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Last update
10 Jul 2002

This page is maintained by
Ed Pavelin

35 GHz Doppler cloud radar

A 35 GHz (9 mm wavelength) cloud radar is under construction at RAL which is designed to complement the existing 94 GHz (3 mm) "Galileo" radar currently operating at Chilbolton. Millimetre-wave cloud radars exploit the fact that the echo intensity of Rayleigh scatter increases with the inverse-fourth power of the wavelength. These radars normally operate in the windows in the atmospheric absorption spectrum at 35 GHz and 94 GHz

The radar, which will be Doppler capable, will have the same pulse repetition frequency and gate length as the 94 GHz radar. It will be based around a 2-m dish. Operating the two radars in parallel will open up the possibility of performing dual-wavelength observations. We also plan to implement pulse compression on this radar, in order to enhance its sensitivity.

The 35 GHz radar will be similar in principle to the MMCR Radars operated at the US ARM (Atmospheric Radiation Measuerement) sites. In addition to its zenith-pointing mode, the Chilbolton radar will have the facility of being attached to the main 25-metre dish, for co-axial measurements with the 3 GHz and 1 GHz radars. It is also planned that the radar will be transportable for use in field campaigns away from Chilbolton.

Dual-wavelength cloud radar measurements make it possible to determine the liquid water content and ice water content of clouds, using the dual-wavelength ratio (DWR). The DWR is defined as the ratio of the radar reflectivities (Z) measured at the two wavelengths.

Liquid water content (LWC) can be derived by exploiting the fact that attenuation by liquid water in clouds is dependent on frequency. The LWC is therefore related to the vertical derivative of the dual-wavelength ratio.

The shorter-wavelength radar (94 GHz) is sensitive to a small amount of Mie scattering from the largest cloud particles. The 35 GHz backscatter, however, is overwhelmingly due to the Rayleigh mechanism. Therefore, the DWR provides a measure of the size of the scattering particles. Together with measurements of radar reflectivity, this allows ice water content to be derived in cirrus clouds.

The new 35 GHz cloud radar is scheduled to be operational by February 2003.

Further reading

  • An unattended cloud-profiling radar for use in climate research
    Moran, K.P. et al., Bulletin of the American Meteorological Society, 79, 443-455, 1998.
  • Cloud characteristics from dual wavelength millimetre-wave radar
    Hogan, R. J., A. J. Illingworth and H. Sauvageot, Proc. 29th Int. Conf. on Radar Meteorol., Montreal, Canada, 457-459, 1999. (Download diffatten.ps.gz)
  • The potential of spaceborne dual-wavelength radar to make global measurements of cirrus clouds
    Hogan, R. J., and A. J. Illingworth, J. Atmos. Oceanic Tech., 16(5), 518-531, 1999.
  • Measuring crystal size in cirrus using 35- and 94-GHz radars
    Hogan, R. J., A. J. Illingworth and H. Sauvageot, J. Atmos. Oceanic Tech., 17(1), 27-37, 2000.

See also...