Transcript Radar and Satellites - UW-Madison Department of

Satellite and Radar
Lecture 5
October 7, 2009
Review from last week
 Pressure Gradient Force
– PGF = CHANGE IN
PRESSURE / DISTANCE
– Direction of PGF – always
pointed from HIGH pressure
toward LOW pressure, directly
perpendicular to an isobar
– Magnitude of PGF- strength is
directly related to how
closely packed the isobars
are at the surface.
Review from last week
 Coriolis Force
– The CF is an apparent force that results from
the constant rotation of the Earth.
– In N. Hemisphere, acts at a 90° angle to the
right of the object in motion (such as the wind)
– We cannot see the planet rotating, so when
something is moving, we perceive it as being
deflected to the right of its intended trajectory in
the N. Hemisphere
Since friction is
directed opposite of
the wind, it slows the
wind. When it slows
the wind, the
magnitude of the CF
is affected and the CF
no longer balances
the PGF. Remember,
CF is always 90° right
of the wind in the
northern hemisphere.
As a result the PGF is
the dominate force
driving the wind and
the wind turns in the
direction of the PGF.
This allows the wind
to cross the isobars
toward low pressure.
996 mb
L
Pressure Gradient
Force
Wind
1000 mb
Frictional Force
Coriolis Force
1004 mb
H
Review from last week
 Geostrophic Balance
– A balance between the pressure gradient force
and the Coriolis force
– Balance allows PGF to be equal and opposite
the CF. This balance will tell use the magnitude
of the geostrophic wind
– The geostrophic wind moves parallel to lines of
constant pressure, with low pressure on the left
 Frictional Force
– Friction affects geostrophic balance by putting a
drag-force on the air: friction always acts in the
direction opposite the direction of the wind
Satellites
 October 4, 1957 – Russia
launched Sputnik 1, the first
satellite in history
– As a result, space science
boomed in America as it led
Americans to fear that the
Soviets would launch missiles
containing nuclear weapons.
 1959 – Scientists at the Space
Science and Engineering
Center (SSEC) at UWMadison conducted pioneering
meteorological satellite
research, revealing the vast http://burro.astr.cwru.edu/stu/a
dvanced/20th_soviet_sputnik.
benefits of meteorological
html
satellites.
Evolution Until Today
 First weather satellite lasted 79 days
 Now many years
 Two distinct types of weather satellites
– GOES - Geostationary Operational Environmental
Satellites
- POES - Polar Operational Environmental Satellites
(also referred to as “LEO” – Low Earth Orbit)
 They are defined by their orbital characteristics
 There are also many other satellites in orbit,
some of which are not functioning and those are
referred to as “space debris”.
Geostationary Vs. Polar Orbiting
http://cimss.ssec.wisc.edu/satmet/modules/sat_basics/images/orbits.jpg
GOES
 GOES: Geostationary Operational Environmental
Satellites
 Orbit as fast as the earth spins
 Maintain constant altitudes (~36,000 km, or 22,300
miles) and momentum over a single point, always over
the equator
 Imagery is obtained approximately every 15 minutes
unless there happens to be an important meteorological
phenomenon worth higher temporal resolution
 Generally has poor spatial resolution- sees large fixed
area and covers polar regions poorly.
 But, good for viewing large scale meteorological
phenomena (cyclones, hurricanes, etc.) at lower and
middle latitudes
GOES
GOES- EAST (GOES- 12)
GOES- WEST (GOES – 11)
GOES COVERAGE
http://goes.gsfc.nasa.gov/pub/goes/global_geosynch_cover
age.gif
Sample Composite
http://www.ssec.wisc.edu/data/comp/latest_moll.gif
POES
• POES: Polar Operational Environmental Satellites
• Rotates around the earth from pole to pole
• Significantly closer to the Earth than geostationary satellites
(879 km above the surface)
• Sees the entire planet twice in a 24 hour period
• Lower altitude gives it a good spatial resolution: Very high
resolution images of the atmosphere and Earth
• Poor temporal resolution: Over any point on Earth, the
satellite only captures two images per day!
• Best resolution over the poles
POES COVERAGE
POES
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More then a few in orbit currently
Two examples are TERRA and AQUA
Have different viewing instruments on them
One example is MODIS: Moderate Resolution
Imaging Spectroradiometer
 Acquires data in 36 spectral bands (groups of
wavelengths)
 As a result, MODIS can create a true color
visible image, which can:
– Show changes in vegetation during fall/spring
– Show smoke plumes, dust plumes, etc.
Example MODIS image
http://www.ssec.wisc.edu/modis-today/images/terra/true_color/2008_02_24_055/t1.08055.USA_Composite.143.4000m.jpg
Wildfires Near Los Angeles Using
MODIS
Types of Satellite Imagery
• VISIBLE
• Measures visible light (solar radiation, 0.6 m) which is
reflected back to the satellite by cloud tops, land, and sea
surfaces.
