Basic Principles of Weather Radar Dr Scott M

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 (http: //www. weathermatrix. net/radar/education/articles/laughlin/images/KDFX. jpg)

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

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

(http: //www. crh. noaa. gov/mkx/radar/part 1/slide 2. html)

(http: //www. crh. noaa. gov/mkx/radar/part 1/slide 3. html)

(University of Illinois WW 2010 Project)

(University of Illinois WW 2010 Project)

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

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

Azimuth Angle • Angle of ‘beam’ with respect to north (University of Illinois WW 2010 Project)

Elevation Angle • Angle of ‘beam’ with respect to ground (University of Illinois WW 2010 Project)

Distance to Target • D = c. T/2 • T pulse’s round trip time (University of Illinois WW 2010 Project)

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-88 D performs PPI scans over several elevation angles to produce 3 D representation of local atmosphere

Plan Position Indicator • Constant elevation angle • Azimuth angle varies (antenna rotates) (University of Illinois WW 2010 Project)

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

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

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

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 WW 2010 Project)

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 (Z )

Choice of Wavelength 1 – Typical weather radar range: 0. 8 -10. 0 cm – WSR-88 D: ~10 cm – TV radar: ~5 cm

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)

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

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

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

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)

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 6 th power - Doubling size yields 64 times the return

(University of Illinois WW 2010 Project)

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

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

Clear-Air Mode Precipitation Mode

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

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 d. BZ

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

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

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

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)

Z-R Relationships 3 Relationship Optimum for: Marshall-Palmer (Z=200 R 1. 6) General stratiform precipitation East-Cool Stratiform (Z=130 R 2. 0) Winter stratiform precipitation - east of continental divide Orographic rain - East West-Cool Stratiform (Z=75 R 2. 0) Winter stratiform precipitation - west of continental divide Orographic rain - West WSR-88 D Convective (Z=300 R 1. 4) Summer deep convection Other non-tropical convection Rosenfeld Tropical (Z=250 R 1. 2) Tropical convective systems (WSR-88 D Operational Support Facility) Also recommended for:

Z-R Relationships 4 Reflectivity Marshall. Palmer (Z=200 R 1. 6) East-Cool Stratiform (Z=130 R 2. 0) West-Cool Stratiform (Z=75 R 2. 0) WSR-88 D Convective (Z=300 R 1. 4) Rosenfeld Tropical (Z=250 R 1. 2) 15 d. BZ 0. 25 mm h-1 0. 51 mm h-1 0. 76 mm h-1 <0. 25 mm h-1 20 d. BZ 0. 76 mm h-1 1. 02 mm h-1 1. 27 mm h-1 0. 51 mm h-1 25 d. BZ 1. 27 mm h-1 1. 52 mm h-1 2. 03 mm h-1 1. 02 mm h-1 1. 27 mm h-1 30 d. BZ 2. 79 mm h-1 3. 56 mm h-1 2. 29 mm h-1 3. 30 mm h-1 35 d. BZ 5. 59 mm h-1 4. 83 mm h-1 6. 60 mm h-1 5. 33 mm h-1 8. 38 mm h-1 40 d. BZ 11. 43 mm h-1 8. 89 mm h-1 11. 68 mm h-1 12. 19 mm h-1 21. 59 mm h-1 45 d. BZ 23. 62 mm h-1 15. 49 mm h-1 20. 58 mm h-1 27. 94 mm h-1 56. 39 mm h-1 50 d. BZ 48. 51 mm h-1 27. 69 mm h-1 36. 58 mm h-1 63. 50 mm h-1 147. 32 mm h-1 55 d. BZ 99. 82 mm h-1 49. 28 mm h-1 65. 02 mm h-1 144. 27 mm h-1 384. 56 mm h-1 60 d. BZ 204. 98 mm h-1 87. 63 mm h-1 115. 57 mm h-1 328. 42 mm h-1 1004. 06 mm h-1 (WSR-88 D Operational Support Facility)

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

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

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

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

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

(http: //www. howstuffworks. com/radar 1. htm)

(Williams 1992)

(http: //www. crh. noaa. gov/mkx/radar/part 1/slide 13. html)

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 smallscale flows/phenomena – fronts and other boundaries – mesoscale circulations – microbursts

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

(http: //virtual. clemson. edu/groups/birdrad/COMMENT. HTM) Base Velocity Storm-Relative Velocity Houston, TX (KHGX) warm colors away from radar cool colors toward radar

mesocyclone Buffalo, NY (KBUF) 1944 UTC 28 April 2002 Storm-Relative Velocity (http: //www. srh. weather. gov/jetstream/remote/srm. htm)

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 700 -3000 Hz (cycles s-1) – key to determining maximum unambiguous range (Rmax) and velocity (Vmax)

The Doppler Dilemma 2 • Maximum Unambiguous Range (Rmax) – Longest distance between target and radar that can be ‘measured’ with confidence – Inversely proportional to PRF

The Doppler Dilemma 3 • Maximum Unambiguous Velocity (Vmax) – Highest radial velocity that can be ‘measured’ with confidence – Directly proportional to radar wavelength and PRF

The Doppler Dilemma 4

The Doppler Dilemma 5 • If Ractual > Rmax, range folding occurs – distant echoes appear close to radar – Rapp = Rmax – Ractual – second-trip echoes • If Vactual > Vmax, velocity folding occurs – radial velocities misreported – Vapp = - (2 Vmax – Vactual) – Sign of Vmax = Vactual

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

Refraction • Radar ‘beam’ typically follows Earth’s curvature http: //academic. amc. edu. au/~irodrigues/LECTURES/Wee k_3_2/sld 006. htm

Subrefraction • Beam tilts upward http: //academic. amc. edu. au/~irodrigues/LECTURES/Wee k_3_2/sld 006. htm

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

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

Cannon AFB, NM (KCVS) Precipitation Mode (http: //virtual. clemson. edu/groups/birdrad/COMMENT. HTM)

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

Boise Mountains Owyhee Mountains Boise, ID (KBOI) Clear-Air Mode (http: //virtual. clemson. edu/groups/birdrad/COMMENT. HTM)

WSR-88 D Network

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

Houston, TX (KHGX) Precipitation Mode (http: //virtual. clemson. edu/groups/birdrad/COMMENT. HTM)

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

National Radar Mosaic Precipitation Mode Sun Strobes (http: //virtual. clemson. edu/groups/birdrad/COMMENT. HTM)

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

Melbourne, FL (KMLB) Clear-Air Mode Anomalous Propagation (AP) (http: //virtual. clemson. edu/groups/birdrad/COMMENT. HTM)

(http: //www. crh. noaa. gov/mkx/radar/part 2/slide 31. html)

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!

(http: //www. aero. und. edu/~rinehart/cartoons. html) (University of Illinois WW 2010 Project)