Satellites and Radar – A primerATMO 203

Satellites

• Two main types of satellite orbits– Geostationary Earth Orbiting• Satellite is 35,786 km (22,236 mi.) above sea level• Stays above a given spot on Earth

– Low Earth Orbiting• Only 500 miles above ground• Also called “polar orbiter”• Allows good view of polar regions• Passes by every part of Earth every 12 hours

Polar Orbit Pathway

Main Types of Satellite Imagery

• Visible Imagery

• Infrared Imagery

• Water Vapor Imagery

Visible Imagery (VIS)

• How it works:– VIS measures the sun’s radiation reflected back to

the satellite by the Earth– Things with high albedo (clouds, ice, snow) show

up brightly on VIS– Things with low albedo (Earth’s surface) show up

as dark– 1 km resolution (North America)– Wavelength: 0.52 to 0.72 μm range

VIS cont’d.

• Advantages:– Can easily identify cloud types– Can see small-scale features• Fronts, outflow boundaries, dust plumes, smoke

– Fog and stratus can easily be seen– Identify developing convection

• Disadvantage– Since VIS is sunlight dependent, can only be used

during the day.

Infrared Imagery (IR)

• How it works:– Measures brightness temperature by thermal radiation

emitted by Earth– Dark areas correspond to high thermal radiation

(warm), and bright areas correspond to low thermal radiation (cold)• i.e. Dark = Warm, Bright = Cold

– Thus, higher cloud tops appear brighter, and low-level clouds appear darker

– Resolution: 4 km (GOES), 1 km (POES)– Wavelength: 11 μm

IR (cont’d)

• Advantages:– Easy to determine a general cloud top height

• i.e. Upper air, mid-level, low-level

– Available 24 hours a day• Esp. useful at night, when VIS is unavailable

• Disadvantages– Low-level clouds (esp. fog) can be invisible on IR, as

they may have the same temperature as the ground– During extreme cold outbreaks, the cold land may

look like a large cirrostratus deck, even if it’s clear

Water Vapor Imagery (WV)

• How it works:– Water vapor absorbs the earth’s emitted radiation

in the 6 to 7 μm wavelength range– WV imagery looks for where the radiation in this

range is weak. Where it is weak, it has been absorbed by water vapor.

– Thus, weak radiation signatures (which appear bright) correspond to moisture, and strong radiation signatures (which appear dark) correspond to dry conditions

– Resolution: 8 km, Main wavelength: 6.7 μm

WV (cont’d)

• Advantages:– Good at assessing mid to upper-level moisture in

warm tropospheric environments– Great for locating subtropical jet

• Disadvantages– Only works well in the middle troposphere (between

350-650 mb)– Works best in warm atmosphere

• Cold surface emits less radiation, so may appear bright

– Says nothing about low-level moisture!!***

Radar

• How it works:– A transmitter creates a high-frequency signal– An antenna sends the signal out into space– The signal bounces off an object, is scattered in all

directions, even back towards the radar– The antenna receives the echo back– A receiver detects and amplifies the signal– A display shows you what the radar detected

Radar

• Weather radars send out short pulses of energy

• Then, they wait for the signal to go out (at the speed of light), hit a target, and return

• Then, after an appropriate wait, another pulse is sent out.

• The cycle continues….

Radar

• This may sound like it takes a while, but keep in mind this process last only milliseconds

• A weather radar can send out and receive anywhere from hundreds to over 2000 pulses per second!

Radar – Detection of “Things”

• The bigger the object, the bigger the echo the radar detects (though there are subtleties that may render this untrue on rare occasions)

• Small rain (mist/drizzle) appears as weak echo (low reflectivity)

• Large objects (heavy rain, hail, buildings) appear as strong echo (high reflectivity)

Radar – What can be detected?

• Precipitation– Rain, snow, sleet, freezing rain, graupel, hail

• Non-precipitation (clear air return)– Birds, insects, bats, solar spikes, anomalous

propagation (AP), ground clutter• AP is caused by the atmosphere bending the radar

beam downward, so the radar is detecting the ground• On a radar image, AP appears splotchy, with high

reflectivities next to low reflectivities

Doppler radar

• Most weather radars are equipped with Doppler capability

• Sparing you the physics, Doppler radar just means that radial velocities of precipitation (or objects, such as bats) can be determined.

• Radial Velocity – The component of an object’s velocity to or from the radar– An object can have zero radial velocity, but have a

non-zero velocity!

Doppler Velocity Image Example

Precipitation

Rain/T-storms

Snow

Notice the higher reflectivities associated with the rain, and the lower reflectivities associated with the snow.

Thunderstorms with hail

Reflectivity values > 55 dBZ are possible hail. Here, we have values upwards of 65 dBZ, indicating probable hail (later confirmed by SPC).

Bugs and Insects

Nighttime in a warm, muggy, southeastern spring night. When it’s warm and muggy, the bugs and insects come out at night, seen here around numerous radar sites.

Solar Spikes

Solar spikes are seen here as the sun sets in the west.

Anomalous Propagation (AP)

Exercise Images

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