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- 1. Weather Radar Kieran Commins Met Éireann Institute of Physics Seminars 2012 1
- 2. RADAR METEOROLOGY • An introduction to the measurement of precipitation by Weather Radar. • Presentation of the fundamentals involved in Radar Meteorology • Understanding of the physical process involved • Appreciation of the errors and problems involved. • Interpretation of data Institute of Physics Seminars 2012 2
- 3. Radar Basics Institute of Physics Seminars 2012 3 • RADAR = RAdio Detection and Ranging • Discovered in Britain • Radio transmissions deflected by passing aircraft • Lead to development of Radar during WW2
- 4. Radar and Weather • Rainfall was seen by radar • Regarded as ‘contamination’ • Many radar have ‘weather filters’ to remove weather signals • BUT some saw the potential is using radar for tracking precipitation DavidAtlas US Airforce & MIT Institute of Physics Seminars 2012 4
- 5. Weather Radar • Early radar needed an operator and hand draw the results • Major development was digitization and automation from early 80’s • Since then, resolution and quality and well as coverage has improved Institute of Physics Seminars 2012 5
- 6. Radar Components • Steerable Antenna • Transmitter • Receiver • Control Unit • Signal processor • Pre-processor Institute of Physics Seminars 2012 6
- 7. Antenna • Directs the transmitted energy in a beam • Collects the returned power • Gain depends on the Diameter and wavelength • A 4.2m antenna has a gain of 48,000 or 47dB • Beam width of 0.82O for the Dublin and Shannon Radars Chilbolton research radar, UK Institute of Physics Seminars 2012 7
- 8. Radome •Protects antenna from wind and rain •Less stress on antenna drives •Shelter of personnel working on the antenna •Sandwich construction ‘tuned’ to the radar wavelength Institute of Physics Seminars 2012 9
- 9. Radar ..Schematic Drawing XMTR Locking MIXER COHO MOD STALO REC MIXER T/R CELL IFAMP Phase Detector Signal Processor Antenna Waveguide Feedhorn Pre-Processor Institute of Physics Seminars 2012 10
- 10. Radar Ranging range = 1/2ct c=speed of light t=time taken for pulse to travel to target and back Institute of Physics Seminars 2012 11
- 11. Radar Scanning • Radar sends out a stream of pulses • Antenna rotates • Builds up a ‘picture’ of the echoes Institute of Physics Seminars 2012 12
- 12. Radar Polar Volume Institute of Physics Seminars 2012 13
- 13. Data Collecting and Sampling Range Element 250m x 0.1O Range Cell 1000 m x 1.0O 64 Range Elements Shannon Radar Institute of Physics Seminars 2012 14
- 14. Shannon Radar 240 km PPI Plan-position Indicator Institute of Physics Seminars 2012 15
- 15. Radar Meteorology Institute of Physics Seminars 2012 16 A little Physics!
- 16. Some definitions Antenna Gain: gain = power with antenna isotropic power or in terms of dB Institute of Physics Seminars 2012 17 isotropic power g = 10 log 10 power with antenna dB
- 17. Point Target • Radar sends out a pulse • Scattered by the target • Some power returns to the receiver Institute of Physics Seminars 2012 18
- 18. Scattering from a Point Target • Radar Pulse is scattered by the target in all directions • Only a small fraction of the power is returned to the receiver Institute of Physics Seminars 2012 19
- 19. Power received from a Point Target. r 2 t t e P = g P A A (4 )2 r4 g = a n t e n n a g a i n Institute of Physics Seminars 2012 20 = R e c e i v e d P o w e r = t r a n s m i t t e d P o w e r w h e r e P r P t At = TargetArea Ae = Antennaeffective area r = range
- 20. Meteorological Targets • Many targets – each raindrop! • Power returned is the SUM of the powers from all the targets Institute of Physics Seminars 2012 21
- 21. Meteorological Targets Power received from Meteorological Targets r t e i P = h P A 8 r2 . FK where Pr = mean power over several pulses. h = pulse length Ae = Effective Antenna Area r range Institute of Physics Seminars 2012 22 F = fraction of the beam filled with targets K = attenuation factor i = scattering cross section of eachdrop
- 22. Pulse Volume Institute of Physics Seminars 2012 23
- 23. Pulse Volume Institute of Physics Seminars 2012 24 Vm = 4 . 2 (r2 2 ) h is the beamwidth and h the pulse length Note the r2 above the line; this partly cancels the 1/r4 rule found in the Power from a Point Target.
