Representing Climate Data II
Satellite Imagery and Radar
Weather Satellites
original used black and white cameras
TIROS, 1960
Radiometers
electronic sensors
• Detect radiation from atmosphere, clouds, surface
• Can sense specific wavelengths of radiation
• “spectral signatures” of gases
• Scans surface
• Scans continuously adjacent squares arranged in scan lines
• Sweep: length of scan line
1. Geostationary
• Moves with earth rotation
• Fixed point
• Altitude 36,000 km (22,240
mi)
Advantage: Real time data
Disadvantage: distorts polar
regions
GOES
Polar-orbitting
• Follows parallel meridian lines
• Altitude 850 km (540 mi)
• Passes poles on every revolution
• Earth rotates eastward and satellite scans successive passes
• Advantage:
• Better coverage of high latitudes
polar-orbitting
Types of weather images:
1. Visible
2. Infrared
3. Water Vapor
4. Radar
satellite
1. visible
• Detects visible
wavelengths.
• Reading shortwave
reflected by earth,
ocean, clouds
• (albedo)
• Daytime only
Albedos of various surfaces:
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Earth’s surface
Cumulonimbus clouds
Stratocumulus clouds
Cirrus clouds
Fresh snow
Melting snow
Sand
Grain crops
Deciduous forest
Coniferous forest
Tropical rainforest
Water bodies
0.31 (31%)
0.9 (90%)
0.6 (60%)
0.4 -0.5 (40 – 50%)
0.8 – 0.9 (80 – 90%)
0.4 – 0.6 (40 – 60%)
0.3 – 0.35 (30 – 35%)
0.18 – 0.25 (18 – 25%)
0.15 – 0.18 (15 – 18%)
0.09 – 0.15 (9 – 15%)
0.07 – 0.15 (7 – 15%)
0.06 – 0.10 (6 – 10%) increases at low sun angles
Visible imagery
• High vs. low albedo :
• High albedo: lighter
• Cloud tops, snow, ice
• Low albedo: darker
• Land, ocean
• Cloud thickness
• Thicker cloud cover is
more reflective:
brighter
• Cloud height (IR better)
• Cumulonimbus : very
bright white
• Low : bright
• High (cirrus) : notbright white
Notice that visible imagery records radiation that passes through
atmospheric window.
2. Infrared
•Detects IR at 10 – 12 μ (thermal)
•Clouds, land, ocean, snow/ice reflect visible
but emit IR
•(visible imagery records reflected shortwave;
IR records emitted IR)
Why 10 – 12 µ?
Infrared imagery
• Detects temperature
• Low temperatures: lighter shades of gray
• High temperatures: darker shades of gray
• Cloud Heights:
• Low clouds warmer than high clouds
• Low clouds: dark
• High clouds: light
• Cumulonimbus clouds: bright white
• Can record at night
• IR images are often color-enhanced to highlight temperature
differences
visible
Infrared (IR)
Enhanced IR
1. What is a disadvantage of geostationary Satellite
imagery?
2. What is “stationary” about a geostationary satellite?
3. Which type of satellite would be better for gathering
information about Artic sea ice changes: polar-orbiting
or geostationary?
4. What is a distinct advantage of infrared imagery
compared to visible?
5. Visible satellite imagery records:
a)Reflected visible (0.4 – 0.7 µ)
b)Emitted infrared (10 – 12 µ)
6. Infrared satellite imagery records:
a)Reflected visible (0.4 – 0.7 µ)
b)Emitted infrared (10 – 12µ)
Types of weather images:
1. Visible
2. Infrared
satellite
3. Water Vapor
4. Radar
BRING YOUR LAPTOPS ON WEDNESDAY!!!!!
3. water vapor
•Visible and IR images:
•Record radiation transmitted through
atmospheric windows
•Water vapor images:
• record IR emitted by water vapor in the
atmosphere
•Water vapor absorbs and emits IR at
6.7- 7.3μ
6.7 – 7.3 µ
a) Does NOT detect water vapor in LOWER
troposphere
•
Because it will be absorbed by water vapor at
higher altitudes and therefore will not go out
to space to be recorded by satellite
b) If upper troposphere is dry, any radiation detected
will be coming from MIDDLE troposphere
lower=warmer=relatively darker gray
c) If upper troposphere is wet, radiation detected is
from HIGH (cold) water vapor
higher = colder = relatively brighter gray/white
(movement of water vapor indicates upper and mid
tropospheric winds)
Enhanced IR of same time period
Enhanced IR of same time period
Water vapor images can also be color
enhanced
• Water Vapor Images are useful for:
• Tracking moisture (at mid and upper levels)
• Locating Low pressure / storm centers
• Identifying the jet stream location
• Can see rising and sinking air regions
• Water vapor
• Current water vapor
• NOAA website
• GOES site
4. radar
(Radio Detection and Ranging)
look inside of clouds
• Now use microwaves instead of radio waves
• Transmitter sends microwave pulses
• Targets scatter energy back to receiver
• Amplified and displayed as echo
• Emission to receipt time shows distance to target
• Shorter microwave wavelengths (~1 cm)
• detect small targets
• (e.g., tiny droplets of water in clouds)
• Longer micro-wavelengths (3 -10 cm)
• detect larger targets
• (e.g.,precipitation)
• Brightness of echo
• Amount of precipitation
Doppler Radar
• Based on principle of Doppler shift:
• Waves moving towards observer have different
frequencies than waves moving away from observer.
• e.g., sound from approaching vs. leaving ambulance
• Doppler radar can measure direction
• Knowing wind speeds and directions within clouds
gives info about vorticity (spin)
158 Doppler stations in US
Tour the radome!
•Makes repeated 360˚scans of atmosphere at
increasing elevation angles.
•2 modes:
•Clear Air mode
•No rain
•Dust, light snow
•VCP 31, 32 (volume coverage pattern)
•Precipitation mode
•rain
Clear Air Mode
Precipitation Mode
Reflectivity units
• dBZ : decibels of Z
• “Z” is energy reflected back to radar
• Values increase with strength of signal
• Clear air
• Precipitation
• dBZ equate to approximate
rainfall rates
Ground clutter
• Ground, buildings, trees, cars
• Within 25 km of radar
• Not moving with respect to radar, so can be detected by radial velocity
• Insects
• Birds
• Turbulence
• Effects density
Base reflectivity
•Reflectivity in lowest elevation “slice”
•Used to survey area close to radar
Composite reflectivity
•Combines all elevation scans
•Shows highest reflectivity
Velocity units
•Radial velocities in knots
•Red: wind moving AWAY
•Green: TOWARD radar
•Need to know where radar is!
Vorticity signature, tornado
Hook echo
• Arkansas
RIDGE
(Radar Integrated Display with Geospatial
Elements)
• Combine radar with topography, roads, county lines, rivers, warnings
• Overlay maps as layers
• Toggle layers on or off
• GIS compatible
NWS Duluth