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5 Apr 2001

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Dual wavelength radar measurement of cirrus microphysical properties

Robin Hogan, Anthony Illingworth and Henri Sauvageot


  • Cirrus clouds play an important role in the earth's radiation budget. Consequently accurate representation of cloud microphysical properties in General Circulation Models is essential for reliable predictions of climate change.
  • To validate ice cloud parameterisations, we need to be able to remotely measure Ice Water Content IWC (the mass of ice present per unit volume), and some measure of crystal size.
  • Unfortunately, in the Rayleigh scattering regime, radar reflectivity Z is proportional to the sixth power of diameter, so is not directly related to either IWC or crystal size.
  • However, if two coincident wavelength radars are used, one which scatters in predominately in the Rayleigh regime and the other which undergoes Mie scattering in the presence of larger crystals, then the Dual Wavelength Ratio DWR, defined as the ratio of Z measured at the longer wavelength to that measured at the shorter, will be directly related to size.

Case Study

  • The figure shows a case study from 22 June 1996 in which thick cirrus was observed by the colocated 35 GHz Rabelais and 94 GHz Galileo radars. Unfortunately due to a hardware fault the 94 GHz radar was around 10 dB less sensitive than the 35 GHz radar during 1996.
  • By assuming a second order gamma distribution for crystal sizes, and that crystals can be approximated by spheres with decreasing density as their size increases, Mie scattering calculations have been carried out to find out how DWR varies with median volume diameter.
  • With both reflectivity and size information available, it is a straightforward matter to calculate IWC also. This is shown in the figure together with that diagnosed by the UK Meteolological Office forecast model for comparison.
  • The technique could be used from a spaceborne platform, although from space one would have the advantage that the attenuating boundary layer does not need to be penetrated, so much higher frequencies can be used. Higher frequencies Mie scatter to a greater extent for the same particle size, so both size and ice water content measurements become more accurate. It has been found that the most suitable wavelength combination to use from space is 79 or 94 GHz for the lower frequency and 215 GHz for the higher frequency.
  • Coincident aircraft meaurements are seen as essential to validate the technique.


  • The potential of spaceborne dual-wavelength radar to make global measurements of cirrus clouds
    Hogan, R. J., and A. J. Illingworth, 1999,J. Atmos. Oceanic Tech., 16(5), 518-531.
  • Measuring crystal size in cirrus using 35- and 94-GHz radars
    Hogan, R. J., A. J. Illingworth and H. Sauvageot, 2000, J. Atmos. Oceanic Tech., 17(1), 27-37.

See also...