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11
RADAR INTER-COMPARISON AS ARADAR INTER-COMPARISON AS AREFLECTIVITY CALIBRATION CHECKREFLECTIVITY CALIBRATION CHECK
RADAR MONITORING WORKSHOPERAD 2010
SIBIU ROMANIA
Mike DixonNational Center for Atmospheric Research
Boulder, Colorado
22
Inter-radar calibration for radar networksInter-radar calibration for radar networks
For effective use of weather radar data, especially For effective use of weather radar data, especially
reflectivity, it is important that the radars be accurately reflectivity, it is important that the radars be accurately
calibrated. calibrated.
In radar networks, when the radar coverage from adjacent In radar networks, when the radar coverage from adjacent
radars overlaps, it is possible to perform an inter-radars overlaps, it is possible to perform an inter-
comparison between the radars, to identify mismatches in comparison between the radars, to identify mismatches in
the reflectivity values.the reflectivity values.
This technique can be helpful in identifying calibration This technique can be helpful in identifying calibration
errors.errors.
33
Radars from BOM, AustraliaRadars from BOM, Australia
As an example, we use data from 3 radars belonging to the As an example, we use data from 3 radars belonging to the
Australian Bureau of Meteorology.Australian Bureau of Meteorology.
The area of interest is Brisbane, Queensland, Australia.The area of interest is Brisbane, Queensland, Australia.
The radar characteristics are:The radar characteristics are:
– CP2CP2, S-band, dual-polarization Doppler, 0.92 degree beam , S-band, dual-polarization Doppler, 0.92 degree beam
widthwidth
– Mt StapletonMt Stapleton, S-band, single-polarization Doppler, 1.0 degree , S-band, single-polarization Doppler, 1.0 degree
beam widthbeam width
– MarburgMarburg, S-band, non-Doppler, 2.0 degree beam width, S-band, non-Doppler, 2.0 degree beam width
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Radar locations south of BrisbaneRadar locations south of BrisbaneRange rings are from CP2, in 50 km intervalsRange rings are from CP2, in 50 km intervals
The spacing between the radars is approximately 35 km
55
Zoomed view of radar locationsZoomed view of radar locations
The green rectangle delineates the area to be used for comparison
66
Comparison caseComparison case2008/11/16, 06:30 to 08:30 UTC2008/11/16, 06:30 to 08:30 UTC
For the comparison, we use data from a squall line which moved For the comparison, we use data from a squall line which moved
through the Brisbane area on 2008/11/16.through the Brisbane area on 2008/11/16.
The period of interest is 06:30 UTC to 08:30 UTC, which is 15:30 to The period of interest is 06:30 UTC to 08:30 UTC, which is 15:30 to
17:30 local time.17:30 local time.
The squall line produced heavy precipitation and large hail in the The squall line produced heavy precipitation and large hail in the
Brisbane metropolitan area.Brisbane metropolitan area.
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CP2 reflectivity, 3.2 degree PPI, 06:30 UTCCP2 reflectivity, 3.2 degree PPI, 06:30 UTC
88
CP2 reflectivity, 3.2 degree PPI, 07:00 UTCCP2 reflectivity, 3.2 degree PPI, 07:00 UTC
99
CP2 reflectivity, 3.2 degree PPI, 07:30 UTCCP2 reflectivity, 3.2 degree PPI, 07:30 UTC
1010
CP2 reflectivity, 3.2 degree PPI, 08:00 UTCCP2 reflectivity, 3.2 degree PPI, 08:00 UTC
1111
CP2 reflectivity, 3.2 degree PPI, 08:30 UTCCP2 reflectivity, 3.2 degree PPI, 08:30 UTC
1212
Reflectivity plots from each radarReflectivity plots from each radar
The following 3 slides show PPI reflectivity plots for CP2, Mt The following 3 slides show PPI reflectivity plots for CP2, Mt
Stapleton and Marburg, respectively, at 08:15 UTC.Stapleton and Marburg, respectively, at 08:15 UTC.
The 3 slides after that show a vertical section from each of the The 3 slides after that show a vertical section from each of the
radars, for a line from CP2 to the north. The location of the vertical radars, for a line from CP2 to the north. The location of the vertical
sections can be seen on the PPIs, from the point marked ‘1’ to the sections can be seen on the PPIs, from the point marked ‘1’ to the
point marked ‘2’.point marked ‘2’.
