clim forecast

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Thunderstorm and Tornado Climatology: 

Thunderstorm and Tornado Climatology Geographic distribution Global Tropics  warm, moist air, sustained uplift E.g. Bogor, Indonesia  322 thunderstorm days per year Amazon rainforest (S. America), Congo basin (Africa)

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US  100,000 thunderstorms, 26 million lightning strikes/yr Central Florida  sea breezes + warm, moist air Eastern Gulf Coast  proximity to warm, moist air from Gulf East slopes of Rockies (Colorado, New Mexico)  orographic influence

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Tornado climatology Tornado Alley: strip of tornado maximum oriented SW-NE from S. Plains to lower Great Lakes Avg. 800/yr in US Codell, Kansas  hit 3 years in a row (1916-18) on same date: May 20!

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Geography of North America Central US  flat, no natural barriers  facilitates movement, collision of air masses (from CA, Gulf of Mexico, desert SW) Dryline: narrow zone of abrupt change in moisture (dew point T°) warm, dry air from desert SW pulled into warm sector of cyclone converges with warm, moist air from Gulf  convection

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Statistics Peak spring, early summer (Apr-Jun)  max air mass contrasts  strongest jet stream mid-afternoon  max heating Majority (74%) are F0-F1

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Seasonal shift in tornado occurrence Feb-Mar  central Gulf states Apr-May  southern Great Plains Jun-Jul  northern Great Plains, southern Great Lakes Increasing northward penetration of warm, moist air  collision zone with cold, dry Canadian air retreats northward

Forecasting Tools: 

Forecasting Tools Doppler radar Comparing frequency of reflected radar pulses  detect air motion within clouds Doppler effect Target moving AWAY from radar  radar returns at LOWER frequency Target moving TOWARDS radar  radar returns at HIGHER frequency E.g. change in pitch of train whistle as it passes by Increased lead times for tornado warnings from < 5 min to 10 min

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Radar detection of tornadoes Hook echo: radar precipitation returns resembling ‘hook’ in SW portion of supercell strong updraft adjacent to strong downdraft  strong wind shear  mesocyclone rotation  development of tornado vortex Velocity echoes “gate-to-gate” shear  red echoes (moving away from radar) adjacent to green (moving towards radar)  rotation

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Atmospheric soundings Data gathered for various heights in atmosphere from weather balloons Vertical profile of wind direction, speed, T°, dew point T°  reveal instability, wind shear

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