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Analysis & Diagnosis 1Radar Palette Home Radar Artifacts
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
Radar Palette Home Radar Artifacts
Radar System LimitationsFactor Situations/causes
Current method used in URP/NRP
Potential/Alternative Solution
Active Canadian or Commercial Groups
System Calibration and StabilityExample: System drift with time
Internal system checks
Monitor accumulation maps for peculiarities or trends. Raingauge adjustment.
NRP/Regional Technicians
Sidelobe ContaminationPrimarily in hail situations at C-band none
Manual adjustment based on pattern recognition.
Reflectivity Outliers (spurious values)
Random system instabilities, non-meteorological targets.
Range ambiguities removed using dual PRF unfolding.
Comparison with surrounding radar data (Nexrad). Comparison with satellite imagery or surface reports (UK).
Attenuation in Precipitation
Significant problem at 5 cm wavelength during heavy precipitation. none
Iterative attenuation correction (difficult to apply:some evidence of numerical instability) Waterloo
Radome Wetting
Moderate-heavy precipitation at radar site
Hydroscopic radome paint
Interpolate using advection correction. Flag data as suspect. Raingauge adjustment (Nexrad). NRP, King
Partial Beamfilling
Convective situations. Larger problem at long range due to beam spreading.
Smaller beamwidth (0.65 degree) in new NRP radars.
User-defined rainrate which is a non-linear function of both the unadjusted rainrate and the range (Nexrad).
Sampling Error due to Cell Motion
Convective situation. More obvious with rapid cell motion. none
Advection correction (spatial-temporal smoother) McGill, King
Radar Palette Home Radar Artifacts
Non-meteorological effects
Factor Situations/causesCurrent method used in URP/NRP
Potential/Alternative Solution
Active Canadian or Commercial Groups
Beam Blockage
Interception of a portion or all of radar beam by nearby obstructions.
Scan above source of blockage. Minimised with 0.65 degree beamwidth.
Add a pre-determined reflectivity value to each blocked sample volume (Nexrad) King, McGill
Ground Clutter
Unwanted detection of nearby obstructions. Worse in complex terrain.
Adaptive filter using FFT processing of Doppler data
Masking (Nexrad). Vertical profile tests:clutter diminishes rapidly with height. VIL - low-level Z correlation methods.
SIGMET software/Doppler processing. McGill
Anomalous Propagation
Low-level inversion bends radar signal downward creating unwanted ground echo.
Adaptive filter using FFT processing of Doppler data
Masking, Bayesian tests using auxiliary data (e.g. satellite, surface stations)
SIGMET software/Doppler processing
Radar Palette Home Radar Artifacts
Meteorological EffectsFactor Situations/causes
Current method used in URP/NRP
Potential/Alternative Solution
Active Canadian or Commercial Groups
Variable Drop Size Distribution
Convective vs stratiform
Z-R lookup table rather than fixed relationship
Variable Z-R relationship tuned to nature of precipitation. Raingauge adjustment. Polarization diversity. King, McGill
Bright Band Effect
Beam passes through melting level. Significant in stratiform rain.
Proposed correction based on vertical profiles.
Limit measurement to regions below or above bright band. King, McGill
Rainfall Intermittency
Poorly understood characteristic of rainfall. Affects sampling accuracy. none
More research required to characterize potential contribution to radar measurement error. Also will affect advection adjustment algorithms.
Anomalously High Power Returns From Wet Ice Targets Wet hail none
Place arbitrary maximum limit on derived rainfall rate. Human intervention based on hail signatures in the imagery (Nexrad). Polarization diversity to recognize hail. McGill
Extrapolation to Surface
24 tilt angles will allow real-time profiles. Vertical profiles King, McGill
Low-Level Rain Growth in Fog/Stratus
When fog/stratus is present
24 tilt angles will allow real-time profiles.
Vertical profiles. Raingauge adjustment (Nexrad, UK).
