10 Mar 2011





Cloud observations using dual wavelength radar

Researchers observe clouds using radar to improve how clouds are represented in weather and climate models. For example, to validate ice cloud parameterisations in General Circulation Models, researchers need to use remote sensing to determine the ice water content of clouds, together with the effective radius of ice crystals in cirrus clouds.

A consortium headed by the University of Reading, together with Officine Galileo, Italy and RCRU have developed a 94 GHz radar for cloud studies. The project was jointly funded by ESTEC and the EU. In June 1996, the 94 GHz radar operated in conjunction with a 35 GHz radar. At that time, the two radars were fixed to the main 3 GHz radar antenna at Chilbolton, pointing in exactly the same direction. The 35 GHz radar scattered mainly in the Rayleigh regime, but the 94 GHz produced Mie scatter from ice crystals.

From these dual wavelength radars, researchers could derive the dual wavelength ratio, or DWR which is related to the effective radius of the ice crystals. The figure above shows data analysed by Reading University, giving the reflectivities from the two radars and the dual wavelength ratio. Since these measurements were taken, the 35 GHz radar has been returned. The 94 GHz radar is no longer mounted on the main radar antenna, but continues to make cloud measurements from its new location at Chilbolton Observatory. Daily images from the 94 GHz radar can be seen on the Chilbolton weather web.

Further details about dual wavelength observations of clouds can be found on the University of Reading website (link opens in a new window).

Coincident observations of cloud by radar and aircraft

Coincident observations of cirrus by radar and aircraft took place in 1997 during the CIRRAD campaign performed in association with Reading University. On 8 October 1997, the Chilbolton radars made measurements of horizontally aligned crystals in a 3 km thick layer of cirrus cloud at the same time as measurements were made with the UK C-130 meteorological research aircraft. Frontal cirrus and stratiform precipitation were present.

Researchers used polarisation parameters to identify the presence of the horizontally aligned crystals. In an area where the cloud sampled by the radar and the aircraft were closest in space and time there were excellent agreements in the measurements. Clouds play an important role in the radiation balance of the earth and the measurements show the potential of radar for ground-based or spaceborne radar observations.

Contact: Hooper, David (STFC,RAL,RALSP)