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Meteo 3: Chapter 5

Meteo 3: Chapter 5. Satellite and Radar Imagery. Remote Sensing. In-situ measurements expensive and lack spatial coverage Need instruments and platforms to observe large portions of the atmosphere quickly Passive remote sensors: Radiometers Active remote sensors: Radar. Satellite Basics.

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Meteo 3: Chapter 5

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  1. Meteo 3: Chapter 5 Satellite and Radar Imagery

  2. Remote Sensing • In-situ measurements expensive and lack spatial coverage • Need instruments and platforms to observe large portions of the atmosphere quickly • Passive remote sensors: Radiometers • Active remote sensors: Radar

  3. Satellite Basics • Used to see clouds (and other phenomena) • Passive remote sensors • observes without being in direct contact • collects energy emitted or scattered by object • Two relevant types (others we won’t worry about) • Geostationary- orbits Earth 22,500 mi above ground over equator at same rate Earth rotates • GOES- Geostationary Operational Environmental Satellite • Polar-orbiting- orbits pass over the poles • Solves GOES problem of being unable to accurately “see” pole • Can only see small swaths of Earth on each pass

  4. Satellite Orbits Current Satellite Orbits

  5. Radiometer- measures visible light reflected off clouds/objects (albedo)…very sensitive to differences in reflected visible light Useful only during day Visible Satellite Imagery

  6. Thin cirrus duller than thick cumulonimbus because cirrus has a much lower albedo Distinguishes between thick (bright) & thin (dull) clouds Thin vs. Thick Clouds

  7. Animations help (show motion & evolution of weather systems) Rivers visible? Clouds vs Snow

  8. Senses infrared radiation emitted by clouds and other objects between 10-12μm Useful at all times Infrared Satellite Imagery

  9. Low clouds appear gray, high clouds bright Temperature decreases with height => E=σT4 => more infrared radiation emitted from low clouds than high clouds Measures emitted radiation, or temperature, differences Cold objects are bright, warm are grey High vs Low Clouds

  10. Measures infrared radiation emitted at 6.7 μm…water vapor emits strongly here Can only sense water vapor in mid-upper troposphere…water vapor emission below will be absorbed Dark spots dry in upper troposphere, bright are moist Water Vapor Imagery

  11. Color Enhanced

  12. Radar Imagery • Radio Detection and Ranging • Active remote sensor • emits pulses of electromagnetic energy (microwaves), then measures how much scattered back off targets (precipitation, insects, mountains, etc.) • Distance of echo from radar determined by considering elapsed time between radar emitting radiation and it returning, as radiation travels at speed of light • Detects precipitation in reflectivity mode • intensity of returned energy depends on number, size, and composition of targets • the more & the larger the targets, the higher the reflectivity…if targets are rain, the rainfall rate is greater

  13. WSR-57 (Weather Surveillance Radar, 1957)

  14. Next Generation Radar D stands for Doppler 175 mi range WSR-88D or NEXRAD

  15. Single image vs composite

  16. Displays including precipitation type

  17. Derive rainfall totals from reflectivity

  18. Wet hail has high reflectivity

  19. Snowflakes have smaller reflectivities than similar-sized raindrops

  20. Radar and severe weather: hook echoes

  21. Bright-banding

  22. Doppler Radar • Johann Christian Doppler noticed change in pitch (frequency) of sound as source moved away or toward stationary observer • frequency of radiation off target is changed if target is moving • faster the speed toward or away, the greater the frequency change • for radar, these frequency changes translate into wind speeds and directions toward or away from radar site

  23. Doppler Schematic www.colorado.edu/physics/2000/applets/doppler2.html

  24. Tornado Vortex Signature • Present as early as 20-30 minutes before tornado

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