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Published on October 7, 2007

Author: Reva

Source: authorstream.com

SUMMER SEVERE WEATHER:  SUMMER SEVERE WEATHER Transport Canada Aviation Safety Seminar March 15, 2006 Nick Czernkovich Outline:  Outline Scales of Motion Thunderstorms Mesoscale Convective Systems Downbursts Flight Planning Weather Systems Why does weather occur?:  Weather Systems Why does weather occur? NATURE LOVES AN EQUILIBRIUM !! Net energy imbalance More incoming energy in the tropics Less incoming energy at the poles Scales Of Motion General:  Scales Of Motion General Can be divided based on: Observations Energy contained within the scales Scales Of Motion Planetary Scale:  Scales Of Motion Planetary Scale Scales Of Motion Synoptic Scale:  Scales Of Motion Synoptic Scale Scales Of Motion Mesoscale:  Scales Of Motion Mesoscale Scales Of Motion Time- vs. Length-Scales:  Scales Of Motion Time- vs. Length-Scales Scales Of Motion Limits of Predictability:  Scales Of Motion Limits of Predictability Theoretical Limit: 2 weeks In General: Synoptic Scales: 2-5 days Mesoscale: 1-6 hours (or less!) Mesoscale Features:  Mesoscale Features Thunderstorms “Garden Variety”/Airmass Multicell Supercell Mesoscale Convective Systems Squall Lines Bow Echoes Downbursts A Note on Humidity:  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 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:  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 >= Td, ALWAYS Td is a measure of water vapour available, not T Thunderstorms:  Thunderstorms Thunderstorms Parcel Theory:  Thunderstorms Parcel 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 900 mb 850 mb 700 mb Parcel Thunderstorms Cloud Formation:  Thunderstorms Cloud Formation Latent Heat Release Rising air expands and cools Thunderstorms An Idealized Example:  Thunderstorms An Idealized Example Altitude Temperature Thunderstorms Capping Inversion:  Thunderstorms Capping Inversion Capping Inversion Parcel Thunderstorms Temperature and Moisture:  Thunderstorms Temperature 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. Thunderstorms Triggering Mechanisms:  Thunderstorms Triggering Mechanisms Heating Moisture Lifting Terrain Mechanical (Turbulence) Frontal Convergence (Southwestern Ontario!) Upper-level divergence Thunderstorms Single Cell:  Thunderstorms Single 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 Thunderstorms Single Cell:  Thunderstorms Single 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 Thunderstorms Single Cell:  Thunderstorms Single Cell Thunderstorms Multicell:  Thunderstorms Multicell 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 Thunderstorms Multicell:  Thunderstorms Multicell 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 Thunderstorms Multicell:  Thunderstorms Multicell Thunderstorms Supercell:  Thunderstorms Supercell 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 Thunderstorms Supercell:  Thunderstorms Supercell T Thunderstorms Supercell:  Thunderstorms Supercell Thunderstorms Supercell:  Thunderstorms Supercell Thunderstorms Supercell:  Thunderstorms Supercell Thunderstorms Supercell:  Thunderstorms Supercell Sunday March 12, 2006 Kansas – Missouri 2.25” Hail Damaging Wind 5 Tornado Reports Thunderstorms Supercell:  Thunderstorms Supercell Thunderstorms Supercell:  Thunderstorms Supercell Thunderstorms Hail:  Thunderstorms Hail 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! Thunderstorms Hail:  Thunderstorms Hail 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) Thunderstorms Hail:  Thunderstorms Hail Thunderstorms Motion:  Thunderstorms Motion 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 Thunderstorms Motion:  Thunderstorms Motion 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 Thunderstorms Motion:  Thunderstorms Motion Angle between Mean Wind and Storm Motion varies Larger angle ~ more organized Often more severe Fast moving storms often more severe Mean Wind Storm Motion Propagation Thunderstorms Flying Considerations:  Thunderstorms Flying Considerations Counter-Clockwise isn’t 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) Thunderstorms Flying Considerations:  Thunderstorms Flying 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 