Particle Fall Speed Research Articles

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

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.

Precipitation mapping with an airborne synthetic aperture imaging radar

Use of an airborne synthetic-aperture radar (SAR) of 3.1-cm wavelength for the detection and mapping of precipitation echoes is described. Difficulties in mapping related to the incoherence of the echoes are discussed; methods of increasing resolution by Doppler filtering are suggested. Observations made with a vertical side-looking SAR system flying above a stratiform storm were found to resemble vertical cross sections obtained with conventional radar showing snow streamers, the bright band and rain. However, the actual section sampled by the SAR after filtering for zero Doppler is shown to depend upon both the relative motion of the particles and the relative winds below the aircraft. Knowledge of the relative winds, deduced from Doppler spectrum variations, and of the particle fall speeds permits the generation of horizontal and vertical cross sections displaced from the aircraft's position.