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Operational Weather Radar
Featuring:WSR-88D Doppler Radar
Transmission Power – 750,000 WGain – 35,481Beam Width <2º @ 125 nmiPulse Length - 0.47 or 1.35 kmTransmission Time – 1.57 of 4.5 μs
DOWs measure record high wind speeds of 301 +/- 20 mph (135 +/- 10 m/s) in 03 May 1999 Oklahoma City tornado.
Abstract Sensors
WSR88 – standard weather service radar 158 Operational Sites Across the Country
Common Radar Bands: K,Ku, C,X, Ka λ=1-10cm, 1-10x size of particles (rain ice) of interest Makes use of Rayleigh Scattering Phased Array Radar
Algorithms Radar Equation Attenuation
Brief Weather Radar History ~1950: RADAR discovered
RADAR: RAdio Detection And Ranging In early use, WWII military operators picked up on what
they thought was noise Really environmental returns, only noise if you want to
see airplanes 1950-1980: Continued development of reflectivity radar 1980-2000: Invention and implementation of Doppler
radar As of 1988 the US started using 10 cm wavelength
radars (WSR-88D) as their standard radar device for radar networks
2000-present: Dual Polarization Radar and Phased Array Radar
Introduction
Key concepts Doppler effect Rayleigh scattering Sensing the return Reflectivity, dbz Vertical cross section
Range Height Indication Scan Constant Azimuth
Improvement with WSR-88D Volume Scanning
Vary azimuth along with elevation Optimally interpolate 3d volume or reflectance
f=fo
V±V x
Radar, Doppler and Raleigh
Radar Equation Whn transmiter and reciever are in the same place,
signal decreases in power to the fourth power
Doppler Equation Where F’ = observed frequency, F = emitted
frequency, v = velocity in medium, vs = velocity of source
Raleigh Scattering EM radiation returned from a field with average
particle size smaller than the wavelength of the radiation
Reflectivity off a target Power of the return signal - reflectivity (z) z has the units mm^6/m^3 -
density of water droplets which would return the reflectivity
emissivity is implicit To cover a large range it is often described in decibels Intensity of the return echo helps determine the
precipitation rate significant precipitation
usually is above 15 dBz Reported reflectivity is an
average of at least 25 pulses from a given azimuth and elevation
Applications Nowcast/ short term forecast
useful for extreme weather Easily deployed in remote locations
Lee Rotors Precipitation Estimation Links to related topics
RUCOOL Codar Maps Radar Altimetry Police Radar Small Aircraft Detection
Benefits from radial movement of parts
Radar Bands and Uses 2-4 S GHz, 7.5-15cm. Sirius and XM radio. Long range
weather, marine weather, ATC 4-8 Ghz - C band – satellite transponders, satellite TV, raw
satellite communications, weather. 18-27 Ghz - K - police, small drizzle/fog research 27-40 Ghz Ka – police 8-12 Ghz - X – airport radar – very long range, missile
guidance
Making a Signal The “listen” time (millisecond) is 1000x the pulse duration This allows the radar an opportunity to be able to receive the
signal again without interference from previous signals Must wait until signal has reached maximum range and
returned. Knowing exact pulse travel time allows for calculation of the
horizontal distance to the target Intensity of the return, or backscatter -> target size Radial Target Motion -> Doppler shift
What happens to the returned signal?
Must orient the returns (power at certain frequencies) into an accurate measure of precipitation
Measurements over a spherical volume are sliced up into horizontally and vertically to produce 2,3 and 4d visualizations
Account for ambient background noise
Algorithms VIL
Vertically Integrated Liquid HAIL
Estimates presence and size of hailstones
Potential Wind Gusts Uses VIL, cloud top height Estimates the winds under the cloud (ie downdraft)
Doppler measurement readily reveals wind shear storm relative velocity
Mesocyclone Detection Algorithm Tornado Detection Algorithm Wind Shear Detection (low level)
Products Base reflectivity
how much precipitation is falling precipitation type assess a storm's structure and dimension
Composite Reflectivity Scans from all elevations, imaging precipitation
intensity and storm structure Base Velocity
radial wind field, speed of fronts/strong wind range of 140 mi
Storm relative motion Track a circulation (show up well in doppler return)
over time to determine storm motion. Removing the storm relative motion from base radials
gives an estimate of the flow with respect to the storm.
“Complications” Migrating Birds Insects Aircraft, solid buildings, large aerosols for air traffic controller – rain for meteorologist – airplanes. Identification of snow, and snow type, modification of observed matter before hitting
the ground (fallstreaks) big enough sample of precip for identification? In precip estimates – Z-R relationship
Phased Array Radar
Multiple beams with variable dwell times scan continuously perform full volume scan every 20-30s track cooperative aircraft track non-cooperative aircraft perform medium dwell scan on heavy cumuliform
convection perform long dwell scan on area of suspected tornadic
development. Expected that tornado warning could in to to 45
min
Proven NAVY technology
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