Thunderstorms Flying Considerations:  Thunderstorms Flying Considerations New Cell Growth Often on the southern flank Look for Towering Cumulus feeder clouds TCu’s can become CB’s VERY quickly TCu’s can be just as turbulent!!! Torrential Rainfall Flameout – Turbines Rapidly reduced visibility Local flooding (airports – landing considerations) Thunderstorms Flying Considerations:  Thunderstorms Flying Considerations Lightning Thunderstorms Flying Considerations:  Thunderstorms Flying 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 … Mesoscale Convective Systems:  Mesoscale Convective Systems Mesoscale Convective Systems Characteristics:  Mesoscale Convective Systems Characteristics Large, organized convection Lifetime ~ 3 hrs to 1 day Basic physics are the same as thunderstorms Considered here: Squall lines Bow Echoes Mesoscale Convective Systems Squall Lines:  Mesoscale Convective Systems Squall Lines Linearity Leading line of thunderstorms & trailing stratiform rain Embedded Supercells and Tornadoes Damaging Winds Mesoscale Convective Systems Squall Lines:  Mesoscale Convective Systems Squall 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 Mesoscale Convective Systems Squall Lines:  Mesoscale Convective Systems Squall Lines Mesoscale Convective Systems Squall Lines:  Mesoscale Convective Systems Squall Lines Mesoscale Convective Systems Bow Echoes:  Mesoscale Convective Systems Bow Echoes Mesoscale Convective Systems Examples:  Mesoscale Convective Systems Examples Mesoscale Convective Systems Examples:  Mesoscale Convective Systems Examples Mesoscale Convective Systems Things Change Fast!:  Mesoscale Convective Systems Things Change Fast! 1042 Z 1142 Z 1242 Z Downbursts:  Downbursts Downbursts:  Downbursts Downburst Defined by Fujita & Caracena in 1977 An exceptionally strong downdraft Vertical speed > 750 ft/min at 3000 ft AGL Areal extent > 800 m Downdraft Cold Outflow Gust Front HW TW DB Downbursts:  Downbursts Macroburst Outflow > 4 km in diameter Damaging winds last 5-20 min Microburst Outflow < 4 km in diameter Peak winds last 2-5 min DRY and WET microbursts Downbursts Environmental Conditions:  Downbursts Environmental Conditions DRY Microburst Elevated cloud bases (elevated CB’s) Light rain (< 35 dBZ) and virga Moist upper-air (~500 mb) Dry sub-cloud layer (dry adiabatic) Most common in U.S. southwest Downbursts Environmental Conditions:  Downbursts Environmental Conditions WET Microburst Typical (lower) cloud bases Heavy rainfall (> 35 dBZ) Dry mid-level (~500 mb) Downbursts Physical Origins of the Microburst:  Downbursts Physical Origins of the Microburst TWO Causes: Precipitation drag Evaporation Evaporation is ~10x more efficient Entrainment: Side of CB cell Overshooting top Cloud top Sides Downbursts Characteristics:  Downbursts Characteristics 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 Downbursts Characteristics:  Downbursts Characteristics Downbursts Characteristics:  Downbursts Characteristics DRY MICROBURST WET MICROBURST Downbursts Characteristics:  Downbursts Characteristics Sometimes they’re hard to see!! Blowing Dust Gust Front Downbursts June 24, 1975:  Downbursts June 24, 1975 Flight Planning:  Flight Planning Flight Planning Synoptic Scale:  Flight Planning Synoptic 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 Flight Planning Synoptic Scale:  Flight Planning Synoptic Scale Flight Planning:  Flight Planning Flight Planning Specifics:  Flight Planning Specifics Temperatures Dew Points Low-level airflow Temperature/Dew Point Advection Check FD’s for vertical winds shear Locations of convergence and lift Lake-Effect, “The Summer Kind”! Flight Planning Final Thoughts …:  Flight Planning Final Thoughts … Shear is the dominant factor in storm organization Even a stable atmosphere can be destabilized by lifting Nocturnal Thunderstorms: TCu’s that last into the overnight can become unstable due to cloud-top-cooling Flight Planning :  Flight Planning Nav Canada Aviation Digital Data Service (ADDS) Environment Canada Watches/Warnings Special Weather Statements Storm Prediction Center Watches/Warnings Mesoscale Discussions Convective Outlooks Hydrometeorological Prediction Center Research Applications Program

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