Robin Hogan
5 July 2002, updated 8 January 2003
This dataset consists of parameters measured by the 3-GHz Chilbolton Advanced Meteorological Radar (CAMRa) situated at Chilbolton, England, 51.1445°N, 1.4370°W (i.e. -1.4370°E). The characteristics of the radar are as follows:
_________________________________________________ Frequency: 3.075 GHz Antenna diameter: 25 m Peak power: 560 kW Pulse width: 0.5 µs Pulse repetition frequency (PRF): 610 Hz System noise figure: 1.3 dB Beamwidth: 0.28° Range resolution: 300 m or 75 m Cross-polar isolation: -34 dB Standard maximum slew rate: 1°/s Unambiguous velocity: 15 m/s _________________________________________________
The data are in NetCDF format. The data from each Chilbolton experiment are assigned a tape (or file) number, within which the data are grouped into a number of rasters, each consisting of one or more scans. Two NetCDF files are present for each scan, and for data held at the British Atmospheric Data Centre they use the filename convention camra_chilbolton_YYYYMMDDHHMMSS_TTTTRRRSS_UUU_XXX.nc, where YYYYMMDDHHMMSS indicates the time and date of the start of the scan, TTTT is the tape number, RRR is the raster number, SS is the scan number. UUU indicates the scan type, where the most common are ppi ("Plan-Position Indicator", i.e. a horizontal scan) and rhi ("Range-Height Indicator", i.e. a vertical scan), although fix (constant azimuth and elevation), s-p (slant plane), cal (calibration) and man (manual) are in principle possible. XXX is either ref or dop, where the ref files contain the parameters Radar Reflectivity Factor, Differential Reflectivity and Linear Depolarisation Ratio, and the dop files contain the parameters Differential Phase Shift, Unfolded Doppler Velocity and Raw Doppler Velocity. In addition, a text file with the name TTTTsummary.txt is provided with the NetCDF files for each experiment, containing information about when and in which direction each scan was taken. Quicklook images are available for each of the netcdf files, with the same filenames except with the nc suffix replaced by png.
The presence of separate ref and dop files is a symptom of the fact that the current DSP-based data acquisition system has separate acquisition PCs for the two sets of paramters, and they each average up different numbers of pulses so their output cannot be easily combined into one data stream. The new PCI-based system will solve this problem.
The source code used to process the data and produce NetCDF files and PNG images may be downloaded from http://www.met.rdg.ac.uk/radar/software.html. Matlab programs for reading and plotting the data are also available at this location.
All NetCDF files contain the following variables:
CAMRa has been calibrated absolutely by exploiting the non-independence of the radar parameters Zh, Zdr and phi_dp in heavy rain, as described by Goddard et al. (1994). The resulting calibration of the Zh parameter should be good to 0.5 dB. It should be noted that, due to the large dish, the far field approximation is strictly valid only beyond 12.5 km, and a correction should be applied to data measured at closer ranges than this (see Sekelsky 2002).
The Zdr parameter is generally calibrated either using observations at vertical incidence where Zdr should be 0 dB, or in light drizzle where the droplets are spherical and again Zdr should be 0 dB. Sometimes in processing an older calibration figure is taken from the "sum" file, which is likely to be less accurate; when this occurs it is reported in the comment attribute of the Zdr parameter. Negative Zdr can occur behind extensive regions of heavy rain where the horizontally polarised signal is attenuated preferentially, but there are also occasions where Zdr is found to drop to -0.5 dB just above the melting level. This could either be due to preferential attenuation of the horizontally polarised signal by melting snow, or to a slight drift of the Zdr calibration (a single Zdr calibration figure is used for each tape number).
The remaining four parameters do not need to be calibrated externally, but there are other issues to be aware of. At close range (particularly within 10 km) the data are often contaminated by ground clutter. Ground clutter usually has an Ldr greater than -10 dB, which provides a fairly effective way for the user to eliminate clutter pixels from the Z, Zdr and Ldr parameters. This does tend to leave gaps in cloud echos at close range.
The velocity measurements by CAMRa have a natural unambiguous range of -15 to +15 m s-1, and velocities outside this range are "folded" back into it. The v parameter represents the raw Doppler output, while v_unfolded has been processed to "unfold" the measurements. The unfolding algorithm works very well for continuous blocks of cloud or precipitation, but for scans containing patches of broken cloud it sometimes does not unfold regions correctly relative to one another. Such cases are usually easy to identify subjectively, and can often be corrected by adding or subtracting 30 m s-1 to some or all of the patches of cloud.
When there is no meteorological signal, v contains random values between -15 and +15 m s-1, and phi_dp is similarly affected. The presence of high variance from one pixel to the next has been used to remove noise from the v_unfolded parameter, and if required this information can be used to eliminate the noisy pixels from the v and phi_dp parameters. However, there are occasions where regions of truly high velocity variance (such as in shear zones) can trigger the algorithm and good data is removed.
Note that when you plot RHIs it is often necessary to make a correction for the curvature of the earth. The quicklooks of the data held at the British Atmospheric Data Centre have been corrected in this way.
Note that the radar is sensitive to insects which can appear in the lowest few km on hot summer days, and are identifiable by their high Zdr.
If data from CAMRa is used in any publication or report then acknowledgement must be given to the Radio Communications Research Unit at the Rutherford Appleton Laboratory for providing the data.