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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

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Satellite and Radar

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    1. Satellite and Radar Lecture 5 October 7, 2009

    2. 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.

    3. 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

    4. 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.  L 996 mb Pressure Gradient Force Wind 1000 mb Frictional Force Coriolis Force 1004 mb H

    5. 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 • Thegeostrophic windmovesparallelto 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

    6. 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 UW-Madison conducted pioneering meteorological satellite research, revealing the vast benefits of meteorological satellites.

    7. 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”.

    8. Geostationary Vs. Polar Orbiting

    9. 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

    10. GOES GOES- EAST (GOES- 12) GOES- WEST (GOES – 11)


    12. Sample Composite

    13. 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


    15. POES • 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.

    16. Example MODIS image

    17. Wildfires Near Los Angeles Using MODIS

    18. 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

    19. 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

    20. 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

    21. 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

    22. Interpreting Visible vs. IR

    23. 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. 

    24. How does RADAR work? • Radar uses electromagnetic radiation to sense precipitation. • Sends out a microwave pulse (wavelength of 4-10 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.”

    25. 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!)

    26. An image showing precipitation intensity is called a “reflectivity image” Intensity measured in decibels (dBZ)


    28. 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!

    29. 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.

    30. 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

    31. 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

    32. 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

    33. 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.

    34. Examples • Birds on radar • •