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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

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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.

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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

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Zoomed view of radar locationsZoomed view of radar locations

The green rectangle delineates the area to be used for comparison

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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

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CP2 reflectivity, 3.2 degree PPI, 07:00 UTCCP2 reflectivity, 3.2 degree PPI, 07:00 UTC

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CP2 reflectivity, 3.2 degree PPI, 07:30 UTCCP2 reflectivity, 3.2 degree PPI, 07:30 UTC

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CP2 reflectivity, 3.2 degree PPI, 08:00 UTCCP2 reflectivity, 3.2 degree PPI, 08:00 UTC

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CP2 reflectivity, 3.2 degree PPI, 08:30 UTCCP2 reflectivity, 3.2 degree PPI, 08:30 UTC

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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’.

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CP2 reflectivity,CP2 reflectivity, 3.2 degree PPI, 08:15 UTC 3.2 degree PPI, 08:15 UTC

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Mt Stapleton reflectivity,Mt Stapleton reflectivity,3.1 degree PPI, 08:15 UTC3.1 degree PPI, 08:15 UTC

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Marburg reflectivity,Marburg reflectivity,3.25 km CAPPI, 08:15 UTC3.25 km CAPPI, 08:15 UTC

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CP2 reflectivity,CP2 reflectivity,Vertical section to the North, 08:15 UTCVertical section to the North, 08:15 UTC

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Mt Stapleton reflectivity,Mt Stapleton reflectivity,Vertical section to the North, 08:15 UTCVertical section to the North, 08:15 UTC

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Marburg reflectivity,Marburg reflectivity,Vertical section to the North, 08:15 UTCVertical section to the North, 08:15 UTC

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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.

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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

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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.

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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.

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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.

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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.

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1-D histogram of CP2 minus Mt Stapleton1-D histogram of CP2 minus Mt Stapleton

The 2 radars are similarly calibrated.

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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

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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.

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THANK YOUTHANK YOU