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SATELLITE
METEOROLOGY
BASICS
• satellite orbits
• EM spectrum
• instrumentation
• interpretation
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
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
Typical weather satellite orbits
LEO = Low Earth Orbit (i.e. polar orbiting)
GEO = Geosynchronous orbit
Typical polar orbit:
ELECTROMAGNETIC RADIATION
For most weather satellites, the following regions of the
electromagnetic spectrum are most important:
Visible light - 0.4 µm to 0.7 µm
Infrared radiation - 0.7 µm to 14 µm
Microwave radiation - 1 mm to 10 cm
http://csep10.phys.utk.edu/astr162/lect/light/spectrum.html
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
In the infrared portion of the spectrum, emitted energy from the earthatmosphere system predominates
From “Weather Satellites”, Rao et. al (1990), AMS publication
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 6 - 7 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.
http://www.ou.nl/open/dja/Klimaat/System/greenhouse_gases.htm
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
1
2
3
4
Approximate spectral bandwidth (µm)
0.5 - 0.7 (visible)
3.7 - 4.0 (infrared)
6.5 - 7.0 (water vapor)
10.2 -11.2 (infrared, atmospheric window)
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
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
• 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
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
• 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