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Lecture

WindLecture5.doc

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Department
Environmental Science
Course
EESA09H3
Professor
Tanzina Mohsin
Semester
Summer

Description
Wind – ESSA09 – Lecture 5: Thunderstorms and Tornadoes Thunderstorms: - Also known as Convective storm  caused by surface heating o Suburban areas around the GTAare getting more warmer than the areas in downtown - Common in Southern Ontario in summer - Only storm that has thunder and lightning - Large thunderstorms can spawn/turn into tornadoes - Four categories: differs in strength of the wind o Ordinary:  Often develop within large air masses  Not necessarily near a frontal system  Little vertical wind shear (don’t need large air masses) • Change of horizontal wind speed with height  Three stages: • Cumulous stage: Differential surface heating induces upward flow in unstable air, updraft, cumulus cloud formation o Warm surface air rises (updraft), condensation starts (formation of cloud), change of phase, and latent heat released which starts to fuel the storm • Mature phase: Development of a downdraft with precipitation o precipitation, cold air starts to sink therefore you have a downdraft that forms at the same location (therefore, easier for DD to cutoff UD) of ordinary thunderstorm  when it is raining, you start to lose latent heat o Gust front develops as downdraft air spreads along horizontal surface o Gust front forces more air up into the updraft o Updraft and downdraft form a convective cell (change of heat) • Dissipation stage: Downdraft cuts off updraft and storm loses energy source and dissipates o When the gust front moves past the updraft, the updraft weakens o Rain starts to fall into the updraft, cutting off the rising humid air  Life Cycle: • Relatively short-lived • Less than 1 hour • Diameter is 1km or less o Multi-cell:  Similar to ordinary except moderate wind shear • Storm tilts • Downdraft forms downwind of updraft (does not form at the same location, therefore, harder for DD to cutoff UD) o As a result DD does not cutoff UD right away, it cuts it off in about 5 or 10 hours  Storm lasts longer • Gust front of one storm initiates or induces another storm o Super-cell:  Form with strong vertical wind shear (need strong large air masses) • Surface winds (mT air) from south/southwest • Upper level winds (cP air) from north/northwest  Along cold front of a midlatitude cyclone • Midlatitude cycle is dying down and at the tail of the cyclone you will see the development of a super-cell thunderstorm  100-600 m in diameter  Tornadoes can spawn • Midlatitude cyclone  super-cell thunderstorm  tornado  Downdraft does not cutoff updraft  Storm can last for several hours • Location of the DD is far away from the location of the UD  Hail can form  Microbursts can also form • i.e., almost like a shower-head, where water drops down straight the ground and then when it hits the ground (bathtub), the water curves up (rises up again) and falls at angles all around • Localized DD (energy is concentrated in one single space) o Radial burst of surface wind (when localized strong wind with formation of localized thunderstorm) • Aviation hazard o Airplane crash,Aug. 1985 at Dallas-Fort WorthAirport, 100 lives lost  Plane view of thunderstorm • Areas of UD and DD • Gust front • Up level blow off (anvil extent) o Mesoscale convective complexes (MCC):  Multiple thunderstorms • Little wind shear ????? • Circular fashion • Covers over 100,000 square km • 12 hours or more, self sustaining • Heavy precipitation - therefore, favourable to farmers  Good for farming, these types usually form over prairies - Squall Line: strong of thunderstorms along a cold front o Nov. 15, 1989 – The Huntsville Tornado is an example o Hail and tornadoes formed o Super-cell thunderstorms??? Lightning: - Special characteristics of thunderstorms - Charge separation occurs in the cloud (charge is carried by the cloud droplets or ice crystals) - Smaller particles tend to go to top of storm with positive charge, larger ones with negative charge to the bottom  negative charges attracts positive charges on the ground o Acts like a magnet to attract positive charge at the surface - Most (90%) lightning starts at cloud base and goes to the surface - First step – Stepped leader  a path of multiple 50m or so is ionized by 3 million volts of electricity o Further steps of 50 to 100m until surface is reached o As surface is neared, positive ions from the surface more upward  When the two connect, the luminous return stroke is seen  Several cm in diameter  Process can repeat leading to forked lightning  Air heats to 30,000deg. C  Generated shock wave  thunder!  Radio waves, “sferics”, are produced allowing for lightning detection worldwide Hail: - Largest form of solid precipitation and most damaging - 2 costliest natural weather disaster in Canada (after 1998 Ice storm) - Largest: 757 grams found in Coffeyville Kansas - Canadian largest: 290 grams in Saskatchewan - Formation: o Formed in cumulonimbus clouds (thunderclouds) o Formed through successive deposition of super-cooled water onto
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