Advanced Weather Radar Technology: Doppler Radar System Overview

1. Operational Weather Radar Featuring: WSR-88D Doppler Radar Transmission Power – 750,000 W Gain – 35,481 Beam Width <2º @ 125 nmi Pulse Length - 0.47 or 1.35 km Transmission 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 • 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

4. 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

5. source: http://www.cocorahs.org/media/docs/radar_basics.pdf

6. source: http://www.cocorahs.org/media/docs/radar_basics.pdf

7. 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

8. 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

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 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

10. 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

11. 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

12. 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

13. 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

14. Source: http://cimms.ou.edu/rvamb/Documents/Report_7.pdf

15. 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)‏

16. 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.

17. “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

18. Phased Array Radar Proven NAVY technology • 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

19. In Closing • Take precipitation rates with caution • Highly accurate under most conditions • Misleading in cases of extreme precipitation • The ultimate in nowcast and high resolution spatial time series for precipitation. • Extremely useful in adaptive forecasting of extreme weather events