstorm

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ThunderstormsFlying Considerations

• New Cell Growth
• Often on the southern flank
• Look for Towering Cumulus feeder clouds
• TCus can become CBs VERY quickly
• TCus can be just as turbulent!!!
• Torrential Rainfall
• Flameout Turbines
• Rapidly reduced visibility
• Local flooding (airports landing considerations)

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ThunderstormsFlying Considerations

• Lightning

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ThunderstormsFlying Considerations

• Local pressure changes
• Inside thunderstorm
• In cold outflow
• Downbursts and Wind Shear
• Extreme local changes in wind speed/direction
• Microbursts and Macrobursts
• More to be discussed

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Mesoscale ConvectiveSystems
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Mesoscale Convective SystemsCharacteristics

• Large, organized convection
• Lifetime 3 hrs to 1 day
• Basic physics are the same as thunderstorms
• Considered here
• Squall lines
• Bow Echoes

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Mesoscale Convective SystemsSquall Lines

• Linearity
• Leading line of thunderstorms trailing
  stratiform rain
• Embedded Supercells and Tornadoes
• Damaging Winds

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Mesoscale Convective SystemsSquall Lines

• Cold fronts
• Ahead of cold fronts (pre-frontal squall)
• 100-300 sm ahead of front
• Between 150-500 sm from the low center
• Dry Lines

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Mesoscale Convective SystemsSquall Lines
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Mesoscale Convective SystemsSquall Lines
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Mesoscale Convective SystemsBow Echoes
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Mesoscale Convective SystemsExamples
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Mesoscale Convective SystemsExamples
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Mesoscale Convective SystemsThings Change Fast!
1042 Z
1142 Z
1242 Z
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Downbursts
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Downbursts

• Downburst
• Defined by Fujita Caracena in 1977
• An exceptionally strong downdraft
• Vertical speed gt 750 ft/min at 3000 ft AGL
• Areal extent gt 800 m

Downdraft
HW
Gust Front
DB
Cold Outflow
TW
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Downbursts

• Macroburst
• Outflow gt 4 km in diameter
• Damaging winds last 5-20 min
• Microburst
• Outflow lt 4 km in diameter
• Peak winds last 2-5 min
• DRY and WET microbursts

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DownburstsEnvironmental Conditions

• DRY Microburst
• Elevated cloud bases (elevated CBs)
• Light rain (lt 35 dBZ) and virga
• Moist upper-air (500 mb)
• Dry sub-cloud layer (dry adiabatic)
• Most common in U.S. southwest

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DownburstsEnvironmental Conditions

• WET Microburst
• Typical (lower) cloud bases
• Heavy rainfall (gt 35 dBZ)
• Dry mid-level (500 mb)

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DownburstsPhysical Origins of the Microburst

• TWO Causes
• Precipitation drag
• Evaporation
• Evaporation is 10x more efficient
• Entrainment
• Side of CB cell
• Overshooting top

Cloud top
Sides
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DownburstsCharacteristics

• Tend to occur in families
• Peak intensity often reached after
  5-10 min
• Can occur from seemingly innocuous clouds

• Downdraft 3000 ft/min
• Local pressure change in outflow
• Extreme Wind Shear
• Extreme turbulence at gust front

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DownburstsCharacteristics
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DownburstsCharacteristics
DRY MICROBURST
WET MICROBURST
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DownburstsCharacteristics
Sometimes theyre hard to see!!
Blowing Dust
Gust Front
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DownburstsJune 24, 1975
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Flight Planning
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Flight PlanningSynoptic Scale

• Review Surface Analysis
• Low Pressure Areas
• Fronts
• Observed winds (look for areas of lift)
• Terrain, Fronts, Convergence, etc.
• Areas of high temperature and dewpoint
• Check Upper Air Charts

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Flight PlanningSynoptic Scale
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Flight Planning
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Flight PlanningSpecifics

• Temperatures
• Dew Points
• Low-level airflow
• Temperature/Dew Point Advection
• Check FDs for vertical winds shear
• Locations of convergence and lift
• Lake-Effect, The Summer Kind!

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Flight PlanningFinal Thoughts

• Shear is the dominant factor in storm
  organization
• Even a stable atmosphere can be destabilized by
  lifting
• Nocturnal Thunderstorms
• TCus that last into the overnight can become
  unstable due to cloud-top-cooling

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Flight Planning

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