Radar Artifacts
• Radar Beam Filling
• Non Uniformity Of Vertical Distribution Of
Precipitation
• Variations In Z-R Relationship
• Attenuation By Intervening Precipitation
• Beam Blocking
• Attenuation Due To A Wet Radome
• Electromagnetic Interference
• Ground and Other Clutter
• Anomalous Propagation
• Antenna Accuracy
• Electronics Stability
• Processing Accuracy
• Radar Range Equation
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Radar Artifacts
Analysis & Diagnosis 1
The Schematic Radar
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Radar Artifacts
Analysis & Diagnosis 2
Radar Sources of Error
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Radar Artifacts
Analysis & Diagnosis 3
Radar System Limitations
Factor
Situations/causes
System Calibration and Stability
Example: System
drift with time
Current method
Potential/Alternative
used in URP/NRP Solution
Monitor accumulation
maps for peculiarities or
Internal system
trends. Raingauge
checks
adjustment.
Primarily in hail
Sidelobe Contamination
situations at C-band none
Range
Random system
ambiguities
instabilities, nonremoved using
meteorological
dual PRF
Reflectivity Outliers (spurious values) targets.
unfolding.
Significant problem
at 5 cm wavelength
during heavy
Attenuation in Precipitation
precipitation.
none
Radome Wetting
Partial Beamfilling
Sampling Error due to Cell Motion
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Moderate-heavy
precipitation at radar
site
Convective
situations. Larger
problem at long
range due to beam
spreading.
Convective situation.
More obvious with
rapid cell motion.
Hydroscopic
radome paint
Smaller
beamwidth (0.65
degree) in new
NRP radars.
none
Active Canadian or
Commercial Groups
NRP/Regional
Technicians
Manual adjustment based
on pattern recognition.
Comparison with
surrounding radar data
(Nexrad). Comparison
with satellite imagery or
surface reports (UK).
Iterative attenuation
correction (difficult to
apply:some evidence of
numerical instability)
Waterloo
Interpolate using
advection correction. Flag
data as suspect.
Raingauge adjustment
(Nexrad).
NRP, King
User-defined rainrate
which is a non-linear
function of both the
unadjusted rainrate and
the range (Nexrad).
Advection correction
(spatial-temporal
smoother)
McGill, King
Radar Artifacts
Non-meteorological effects
Factor
Beam Blockage
Current method used
Situations/causes
in URP/NRP
Interception of a
Scan above source of
portion or all of radar blockage. Minimised
beam by nearby
with 0.65 degree
obstructions.
beamwidth.
Unwanted detection
of nearby
obstructions. Worse
Ground Clutter
in complex terrain.
Low-level inversion
bends radar signal
downward creating
unwanted ground
Anomalous Propagation echo.
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Adaptive filter using
FFT processing of
Doppler data
Potential/Alternative
Solution
Add a pre-determined
reflectivity value to
each blocked sample
volume (Nexrad)
Masking (Nexrad).
Vertical profile
tests:clutter
diminishes rapidly with
height. VIL - low-level Z
correlation methods.
Adaptive filter using
FFT processing of
Doppler data
Masking, Bayesian
tests using auxiliary
data (e.g. satellite,
surface stations)
Radar Artifacts
Active Canadian or
Commercial Groups
King, McGill
SIGMET
software/Doppler
processing. McGill
SIGMET
software/Doppler
processing
Meteorological Effects
Current method used
in URP/NRP
Potential/Alternative
Active Canadian or
Solution
Commercial Groups
Variable Z-R relationship
tuned to nature of
precipitation. Raingauge
Z-R lookup table rather adjustment. Polarization
than fixed relationship diversity.
King, McGill
Factor
Situations/causes
Variable Drop Size
Distribution
Convective vs
stratiform
Beam passes
through melting level. Proposed correction
Significant in
based on vertical
stratiform rain.
profiles.
Bright Band Effect
Poorly understood
characteristic of
rainfall. Affects
Rainfall Intermittency sampling accuracy.
