Basic Principles of Weather Radar

Transcription

1 Basic Principles of Weather Radar

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

3 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

4 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

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7 (University of Illinois WW2010 Project)

8 (University of Illinois WW2010 Project)

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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)

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18 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

19 ( Red numbers are elevation angles Note how beam (generally) expands with increasing distance from radar

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

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22 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)

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24 Choice of Wavelength 1 P r 1 2 Typical weather radar range: cm WSR-88D: ~10 cm TV radar: ~5 cm

25 Choice of Wavelength 2 P r 1 2 P r 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)

26 Radar Equation for Distributed Targets 2 P r R Z c 2 where P r average returned power R c radar constant Z e equivalent radar reflectivity factor ( reflectivity ) r distance from radar to target r e

27 Radar Equation for Distributed Targets 3 P r R Z c 2 P r is: - directly proportional to reflectivity - inversely proportional to square of distance between radar and target(s) r e

28 Equivalent Radar Reflectivity Factor 1 Z e N D where N i number of scattering targets D i diameter of scattering targets v pulse volume k i 1 v i 6 i

29 Equivalent Radar Reflectivity Factor 2 Z e 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)

30 Equivalent Radar Reflectivity Factor 3 Z e k i 1 N D Z e is: - directly proportional to number of scatterers - inversely proportional to sample volume - directly proportional to scatterer diameter raised to 6 th power - Doubling size yields 64 times the return v i 6 i

31 (University of Illinois WW2010 Project)

32 dbz dbz 10 Z log e mm m Typical units used to express reflectivity Range: 30 dbz for fog +75 dbz for very large hail

33 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

34 Clear-Air Mode Precipitation Mode

35 Clear-Air Mode Precipitation Mode Greer, SC (KGSP) (

36 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

37 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

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

39 Z-R Relationships 1 Z ar b where Z reflectivity (mm 6 m -3 ) R rainfall rate (mm h -1 ) a and b are empirically derived constants

40 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)

41 Z-R Relationships 3 Relationship Optimum for: Also recommended for: Marshall-Palmer (Z=200R 1.6 ) East-Cool Stratiform (Z=130R 2.0 ) West-Cool Stratiform (Z=75R 2.0 ) WSR-88D Convective (Z=300R 1.4 ) Rosenfeld Tropical (Z=250R 1.2 ) General stratiform precipitation Winter stratiform precipitation - east of continental divide Winter stratiform precipitation - west of continental divide Summer deep convection Tropical convective systems (WSR-88D Operational Support Facility) Orographic rain - East Orographic rain - West Other non-tropical convection

42 Z-R Relationships 4 Reflectivity Marshall- Palmer (Z=200R 1.6 ) East-Cool Stratiform (Z=130R 2.0 ) West-Cool Stratiform (Z=75R 2.0 ) WSR-88D Convective (Z=300R 1.4 ) Rosenfeld Tropical (Z=250R 1.2 ) 15 dbz 0.25 mm h mm h mm h -1 <0.25 mm h -1 <0.25 mm h dbz 0.76 mm h mm h mm h mm h mm h dbz 1.27 mm h mm h mm h mm h mm h dbz 2.79 mm h mm h mm h mm h mm h dbz 5.59 mm h mm h mm h mm h mm h dbz mm h mm h mm h mm h mm h dbz mm h mm h mm h mm h mm h dbz mm h mm h mm h mm h mm h dbz mm h mm h mm h mm h mm h dbz mm h mm h mm h mm h mm h -1 (WSR-88D Operational Support Facility)

43 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

44 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

45 1-h Total Precipitation (ending at 2009 UTC 11 June 2003) Storm Total Precipitation ( June 2003 to 2009 UTC 11 June 2003) St. Louis, MO (KLSX)

46 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

47 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

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51 Radial Velocity 1 Hydrometeors moving toward/away from radar Positive values targets moving away from radar Negative values targets moving toward radar Can be used to ascertain large-scale and small-scale flows/phenomena fronts and other boundaries mesoscale circulations microbursts

