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Basic Principles of Weather Radar

Basic Principles of Weather Radar. Dr. Scott M. Rochette. Basis of Presentation. Introduction to Radar Basic Operating Principles Reflectivity Products Doppler Principles Velocity Products Non-Meteorological Targets Summary. Laughlin AFB, TX (KDFX) 0612 UTC 26 May 2001.

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Basic Principles of Weather Radar

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  1. Basic Principles of Weather Radar Dr. Scott M. Rochette

  2. Basis of Presentation • Introduction to Radar • Basic Operating Principles • Reflectivity Products • Doppler Principles • Velocity Products • Non-Meteorological Targets • Summary

  3. Laughlin AFB, TX (KDFX) 0612 UTC 26 May 2001 (http://www.weathermatrix.net/radar/education/articles/laughlin/images/KDFX.jpg)

  4. Radar • RAdio Detection And Ranging • Developed during WWII for detecting enemy aircraft • Active remote sensor • Transmits and receives pulses of E-M radiation • Satellite is passive sensor (receives only) • Numerous applications • Detection/analysis of meteorological phenomena • Defense • Law Enforcement • Baseball

  5. Weather Surveillance Radar • Transmits very short pulses of radiation • Pencil beam (narrow cone) expands outward • Pulse duration ~ 1 μs (7 seconds per hour) • High transmitted power (~1 megawatt) • ‘Listens’ for returned energy (‘echoes’) • Listening time ~ 1 ms (59:53 per hour) • Very weak returns (~10-10 watt) • Transmitted energy is scattered by objects on ground and in atmosphere • Precipitation, terrain, buildings, insects, birds, etc. • Fraction of this scattered energy goes back to radar

  6. (http://www.crh.noaa.gov/mkx/radar/part1/slide2.html)

  7. (http://www.crh.noaa.gov/mkx/radar/part1/slide3.html)

  8. (University of Illinois WW2010 Project)

  9. (University of Illinois WW2010 Project)

  10. http://weather.noaa.gov/radar/radinfo/radinfo.html

  11. Determining Target Location • Three pieces of information • Azimuth angle • Elevation angle • Distance to target • From these data radar can determine exact target location

  12. Azimuth Angle • Angle of ‘beam’ with respect to north (University of Illinois WW2010 Project)

  13. Elevation Angle • Angle of ‘beam’ with respect to ground (University of Illinois WW2010 Project)

  14. Distance to Target • D = cT/2 • T  pulse’s round trip time (University of Illinois WW2010 Project)

  15. Scanning Strategies 1 • Plan Position Indicator (PPI) • Antenna rotates through 360° sweep at constant elevation angle • Allows detection/intensity determination of precipitation within given radius from radar • Most commonly seen by general public • WSR-88D performs PPI scans over several elevation angles to produce 3D representation of local atmosphere

  16. Plan Position Indicator • Constant elevation angle • Azimuth angle varies (antenna rotates) (University of Illinois WW2010 Project)

  17. Elevation Angle Considerations • Radar usually aimed above horizon • minimizes ground clutter • not perfect • Beam gains altitude as it travels away from radar • Radar cannot ‘see’ directly overhead • ‘cone of silence’ • appears as ring of minimal/non-returns around radar, esp. with widespread precipitation • Sample volume increases as beam travels away from radar

  18. Red numbers are elevation angles • Note how beam (generally) expands with increasing distance from radar (http://weather.noaa.gov/radar/radinfo/radinfo.html)

  19. Blue numbers are heights of beam AGL at given ranges • Most effective range: 124 nm

  20. Scanning Strategies 2 • Range Height Indicator (RHI) • Azimuth angle constant • Elevation angle varies (horizon to near zenith) • Cross-sectional view of structure of specific storm (University of Illinois WW2010 Project)

  21. Radar Equation for Distributed Targets 1 where Pr  average returned power A  numerical constants B  radar characteristics C  target scatter efficiency characteristics D  equivalent radar reflectivity factor (Ze)

  22. Choice of Wavelength 1 • Typical weather radar  range: 0.8-10.0 cm • WSR-88D: ~10 cm • TV radar: ~5 cm

  23. Choice of Wavelength 2 • Pr inversely proportional to square of wavelength (i.e., short wavelength  high returned power) • However, shorter wavelength energy subject to greater attenuation (i.e., weaker return signal) • Short wavelength radar better for detecting smaller targets (cloud/drizzle droplets) • Long wavelength radar better for convective precipitation (larger hydrometeors)

