radcart5 −− interpolate airborne or ground-based Doppler radar data to a Cartesian grid for dual Doppler analysis
radcart5 [-p r0:dbz0] [-c zmin0:zmincoef] [-s svx:svy:reftime] [-e el] [-r minrange] -- lon0 dx xsize lat0 dy ysize alt0 dz zsize
Radcart5 reads a Candis file on the standard input consisting of a volume of radar data from one or more radars. A Candis file is written to the standard output with these data interpolated to a rectangular Cartesian grid. Corrections are made for the earth’s curvature.
Positions are adjusted for storm propagation to a specified time if the -s flag with the arguments reftime, svx, and svy are supplied. The reference time, reftime, should be in kiloseconds and the storm propagation velocity components should be in meters per second. If these arguments are not given, the assumed propagation velocity is zero. Note that svx and svy are not used to make the Cartesian velocity components storm-relative; they are Earth-relative.
Interpolation of the ray to the Cartesian grid is made after corrections for the storm velocity and the curvature of the earth are made. The actual interpolation is made taking the corrected position of each gate and distributing the values to all eight corners of the box by a weighting system based upon how close the gate is to each corner.
The other flags have the following meanings: -p invokes returned power thresholding, with gates omitted when the returned power is less than a certain value. Returned power is expressed for convenience as reflectivity dbz0 at a specified range r0, which should be expressed in kilometers. -c suppresses the use of gates less than a certain elevation h = zmin0 + zmincoef*range (in kilometers) if the ray is pointing downward. This is used to eliminate surface clutter. -e limits the elevation angle to the range [-el, el]. -r limits the gates accepted in the synthesis to beyond a minimum range in kilometers.
The nine mandatory arguments define the grid. Lon0, lat0, and alt0 define the lower southwest corner of the grid in degrees (longitude and latitude) and kilometers (altitude). Dx, dy, and dz define the grid interval in kilometers. Xsize, ysize, and zsize define the size of the domain in kilometers. The number of grid points in the x dimension is calculated from nx = xsize/dx + 1.5, with similar calculations in the y and z directions.
The input file must meet certain requirements. Each variable slice must contain data associated with a single ray. Certain assumptions are made about the names of input fields. These are easily changed by editing the source file before compilation. The assumed names are all located in defines near the beginning of the program. Default names of required input fields for each variable slice are "vr" (radial velocity in meters per second), "dbz" (reflectivity in dBZ), "lon" (longitude of radar in degrees), "lat" (latitude of radar in degrees), "alt" (altitude of radar in kilometers), "monotime" or "time" (time in kiloseconds), "az" (azimuth angle of the ray in degrees), and "el" (elevation of the ray in degrees). The first two fields are assumed to be one-dimensional with index field "range" (radar range in kilometers). The other fields are scalar. Note that "lon", "lat", and "alt" are not normally supplied for every ray for ground-based radars. These need to be somehow created (using, say, cdfmath) before applying radcart5.
The output static slice contains the three index fields x, y, and z. The reflectivity is interpolated to the Cartesian grid, and an average is taken of the actual reflectivity, not the log-reflectivity. All other fields are dropped.
Additional Cartesian fields resulting from computations on input fields are added to the output. These fields are ux0, uy0, gx, gy, eur, evr, eup, evp, pcount, chisq, dual, dual2, and avgtime, and are created to help in subsequent dual Doppler synthesis. ux0 and uy0 are the horizontal winds assuming that the vertical particle velocity wp is zero. The actual horizontal winds can be constructed from ux = ux0 + gx*wp and uy = uy0 + gy*wp once wp is somehow determined.
Errors in ux and uy can be determined from the formulas dux^2 = eur*dvr^2 + eup*dwp^2 and duy^2 = evr*dvr^2 + evp*dwp^2, where dvr is the error in the radial velocity measurement and dwp is the error in the vertical particle velocity determination. The quantities eur and evr have to do with detecting bad geometry for dual Doppler synthesis, while eup and evp have to do with sensitivity to errors in particle fall speed, e. g., when ray angles from the horizontal are large. Thresholding may be done on the values of eur, evr, eup, and evp to eliminate bad data. Suggested thresholding values are eur, evr < 0.3 and eup, evp < 1, though trial and error is definitely in order.
Chisq is the chi squared in the fit used to determine the Cartesian velocities from the radial velocities. Large values of chisq indicate one or more possible bad radial velocities, originating perhaps from bad unfolding. Thresholding on chisq can remove bad data, with a maximum allowed value of 3 suggested. Thresholding on pcount, the number of data samples per grid point, is also warranted, as having just a few radial velocity estimates can lead to less confidence in the data. Experimentation is in order here, but setting a minimum value of pcount = 10 has worked for large (10 km) horizontal grid boxes.
Two additional variables proportional to the inverse of the eigenvalues of the two-dimensional, horizontal error covariance matrix are included in the output. These are called dual1 and dual2, with dual1 always being less than dual2. A small or zero value of dual1 indicates poor dual Doppler radar geometry. Experimentation is in order on setting the proper threshold here, though a starting point is dual1 > 0.07. In the calculation of this quantity and the chi squared itself, the typical error in the radial velocities is assumed to be 1 m/s.
RADUF(1), RADUFFIX(1), RTAPE(1), HRDRAD(1), RADVERT3(1), CDFTHRESH(1)
This program contains contributions from Carlos Lopez Carrillo, Lucio Lopez Cavazos, Sharon Lewis, and David Raymond