- 24. Cross-section of Meteorological Targets Cross-section: Institute of Physics Seminars 2012 27 i 4 2 6 i = |K| D 5 where K = and m is a complex refractive index m2 -1 m2 + 2 K 2 = 0.93 for water and 0.197 for ice. Di = Diameter of a drop
- 25. The Radar Equation Institute of Physics Seminars 2012 28 • Power Received from Precipitation r 3 2 6 i p = p g D h |K|2 l t 128 8 ln(2) 2 r2 . . . l = atmosphericattenuationfactor Di = dropdiameter = wavelength Pt = transmitted power g = antenna gain h = pulsevolume
- 26. Radar Equation Institute of Physics Seminars 2012 29 • For a given radar, most elements are constants • Equation simplifies to: r p C z r2 z = reflectivity C = constant
- 27. Scattering from large and small targets Institute of Physics Seminars 2012 30
- 28. Reflectivity • Radar measures the reflectivity of precipitation • The reflectivity (z) is related to the size of drops Institute of Physics Seminars 2012 31 6 i D z
- 29. Reflectivity and Rainfall..2 Effect of drop size. Institute of Physics Seminars 2012 32 • 1 Drop, 1mm diameter in 1 m3: • 16 = 1 • z = 0 dBZ • 1000 Drops, 0.1 mm diameter • z = 1000 x (0.1)6 = 0.001 • z = -30dBz • Same amount of liquid in both cases!
- 30. Reflectivity and Rainfall..3 Impossible Problem? Institute of Physics Seminars 2012 33 • NO! • Rainfall depends on drop size • large drops --> heavy rain • small drops -- > drizzle • large drops have higher vertical velocities • more water reached the surface • small drops low vertical velocities • remain suspended for much longer
- 31. Reflectivity and Rainfall..4 Z-R Relationship Institute of Physics Seminars 2012 34 • A definite relationship between Z and R has been found by empirical methods: • z = a R b • Standard relationship used in Ireland, UK and many other countries is: • z = 200 R 1.6 • Marshall Palmer Relationship. • Applies particularly to frontal rain.
- 32. Summary Institute of Physics Seminars 2012 35 • Radar measures REFLECTIVITY not rainfall • Reflectivity is proportional to SIXTH power of the drop size • Rainfall is deduced from reflectivity by an empirical relationship • Heavy rainfall is over-estimated • Light rainfall/drizzle is under-estimated
- 33. End of the Physics…. Institute of Physics Seminars 2012 36
- 34. Radars in Met Éireann Institute of Physics Seminars 2012 37
- 35. Radars in Met Éireann Institute of Physics Seminars 2012 38 • Two radars: At Dublin and ShannonAirports • Using radar since the 60’s. • First radar were manually operated and reports generated every 3 hours • Shannon radar digitized in 1984, Dublin in 1991. • Both upgraded in 2010-2011
- 36. Radar in Met Éireann • Modern radars installed in the 90’s • Scan every 5 minutes to a range of 250 km with a 1km resolution. • Archives dating from 1997. • Accumulation data from 1998. • Do not use telemetered raingauges or rain gauge corrections at present. Institute of Physics Seminars 2012 39
- 37. Single Site Radar Images Institute of Physics Seminars 2012 40
- 38. Irish Radar Composite Institute of Physics Seminars 2012 41
- 39. Animation -High Resolution Composite • 2 radars • Shannon • Dublin • Every 15 minutes • Made in Dublin Institute of Physics Seminars 2012 42
- 40. Ireland Composite Radars Included: Shannon Dublin Castor Bay Crugygorllwyn Cobbacombe Cross Institute of Physics Seminars 2012 43
- 41. UK-Ireland Composite • 12 Radars in • Ireland, Jersey and • the UK Institute of Physics Seminars 2012 44
- 42. 24 hour accumulation Short range 75km 1000m height Distribution is even Institute of Physics Seminars 2012 46
- 43. 24 hour accumulation 240km range Intensities fall off at long ranges Institute of Physics Seminars 2012 47
- 44. 14 Nov 2002 • Major rainfall event • Flooding of Tolka and Liffey. • 24 Hour accumulation Institute of Physics Seminars 2012 48
- 45. 14th Nov2002 • Close up of Dublin Area • Intense areas to the West of Dublin Institute of Physics Seminars 2012 49
- 46. Future Institute of Physics Seminars 2012 50 • Much research is being carried out on improving radar rainfall measurements. • New techniques involve dual-polarisation or even dual frequency radars but these are much more expensive. • The next 10 years should see some major improvements.
