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Radar scattering by aggregate snowflakes

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Westbrook, C. D., Ball, R. C. and Field, P. R.. (2006) Radar scattering by aggregate snowflakes. Quarterly Journal of the Royal Meteorological Society, Vol.132 (No.616, Part A). pp. 897-914. ISSN 0035-9009

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Official URL: http://dx.doi.org/10.1256/qj.05.82

Abstract

The radar scattering properties of realistic aggregate snowflakes have been calculated using the Rayleigh-Gans theory. We find that the effect of the snowflake geometry on the scattering may be described in terms of a single universal function, which depends only on the overall shape of the aggregate and not the geometry or size of the pristine ice crystals which compose the flake. This function is well approximated by a simple analytic expression at small sizes; for larger snowflakes we fit a curve to Our numerical data. We then demonstrate how this allows a characteristic snowflake radius to be derived from dual wavelength radar measurements without knowledge of the pristine crystal size or habit, while at the same time showing that this detail is crucial to using such data to estimate ice water content. We also show that the 'effective radius'. characterizing the ratio of particle volume to projected area, cannot be inferred from dual wavelength radar data for aggregates. Finally, we consider the errors involved in approximating snowflakes by 'air-ice spheres', and show that for small enough aggregates the predicted dual wavelength ratio typically agrees to within a few percent, provided some care is taken in choosing the radius of the sphere and the dielectric constant of the air-ice mixture; at larger sizes the radar becomes more sensitive to particle shape, and the errors associated with the sphere model are found to increase accordingly.

Item Type:	Journal Article
Subjects:	Q Science > QC Physics
Divisions:	Faculty of Science > Physics
Library of Congress Subject Headings (LCSH):	Snowflakes, Radar, Microphysics, Scattering (Physics) , Aggregation (Chemistry), Wavelengths
Journal or Publication Title:	Quarterly Journal of the Royal Meteorological Society
Publisher:	Royal Meteorological Society
ISSN:	0035-9009
Date:	April 2006
Volume:	Vol.132
Number:	No.616, Part A
Number of Pages:	18
Page Range:	pp. 897-914
Identification Number:	10.1256/qj.05.82
Status:	Peer Reviewed
Publication Status:	Published
Access rights to Published version:	Open Access
References:	Bohren, C. F. and Huffman, D. R. 1983. Absorption and scattering of light by small particles. John Wiley and Sons, New York. Berry, M. V. and Percival, I. C. 1986. Optics of fractal clusters such as smoke. Opt. Acta, 33, 577–591. Draine, B. T. and Flatau, P. J. 1994. Discrete dipole approximation and its application for scattering calculations. J. Opt. Soc. Am. A, 11, 1491–1499. Heymsfield, A. J., Lewis, S., Bansemer, A., Iaquinta, J., Miloshevich, L. M., Kajikawa, M., Twohy, C. and Poellot, M. R. 2002. A general approach for deriving the properties of cirrus and stratiform ice cloud particles.. J. Atmos. Sci., 59, 3–29. Hogan, R. J., Illingworth, A. J. and Sauvageot, H. 2000. Measuring crystal size in cirrus using 35- and 94-Ghz radars. J. Atmos. & Ocean. Tech., 17, 27–37. Locatelli, J. D. and Hobbs, P. V. 1974. Fall speeds andmasses of solid precipitation particles. J. Geophys. Res., 79, 2185–2197. Matrosov, S. Y. 1992. Radar reflectivity in snowfall. IEEE. Trans. Geosci. & Rem. Sens., 30, 454–461. 1998. A dual-wavelength radar method to measure snowfall rate. J. Appl. Met., 37, 1510–1521. Mitchell, D. M. 1996 Use of mass- and area- dimensional power laws for determinating precipitation particle terminal velocities. J. Atmos. Sci., 53, 1710–1723 Purcell, E. M. and Pennypacker, C. R. 1973. Scattering and absorption of light by nonspherical grains. Astrophys. J., 186, 705–715 Westbrook, C. D., Ball, R. C., Field, P. R. and Heymsfield, A. J. 2004a. Universality in snowflake aggregation. Geophys. Res. Lett., 31, L15104–15107 2004b. A theory of growth by differential sedimentation, with application to snowflake formation. Phys. Rev. E, 70, 021403.
URI:	http://wrap.warwick.ac.uk/id/eprint/33392