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Some frequencies travel through clouds with virtually no attenuation. ALL em waves move at the speed of lightSlide4: How does a radar work?Slide6: hello Compare to: Acoustic Echo-locationSlide7: hello Acoustic Echo-locationSlide8: hello distance Acoustic Echo-location Slide9: Hi !! t = 2 x range / speed of sound Example: range = 150 m Speed of sound ≈ 340 meters/second t = 2 X 150 / 340 ≈ 1 secondRADAR Echolocation(RADAR ~ RAdio Detection And Ranging)“Microwave Echo-Location”: RADAR Echolocation (RADAR ~ RAdio Detection And Ranging) “Microwave Echo-Location” Microwave Transmitter Receiver Tx RxTarget Range: Target Range time t = 2 x range / speed of light measure t, then determine Range Example: t = .001 sec Speed of light = c = 3x108 meters/second Range = .001 x 3x108 / 2 = 150,000 m = 150 km Tx RxThresholding: Thresholding time Measure time elapsed between transmit pulse and target crossing a threshold voltage. Then calculate range. Don’t “report back” any information from targets that don’t cross the threshold Threshold Voltage Range-Gating: Range-Gating time Range GatesSlide14: We will see that Radars work by… Transmitting microwave pulses…. and measuring the … Time delay (range) Amplitude Frequency Polarization … of the microwave echo in each range gateTarget Size: Target Size time Scattered wave amplitude conveys size of the scattering objects. Measure amplitude, determine size.Target Radial Velocity: Target Radial Velocity Frequency ft Frequency ft+ fdTarget Radial Velocity: Target Radial Velocity Frequency ft Frequency ft+ fdZero Velocity for “Crossing Targets”: Zero Velocity for “Crossing Targets” Frequency ft Frequency ft+ fd Doppler FrequencyTarget Spatial Orientation: Target Spatial Orientation Polarization Pt Polarization Ps Large Drops Small Drops Example: Weather Echoes: Example: Weather Echoes Microwave Transmitter Receiver Echo versus Range(range profile): Echo versus Range (range profile) time Transmitted Pulse #1 Cloud Echo Slide22: In summary, radars work by… Transmitting microwave pulses…. and measuring the … … of the microwave echo in each range gate Time delay (range) Amplitude (size) Frequency (radial velocity) Polarization (spatial orientation & “oblateness”)Other concepts of Radars: Other concepts of RadarsColors in radar images: Colors in radar images The colors in radar images indicate the amount of rain falling in a given area. Each raindrop reflects the energy from the radar. Therefore, the more raindrops in a certain area, the brighter the color in the radar image of that area. The bright red color around the eye of a hurricane radar image indicates the area of heaviest rainfall. The green colored area has a moderate amount of rain, while the blue areas represent the least amount of rain. Hurricane Andrew, 1992Slide25: 0.1 mm/hr 1 mm/hr 15 mm/hr 100 mm/hr >150 mm/hr QPE – Quantitative Precipitation EstimationWhy Radar Can't (Usually) See Tornadoes: Why Radar Can't (Usually) See Tornadoes The network of WSR-88D Doppler radars across the US has certainly proven itself for the ability to detect severe weather. Tornado warnings, in particular, are much better now that National Weather Service forecasters have this fantastic new (new as of the early 1990s) tool. But did you know that Doppler radar (usually) can't see an actual tornado? When Doppler radar is cited in a tornado warning it is generally because meteorologists see evidence the storm itself is rotating. It is a supercell thunderstorm or at least contains an area of rotation called a mesocyclone. When can and when can't Doppler radar see a tornado? It's math! Let's figure it out. We'll be looking into two factors: 1) the first is something you learned in school a loooong time ago - the earth is curved, and 2) the radar "beam" is 1 degree wide.Slide27: NEXRAD System Today GapSlide28: May 3, 1999 Tornado Outbreak in Oklahoma NWS has ~150 NEXrad radars in US;1 in Cayey, PR: NWS has ~150 NEXrad radars in US; 1 in Cayey, PRProposed CASA radar network: Proposed CASA radar networkCASA radars will complement NWS radars: CASA radars will complement NWS radars Water spout at Mayaguez Beach, PR- Sept 2005 –unseen by NEXRADRadar "Beamwidth": Radar "Beamwidth" The geometry of the dish and a few other factors help determine the pulse volume, which can be specified in degrees. NEXRAD radar sends discrete pulses (and spends 99.57% of the time listening for return echoes) Meteorologists like to use the convenient terms "beam" and "beamwidth" to describe where the radar is pointing and the effective resolution of the air being sampled.Antennas: Antennas Antenna is a transition passive device between the air and a transmission line that is used to transmit or receive electromagnetic waves.Antenna Beamwidth: Antenna Beamwidth radians D is the antenna diameter λ is the wavelength of signal in air Tradeoff: Small wavelengths (high frequencies) = small antennas But small wavelengths attenuate moreBeamwidth Size vs. Object Size: Beamwidth Size vs. Object Size Beamwidth What can a radar see? Beamwidth is one consideration. Earth curvature and height of the feature is another (addressed on the next page). For the moment, we'll keep the problem in two dimensions and ignore height above ground. The geometry is an isosceles triangle. Be sure to note which beamwidth you are calculating for (i.e. 1 degree). Beamwidth: Beamwidth 0.7 mi 1.4 mi 2.1 mi 2.8 mi Object Size: Object Size How wide and tall are various things we want to see? Width of Meteorological Objects (i.e. Storms, Tornadoes) Earth Curvature: Earth Curvature Fill in the table with values you calculate 0.17 mi 0.35 mi 0.52 mi 0.70 mi 7 mi 16 mi 23 mi 31 miPlay related games: Play related gamesPlay the games to learn the basics: Play the games to learn the basics http://whyfiles.org http://meted.ucar.edu/hurrican/strike/index.htm http://meted.ucar.edu/hurrican/strike/ http://meted.ucar.edu/hurrican/strike/info_3.htm# http://www.nws.noaa.gov/om/hurricane/index.shtml http://www.nws.noaa.gov/om/edures.htm More Games for Kids 4-104: More Games for Kids 4-104 http://www.nws.noaa.gov/om/reachout/kidspage.shtmlReferences: References The COMET project [http://www.comet.ucar.edu/] NASA TRMM NCAR (National Center for Atmospheric Research) - University Corporation for Atmospheric Research (UCAR) NOAA Educational Page [http://www.nssl.noaa.gov/edu/ideas/radar.html] Dave McLaughlin Basics of Radars presentation NWS [http://www.crh.noaa.gov/fsd/soo/doppler/doppler.htm] You do not have the permission to view this presentation. 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