- 47. Differences between Meteorological and ATC Radars Institute of Physics Seminars 2012 51 • Elliptical Antenna giving a “fan” beam. • High rotation speeds - 15 rpm • Wide beam width • Suppression of all slowing moving targets especially weather. • No vertical information - rely on transponders for height information • Some have a “Weather Channel” especially in USA
- 48. Radar Problems! Institute of Physics Seminars 2012 52
- 49. Radar problems Institute of Physics Seminars 2012 53 • Earth’s curvature • Ground Clutter • Permanent Echoes • Occultation by High Ground • Bright Band • ANAPROP
- 50. The Earth is Curved The following does not apply if the Earth is flat!!! Institute of Physics Seminars 2012 54
- 51. Curved Earth • The Earth is curved! • This curvature is very significant for radars. • At a distance of 250km, the lowest point visible to the radar is 12,000 – 14,000 ft above the surface. • An entire rain system can exist below this level and may not be seen. Institute of Physics Seminars 2012 55
- 52. Earth’sCurvature • Even though the beam is horizontal, it still rises above the Earth’s surface Institute of Physics Seminars 2012 56
- 53. Lowest Radar beam Height Vs Distance 0km 120km 240km 5000’ Institute of Physics Seminars 2012 57 10,000’
- 54. Shannon Radar 240 km Weaker intensities at the longer ranges. Radar only “sees” Tops of clouds. Best signals near the radar. Institute of Physics Seminars 2012 58
- 55. Radar problems Ground Clutter Institute of Physics Seminars 2012 59 • caused by power in the antenna side lobes • about 5% total power is in side lobes • signal reflected by the ground and buildings • worst within 10km of radar • Permanent echoes from hills mountains or tall buildings • Very evident on Dublin Radar display
- 56. Side Lobes Main Beam 1st Side Lobe 2nd Side Lobe Institute of Physics Seminars 2012 60
- 57. Permanent Echoes Institute of Physics Seminars 2012 61
- 58. Radar problems Ground Clutter Solutions Institute of Physics Seminars 2012 62 • Radar site • low flat site : good • hill top good for permanent echoes but clutter more of a problem • Clutter Maps • subtract clutter recorder on a fine day • can be used at Dublin and Shannon • not very effective
- 59. Radar problems Ground Clutter Solutions Institute of Physics Seminars 2012 63 • Doppler Suppression • remove echoes with a velocity near zero • loose some real data! • Statistical MTI • examine a large number of echoes in each range cell • clutter has smaller spread that echoes from hydrometeors
- 60. Radar problems Occultation by High Ground • Radar beam is partially or Totally blocked by hills. • Get “shadows” in the rainfall patterns especially in Accumulation products Institute of Physics Seminars 2012 64
- 61. Occultation No Screening Partial Screening Total Screening Partial and Total Screening by high ground Institute of Physics Seminars 2012 65
- 62. Radar problems Occultation by High Ground Institute of Physics Seminars 2012 67 • Partially Blocked Beam: • correct affected cells by a % correction • effective up to 50% blockage • Totally Blocked Beam: • use nearest unblocked cells • use weighted mean of nearest neighbours • mark as blocked • 3D Clutter and Occultation Scheme
- 63. Radar problems Bright Band Institute of Physics Seminars 2012 68 • Caused by melting snow • Wet snow has higher reflectivity than either snow or rain • Can be seen in vertical cross-sections • Can lead to over-estimation of rainfall • Solutions • visual examination • computer algorithms (experimental)
- 64. Bright Band Rain Melting Snow Bright Band Freezing Level Snow Institute of Physics Seminars 2012 69
- 65. Bright Band Example 24th April 1998 UK Composite Institute of Physics Seminars 2012 70 Dublin CAPPI Dublin MAX CAPPI
