1 Operational Weather Radar Featuring: WSR-88D Doppler Radar Transmission Power – 750,000 WGain – 35,481Beam Width 125 nmiPulse Length or 1.35 kmTransmission Time – 1.57 of 4.5 μs
2 DOWs measure record high wind speeds of 301 +/- 20 mph (135 +/- 10 m/s) in 03 May 1999 Oklahoma City tornado.
3 Abstract Sensors Algorithms WSR88 – standard weather service radar 158 Operational Sites Across the CountryCommon Radar Bands: K,Ku, C,X, Kaλ=1-10cm, 1-10x size of particles (rain ice) of interestMakes use of Rayleigh ScatteringPhased Array RadarAlgorithmsRadar EquationAttenuation
4 Brief Weather Radar History ~1950: RADAR discoveredRADAR: RAdio Detection And RangingIn early use, WWII military operators picked up on what they thought was noiseReally environmental returns, only noise if you want to see airplanes: Continued development of reflectivity radar: Invention and implementation of Doppler radarAs of 1988 the US started using 10 cm wavelength radars (WSR-88D) as their standard radar device for radar networks2000-present: Dual Polarization Radar and Phased Array Radar
7 Introduction Key concepts Doppler effect Rayleigh scattering Sensing the returnReflectivity, dbzVertical cross sectionRange Height Indication ScanConstant AzimuthImprovement with WSR-88DVolume ScanningVary azimuth along with elevationOptimally interpolate 3d volume or reflectance
8 Radar, Doppler and Raleigh Radar EquationWhn transmiter and reciever are in the same place, signal decreases in power to the fourth powerDoppler EquationWhere F’ = observed frequency, F = emitted frequency, v = velocity in medium, vs = velocity of sourceRaleigh ScatteringEM radiation returned from a field with average particle size smaller than the wavelength of the radiation
9 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 reflectivityemissivity is implicitTo cover a large range it is often described in decibelsIntensity of the return echo helps determine the precipitation ratesignificant precipitation usually is above 15 dBzReported reflectivity is an average of at least 25 pulses from a given azimuth and elevation
10 Applications Nowcast/ short term forecast useful for extreme weatherEasily deployed in remote locationsLee RotorsPrecipitation EstimationLinks to related topicsRUCOOL Codar MapsRadar AltimetryPolice RadarSmall Aircraft DetectionBenefits from radial movement of parts
11 Radar Bands and Uses2-4 S GHz, cm. Sirius and XM radio. Long range weather, marine weather, ATC4-8 Ghz - C band – satellite transponders, satellite TV, raw satellite communications, weather.18-27 Ghz - K - police, small drizzle/fog research 27-40 Ghz Ka – police8-12 Ghz - X – airport radar – very long range, missile guidance
12 Making a SignalThe “listen” time (millisecond) is 1000x the pulse durationThis allows the radar an opportunity to be able to receive the signal again without interference from previous signalsMust wait until signal has reached maximum range and returned.Knowing exact pulse travel time allows for calculation of the horizontal distance to the targetIntensity of the return, or backscatter -> target sizeRadial Target Motion -> Doppler shift
13 What happens to the returned signal? Must orient the returns (power at certain frequencies) into an accurate measure of precipitationMeasurements over a spherical volume are sliced up into horizontally and vertically to produce 2,3 and 4d visualizationsAccount for ambient background noise
15 Algorithms VIL HAIL Potential Wind Gusts Vertically Integrated LiquidHAILEstimates presence and size of hailstonesPotential Wind GustsUses VIL, cloud top heightEstimates the winds under the cloud (ie downdraft)Doppler measurement readily reveals wind shear storm relative velocityMesocyclone Detection AlgorithmTornado Detection AlgorithmWind Shear Detection (low level)
16 Products Base reflectivity Composite Reflectivity Base Velocity how much precipitation is fallingprecipitation typeassess a storm's structure and dimensionComposite ReflectivityScans from all elevations, imaging precipitation intensity and storm structureBase Velocityradial wind field, speed of fronts/strong windrange of 140 miStorm relative motionTrack 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.
17 “Complications” Migrating Birds Insects Aircraft, solid buildings, large aerosolsfor air traffic controller – rainfor 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
18 Phased Array RadarProven NAVY technologyMultiple beams with variable dwell times scan continuouslyperform full volume scan every 20-30strack cooperative aircrafttrack non-cooperative aircraftperform medium dwell scan on heavy cumuliform convectionperform long dwell scan on area of suspected tornadic development.Expected that tornado warning could in to to 45 min
19 In Closing Take precipitation rates with caution Highly accurate under most conditionsMisleading in cases of extreme precipitationThe ultimate in nowcast and high resolution spatial time series for precipitation.Extremely useful in adaptive forecasting of extreme weather events