Anomalously High
Power Returns From
Wet Ice Targets
Wet hail
Extrapolation to
Surface
Low-Level Rain
Growth in
When fog/stratus is
Fog/Stratus
present
Strongest in
mountainous terrain,
but significant over
Orographic
gentle slopes as well
Enhancement
(e.g. Prairies)
Evaporation in Low
Strongest in drier
Level
climates (Prairies)
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none
Limit measurement to
regions below or above
bright band.
King, McGill
More research required
to characterize potential
contribution to radar
measurement error. Also
will affect advection
adjustment algorithms.
Place arbitrary maximum
limit on derived rainfall
rate. Human intervention
based on hail signatures
in the imagery (Nexrad).
Polarization diversity to
recognize hail.
McGill
none
24 tilt angles will allow
real-time profiles.
Vertical profiles
Vertical profiles.
24 tilt angles will allow Raingauge adjustment
real-time profiles.
(Nexrad, UK).
none
Spatially-variable
correction zones (UK)
based on gauge
comparisons.
none
Raingauge adjustment
King, McGill
Radar Artifacts
Radar sensitivity and blockage
• Not all radars are equal
• Most are retrofitted Enterprise radars with 1.1° beam width and 4 m
dishes
• Some are Andrews radars with 0.65° beam width and 6 m dishes
• Spirit River AB, WWW, has only 1.6° beam width!
• Many have obvious issues
– differences in sensitivity between each other
– blockages
• But many have less obvious, but significant problems that only
become apparent with long-term precipitation accumulations
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Radar Artifacts
Analysis & Diagnosis 7
BC
15 Day Precip Accum
CAPPI 1.5 km
WWW 1015m
XPG 1118m
XSS 1829m
XSI 700m
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WUJ 60m
Radar Artifacts
Analysis & Diagnosis 8
Prairies
30 Day Precip Accum
PRECIPET
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Radar Artifacts
Analysis & Diagnosis 9
Nrn
Ontario
2 Day Precip Accum
CAPPI 1.0 km
Feb 16-17 2008
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Radar Artifacts
Analysis & Diagnosis 10
2 Day Precip Accum
CAPPI 1.0 km
Mar 8-9 2008
Srn
Ontario
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Radar Artifacts
Analysis & Diagnosis 11
Srn
Quebec
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2 Day Precip Accum
CAPPI 1.0 km
Mar 8-9 2008
Radar Artifacts
Analysis & Diagnosis 12
2 Day Precip Accum
CAPPI 1.0 km
Atlantic
Jan 28-30 2008
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Radar Artifacts
Analysis & Diagnosis 13
Sources of Error - Radar Dome Wetting
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Radar Artifacts
Analysis & Diagnosis 14
Sources of Error
• Intervening precipitation
• Wet radome
• Beam blocking
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Radar Artifacts
Attenuation: What lies beyond?
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Radar Artifacts
Analysis & Diagnosis 16
Example of Attenuation in Precipitation
Wedge of attenuation
Foreground
Storm
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Radar Artifacts
Cross Section
constructed
along this line
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Radar Artifacts
60 dbZ core
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Radar Artifacts
Sources of Error - Cell Blocking
Cell blocking
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Radar Artifacts
Analysis & Diagnosis 20
BEAM BLOCKING
Spirit River, AB/BC
Persistent
???
Ground clutter
Beam blockage
N Clear Hills
S Rockies
???
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Radar Artifacts
Attenuation
Wedge of attenuation
Foreground
Storm
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Radar Artifacts
Attenuation
• Warm
Frontal
Supercell
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Radar Artifacts
Attenuation
•
•
•
•
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Radar Artifacts
north
end
of
derecho?
Radome
Wetting
You just
Checked
The radar…
NO
PROBLEM…
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PROBLEM
!