- 66. Radar problems ANAPROP • Anomalous Propagation • Normally refractive index decreases with height and bends beam down • In ANAPROP refractive index changes and the beam can: • bend back to the ground OR • away from the ground Institute of Physics Seminars 2012 71
- 67. Anaprop Anomalous Propagation; reflection of the radar beam from the Earth’s Surface Institute of Physics Seminars 2012 72
- 68. Radar problems ANAPROP Institute of Physics Seminars 2012 73 • Sometimes distant coasts can be seen - Welsh coast • Solutions: • Doppler and Statistical suppression helps • usually visually identifiable
- 69. ANAPROP Example Institute of Physics Seminars 2012 74
- 70. Effect of the Sun • Radar Receiver detects noise from the sun. • Shows as a beam • Sun is used to align the antenna • Also strong ANAPROP on image Institute of Physics Seminars 2012 75
- 71. Radar Problems Incomplete Beam Filling Above prec. Partially within precipitation Totally filled 100km Institute of Physics Seminars 2012 76 240km
- 72. Radar Problems Rainfall Attenuation Radar Display Reduced beam Normal Beam Institute of Physics Seminars 2012 77
- 73. Accumulation problems • Instant values every 15 minutes • Accuracy reduced with fast moving showers • Some areas may be missed! Institute of Physics Seminars 2012 78
- 74. Accuracy Institute of Physics Seminars 2012 79 • Due to high dependence on the drop size distribution, accuracy in limited • Best for frontal rainfall - 50% accuracy • Over estimates showers - 50 - 200% • Under estimates drizzle - 50 - 100% • Accuracy falls off badly at longer ranges, perhaps 5 - 10% > 200km • Subject to contamination/occultation
- 75. Radar Raingauges Vs Institute of Physics Seminars 2012 80
- 76. Raingauges Institute of Physics Seminars 2012 81 • These measure surface rainfall at a point. • They are subject to errors – wind causes loss of rainfall. • But they much more accurate than radar. • Modern raingauges can be telemetered automatically reporting data.
- 77. Raingauges Institute of Physics Seminars 2012 82 • Rainfall varies widely even over short distances. • Even 600+ rainfall stations in Ireland cannot fully measure rainfall variations. • “Hot spots” or areas of intense rainfall can be missed by gauges.
- 78. Shannon Radar Close-up Small scale but extreme variations in intensity. Raingauges can easily miss these. Institute of Physics Seminars 2012 83
- 79. Radar and Raingauges Institute of Physics Seminars 2012 85 • The combination of Radar and Raingauges can significantly improve the accuracy of radar rainfall measurements. • Radar data can be corrected by ground truth and the corrections interpolated to the rest of the data. • Get a map with high spatial resolution with reasonable accuracy.
- 80. And finally Institute of Physics Seminars 2012 86 Summary
- 81. Some Pros and Cons of Radar Institute of Physics Seminars 2012 87 • Advantages • Good spatial resolution - 1km x 1km • Good temporal resolution - 15 minute • Good for surveillance esp. when networked • Disadvantages • Poor rainfall rate accuracy • Poor rainfall accumulation accuracy • Interference from ground and other problems
- 82. Advantages of Radar Institute of Physics Seminars 2012 88 • Good spatial coverage – wide area at good resolution • Good temporal coverage – typically 5 to 15 minutes intervals • Possible to make accumulation maps • Combine several radars to form regional composites
- 83. Uses of radar data Institute of Physics Seminars 2012 89 • Nowcasting • River managment in conjunction with flow and hydrometric gauges. Well developed in the UK (Environment Agency and Met Office). • Flood warnings. • Hydrological studies. • Climatological enquiries – did it rain at a particular place?
- 84. Thank you Institute of Physics Seminars 2012 90