Global options

This page describes the processing options that are used by chilplot, chil2a, chil2nc and loadchil. Hereafter, {program} is used to mean any of these programs. The recommended values for each radar system are given at the end.

Usage

{program}
With no options, some basic usage information is returned.
{program} file1.dat rasterrange1 [file2.dat rasterrange2 ...] [options]
Read the specified rasters from a set of files, where `rasterrange' can be a single raster number (e.g. `1'), a range of rasters (e.g. `1-10') or a single hyphen to indicate all rasters in the file (`-').
{program} -all file1.dat [file2.dat ...] [options]
Read all rasters from the files specified.
{program} -date date filepattern1.dat [filepattern2.dat ...] [options]
Search the summary files corresponding to the files that match the specified patterns for rasters recorded on the date specified (which can be of the form YYYYMMDD or YYMMDD. A file pattern can contain the wildcards `*' and `?'.

General options

These options can be used with both scanning and cloud data. Note that n means an integer, f means a floating-point number and s means a string.
-verbose
Display information on files and rasters as they are opened.
-quiet
Suppress progress messages; display only warnings and errors.
-version
Display version and quit.
-Zcal f
Add f dB to all the Zh values.
-averays n
Average groups of n rays together; the parameters Zh and Ldr are converted into linear units before taking the average. If the radar is measuring v in cloud mode and averaging is performed then sigma_v, the standard deviation of the velocity, is calculated as well.
-avegates n
Average every n gates. The comments for -averays above apply here.
-day n
Accept data only with a day-of-the-month n.
-sumdir s
Look in directory s for summary files if they are not found in the same directory as the data files. By default, s is set to `.', the current working directory.
-maxfields n
Retrieve at most n radar parameters; for example, if a file contained Zh, Zdr and Ldr then specifying -maxfields 2 would result in only Zh and Zdr being retrieved.
-rangeoffset f
Perform or re-perform range correction with a range offset of f metres. Note that in recent cloud data the appropriate range correction figure is given in the information record of each raster so that only -correctrange need be used.
-clean
This option will remove any cloudy/precipitation pixel that does not have a signal above or below it, and thus reduce speckle noise. It is also particularly useful for removing anomalous `lines' of cloud in vertically-pointing cloud data.
-doubleclean
This option is most useful with vertically-pointing cloud data; often when vertical averaging is not applied (-avegates is not selected), the anomalous `lines' of cloud mentioned above can be two pixels thick. This option will remove any cloudy pixel or pair of cloudy pixels that do not have cloud above or below them. Provided you don't mind losing any real cloud that is 2 pixels thick, and are not doing any vertical averaging, you should probably select this option.

Processing options specific to scanning data

-scan n
Retrieve only scan n of the raster.
-ZDRcal f
Add f dB to all the Zdr values.
-LDRcal f
Add f dB to all the Ldr values.
-PHIDPcal f
Add f degrees to all the phi_dp values.
-maxLDR f
Remove all data (probably ground clutter) with an LDR greater than f dB (-10 is a good value).
-polar fZH fZV fZHV
Subtract noise from Zh, Zdr and Ldr using noise-equivalent reflectivities at 1 km in the H, V and cross-polar channels of fZH, fZV and fZHV (dBZ) respectively. This is important if these parameters are to be used at low signal-to-noise ratios. Between October 1998 and July 2000, the correct values for the CAMRa radar were -41.7, -43.7 and -43.7 respectively. These figures are likely to change whenever the hardware setup is modified. If you are missing Zdr or Ldr then fZV or fZHV are ignored (respectively).
-threshold f
Set the noise-elimination threshold to f, when the -polar option is in use. The default value is 1.19, which essentially means that any signal less than 19% (in a linear sense) above the mean noise floor is rejected. If anomalous echos appear in obviously cloud-free regions then use a higher value.
-doppler f
Reject noisy data by looking for big jumps in v2 or phi_dp. The threshold parameter f should be around 400: use higher values to accept larger jumps in these two parameters. This option is best used in combination with -doubleclean.

