Abstract
Radar reflectivity measurements from three different wavelengths are used to retrieve information about the shape of aggregate snowflakes in deep stratiform ice clouds. Dual-wavelength ratios are calculated for different shape models and compared to observations at 3, 35 and 94 GHz. It is demonstrated that many scattering models, including spherical and spheroidal models, do not adequately describe the aggregate snowflakes that are observed. The observations are consistent with fractal aggregate geometries generated by a physically-based aggregation model. It is demonstrated that the fractal dimension of large aggregates can be inferred directly from the radar data. Fractal dimensions close to 2 are retrieved, consistent with previous theoretical models and in-situ observations.
Highlights
Most ice phase precipitation at the surface is composed of aggregates of ice crystals [Hobbs et al, 1974], and aggregation often dominates the formation of large particles in deep ice clouds aloft [Field and Heymsfield, 2003; Westbrook et al, 2007]
We have shown that two different case studies had a similar relationship between DWR3−35 and DWR35−94, consistent with the fractal model proposed by W04
The observation that DWR35−94 approaches a limiting value is consistent with fractal geometry and allows the fractal dimension of the aggregates to be estimated from observations: df ≈ 2 in our cases
Summary
Most ice phase precipitation at the surface is composed of aggregates of ice crystals [Hobbs et al, 1974], and aggregation often dominates the formation of large particles in deep ice clouds aloft [Field and Heymsfield, 2003; Westbrook et al, 2007]. The ratio of Z measured at a pair of frequencies is a function of particle size and particle shape; Kneifel et al [2011] showed that the relationship between two such ratios (derived from measurements at three frequencies) allows us to distinguish between different particle shapes present in a given sample of cloud and validate or disprove theoretical models of their structure and scattering properties. Leinonen et al [2012] analyzed a selection of vertical profiles from airborne radar measurements in convective clouds at 13, 35, and 94 GHz from the Wakasa Bay field campaign in 2003 and concluded the data were inconsistent with spherical/spheroidal models of the snowflakes. The new measurements of aggregate snowflakes presented in this letter were obtained using three sensitive ground-based radars as part of the Chilbolton Triple-Wavelength Snowflake Experiment at the Chilbolton Observatory in southern England during 2014.
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