Radar Artifacts
Sources of Error - Sidelobes
Orographic Enhancement
3D View of Real Beam
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Radar Artifacts
Analysis & Diagnosis 26
Sources of Error - Blocking Impacts on Precipitation Estimates
Why no
precipitation
amount
maximum on
the peaks?
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Radar Artifacts
Analysis & Diagnosis 27
Bright Band
• Melting snowflakes are large bright radar targets
• Reflectivity from melting snow is larger than that
of the rain below or the snow above as falling
snow passes through the melting layer
• Huge impact on quantitative precipitation
estimates
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Radar Artifacts
Analysis & Diagnosis 28
Bright band shows up as ring (or partial ring) of high Z centred on the radar
Bright band
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Radar Artifacts
Melting of Snow Flakes starts at 2.5 km height Bright
Band
• Freezing Level
~2.5km AGL
• Either
associated
with Rain or
Freezing Rain
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Radar Artifacts
Bright Band
Time/height Display
Melting Starts
Melting
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Time
Radar Artifacts
Analysis & Diagnosis 31
But not on the lower elevation scan, in this case
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Radar Artifacts
Melting layer shows as peak in Z (note it shows up best in data for the 3.5 degree
scan. Why?)
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Radar Artifacts
Sources of Error - Conventional Ground Clutter
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Radar Artifacts
Analysis & Diagnosis 34
Multi-path Echoes
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Radar Artifacts
Spot the AP
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Radar Artifacts
Corresponding Radial Velocity Image
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Radar Artifacts
AP versus Real - Compared
Doppler Reflectivity
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Radar Artifacts
Analysis & Diagnosis 38
Sources of Error - Anomalous Propagation AP
Strong inversions
Typical early morning after clear (long) nights – fall maximum
Over cold water surface – spring maximum
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Radar Artifacts
Analysis & Diagnosis 39
Anomalous Propagation of the Radar Beam
Only Real Weather
AP and Real Weather
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The
Doppler Velocity View
Radar Artifacts
Analysis & Diagnosis 40
AP and Frontal Analysis
Anomalous Precipitation
AP
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Radar Artifacts
Analysis & Diagnosis 41
AP … Water Vapour and Fog
VISIBILITY Probably still OK
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Radar Artifacts
Analysis & Diagnosis 42
Ground Clutter
• Direct reflections from ground targets, either in
the main lobe or sidelobes
• Echoes are unusually variable in space, but
constant in time (in contrast to AP, which is
variable in time)
• Well handled by clutter filters during signal
processing
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Radar Artifacts
Ground Clutter
• Rockie
Mountains
• persistent
• even on
EchoTop
product
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Radar Artifacts
Ground Clutter
Bethune, Saskatchewan
• Missouri
Couteau
• only seen
on 0.5PPI
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Radar Artifacts
Radar Beam Overshooting
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Radar Artifacts
Analysis & Diagnosis 46
PPI and CAPPI
4.0 km CAPPI
0.3 Degree PPI
1.5km CAPPI
0.3 Degree PPI
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Radar Artifacts
Analysis & Diagnosis 47
Illustration Using King Radar
NORTH
Georgian
Bay
Lake Simcoe
Lake
Huron
Lake Ontario
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Radar Artifacts
Analysis & Diagnosis 48
Calculated horizon in Panorama
Far side of LkOntario
Lake itself
To Lk Huron
To
G.Bay
Lk Simcoe
North
East
South
West
North
Shaded by distance to horizon. Red is basically the neighbourhood
(“look down”) and mauve is see forever.
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Radar Artifacts
The Digital Elevation Map isn’t 100% complete
or up-to-date.
Largest Landfill
in Canada
Buildings
Behind the trees
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Radar Artifacts
Trees
Blocking and Overshooting Compromise
Approximate current lowest beam location BALD
North
East
South
West
North
Approximate proposed lowest beam location LOAA
North
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East
South
West
Radar Artifacts
North
Sources of Error - Virga
Typical of strong overrunning WCB with equally strong and dry CCB
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Radar Artifacts
Analysis & Diagnosis 52
Cross section to the southwest
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Radar Artifacts
Analysis & Diagnosis 53
Low-elevation angle Doppler Z PPI scan
Let’s look at
the same
case
observed by
the Doppler
scans
Low level
reflectivities
to the west
of the radar
– rain?
