Determination of Cirrus Microphysical Properties
Using Dual Wavelength Radar
R. J. Hogan and A. J. Illingworth
- 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
- 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 effective radius re
(a representative crystal size for radiation calculations).
- 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 re.
- 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 re.
- In June 1996 a new ESTEC 94 GHz (3.2 mm) radar
was installed at Chilbolton to
accompany the existing French 35 GHz (8.6 mm) radar in the study of clouds.
Currently the radars operate in a fixed zenith-pointing configuration.
- The following figure is a time series of reflectivity at both
frequencies (Z35 and Z94) and the corresponding DWR
taken through cirrus on 22 June 1996. To cope with the large dynamic
range, logarithmic units are used.
- 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 re.
- With both reflectivity and size
information available, it is a straightforward matter to
calculate IWC also.
The two parameters and IWC for this case are shown
- To indicate the ability of the Meteorological Office Unified
Model to locate the cloud correctly, model relative
humidity is shown superimposed on the 35 GHz reflectivity in the
following figure, for the period during which the cloud was overhead.
The Next Step
- We have demonstrated the potential of this technique to systematically
measure the relevant cloud properties. In the near future dual
wavelength observations will be validated by comparison with in situ
measurements taken by the Meteorological Office C-130 aircraft.
- In liquid water clouds the 94 GHz radar experiences stronger
attenuation than the 35 GHz radar, and hence DWR could also
provide a direct measure of Liquid Water Content.
- By operating the radars continuously for around a month, and
comparing with Unified Model analyses over Chilbolton, the ability
of the model to realistically simulate both liquid and ice phase
clouds will be objectively assessed.
- There is also a need to test cloud retrieval algorithms for the
planned spaceborne 94GHz cloud radar.
Cloud radar research at Reading
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