  24. Radar Equation for Distributed Targets 2 where Pr  average returned power Rc  radar constant Ze  equivalent radar reflectivity factor (‘reflectivity’) r  distance from radar to target

  25. Radar Equation for Distributed Targets 3 • Pr is: • directly proportional to ‘reflectivity’ • inversely proportional to square of distance between radar and target(s)

  26. Equivalent Radar Reflectivity Factor 1 where Ni  number of scattering targets Di  diameter of scattering targets v  pulse volume

  27. Equivalent Radar Reflectivity Factor 2 • Ze relates rainfall intensity to average returned power • ‘Equivalent’ acknowledges presence of numerous scattering targets of varying: • sizes/shapes • compositions (water/ice/mixture) • distributions • Several assumptions made (not all realistic)

  28. Equivalent Radar Reflectivity Factor 3 • Ze is: • directly proportional to number of scatterers • inversely proportional to sample volume • directly proportional to scatterer diameter raised to 6th power • Doubling size yields 64 times the return

  29. (University of Illinois WW2010 Project)

  30. dBZ • Typical units used to express reflectivity • Range: • –30 dBZ for fog • +75 dBZ for very large hail

  31. Scanning Modes • Clear-Air Mode • slower antenna rotation • five elevation scans in 10 minutes • sensitive to smaller scatterers (dust, particulates, bugs, etc.) • good for snow detection • Precipitation Mode • faster antenna rotation • 9-14 elevation scans in 5-6 minutes • less sensitive than clear-air mode • good for precipitation detection/intensity determination • trees • Livestock • birds • bats • insects • Other • sun strobes • anomalous propagation

  32. Clear-Air Mode Precipitation Mode

  33. Clear-Air Mode Precipitation Mode Greer, SC (KGSP) (http://virtual.clemson.edu/groups/birdrad/COMMENT.HTM)

  34. Reflectivity Products 1 • Base Reflectivity • single elevation angle scan (5-14 available) • useful for precipitation detection/intensity • Usually select lowest elevation angle for this purpose • high reflectivities  heavy rainfall • usually associated with thunderstorms • strong updrafts  larger raindrops • large raindrops have higher terminal velocities • rain falls faster out of cloud  higher rainfall rates • hail contamination possible > 50 dBZ

  35. Reflectivity Products 2 • Composite Reflectivity • shows highest reflectivity over all elevation scans • good for severe thunderstorms • strong updrafts keep precipitation suspended • drops must grow large enough to overcome updraft

  36. Base Reflectivity Composite Reflectivity Little Rock, AR (KLZK) Precipitation Mode

  37. Z-R Relationships 1 where Z  ‘reflectivity’ (mm6 m-3) R  rainfall rate (mm h-1) a and b are empirically derived constants

  38. Z-R Relationships 2 • Allow one to estimate rainfall rate from reflectivity • Numerous values for a and b • determined experimentally • dependent on: • Precipitation character (stratiform vs. convective) • Location (geographic, maritime vs. continental, etc.) • Time of year (cold-season vs. warm season)

  39. Z-R Relationships 3 (WSR-88D Operational Support Facility)

  40. Z-R Relationships 4 (WSR-88D Operational Support Facility)

  41. Radar Precipitation Estimation 1 • 1-/3-h Total Precipitation • covers 1- or 3-h period ending at time of image • can help to track storms when viewed as a loop • highlights areas for potential (flash) flooding • interval too short for some applications

  42. Radar Precipitation Estimation 2 • Storm Total Precipitation • cumulative precipitation estimate at time of image • begins when radar switches from clear-air to precipitation mode • ends when radar switches back to clear-air mode • can help to track storms when viewed as a loop • helpful in estimating soil saturation/runoff • post-storm analysis highlights areas of R+/hail • no control over estimation period

  43. 1-h Total Precipitation (ending at 2009 UTC 11 June 2003) Storm Total Precipitation (0708 10 June 2003 to 2009 UTC 11 June 2003) St. Louis, MO (KLSX)

  44. Radar Precipitation Estimation Caveats • No control over STP estimation interval • Based on empirically-derived formula • not always ideal for given area/season/character • Hail contamination • (large) water-covered ice pellets very reflective • causes overestimate of precip intensity/amount • Mixed precipitation character in same area • convective and stratiform precipitation falling simultaneously • which Z-R relationship applies? • Patterns generally good, magnitudes less so

  45. Doppler Effect • Based on frequency changes associated with moving objects • E-M energy scattered by hydrometeors moving toward/away from radar cause frequency change • Frequency of return signal compared to transmitted signal frequency  radial velocity

  46. (http://www.howstuffworks.com/radar1.htm)

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