1313
CP2 reflectivity,CP2 reflectivity, 3.2 degree PPI, 08:15 UTC 3.2 degree PPI, 08:15 UTC
1414
Mt Stapleton reflectivity,Mt Stapleton reflectivity,3.1 degree PPI, 08:15 UTC3.1 degree PPI, 08:15 UTC
1515
Marburg reflectivity,Marburg reflectivity,3.25 km CAPPI, 08:15 UTC3.25 km CAPPI, 08:15 UTC
1616
CP2 reflectivity,CP2 reflectivity,Vertical section to the North, 08:15 UTCVertical section to the North, 08:15 UTC
1717
Mt Stapleton reflectivity,Mt Stapleton reflectivity,Vertical section to the North, 08:15 UTCVertical section to the North, 08:15 UTC
1818
Marburg reflectivity,Marburg reflectivity,Vertical section to the North, 08:15 UTCVertical section to the North, 08:15 UTC
1919
Inter-comparison methodInter-comparison method
We compare the radars in pairs, as follows:We compare the radars in pairs, as follows:
– CP2CP2 (polar) (polar) vs. Mt Stapletonvs. Mt Stapleton (polar), comparing the max reflectivity at any level (polar), comparing the max reflectivity at any level
above 2 degrees.above 2 degrees.
– CP2CP2 (polar) (polar) vs. Marburgvs. Marburg (Cartesian), comparing the max reflectivity at any level (Cartesian), comparing the max reflectivity at any level
above 2 degrees for polar and 2 km for Cartesian.above 2 degrees for polar and 2 km for Cartesian.
– Mt StapletonMt Stapleton (Cartesian) (Cartesian) vs. Marburgvs. Marburg (Cartesian), comparing values at each (Cartesian), comparing values at each
available CAPPI height.available CAPPI height.
CP2 has only polar data, Marburg has only Cartesian data, and Mt. CP2 has only polar data, Marburg has only Cartesian data, and Mt.
Stapleton has both.Stapleton has both.
We perform the comparison in this way to show that it is reasonably We perform the comparison in this way to show that it is reasonably
robust, and can handle varying geometry.robust, and can handle varying geometry.
2020
Inter-comparison methodInter-comparison method
We accumulate data for all available times between 06:30 and We accumulate data for all available times between 06:30 and
08:30 UTC.08:30 UTC.
For each of the radar pairs, we identify scans which are close For each of the radar pairs, we identify scans which are close
enough in time for a reasonable comparison – i.e. within 5 minutes enough in time for a reasonable comparison – i.e. within 5 minutes
of each other.of each other.
We compute the difference in reflectivity between each pair, for We compute the difference in reflectivity between each pair, for
each grid point. We discard differences with an absolute value each grid point. We discard differences with an absolute value
greater than 15 dB.greater than 15 dB.
We perform the comparison within the bounding box shown We perform the comparison within the bounding box shown
earlier, and repeated on the next slide.earlier, and repeated on the next slide.
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Bounding box for inter-comparisonBounding box for inter-comparison
The green rectangle delineates the area to be used for comparison
2222
Inter-comparison resultsInter-comparison results
The following plots show the results of the pair-wise inter-The following plots show the results of the pair-wise inter-
comparisons.comparisons.
There are 2 plot types:There are 2 plot types:
– 2-D histograms, showing the reflectivity for one radar plotted against 2-D histograms, showing the reflectivity for one radar plotted against
the other;the other;
– 1-D histograms, shows the distribution of the differences between the 1-D histograms, shows the distribution of the differences between the
reflectivity values at each radar.reflectivity values at each radar.
2323
2-D histogram of CP2 vs. Mt Stapleton2-D histogram of CP2 vs. Mt Stapleton
The magenta line is the 1:1 line. The 2 radars are similarly calibrated.
2424
2-D histogram of CP2 vs. Marburg2-D histogram of CP2 vs. Marburg
The magenta line is the 1:1 line. CP2 has higher values than Marburg.
2525
2-D histogram of Mt Stapleton vs. Marburg2-D histogram of Mt Stapleton vs. Marburg
The magenta line is the 1:1 line. Mt Stapleton has higher values than Marburg.
2626
1-D histogram of CP2 minus Mt Stapleton1-D histogram of CP2 minus Mt Stapleton
The 2 radars are similarly calibrated.
2727
1-D histogram of CP2 minus Marburg1-D histogram of CP2 minus Marburg
CP2 has higher values than Marburg.
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1-D histogram of Mt Stapleton minus Marburg1-D histogram of Mt Stapleton minus Marburg
Mt Stapleton has higher values than Marburg.
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Results - reflectivity differencesResults - reflectivity differences
MeanMean MedianMedian
CP2 – Mt StapletonCP2 – Mt Stapleton -0.04-0.04 -0.04-0.04
CP2 - MarburgCP2 - Marburg +4.88+4.88 +5.10+5.10
Mt Stapleton - MarburgMt Stapleton - Marburg +4.80+4.80 +5.00+5.00
3030
ConclusionsConclusions
These results show that CP2 and Mt Stapleton are, for practical purposes,
calibrated identically, while Marburg is about 4.8 dB lower.
It seems likely that Marburg is mis-calibrated, although this could be a
combination of low calibration for Marburg and a high calibration for the
other two..
NOTE: although this result shows agreement between the 3 comparison
pairs, this is not always the case. The technique must be repeated for a
number of weather events, to ensure an accurate result.
3131
THANK YOUTHANK YOU