1. SATELLITE METEOROLOGY BASICS satellite orbits EM spectrum
instrumentation
interpretation

2. WHY USE SATELLITES? Spectral, spatial, and temporal resolution
follow severe weather outbreaks, and allow advance warning
give real-time cloud photographs
information on the Earth-atmosphere energy budget
temperature soundings in atmosphere
sea-surface temperatures
distribution of water vapor in the atmosphere
monitor volcanic eruptions and motion of ash clouds
chemical composition of atmosphere; ozone, etc

3. SATELLITE ORBITS Geostationary (geosynchronous) orbits
orbit in the equatorial plane at a rate matching Earth’s rotation
provide constant view of a given location
orbit at about 35,800 km
high enough to allow a full-disc view of the planet
polar regions are distorted because of low viewing angle
Polar orbits
circle the planet in an almost north-south orbit, passing close to both poles
orbits are typically less than 1000 km above the surface of Earth
orbits are usually “sun synchronous” - pass over the same location at the same local time each day
have the advantage of viewing Earth directly beneath them

4. Typical weather satellite orbits
LEO = Low Earth Orbit (i.e. polar orbiting)
GEO = Geosynchronous orbit

5. Typical polar orbit:

6. ELECTROMAGNETIC RADIATION
For most weather satellites, the following regions of the electromagnetic spectrum are most important:
Visible light µm to 0.7 µm
Infrared radiation µm to 14 µm
Microwave radiation - 1 mm to 10 cm

7. In the visible portion of the spectrum, features are observed by virtue of reflected and scattered solar radiation
From “Weather Satellites”, Rao et. al (1990), AMS publication

9. In the infrared portion of the spectrum, emitted energy from the earth-atmosphere system predominates
From “Weather Satellites”, Rao et. al (1990), AMS publication

11. The special role of water vapor in the atmosphere
water vapor makes up 1 - 4% (by volume) of the atmosphere
plays a critical role in Earth’s energy budget
absorbs and emits EM radiation throughout most of the IR
one of the strongest water vapor bands is between µm
“moist” and “dry” features seen on water vapor satellite images result from combinations of vertical motion, horizontal deformation, and moisture advection in the middle and upper troposphere
water vapor imagery can be used to track moisture gradients
can be used to locate and define synoptic features such as shortwave troughs, ridges, the jet stream, etc.

14. INSTRUMENTATION
In order to collect enough radiant energy to provide reliable measurements, satellite sensors are designed to measure energy over an interval of wavelengths called a “band”
Most satellites utilize a number of distinct bands, centered on selected frequencies, referred to as the sensor’s “channels”
For example, most Geostationary Operational Environmental Satellites (GOES) sensors have at least 3 channels:
Channel Approximate spectral bandwidth (µm)
(visible)
(infrared)
(water vapor)
(infrared, atmospheric window)

15. Examples of other instrumentation:
“Advanced Very High Resolution Radiometer” (AVHRR) - on polar orbiting satellites
Microwave Sounding Unit
Stratospheric Sounding Unit
Vertical Temperature Profile Radiometer
Solar Backscatter UV sensor
Temperature-Humidity Infrared Radiometer

16. INTERPRETATION
visible imagery is derived from reflected solar radiation from Earth-atmosphere system
intensity of image brightness depends on the albedo of the underlying surface or cloud
most reflective surfaces are light or nearly white, least reflective are black
Clouds are typically seen as white objects against the darker background of Earth’s surface

17. IR imagery is derived from terrestrial radiation emitted from the Earth-atmosphere system
areas with low emitted energy have low brightness values
HOWEVER, by convention, emitted energy and brightness values are inverted for thermal IR imagery
Cold cloud tops and cold surfaces appear bright
Since temperature typically decreases with height in the troposphere, IR radiation with the lowest intensity is emitted by highest and coldest clouds

18. Water vapor satellite imagery is derived from radiation emitted by water vapor at wavelengths between about 6 to 7 µm
If the upper troposphere is moist, radiation reaching the satellite originates from this cold region, and is displayed at light to white shades
If the upper troposphere is dry, radiation will originate from water vapor at warmer (lower) levels, and is displayed in darker shades

19. Go through the “Satellite Primer” on the course web site
Homework #3 will be available on the web site tomorrow. Due next Wednesday.
Quiz; handed out soon; notes on the floor;
Don’t start writing until you are told to !!
15 minutes
Hand in at the end of class