Lectures_11a - Severe Weather A roll cloud forming behind a...

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Fig. 10-8, p.264 Severe Weather A roll cloud forming behind a gust front
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Severe Weather We shall look at severe weather on a number of scales Mesoscale Thunderstorms Severe thunderstorms Mesoscale convective complex systems Tornadoes Hurricanes
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Thunderstorms Form when warm, humid air rises in an unstable environment, condensation of the water occurs, and precipitation results. Ordinary (Air-Mass) thunderstorms : – Summer (mostly) – Away from fronts. – Short-lived (30min – 1 hour) – No strong winds. No hail. Severe thunderstorms : – High winds. – Heavy rain/Flash floods. Hail. – sometimes tornadoes Mesoscale convective complex – Extensive (spatially) severe weather Storms : 40,000/day around the world. 100 lightning flashes/sec around the world.
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Ingredients for a thunderstorm Unstable air mass Moisture (particularly at low-levels) needed for clouds to form A mechanism to lift air parcels past the level of free convection (LFC) can be provided by synoptic-scale fronts, mesoscale boundaries (gust fronts, sea breeze fronts), etc.
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Fig. 10-1, p.259 Simplified model depicting the life cycle of an ordinary thunderstorm that is nearly stationary. (Arrows show vertical air currents. Dashed line represents freezing level, 0°C isotherm.) Life Cycle of Thunderstorm
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Fig. 10-1a, p.259 Cumulus or initiation stage Air is unstable Lifted above the LCL into Level of free convection Life Cycle of Thunderstorm
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Fig. 10-1b, p.259 Mature Stage Condensation Intense rainfall Falling rain drops drag air down Evaporation cools air (heavier) Tends to cut off upwelling circulation Anvil formation Overshooting at top Gust front (cool air) pushes up warm, humid air to help fuel the storm This may initiate another TS Life Cycle of Thunderstorm
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Fig. 10-1c, p.259 Dissipating Stage Weak downdraft Anvil extends horizontally with strong winds aloft Life Cycle of Thunderstorm
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Cumulus stage single or cluster of clouds form as humid air rises and condenses. Cloud grows vertically in the unstable environment (can be just a few minutes). The latent heat released during the condensation maintains the air parcel at a warmer temperature, which helps it to rise higher. Cloud drops begin to grow larger, heavier. There is entrainment of dry air inside the cloud. Drops begin to fall in a downdraft. Thunderstorms
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Mature Stage: Appearance of downdraft . Up/down drafts forming dynamical cell May have many cells (may last an hour or so each). Most intense, heavy rain during the mature phase (sometimes hail) Tops takes on anvil shape due to high winds. Lightning and thunder can occur. There is a lot of turbulence in the centre of the cloud.
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This note was uploaded on 06/05/2010 for the course EATS 1011 taught by Professor Johnm during the Winter '10 term at York University.

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Lectures_11a - Severe Weather A roll cloud forming behind a...

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