A New Aspect of the Vertical Incidence Doppler Radar Spectrum of Ice Particle Fallspeeds

Richard E. Passarelli Jr. Massachusetts Institute of Technology, Cambridge, MA 02138

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Ramesh C. Srivastava The University of Chicago, Chicago, IL 60637

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Abstract

Unlike raindrops, ice particles of the same mass can have different fallspeeds, due to variations in the particle shape and bulk density. (This is an extension of the popular axiom that no two snowflakes are alike.) This is an additional source of variance for vertical incidence Doppler (VID) spectra taken in snow that has been neglected in previous studies which assume a one-to-one fallspeed-mass relationship. The total VID spectral variance due to the dispersion of ice particle fallspeeds can be broken down into two components, i.e., that due to the mean fallspeed-mass relationship and that due to fluctuations about the mean. Existing data on ice particle fallspeeds are not adequate for a thorough evaluation, but do indicate that these two sources of fallspeed variance can be of the same order. These results suggest that the task of deducing snow-size spectra from VID measurements is more difficult than has been recognized.

Abstract

Unlike raindrops, ice particles of the same mass can have different fallspeeds, due to variations in the particle shape and bulk density. (This is an extension of the popular axiom that no two snowflakes are alike.) This is an additional source of variance for vertical incidence Doppler (VID) spectra taken in snow that has been neglected in previous studies which assume a one-to-one fallspeed-mass relationship. The total VID spectral variance due to the dispersion of ice particle fallspeeds can be broken down into two components, i.e., that due to the mean fallspeed-mass relationship and that due to fluctuations about the mean. Existing data on ice particle fallspeeds are not adequate for a thorough evaluation, but do indicate that these two sources of fallspeed variance can be of the same order. These results suggest that the task of deducing snow-size spectra from VID measurements is more difficult than has been recognized.

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