Transcript How (not) to Make a Good PowerPoint Presentation

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
w.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/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
(http://weather.noaa.gov/radar/radinfo/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 WW2010 Project)
Radar Equation for
Distributed Targets 1
k
ND
6
i
i
P
g

 c
l K i 1
t
Pr 
2
2


1024
ln
2
v

r












C
3
2
A
B
2
D
where Pr  average returned power
A  numerical constants
B  radar characteristics
C  target scatter efficiency characteristics
D  equivalent radar reflectivity factor (Ze)
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
Rc Z e
Pr 
2
r
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
Rc Z e
Pr 
2
r
• Pr is:
- directly proportional to ‘reflectivity’
- inversely proportional to square of distance
between radar and target(s)
Equivalent Radar
Reflectivity Factor 1
k
Ze 
N D
i
i 1
6
i
v
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
k
Ze 
N D
i
i 1
6
i
v
• 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
(University of Illinois WW2010 Project)
dBZ
Ze
dBZ  10 log10
6 3
1mm m
• Typical units used to express reflectivity
• Range:
• –30 dBZ for fog
• +75 dBZ 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 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
Base Reflectivity
Composite Reflectivity
Little Rock, AR (KLZK)
Precipitation Mode
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:
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)
Also recommended for:
Z-R Relationships 4
Reflectivity
MarshallPalmer
(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
(WSR-88D 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/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
(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
Rmax
c

2 PRF
• Maximum Unambiguous Range (Rmax)
– Longest distance between target and radar
that can be ‘measured’ with confidence
– Inversely proportional to PRF
The Doppler Dilemma 3
Vmax 
PRF
4
• Maximum Unambiguous Velocity (Vmax)
– Highest radial velocity that can be
‘measured’ with confidence
– Directly proportional to radar wavelength
and PRF
The Doppler Dilemma 4
Vmax 
Rmax
PRF
4
c

2 PRF
Vmax Rmax 
c
8
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!
Non-Meteorological Targets
• Ground Clutter
– trees
– mountains
– buildings
• Livestock
– insects
– birds
– bats
• Other ‘Targets’
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)
Livestock
• Radar can be used to track migrations
• Insects tend to be ‘carried’ by low-level
flow
• Bats and birds can travel ‘against’ flow
Greer, SC (KGSP)
Clear-Air Mode
Insects flying on NE winds
(http://virtual.clemson.edu/groups/birdrad/COMMENT.HTM)
(http://virtual.clemson.edu/groups/birdrad/COMMENT.HTM)
Blue: Low Bird Density
Green: High Bird Density
Fort Polk, LA (KPOE)
Precipitation Mode
Brownsville, TX (KBRO)
Clear-Air Mode
Raptor Migrations (S N)
(http://virtual.clemson.edu/groups/birdrad/COMMENT.HTM)
Laughlin, TX (KDLF)
Precipitation Mode
Bat Roost Rings
(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)
Other Targets 3
• Chaff
– metallic dust deployed by aircraft
– used to diffuse radar signatures
– often seen in vicinity of military bases
• Aircraft
– large metallic objects
– excellent scatterers
– original intended radar target
Melbourne, FL (KMLB)
Clear-Air Mode
Chaff
(http://virtual.clemson.edu/groups/birdrad/COMMENT.HTM)
Shreveport, LA (KSHV)
Clear-Air Mode
0954 CST 1 February 2003
Summary
• Weather surveillance radar has varied uses
–
–
–
–
short-term weather forecasting
hazardous weather warnings
hydrologic applications
tracking bird migration patterns
• 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 WW2010 Project)