52 Radial Velocity 2 Base Velocity ground-relative good for large-scale flow and straight-line winds Storm-Relative Velocity storm motion subtracted from radial velocity good for detecting circulations and divergent/convergent flows

53 ( Base Velocity Storm-Relative Velocity Houston, TX (KHGX) warm colors away from radar cool colors toward radar

54 mesocyclone Buffalo, NY (KBUF) 1944 UTC 28 April 2002 Storm-Relative Velocity (

55 The Doppler Dilemma 1 Pulse can only travel so far and return in time before next pulse is transmitted Distant targets may be reported as close, and/or Velocities may be aliased Pulse Repetition Frequency (PRF) transmission interval typical values Hz (cycles s -1 ) key to determining maximum unambiguous range (R max ) and velocity (V max )

56 The Doppler Dilemma 2 R max c 2PRF Maximum Unambiguous Range (R max ) Longest distance between target and radar that can be measured with confidence Inversely proportional to PRF

57 The Doppler Dilemma 3 V max PRF 4 Maximum Unambiguous Velocity (V max ) Highest radial velocity that can be measured with confidence Directly proportional to radar wavelength and PRF

58 The Doppler Dilemma 4 V R max max PRF 4 c 2PRF V max R max c 8

59 The Doppler Dilemma 5 If R actual > R max, range folding occurs distant echoes appear close to radar R app = R max R actual second-trip echoes If V actual > V max, velocity folding occurs radial velocities misreported V app = - (2V max V actual ) Sign of V max = V actual

60 The Doppler Dilemma 6 If V max = ± 25 m s -1 and target is moving away from radar at 30 m s -1 i.e., V actual = +30 m s -1 V app = - (50 30) = -20 m s -1 toward radar at slower speed! What about target moving at - 50 m s -1? V app = - [-50 (- 50)] = 0 m s -1 Target would appear to be stationary! If R max is large, then V max has to be small (and vice versa) cannot be large simultaneously!

61 Refraction Radar beam typically follows Earth s curvature ES/Week_3_2/sld006.htm

62 Subrefraction Beam tilts upward ES/Week_3_2/sld006.htm

63 Non-Meteorological Targets Ground Clutter trees mountains buildings Other Targets sun strobes anomalous propagation (AP)

64 Ground Clutter Stationary objects usually filtered out Swaying trees or towers may show up Look for drifting high reflectivity returns near radar

65 Cannon AFB, NM (KCVS) Precipitation Mode (

66 Mountain Blockage Low elevation angle scans blocked by terrain Shadows appear consistently in imagery Mainly a problem in western U.S.

67 Boise Mountains Owyhee Mountains Boise, ID (KBOI) Clear-Air Mode (

68 WSR-88D Network

69 Building Blockage Nearby building blocks beam if building is taller than antenna (~100 ft) Narrow shadows appear consistently in imagery Occurs in/near metropolitan areas

70 Houston, TX (KHGX) Precipitation Mode (

71 Other Targets 1 Sun strobes occur typically around dawn/dusk radar receives intense dose of E-M radiation along narrow radials similar strobes occur if beam intercepts intense source of microwave radiation other radars microwave repeaters

72 National Radar Mosaic Precipitation Mode Sun Strobes (

73 Other Targets 2 Anomalous propagation (AP) beam refracted into ground under very stable atmospheric conditions inversions near large bodies of water behind thunderstorms appear similar to intense precipitation compare to surface observations check satellite imagery examine higher elevation scans

74 Melbourne, FL (KMLB) Clear-Air Mode Anomalous Propagation (AP) (

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76 Summary Weather surveillance radar has varied uses short-term weather forecasting hazardous weather warnings hydrologic applications Must be aware of radar s limitations WYSINAWYG What You See Is NOT ALWAYS What You Get!