SATELLITE METEOROLOGYBASICS

• 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:

http://csep10.phys.utk.edu/astr162/lect/light/spectrum.html

ELECTROMAGNETIC RADIATION

For most weather satellites, the following regions of the electromagnetic spectrum are most important:

Visible light - 0.4 µm to 0.7 µmInfrared radiation - 0.7 µm to 14 µmMicrowave radiation - 1 mm to 10 cm

From “Weather Satellites”, Rao et. al (1990), AMS publication

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 earth-atmosphere system predominates

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 Approximate spectral bandwidth (µm) 1 0.5 - 0.7 (visible) 2 3.7 - 4.0 (infrared) 3 6.5 - 7.0 (water vapor) 4 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

•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

Water vapor satellite imagery is derived from radiation emitted by water vapor at wavelengths between about 6 to 7 µm

• 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