Processing options specific to cloud data

-process
Subtract noise using the Zh values in gates at the top of the ray (which are assumed to be free from cloud) to characterise the noise.
-noisegates n
Use n gates at the top of the ray to characterise the noise. The default is 25.
-oscillator
Subtract noise assuming that the there is a weak receiver interference of an oscillatory nature, and use the gates at the top of the ray to characterise this oscillation. Hence it is important that the correct number of gates (specified with the -noisegates option) are used, as they must match the frequency of the oscillation. If this option is not used with 94 GHz data then anomalous `lines' of cloud, around 1 pixel thick, will appear in the data around 25 gates apart.
-noisefactor f
When performing noise subtraction, eliminate all signals which are less than f standard deviations above the mean noise floor. The default value is 3, but for cloud radars such as the Rabelais, a value closer to 5 is necessary to account for the strangely range-dependent level of the noise floor.
-correctrange
Perform range correction using the range offset value encoded in the information record (which has been estimated at Chilbolton from an oscilloscope). If range correction has already been performed then it will be recalculated using the new range.
-noclutter n f
Attempt to remove clutter using the v and sigma_v variables; basically any data up to gate n with both a v and sigma_v of less than f m/s is removed. For the 94 GHz radar, the options `-noclutter 15 0.1 -clean' will get rid of most clutter, although insects will not be removed. Since sigma_v can be calculated only if some averaging is performed, this option can take effect only if some averaging is performed.
-v_floor f
Often the velocity information at low signal-to-noise levels is garbage; this option sets the equivalent noise floor for the v parameter to f dB above that used for Zh.
-sv_floor f
Similarly, this option sets the equivalent noise floor for the sigma_v parameter to f dB above that used for Zh.
-reject n f
Sometimes the 94-GHz receiver and transmitter can get out of sync, in which case the range to each gate is completely wrong. Since the outgoing transmit pulse is detected strongly by the receiver at zero range, this problem can be detected by looking for occasions when the very large and abrupt echo from the transmit pulse occurs several km above the surface. Selecting this option will skip rays whenever there is a jump in reflectivity of more than f dB over two range gates at the nth gate (zero based) or above. In normal 94 GHz reflectivity data there is a jump of around 50 dB over two range-gates when the transit pulse is sent out, and this should normally occur centred on gate 4 (zero based). Thus `-reject 10 30' should be OK for this radar (changes in reflectivity of 30 dB over a vertical distance of 120 m never occur naturally). Note that this test will be performed after any averaging of gates, so if you use -avegates 2 then you would have to change this option to `-reject 5 30'.
-minelev f
Skip any rays with an elevation less than f degrees. This is to screen out occasions when the dish is scanning but the operator has forgotten to turn the cloud data acquisition program off.
-nobreak
Don't split cloud data up into separate days.

Recommended options for scanning 3 GHz CAMRa data

Data recorded since October 1998

-Zcal +5 -PHIDPcal +42 -ZDRcal +1.5 -polar -41.7 -43.1 -43.1 -maxLDR -10 -clean

Data recorded since around October 2000

-Zcal +6.8 -PHIDPcal +42 -ZDRcal +2 -polar -41.6 -43.1 -43.6 -maxLDR -10 -clean

Recommended options for 3 GHz Singapore data

This is rather preliminary...

-Zcal 18 -LDRcal -11 -polar -18.35 0 -7.43 -clean -rangeoffset -500

Recommended options for 35 GHz Rabelais cloud data

The is an abundance of artifacts in 35 GHz data that my program can't remove - in particular look out for that big band between 2 and 3 km. The system also has a poor recovery time so that anomalous echos are often observed directly above strong signals, where from the 94 GHz radar it is clear that there is no cloud. I've selected an increased noisefactor of 4.5 to get rid of some of this, but you might want to go higher.

Data recorded from 1 to 28 Oct 1998

-process -noisegates 25 -noisefactor 4.5 -clean -Zcal -15.5 -rangeoffset -480

Data recorded from 29 Oct 1998 to Jan 1999

On the evening of 28 October an attenuator setting was changed by 9.5 dB - calibration changed accordingly.

-process -noisegates 25 -noisefactor 4.5 -clean -Zcal -25 -rangeoffset -480

Data recorded since Feb 1999

Range offset encoded into the information record so doesn't need to be specified manually. Note that early on in these data I and Q were not centred every ray before calculating velocity so some of the Doppler data may be very dubious.

-process -noisegates 25 -noisefactor 4.5 -clean -Zcal -25 -correctrange

Recommended options for 94 GHz cloud data

Data recorded between Oct 1998 and Jan 1999

The -oscillator option should get rid of most of those annoying lines, but data was only recorded up to 12 km and you need 25 gates at the top of the ray to characterise the oscillator interference because of it's frequency - hence when cloud extends well above 10 km this affects noise subtraction further down the ray.

-oscillator -noisegates 25 -noisefactor 3.0 -doubleclean -Zcal +16 -rangeoffset -190

Data from Feb 1999

The radar was put on the side of the main CAMRa dish, so the calibration figure will have changed - from the noise level I estimate it to be around 7 dB more, and this gives much better visual agreement with the 35 GHz from the same period. There are more gates to get the noise from (if you're not averaging any gates you can use -noisegates 50 if you want). The range offset is encoded into the information record as -480 m so doesn't need to be specified (you can use the -correctrange option). However, looking at the data suggests that this is out by about one range gate, so you might find that -rangeoffset -420 makes rain look better very near the ground where the values of reflectivity are most sensitive to errors in range correction. You may not want to use the -noclutter option as it can make plots look a little odd when insects are present (which it cannot remove). The -maxfields 1 option suppresses the v and sigma_v parameters which we don't trust.

-oscillator -noisegates 25 -noisefactor 3.0 -doubleclean -Zcal +23 -correctrange -minelev 85 -reject 10 30 -noclutter 15 0.1 -maxfields 1

This is suitable for no averaging of gates; if you specify `-avegates 2' then you ought to change the rejection option to `-reject 5 30' and use `-clean' rather than `-doubleclean'.