Index for this report.
Processing the Radar Data
Radar Geometry Issues (including scan gaps)
by F. Merceret
Differences in Radars (74C vs 88D)
by J. Dye
Distance from Radar
Attenuation
Radar Boundary Layer 'noise'
No Data Cone
Reflectivity Averaging: A Monte Carlo Study
by F. Merceret
Why we average dBZ rather than Z
by S. Lewis
Radar Issues: A Presentation at the Nov 2002 Workshop
by F. Merceret
Towards our goal of a radar based parameter to be used in a Lightning Launch Commit Criteria we must keep in mind the limitations of the radar data we are currently using. This report will try to summarize those limitations and provide links to reports done by other investigators.
The bottom line is: never believe a single 1x1x1 km cube of radar data. This is a temtation when you want to "fly" the aircraft through the volume and compare the radar data to the aircraft in situ data.
Processing the Radar Data
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The processing used for the WSR74C for this project consisted of
tranfering the raw (SIGMET format) radar sweep data to universal
format (which will be refered to as "uf data") using the
TRMM-RSL
library.
The 3D gridded radar data has been processed by using the
MMM-SPRINT program.
In this report a "volume" of radar data will refer to this gridded
format. The grid spacing used was 1x1x1 km. (SPRINT was used to grid
both the WSR74C and the NEXRAD radar data.)
The error associated with processing the radar data from raw sweeps to gridded format should be considered. F. Merceret investigated the "sigmaZ" for the WSR74C to be about 1.1 dB without averaging the bins. He also noted that the 74C averages 4 bins radially. (He did not persue the result of the averaging.) The exact number is not as important here as the recognition of its existance. This error is compounded by the fact that it is necessary to do all of the processing using a singel operating system. Different systems use different algorithms for floating point round off. It is typically on the order of 2dBZ (LJ Miller personal communication).
The figure shows one example of a single volume that was processed using different machines. These differences are not large, but there is a large tail with a very, very small fraction having differences as large as 8 to 10 dBZ.
So it is not surprising to find that differences have been found when the processing is done on different machines using different operating systems. Therefore, in the summer of 2003 it was necessary to regrid all of the WSR74C data to make sure it was all done on the same machine and to add some new grid coordinates.
When a radar rule is applied to gurture work the following uncertainties must be considered:
Distance from Radar
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As the system being studied is further away from the radar, the beamwidth and gate spacing increase until the radar values exceed the 1km grid spacing used. Valuable information can still be obtained under these conditions, but it must be recognized that a "single 1km cube" cannot be used for analysis.
No Data Cone
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The following scan pattern plots were taken from F. Merceret's report.
(
Radar Geometry Issues ). Note in particular the zenith of the
radar contains no data.
The resulting CAPPIs thereby contain holes or "data gaps". This is a
particular concern when the system being studied is immediately over the
radar.
