Radar Backscattering from Snowflakes: Comparison of Fractal, Aggregate, and Soft Spheroid Models

Abstract

The sensitivity of radar backscattering cross sections on different snowflake shapes is studied at C, Ku, Ka, and W bands. Snowflakes are simulated using two complex shape models, namely, fractal and aggregate, and a soft spheroid model. The models are tuned to emulate physical properties of real snowflakes, that is, the mass–size relation and aspect ratio. It is found that for particle sizes up to 5 mm and for frequencies from 5 to 35 GHz, there is a good agreement in the backscattering cross section for all models. For larger snowflakes at the Ka band, it is found that the spheroid model underestimates the backscattering cross sections by a factor of 10, and at W band by a factor of 50–100. Furthermore, there is a noticeable difference between spheroid and complex shape models in the linear depolarization ratios for all frequencies and particle sizes.

G band atmospheric radars: new frontiers in cloud physics

“…2, we discuss in this section the benefit of introducing G band radar observations to multi-wavelength observations. (Liu, 2008a;Petty and Huang, 2010;Tyynelä et al, 2011), for soft spheres and 0.6 axial ratio spheroids (following the snow densities proposed in Hogan et al (2012) (dashed) and in Matrosov (2007) (continuous lines)) and the Rayleigh-Gans approximation according to Westbrook et al (2006) (magenta dash-dotted) have been included.…”

“…However, recent studies (Kim, 2006;Liu, 2008a;Tyynelä et al, 2011) have shown that for size parameters larger than 2 (which roughly correspond to maximum sizes of 5, 2, 1.4 and 0.9 mm at 35, 94, 140 and 220 GHz, respectively) the details of the crystal shapes become increasingly important. Above such size parameters, backscattering cross sections for aggregate and fractal snowflakes can easily deviate by one (two) orders of magnitude at 35 GHz (94 GHz) from the soft-spheroid model.…”

Performance assessment of a triple-frequency spaceborne cloud–precipitation radar concept using a global cloud-resolving model

“…Additionally, studies over the recent years have shown that the use of homogeneous spherical and spheroidal shapes to model radar observations of snowflakes can lead to an underestimation of the backscattering cross section by up to an order of magnitude (e.g., Petty and Huang, 2010;Tyynelä et al, 2011) compared to those derived from models with detailed snowflake structure. In order to use more realistic snowflake scattering properties, we obtained them from the database published by Nowell et al (2013), which was generated by using the discrete dipole approximation (DDA) to compute the scattered radiation from aggregates of bullet rosettes.…”

Ice clouds microphysical retrieval using 94‐GHz Doppler radar observations: Basic relations within the retrieval framework

“…In their theoretical study, Westbrook et al [2004a] showed that during the growth by aggregation, the aspect ratio of aggregates asymptotically approaches the value of 0.65. Recently published the results of snowflake simulations [Tyynelä et al, 2011] show the aspect ratios between 0.5 and 0.95 in the range of sizes the most important for the W-band total reflectivity, i.e., below about 2 mm. The value of 0.6 representing average aspect ratio measured with no taking into account the orientation is derived from the observational studies of Korolev and Isaac [2003].…”