Basic Principles of Weather Radar - Davis School · PDF file(University of Illinois WW2010 Project) Elevation Angle Considerations •Radar usually aimed above horizon –minimizes

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)


(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


Elevation Angle

• Angle of ‘beam’ with respect to ground


Distance to Target

• D = cT/2

• T pulse’s round trip time


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)


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


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


• 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)


• 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


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









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)


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)


• Maximum Unambiguous Range (Rmax)

– Longest distance between target and radar that can be ‘measured’ with confidence

– Inversely proportional to PRF

PRF

cR

2max


• Maximum Unambiguous Velocity (Vmax)

– Highest radial velocity that can be ‘measured’ with confidence

– Directly proportional to radar wavelength and PRF

4max

PRFV


4max

PRFV

PRF

cR

2max

8maxmax

cRV


• 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


• 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




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)


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!

Documents

Basic Principles of Weather Radar - Davis School · PDF file(University of Illinois WW2010 Project) Elevation Angle Considerations •Radar usually aimed above horizon –minimizes