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

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

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

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

9. High vs Low Clouds 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

11. Water Vapor Imagery
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

12. Color Enhanced

13. 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

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

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

16. Single image vs composite

17. Displays including precipitation type

18. Derive rainfall totals from reflectivity

19. Wet hail has high reflectivity

20. Snowflakes have smaller reflectivities than similar-sized raindrops

21. Radar and severe weather: hook echoes

22. Bright-banding

23. 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

24. Doppler Schematic

25. Tornado Vortex Signature
Present as early as minutes before tornado