Radar Palet e Home Radar Artifacts Analysis & Diagnosis 1 Radar Artifacts Radar Beam Filling Non...

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


The Schematic Radar


Radar Sources of Error

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


Non-meteorological effects

Factor Situations/causesCurrent method used in URP/NRP



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


Meteorological EffectsFactor Situations/causes

Current method used in URP/NRP



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


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


15 Day Precip Accum CAPPI 1.5 km

XSS 1829m

WUJ 60mXSI 700m

XPG 1118m

WWW 1015m

BC


30 Day Precip Accum PRECIPET

Prairies



Feb 16-17 2008

Nrn Ontario



Mar 8-9 2008

Srn Ontario



Mar 8-9 2008

Srn Quebec

N



Jan 28-30 2008

Atlantic


Sources of Error - Radar Dome Wetting


Sources of Error

• Intervening precipitation• Wet radome

• Beam blocking


Attenuation: What lies beyond?


Foreground Storm

Wedge of attenuation

Example of Attenuation in Precipitation


Cross Sectionconstructedalong this line


60 dbZ core


Sources of Error - Cell Blocking

Cell blocking


Spirit River, AB/BCBEAM BLOCKING

???

???

Persistent

Ground clutter

Beam blockage

N Clear Hills

S Rockies


Foreground Storm

Wedge of attenuationAttenuation


Attenuation

• Warm Frontal Supercell


Attenuation

• north • end• of • derecho?


RadomeWetting

You just

Checked

The radar…

NO

PROBLEM…

PROBLEM !


Sources of Error - Sidelobes

3D View of Real Beam

Orographic Enhancement


Sources of Error - Blocking Impacts on Precipitation Estimates

No PCPN?

No PCPN? Why no precipitation

amount maximum on the peaks?


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


Bright band shows up as ring (or partial ring) of high Z centred on the radar

Bright band


Bright Band

• Freezing Level ~2.5km AGL

• Either associated with Rain or Freezing Rain

Melting of Snow Flakes starts at 2.5 km height


Bright BandTime/height Display

Time

Melting StartsMelting


But not on the lower elevation scan, in this case


Melting layer shows as peak in Z (note it shows up best in data for the 3.5 degree scan. Why?)


Sources of Error - Conventional Ground Clutter


Multi-path Echoes


Spot the AP


Corresponding Radial Velocity Image


AP versus Real - Compared

Doppler Reflectivity


Sources of Error - Anomalous Propagation AP

Strong inversionsTypical early morning after clear (long) nights – fall maximumOver cold water surface – spring maximum


Anomalous Propagation of the Radar Beam

AP and Real Weather

Only Real Weather

The Doppler Velocity View


AP and Frontal Analysis

AP

Anomalous Precipitation


AP … Water Vapour and Fog

VISIBILITY Probably still OK


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


Ground Clutter

• Rockie Mountains

• persistent • even on EchoTop

product


Bethune, Saskatchewan

• Missouri Couteau

• only seen on 0.5PPI

Ground Clutter


Radar Beam Overshooting


PPI and CAPPI

1.5km CAPPI

4.0 km CAPPI

0.3 Degree PPI

0.3 Degree PPI


Illustration Using King Radar

NORTH

Lake Simcoe

Lake Ontario

Lake

Huron

Georgian

Bay


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


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




Blocking and Overshooting CompromiseApproximate current lowest beam location BALD

Approximate proposed lowest beam location LOAA


Sources of Error - Virga

Typical of strong overrunning WCB with equally strong and dry CCB


Cross section to the southwest


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?


3.5 Degree Doppler Z PPI scan

Echoes at higher altitude precedeprecipitation at low Levels – virga?


Sources of Error - ECHO TOP - Beamwidth and Beamfilling


ECHO TOP Limiting by Maximum Elevation


Beam Spreading

At 240 km

3 km diameter

Radar beam

At 30 km

0.5 km diameter

Radar beam


Partial Beam Filling


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


Radar Dome Wetting


“Clear” Air Radar Returns


“Clear” Air Radar Returns and Lake Breeze


Radar Detection of a Lake Breeze


“Clear” Air Thunderstorm Outflow Gust


Rain Rate vs Snow Rate

Snow being depicted using the Rain Z-R Relationship

Snow depicted using the Snow Z-R Relationship


Bird Migration and Radar


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


Multi-trip Echoes


Second Trip Echoes

to a distant target...first pulse

returning first pulse

second pulse


Second Trip Echo

actual position of echo beyond range

Echo within range

mapped position of 2nd trip echo

Pulse OnePulse Two


Second Trip Echo

Elong

ated R

adial

Weaker

Far AwayRandon

Doppler

Changing the PRF changes Location

Check the Satellite


Second Trip Processing

Conventional CAPPI

Doppler Reflectivity Doppler Reflectivity with

Second Trip Echoes

Doppler Reflectivity with

Processed Second Trip Echoes



Note that the geography is wrong...Conventional Radar Cappi

Doppler Radar PPI RAW 2nd Trip Echoes

Doppler Radar PPI Processed for 2nd Trip Echoes



Conventional CAPPI

Doppler Reflectivity Doppler Reflectivity with

Second Trip Echoes

Doppler Radial Velocity with

Processed Second Trip Echoes


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


Sunrise-Sunset

April Sunrise

Radar Palet e Home Radar Artifacts Analysis & Diagnosis 1 Radar Artifacts Radar Beam Filling Non...

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