SUMMER SEVERE WEATHER Transport Canada Aviation Safety Seminar March 15, 2006 Nick Czernkovich

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

Weather Systems Why does weather occur? NATURE LOVES AN EQUILIBRIUM !! NATURE LOVES AN EQUILIBRIUM !! Net energy imbalance Net energy imbalance More incoming energy in the tropics More incoming energy in the tropics Less incoming energy at the poles Less incoming energy at the poles

Scales Of Motion General Can be divided based on: Can be divided based on: Observations Observations Energy contained within the scales Energy contained within the scales

Scales Of Motion Planetary Scale

Scales Of Motion Synoptic Scale

Scales Of Motion Mesoscale

Scales Of Motion Time- vs. Length-Scales

Scales Of Motion Limits of Predictability Theoretical Limit: 2 weeks Theoretical Limit: 2 weeks In General: In General: Synoptic Scales: 2-5 days Synoptic Scales: 2-5 days Mesoscale: 1-6 hours (or less!) Mesoscale: 1-6 hours (or less!)

Mesoscale Features Thunderstorms Thunderstorms “Garden Variety”/Airmass “Garden Variety”/Airmass Multicell Multicell Supercell Supercell Mesoscale Convective Systems Mesoscale Convective Systems Squall Lines Squall Lines Bow Echoes Bow Echoes Downbursts Downbursts

A Note on Humidity Relative Humidity = % saturation Relative Humidity = % saturation Temperature – Dew Point spread is a measure of RH Temperature – Dew Point spread is a measure of RH Smaller T-Td spread = Higher RH Smaller T-Td spread = Higher RH Ok so far? Airmass #1 has T=20C and Td=5C Airmass #2 has T=8C and Td=5C Which has a higher RH? Which contains more water vapour?

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

Thunderstorms

Thunderstorms Parcel Theory Parcel Theory: Considers a lifted parcel to be a closed system Considers a lifted parcel to be a closed system No mass is exchanged with the environment No mass is exchanged with the environment As parcel rises, it expands to equalize pressure with surroundings As parcel rises, it expands to equalize pressure with surroundings 900 mb 850 mb 700 mb Parcel

Thunderstorms Cloud Formation Latent Heat Release Latent Heat Release Rising air expands and cools Parcel cools at 3 C / 1000 ft Lifting Condensation Level RH = 100% Parcel cools at ~1.5 C / 1000 ft Phase change Vapour  Liquid

Thunderstorms An Idealized Example Altitude Temperature LFC LCL Environment 2 C/1000 ft 1.5 C/1000 ft 3 C/1000 ft Parcel

Thunderstorms Capping Inversion Capping Inversion Parcel LCL

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

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

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

Thunderstorms Single Cell Often referred to as “popcorn convection” or “pulse storms” Often referred to as “popcorn convection” or “pulse storms” Lifetimes ~ 30 min to 1 hr Lifetimes ~ 30 min to 1 hr Difficult to predict location of formation Difficult to predict location of formation Usually disorganized Usually disorganized Form in low shear environments Form in low shear environments

Thunderstorms Single Cell

Thunderstorms Multicell Organized group of single cells Organized group of single cells Self sustaining Self sustaining Each cell goes through the typical single cell lifecycle Each cell goes through the typical single cell lifecycle Outflow from old cells generates new cells Outflow from old cells generates new cells Usually on the southern flank Usually on the southern flank Warm Moist Inflow

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

Thunderstorms Multicell

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

Thunderstorms Supercell T

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

Thunderstorms Supercell

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

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

Thunderstorms Hail

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

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

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

Thunderstorms Flying Considerations Counter-Clockwise isn’t always best! Counter-Clockwise isn’t always best! Storm systems tend to move SOUTHWEST to NORTHEAST Storm systems tend to move SOUTHWEST to NORTHEAST Organized storms tend to move to the right of the mean wind Organized storms tend to move to the right of the mean wind Mean wind ~700 mb (9000 ft) Mean wind ~700 mb (9000 ft)

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

Thunderstorms Flying Considerations New Cell Growth New Cell Growth Often on the southern flank Often on the southern flank Look for Towering Cumulus feeder clouds Look for Towering Cumulus feeder clouds TCu’s can become CB’s VERY quickly TCu’s can become CB’s VERY quickly TCu’s can be just as turbulent!!! TCu’s can be just as turbulent!!! Torrential Rainfall Torrential Rainfall Flameout – Turbines Flameout – Turbines Rapidly reduced visibility Rapidly reduced visibility Local flooding (airports – landing considerations) Local flooding (airports – landing considerations)