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Radar Artifacts
Analysis & Diagnosis 54
3.5 Degree Doppler Z PPI scan
Echoes at higher
altitude precede
precipitation at low
Levels – virga?
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Radar Artifacts
Analysis & Diagnosis 55
Sources of Error - ECHO TOP - Beamwidth and
Beamfilling
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Radar Artifacts
Analysis & Diagnosis 56
ECHO TOP
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Limiting by Maximum Elevation
Radar Artifacts
Analysis & Diagnosis 57
Beam Spreading
At 240 km
3 km diameter
Radar beam
At 30 km
0.5 km diameter
Radar beam
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Radar Artifacts
Analysis & Diagnosis 58
Partial Beam Filling
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Radar Artifacts
Analysis & Diagnosis 59
ECHO TOPS
- Within 36 km of the radar, echo tops will be underestimated
- Within 15 km precipitation can easily go undetected
- Layers of precipitation not detected: only the max tops
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Radar Artifacts
Analysis & Diagnosis 60
Radar Dome Wetting
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Radar Artifacts
Analysis & Diagnosis 61
“Clear” Air Radar Returns
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Radar Artifacts
Analysis & Diagnosis 62
“Clear” Air Radar Returns and Lake Breeze
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Radar Artifacts
Analysis & Diagnosis 63
Radar Detection of a Lake Breeze
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Radar Artifacts
Analysis & Diagnosis 64
“Clear” Air Thunderstorm Outflow Gust
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Radar Artifacts
Analysis & Diagnosis 65
Rain Rate vs Snow Rate
Snow being depicted using the Rain Z-R
Relationship
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Snow depicted using the Snow Z-R Relationship
Radar Artifacts
Analysis & Diagnosis 66
Bird Migration and Radar
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Radar Artifacts
Analysis & Diagnosis 67
New Radar Techniques
Second Trip processing… doubling the doppler
range... out to 220 km…
Bistatic Radar… multiple antennas for radiating
and reception to get two components of the
wind… needs GPS
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Radar Artifacts
Analysis & Diagnosis 68
Multi-trip Echoes
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Radar Artifacts
Second Trip Echoes
to a distant
target...
first pulse
second pulse
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returning
Radar Artifacts
first pulse
Second Trip Echo
Pulse One
Pulse Two
mapped
position of
2nd trip echo
actual position
of echo
beyond range
Echo within
range
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Radar Artifacts
Second Trip Echo
Weaker
Far Away
Randon
Doppler
Changing the PRF changes Location
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Radar Artifacts
Check the Satellite
Second Trip Processing
Doppler Reflectivity with
Conventional CAPPI
Doppler Reflectivity
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Processed Second Trip Echoes
Doppler Reflectivity with
Radar
Artifacts
Analysis & Diagnosis 73
Second
Trip Echoes
Second Trip Processing
Conventional Radar Cappi
Doppler Radar PPI RAW 2nd
Trip Echoes
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Note that the geography is
wrong...
Doppler Radar PPI Processed for
2nd Trip Echoes
Radar Artifacts
Analysis & Diagnosis 74
Second Trip Processing
Doppler Radial Velocity with
Conventional CAPPI
Doppler Reflectivity
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Processed Second Trip Echoes
Doppler Reflectivity with
Radar
Artifacts
Analysis & Diagnosis 75
Second
Trip Echoes
Bistatic Doppler Radar
Real Wind
X
Radar B
Radar A
GPS …. Radar A and Radar B location
GPS …. Point X location
Solve for the Real wind knowing the geometry
and the Radar A and Radar B radial components..
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Radar Artifacts
Analysis & Diagnosis 76
Sunrise-Sunset
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Radar Artifacts