Orographic Enhancement
Strongest in mountainous terrain, but significant over gentle slopes as well (e.g. Prairies) none
Spatially-variable correction zones (UK) based on gauge comparisons.
Evaporation in Low Level
Strongest in drier climates (Prairies) none Raingauge adjustment
Analysis & Diagnosis 7Radar Palette Home 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
Analysis & Diagnosis 8Radar Palette Home Radar Artifacts
15 Day Precip Accum CAPPI 1.5 km
XSS 1829m
WUJ 60mXSI 700m
XPG 1118m
WWW 1015m
BC
Analysis & Diagnosis 10Radar Palette Home Radar Artifacts
2 Day Precip Accum CAPPI 1.0 km
Feb 16-17 2008
Nrn Ontario
Analysis & Diagnosis 11Radar Palette Home Radar Artifacts
2 Day Precip Accum CAPPI 1.0 km
Mar 8-9 2008
Srn Ontario
Analysis & Diagnosis 12Radar Palette Home Radar Artifacts
2 Day Precip Accum CAPPI 1.0 km
Mar 8-9 2008
Srn Quebec
N
Analysis & Diagnosis 13Radar Palette Home Radar Artifacts
2 Day Precip Accum CAPPI 1.0 km
Jan 28-30 2008
Atlantic
Radar Palette Home Radar Artifacts
Sources of Error
• Intervening precipitation• Wet radome
• Beam blocking
Radar Palette Home Radar Artifacts
Foreground Storm
Wedge of attenuation
Example of Attenuation in Precipitation
Analysis & Diagnosis 20Radar Palette Home Radar Artifacts
Sources of Error - Cell Blocking
Cell blocking
Radar Palette Home Radar Artifacts
Spirit River, AB/BCBEAM BLOCKING
???
???
Persistent
Ground clutter
Beam blockage
N Clear Hills
S Rockies
Analysis & Diagnosis 26Radar Palette Home Radar Artifacts
Sources of Error - Sidelobes
3D View of Real Beam
Orographic Enhancement
Analysis & Diagnosis 27Radar Palette Home Radar Artifacts
Sources of Error - Blocking Impacts on Precipitation Estimates
No PCPN?
No PCPN? Why no precipitation
amount maximum on the peaks?
Analysis & Diagnosis 28Radar Palette Home Radar Artifacts
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
Radar Palette Home Radar Artifacts
Bright band shows up as ring (or partial ring) of high Z centred on the radar
Bright band
Radar Palette Home Radar Artifacts
Bright Band
• Freezing Level ~2.5km AGL
• Either associated with Rain or Freezing Rain
Melting of Snow Flakes starts at 2.5 km height
Analysis & Diagnosis 31Radar Palette Home Radar Artifacts
Bright BandTime/height Display
Time
Melting StartsMelting
Radar Palette Home Radar Artifacts
Melting layer shows as peak in Z (note it shows up best in data for the 3.5 degree scan. Why?)
Analysis & Diagnosis 34Radar Palette Home Radar Artifacts
Sources of Error - Conventional Ground Clutter
Analysis & Diagnosis 38Radar Palette Home Radar Artifacts
AP versus Real - Compared
Doppler Reflectivity
Analysis & Diagnosis 39Radar Palette Home Radar Artifacts
Sources of Error - Anomalous Propagation AP
Strong inversionsTypical early morning after clear (long) nights – fall maximumOver cold water surface – spring maximum
Analysis & Diagnosis 40Radar Palette Home Radar Artifacts
Anomalous Propagation of the Radar Beam
AP and Real Weather
Only Real Weather
The Doppler Velocity View
Analysis & Diagnosis 41Radar Palette Home Radar Artifacts
AP and Frontal Analysis
AP
Anomalous Precipitation
Analysis & Diagnosis 42Radar Palette Home Radar Artifacts
AP … Water Vapour and Fog
VISIBILITY Probably still OK
Radar Palette Home Radar Artifacts
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
Radar Palette Home Radar Artifacts
Ground Clutter
• Rockie Mountains
• persistent • even on EchoTop
product
Radar Palette Home Radar Artifacts
Bethune, Saskatchewan
• Missouri Couteau
• only seen on 0.5PPI
Ground Clutter
Analysis & Diagnosis 47Radar Palette Home Radar Artifacts
PPI and CAPPI
1.5km CAPPI
4.0 km CAPPI
0.3 Degree PPI
0.3 Degree PPI
Analysis & Diagnosis 48Radar Palette Home Radar Artifacts
Illustration Using King Radar
NORTH
Lake Simcoe
Lake Ontario
Lake
Huron
Georgian
Bay
Radar Palette Home Radar Artifacts
Calculated horizon in Panorama
Shaded by distance to horizon. Red is basically the neighbourhood (“look down”) and mauve is see forever.
