5 Nov 1998 data

• Scatter Plots of Z versus ZDR

  Data presented in these scatter plots are decoded from the raw recorded polar radar volumes. Each point represents a range bin along a ray of a particular azimuth and elevation in the polar data. These data are subsetted to include only those points with reflectivity values > 15 dBZ and with range > 25 km. The range criterion is used to eliminate points near the radar which may be subject to clutter. The actual range criterion in the TRMM products is 15 km, 25 km is more conservative.

  981105.2231:ice region (altitudes > 6 km) [numpts=364]

  981105.2241:ice region (altitudes > 6 km) [numpts=133]

  981105.2231:rain region (altitudes < 4 km) [numpts=45322]

  981105.2241:rain region (altitudes < 4 km) [numpts=37792]

  ZDR values are expected to be near 0 dB in ice, and are usually between -1 and 5 dB in rain with the negative values down to ~-1 corresponding to vertically-oriented graupel. ZDR values in rain are near 0 for small spherical drops and increase with increasing drop size (and oblateness).

  Obvious problem: The distribution of ZDRs in the ice region with a mode of ~2.5 dB while expected mode is near 0 dB.

  Close range 0-15 km rain region < 4 km altitude

  Medium range 15-25 km rain region < 4 km altitude

  Since data within 15 km of the radar are not used in TRMM products, noisy data within this range are not a problem.

• PHIDP and Z as a function of range along radials

  Data presented in these plots are decoded from the raw recorded polar radar volumes. Each point represents a range bin along a ray of a particular azimuth and elevation in the polar data. These data are subsetted to include only those points with reflectivity values > 30 dBZ and with range > 25 km. PHIDP plotted as dots and corresponding Z plotted as connected lines. Since regions along the ray where Z > 30 are discontinuous, the data are discontinuous in range.

  volume and tilt	Azimuth (deg) of ray
  0.9 deg tilt at 2231 981105	0	1	2	3	42	45	46	61	64	66

  The relation of KDP (and hence PHIDP) to rainrate is strongest in regions with reflecitivity > 40 dBZ. Since reflectivities tend to be lower in this example, it is difficult to access the quality of the PHIDP data. Once nature cooperates and provides an intensive, areal extensive storm near Kwajalein, we will be in a better position to evaluation PHIDP.

  9 Dec 1998 data (after ROM upgrade)

  
  
  • Scatter Plots of Z versus ZDR

    These data are subsetted to include only those points with reflectivity values > 15 dBZ and with range > 25 km except where noted.

    981209 1950 UTC:ice region (altitudes > 6 km) [numpts=349], rain region (altitudes < 4 km) [numpts=21961]

    Range 70-150 km for 981209 1950 UTC:ice region (altitudes > 6 km) [numpts=329], rain region (altitudes < 4 km) [numpts=13331]

  Noisy ZDR data is present in all elevation angles. Effected range is 0-70 km at 0.4 deg and 0-50 km at 11.1 deg. Note: 1.5 deg and certainly 11.1 deg should not contain surface returns from sea clutter. This problem was not present in the data before the ROM upgrade. Examination of the Z versus ZDR plots in the ice region shows that ZDR values are still too high (should be near 0), but the modal value is now closer to 0.8 dB rather than the 2.5 dB it was in the 5 Nov 1998 example data. Note: that the mode in ice does not change when the data are restricted to beyond 70 km. Most of the noisy ZDR values have corresponding weak radar reflectivity and do not appear in the scatter plots.

  
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Sandra Yuter yuter@atmos.washington.edu