SHW Ch. 2 - Radar & Satellite

• RADAR (Radio Detection And Ranging):

  • a transmitter emits pulses of microwave radiation and a receiver measures those RADAR echoes that are back-scattered to the RADAR dish antenna; Figure 2.8

  • RADAR base reflectivity is related to the 6^th power of the diameter of the raindrops;

  • RADAR is affected by GROUND CLUTTER (i.e., buildings, trees, mountains, etc.).

• Conventional RADAR:

  • developed in World War II and used by the NWS until the 1990s;

  • measures base reflectivity and can determine cloud tops;

  • has six (6) levels of reflectivity (dBZ);

  • cannot detect drizzle or light snow; can be "fooled" by large hail and sleet.

• Doppler RADAR (WSR-88D or NEXRAD) Figure 2.10:

  • can measure base reflectivity and clouds tops, as well as wind movement (radial velocity and vertical wind profiles);

  • has 16 levels of reflectivity (dBZ); two modes: precipitation mode and clear air mode; Figure 2.9

  • can detect drizzle and light snow (can actually detect insects!), as well as strong wind gusts and meso-cyclones;

  • in the radial velocity mode, wind coming toward the RADAR site is, by convention, a "cool color" (greens & blues), whereas wind going away from the RADAR is, by convention, a "warm color" (reds & oranges);

  • a tornado would appear as a small area, that rapidly changes from green/blue to red/orange;

  • can "integrate" the base reflectivity to estimate how much precipitation (rain or snow) has fallen over a given area; can be "fooled" by large hail and sleet;

  • ground clutter can be "removed" from the RADAR display; can still be "fooled" by large hail and sleet.

(SHW Ch. 2 - Radar & Satellite continued)

• Wind Profilers:

  • a type of Doppler RADAR that operates in very high frequency (VHF) and ultra high frequency (UHF) radio bands, using a phased array antenna; Figure 2.12 & 2.13

  • profiler senses the motion of the air along the beam by determining the Doppler shift that occurs in the returned signal, as the turbulent regions of air move toward or away from the profiler; through triangulation from three beams, a vertical wind profile (sounding) can be obtained every 6 minutes; Figure 2.14

  • the wind profilers compliment the WSR-88D network, which operates best in precipitation, as the wind profilers operate best in clear air

  • profilers allow meteorologists to rapidly detect changes occurring in the atmosphere.

• Commercial Aircraft Measurements (ACARS):

  • the Aeronautical Radio, Inc. Communications Addressing and Reporting System (ACARS) is a system that commercial aircraft use to transmit airline operational information, as well as meteorological data; Figure 2.17

  • every time an airplane carrying an ACARS system takes off or lands at an airport, a sounding is obtained in the vicinity of the airport;

  • provide valuable data at cruising altitudes in the upper troposphere and lower stratosphere.

• Lightning Detection:

  • National Lightning Detection Network (NLDN) was largely installed by the University at Albany in the 1980s, and is now operated by Global Atmospherics, Inc. in Tucson, AZ); Figure 2.18

  • the NLDN monitors Earth's magnetic field and by using three or more "direction finders", triangulates cloud-to-ground (LTGCG) lightning strikes across the contiguous lower 48 states; Figure 2.19

  • the NLDN also measures the strength, polarity and multiplicity of each cloud-to-ground lightning stroke;

  • the NLDN was initially installed out west to monitor potential forecast fires begun by lightning, due to high cloud based thunderstorms.

(SHW Ch. 2 - Radar & Satellite continued)

Weather Satellites

• TIROS I: the first weather satellite, launched on April 1, 1960; used television cameras to photograph clouds.

• Polar Orbiting Satellite: these satellites have an orbit that passes directly over the north and south poles, closely paralleling the earth's meridian lines; the earth rotates under the satellite, eventually allowing for a composite photograph of the entire earth (full disk) in great detail;

• Geostationary Satellite: they orbit the equator at the same rate the earth spins and as a result, remain above a fixed point on the earth's surface; must be in orbit 22,300 miles above Earth to maintain geosynchronous orbit;

• Geostationary Operational Environmental Satellite (GOES): uses radiometers to "see" the clouds, such as an imager (which provides better resolution ... down to 1 nm) and a sounder (which gives a more accurate profile of temperature and moisture at different levels in the atmosphere); visible, infrared, and water vapor; Figure 2.15 & 2.16

• VISIBLE: the radiometer is set to "see" reflected sunlight ... so that clouds appear to be WHITE; object with a low "albedo" appear dark, such as the oceans and forests; SNOWCOVER will give the appearance of clouds.

• INFRARED: the radiometer is set to "see" outgoing infrared radiation ... so that warm clouds (land & oceans) will appear DARK and the colder, cloud tops will appear WHITE, resulting in a "gray scale" for in-between temperatures.

• ENHANCED INFRARED: the "gray scale" is color enhanced to increase the contrast between features and their backgrounds.

• WATER VAPOR: the radiometer is set to "see" just 6.7 micro-meters, which is a unique absorptivity wavelength of water vapor; the areas of high moisture will appear WHITE and the dry areas will be DARK; due to re-radiation in all directions ... this method of sampling only "sees" the middle and upper troposphere ... it cannot "see" to the ground; sometimes, the water vapor pictures are color-enhanced to bring out details.