Thunderstorms Flying Considerations Lightning Lightning

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

Mesoscale Convective Systems

Mesoscale Convective Systems Characteristics Large, organized convection Large, organized convection Lifetime ~ 3 hrs to 1 day Lifetime ~ 3 hrs to 1 day Basic physics are the same as thunderstorms Basic physics are the same as thunderstorms Considered here: Considered here: Squall lines Squall lines Bow Echoes Bow Echoes

Mesoscale Convective Systems Squall Lines Linearity Linearity Leading line of thunderstorms & trailing stratiform rain Leading line of thunderstorms & trailing stratiform rain Embedded Supercells and Tornadoes Embedded Supercells and Tornadoes Damaging Winds Damaging Winds

Mesoscale Convective Systems Squall Lines Cold fronts Cold fronts Ahead of cold fronts (pre-frontal squall) Ahead of cold fronts (pre-frontal squall) sm ahead of front sm ahead of front Between sm from the low center Between sm from the low center Dry Lines Dry Lines

Mesoscale Convective Systems Squall Lines

Mesoscale Convective Systems Bow Echoes

Mesoscale Convective Systems Examples

Mesoscale Convective Systems Things Change Fast! 1042 Z1142 Z1242 Z

Downbursts

Downbursts Downburst Downburst Defined by Fujita & Caracena in 1977 Defined by Fujita & Caracena in 1977 An exceptionally strong downdraft An exceptionally strong downdraft Vertical speed > 750 ft/min at 3000 ft AGL Vertical speed > 750 ft/min at 3000 ft AGL Areal extent > 800 m Areal extent > 800 m Downdraft Cold Outflow Gust Front HW TW DB

Downbursts Macroburst Macroburst Outflow > 4 km in diameter Outflow > 4 km in diameter Damaging winds last 5-20 min Damaging winds last 5-20 min Microburst Microburst Outflow < 4 km in diameter Outflow < 4 km in diameter Peak winds last 2-5 min Peak winds last 2-5 min DRY and WET microbursts DRY and WET microbursts

Downbursts Environmental Conditions DRY Microburst DRY Microburst Elevated cloud bases (elevated CB’s) Elevated cloud bases (elevated CB’s) Light rain (< 35 dBZ) and virga Light rain (< 35 dBZ) and virga Moist upper-air (~500 mb) Moist upper-air (~500 mb) Dry sub-cloud layer (dry adiabatic) Dry sub-cloud layer (dry adiabatic) Most common in U.S. southwest Most common in U.S. southwest

Downbursts Environmental Conditions WET Microburst WET Microburst Typical (lower) cloud bases Typical (lower) cloud bases Heavy rainfall (> 35 dBZ) Heavy rainfall (> 35 dBZ) Dry mid-level (~500 mb) Dry mid-level (~500 mb)

Downbursts Physical Origins of the Microburst TWO Causes: TWO Causes: Precipitation drag Precipitation drag Evaporation Evaporation Evaporation is ~10x more efficient Evaporation is ~10x more efficient Entrainment: Entrainment: Side of CB cell Side of CB cell Overshooting top Overshooting top Cloud top Sides

Downbursts Characteristics Tend to occur in families Tend to occur in families Peak intensity often reached after 5-10 min Peak intensity often reached after 5-10 min Can occur from seemingly innocuous clouds Can occur from seemingly innocuous clouds Downdraft ft/min Downdraft ft/min Local pressure change in outflow Local pressure change in outflow Extreme Wind Shear Extreme Wind Shear Extreme turbulence at gust front Extreme turbulence at gust front

Downbursts Characteristics Fast Moving Families of Microbursts Extreme Turbulence Shear Line

Downbursts Characteristics DRY MICROBURSTWET MICROBURST

Downbursts Characteristics Sometimes they’re hard to see!! Blowing Dust Gust Front

Downbursts June 24, 1975

Flight Planning

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

Flight Planning Synoptic Scale

Flight Planning

Flight Planning Specifics Temperatures Temperatures Dew Points Dew Points Low-level airflow Low-level airflow Temperature/Dew Point Advection Temperature/Dew Point Advection Check FD’s for vertical winds shear Check FD’s for vertical winds shear Locations of convergence and lift Locations of convergence and lift Lake-Effect, “The Summer Kind”! Lake-Effect, “The Summer Kind”!

Flight Planning Final Thoughts … Shear is the dominant factor in storm organization Shear is the dominant factor in storm organization Even a stable atmosphere can be destabilized by lifting Even a stable atmosphere can be destabilized by lifting Nocturnal Thunderstorms: Nocturnal Thunderstorms: TCu’s that last into the overnight can become unstable due to cloud-top-cooling TCu’s that last into the overnight can become unstable due to cloud-top-cooling

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