• Thus, visible images can only be seen during daylight
hours!
• Dark areas: Regions where small amounts of visible
light are reflected back to space, such as forests and
oceans
• Light areas: Regions where large amounts of visible
light are reflected back to space, such as snow or clouds
Visible Pros/Cons
 Pros:
– Seeing basic cloud patterns and storm systems
– Monitoring snow cover
– Shows nice shadows of taller clouds (has a 3-D look
to it)
 Cons:
– Only useful during the daylight hours
– Difficult to distinguish low clouds from high clouds
since all clouds have a similar albedo (reflect a similar
amount of light)
– Hard to distinguish snow from clouds in winter
Types of Satellite Imagery
• WATER VAPOR (WV)
• Displays infrared radiation emitted by the water vapor
(6.5 to 6.7 m) in the atmosphere
• Bright, white shades represent radiation from a moist
layer or cloud in the upper troposphere
• Dark, grey or black shades represent radiation from
the Earth or a dry layer in the middle troposphere
Types of Satellite Imagery
• INFRARED (IR)
• Displays infrared radiation (10 to 12 m) emitted
directly by cloud tops, land, or ocean surfaces
• Wavelength of IR depends solely on the temperature
of the object emitting the radiation
• Cooler temperatures (like high cloud tops) are shown
as light gray, or white tones
• Warmer temperatures (low clouds, ocean/lake
surfaces) are shown dark gray
• Advantage: You can always see the IR satellite
image
Interpreting Visible vs. IR
RADAR
 What does Radar mean?
– Radio Detection and Ranging
 During World War II, this Radio Detection
and Ranging technique was developed to
track enemy ship and aircraft. However, it
was soon noted that precipitation, of any
kind, would obstruct this remote detection.
At first this was a problem, but the potential
benefits were soon seen. This was the birth
of weather Radar.
How does RADAR work?
 Radar uses electromagnetic radiation to sense
precipitation.
 Sends out a microwave pulse (wavelength of 410 cm) and listens for a return echo.
 If the radiation pulse hits precipitation particles,
the energy is scattered in all directions
 The RADAR has a “listening” period. When it
detects radiation scattered back, the radiation
is called an “echo.”
How does RADAR work?
• The RADAR beam is typically
0.5o above the horizon and
1.5o wide.
• It rotates in a full circle, with a
radius of ~200 miles
•
Time difference between
transmission and return of
signal = distance to the storm
• The intensity of precipitation is
measured by the strength of
the echo, in units of decibels
(just like intensity of sound
waves!)
•
An image showing precipitation intensity is called a
“reflectivity image”
Intensity measured in decibels (dBZ)
 www.radar.weather.gov/graphics/ridge_sitemap.gif
Types of RADAR
 Conventional Radar
– Echoes are simply displayed on radar screen.
– Only produces reflectivity images.
– Circular sweeps and vertical sweeps, to attempt to
reconstruct the precipitation type and intensity
throughout the atmosphere
– Can identify storm structure, locations of
tornadoes, and even non-meteorological objects!
Good/Bad of Conventional Radar
 Good for
– Seeing bands/location of precip and their
intensity
– Hook echoes
– Bow echoes
 Bad for
– Ground clutter, bouncing off things other than
precipitation
– Overestimation/Underestimation of precip
– Cannot tell type of precipitation by radar alone
(Have to use temperatures, actual observations,
etc.
Doppler Radar
 One of the most advanced versions of radar
 Does everything a conventional radar can do,
PLUS more...
 In addition to conventional techniques, the
Doppler Radar has a scan that operates on
principle of the Doppler Effect
– Usually described using sound waves
– Definition: The change in the observed frequency of
waves produced by the motion of the wave source
Doppler Radar in Meteorology
• Measures changes in wavelength of the
RADAR beam after it is scattered from a
travelling object
• Wavelength of the beam changes after it
“strikes” the object
• Thus, wind direction AND speed can be
measured by RADAR
Doppler RADAR in Meteorology
• This is VERY useful in detecting tornado signatures!
•Doppler can measure wind speed and direction in a storm
and can be viewed in a storm-relative velocity image
• Red: Winds away from RADAR site, Green: Winds toward
RADAR site
• This is how the National Weather Service issues tornado
warnings
Phased-array radar
 Next generation of radar.
 Can scan multiple levels at
once using multiple radar
beams sent out at one time.
 Scanning only takes 30
secs compared to ~6
minutes for the Doppler
 Gives instantaneous profile
of atmosphere for winds and
precipitation intensity.
Examples
 Birds on radar
– http://www.crh.noaa.gov/images/mkx/radar/bird
animation.gif
 http://www.crh.noaa.gov/mkx/?n=usingradar