Far side of LkOntarioLake itself To Lk Huron
ToG.Bay
Lk Simcoe
North East South West North
Radar Palette Home Radar Artifacts
The Digital Elevation Map isn’t 100% complete or up-to-date.
BuildingsLargest Landfillin Canada Trees
YO
U a
re H
ERE
Behind the trees
Radar Palette Home Radar Artifacts
North East South West North
North East South West North
Blocking and Overshooting CompromiseApproximate current lowest beam location BALD
Approximate proposed lowest beam location LOAA
Analysis & Diagnosis 52Radar Palette Home Radar Artifacts
Sources of Error - Virga
Typical of strong overrunning WCB with equally strong and dry CCB
Analysis & Diagnosis 54Radar Palette Home Radar Artifacts
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?
Analysis & Diagnosis 55Radar Palette Home Radar Artifacts
3.5 Degree Doppler Z PPI scan
Echoes at higher altitude precedeprecipitation at low Levels – virga?
Analysis & Diagnosis 56Radar Palette Home Radar Artifacts
Sources of Error - ECHO TOP - Beamwidth and Beamfilling
Analysis & Diagnosis 58Radar Palette Home Radar Artifacts
Beam Spreading
At 240 km
3 km diameter
Radar beam
At 30 km
0.5 km diameter
Radar beam
Analysis & Diagnosis 60Radar Palette Home Radar Artifacts
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
Analysis & Diagnosis 66Radar Palette Home Radar Artifacts
Rain Rate vs Snow Rate
Snow being depicted using the Rain Z-R Relationship
Snow depicted using the Snow Z-R Relationship
Analysis & Diagnosis 68Radar Palette Home Radar Artifacts
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
Radar Palette Home Radar Artifacts
Second Trip Echoes
to a distant target...first pulse
returning first pulse
second pulse
Radar Palette Home Radar Artifacts
Second Trip Echo
actual position of echo beyond range
Echo within range
mapped position of 2nd trip echo
Pulse OnePulse Two
Radar Palette Home Radar Artifacts
Second Trip Echo
Elong
ated R
adial
Weaker
Far AwayRandon
Doppler
Changing the PRF changes Location
Check the Satellite
Analysis & Diagnosis 73Radar Palette Home Radar Artifacts
Second Trip Processing
Conventional CAPPI
Doppler Reflectivity Doppler Reflectivity with
Second Trip Echoes
Doppler Reflectivity with
Processed Second Trip Echoes
Analysis & Diagnosis 74Radar Palette Home Radar Artifacts
Second Trip Processing
Note that the geography is wrong...Conventional Radar Cappi
Doppler Radar PPI RAW 2nd Trip Echoes
Doppler Radar PPI Processed for 2nd Trip Echoes
Analysis & Diagnosis 75Radar Palette Home Radar Artifacts
Second Trip Processing
Conventional CAPPI
Doppler Reflectivity Doppler Reflectivity with
Second Trip Echoes
Doppler Radial Velocity with
Processed Second Trip Echoes
Analysis & Diagnosis 76Radar Palette Home Radar Artifacts
Bistatic Doppler Radar
Radar A
Radar B
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..
Real Wind
X