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.
- 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
- 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 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),
- Measuring crystal size in cirrus using 35- and 94-GHz
Hogan, R. J., A. J. Illingworth and H. Sauvageot, 2000,
J. Atmos. Oceanic Tech., 17(1), 27-37.