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Basic Principles of Weather Radar
Basis of Presentation
• Introduction to Radar
• Basic Operating Principles
• Reflectivity Products
• Doppler Principles
• Velocity Products
• Non-Meteorological Targets
• Summary
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/part1/slide2.html)
(http://www.crh.noaa.gov/mkx/radar/part1/slide3.html)
(University of Illinois WW2010 Project)
(University of Illinois WW2010 Project)
http://weather.noaa.gov/radar/radinfo/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 WW2010 Project)
Elevation Angle
• Angle of ‘beam’ with respect to ground
(University of Illinois WW2010 Project)
Distance to Target
• D = cT/2
• T pulse’s round trip time
(University of Illinois WW2010 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-88D performs PPI scans over several elevation angles to produce 3D representation of local atmosphere
Plan Position Indicator
• Constant elevation angle
• Azimuth angle varies (antenna rotates)
(University of Illinois WW2010 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
• Red numbers are elevation angles
• Note how beam (generally) expands with increasing distance from radar
(http://weather.noaa.gov/radar/radinfo/radinfo.html)
• 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 WW2010 Project)
Choice of Wavelength 1
Pr 1
2– Typical weather radar range: 0.8-10.0 cm
– WSR-88D: ~10 cm
– TV radar: ~5 cm
Choice of Wavelength 2
Pr 1
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
PR Z
rr
c e 2
Radar Equation for Distributed Targets 3
• Pr is:
- directly proportional to ‘reflectivity’
- inversely proportional to square of distance between radar and target(s)
PR Z
rr
c e 2
Equivalent Radar Reflectivity Factor 1
where Ni number of scattering targets
Di diameter of scattering targets
v pulse volume
v
DN
Z
k
i
ii
e
1
6
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 6th power
- Doubling size yields 64 times the return
v
DN
Z
k
i
ii
e
1
6
(University of Illinois WW2010 Project)
dBZ
• Typical units used to express reflectivity
• Range: • –30 dBZ for fog
• +75 dBZ for very large hail
dBZZ
mm m
e 10
110 6 3log
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
Clear-Air Mode
Precipitation Mode
Greer, SC (KGSP) (http://virtual.clemson.edu/groups/birdrad/COMMENT.HTM)
Clear-Air Mode Precipitation Mode
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
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
Little Rock, AR (KLZK)
Precipitation Mode
Base Reflectivity Composite Reflectivity
Z-R Relationships 1
Z aR b
where Z ‘reflectivity’ (mm6 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: Also recommended for:
Marshall-Palmer (Z=200R1.6)
General stratiform precipitation
East-Cool Stratiform (Z=130R2.0)
Winter stratiform precipitation - east of
continental divide
Orographic rain - East
West-Cool Stratiform (Z=75R2.0)
Winter stratiform precipitation - west of
continental divide
Orographic rain - West
WSR-88D Convective (Z=300R1.4)
Summer deep convection
Other non-tropical convection
Rosenfeld Tropical (Z=250R1.2)
Tropical convective systems
(WSR-88D Operational Support Facility)
(WSR-88D Operational Support Facility)
Z-R Relationships 4
Reflectivity Marshall-Palmer
(Z=200R1.6)
East-Cool Stratiform (Z=130R2.0)
West-Cool
Stratiform (Z=75R2.0)
WSR-88D Convective (Z=300R1.4)
Rosenfeld Tropical
(Z=250R1.2)
15 dBZ 0.25 mm h-1 0.51 mm h-1 0.76 mm h-1 <0.25 mm h-1 <0.25 mm h-1
20 dBZ 0.76 mm h-1 1.02 mm h-1 1.27 mm h-1 0.51 mm h-1 0.51 mm h-1
25 dBZ 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 dBZ 2.79 mm h-1 2.79 mm h-1 3.56 mm h-1 2.29 mm h-1 3.30 mm h-1
35 dBZ 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 dBZ 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 dBZ 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 dBZ 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 dBZ 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 dBZ 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
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
St. Louis, MO (KLSX)
1-h Total Precipitation (ending at 2009 UTC 11 June 2003)
Storm Total Precipitation (0708 10 June 2003 to 2009 UTC 11 June 2003)
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/radar1.htm)
(Williams 1992)
(http://www.crh.noaa.gov/mkx/radar/part1/slide13.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 small-scale 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
Houston, TX (KHGX)
warm colors away from radar
cool colors toward radar
Base Velocity Storm-Relative Velocity
(http://virtual.clemson.edu/groups/birdrad/COMMENT.HTM)
Buffalo, NY (KBUF)
1944 UTC 28 April 2002
Storm-Relative Velocity
mesocyclone
(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
PRF
cR
2max
The Doppler Dilemma 3
• Maximum Unambiguous Velocity (Vmax)
– Highest radial velocity that can be ‘measured’ with confidence
– Directly proportional to radar wavelength and PRF
4max
PRFV
The Doppler Dilemma 4
4max
PRFV
PRF
cR
2max
8maxmax
cRV
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 = - (2Vmax – 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/Week_3_2/sld006.htm
Subrefraction
• Beam tilts upward
http://academic.amc.edu.au/~irodrigues/LECTURES/Week_3_2/sld006.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-88D 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/part2/slide31.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!