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Measuring Crystal Size in Cirrus Using 35- and 94-GHz Radars

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  • 1 Department of Meteorology, University of Reading, Reading, United Kingdom
  • | 2 Université Paul Sabatier, Toulouse, France
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Abstract

Results are presented from a case study in which coincident 35- and 94-GHz radars located at Chilbolton, England, were used to measure crystal size in cirrus clouds. In the presence of larger crystals the 94-GHz radar scatters sufficiently beyond the Rayleigh regime that the difference in reflectivity factor measured by the two can be directly related to size. This enables more accurate estimation of ice water content than would be possible using a single radar. The small crystals at the top of the cloud scatter in the Rayleigh regime at both wavelengths, which provides a valuable method of calibration, but also means that sizing is not possible in this part of the cloud. Ice water content and median volume diameter were derived and compared with the analyses of the U.K. Meteorological Office Unified Model. The smallest measurable median volume diameter in this case study was around 200 μm, although it is believed that with both radars sensitive down to −35 dBZ, it should be possible to measure median diameters of 100 μm. Scattering calculations have been carried out to determine the sensitivity of these measurements to crystal density, crystal aspect ratio, and the shape of the size distribution. Density is found to be the most significant source of uncertainty, possibly introducing errors of 20% into retrieved diameter.

Corresponding author address: Robin Hogan, Department of Meteorology, P.O. Box 243, Earley Gate, Reading RG6 6BB, United Kingdom.

Email: R.J.Hogan@reading.ac.uk

Abstract

Results are presented from a case study in which coincident 35- and 94-GHz radars located at Chilbolton, England, were used to measure crystal size in cirrus clouds. In the presence of larger crystals the 94-GHz radar scatters sufficiently beyond the Rayleigh regime that the difference in reflectivity factor measured by the two can be directly related to size. This enables more accurate estimation of ice water content than would be possible using a single radar. The small crystals at the top of the cloud scatter in the Rayleigh regime at both wavelengths, which provides a valuable method of calibration, but also means that sizing is not possible in this part of the cloud. Ice water content and median volume diameter were derived and compared with the analyses of the U.K. Meteorological Office Unified Model. The smallest measurable median volume diameter in this case study was around 200 μm, although it is believed that with both radars sensitive down to −35 dBZ, it should be possible to measure median diameters of 100 μm. Scattering calculations have been carried out to determine the sensitivity of these measurements to crystal density, crystal aspect ratio, and the shape of the size distribution. Density is found to be the most significant source of uncertainty, possibly introducing errors of 20% into retrieved diameter.

Corresponding author address: Robin Hogan, Department of Meteorology, P.O. Box 243, Earley Gate, Reading RG6 6BB, United Kingdom.

Email: R.J.Hogan@reading.ac.uk

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