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Scattering models of precipitation-size ice particles have shown that aggregates and spheroidal particles occupy distinct regions of the K-u-K-a-W-band dual-frequency ratio (DFR) plane. Furthermore, past ground-based observations suggest that particle bulk density and characteristic size can be retrieved from the DFR plane. This study, for the first time, evaluates airborne DFR observations with coincident airborne microphysical measurements. Over 2hr of microphysical data collected aboard the University of North Dakota Citation from the Olympic Mountains Experiment are matched with Airborne Precipitation and cloud Radar Third Generation triple-frequency radar observations. Across all flights, 31% (63%) of collocated data points show nonspheroidal (spheroidal) particle scattering characteristics. DFR observations compared with in situ observations of effective density and particle characteristic size reveal relationships that could potentially be used to develop quantitative dual- and triple-frequency DFR ice property retrievals.

Plain Language Summary Currently, remote sensing retrievals of ice clouds require assumptions since particle shape and size vary greatly in the atmosphere. Additionally, particle shape and size constrain relationships of mass and fall velocity of ice within a cloud, which affect remote sensing retrievals. Modeling studies have shown that the scattering characteristics of complex ice particles (e.g., aggregates) have a distinct signature compared to spherical representations of the same particles when using three frequencies under the following conditions: (1) at least one radar with its wavelength close to the size of the particle and (2) particles have low effective densities. Thus, there is potential to retrieve information about particle shape using triple-frequency radar observations to constrain the assumptions of particle shape in the ice cloud retrieval. This paper is the first study to use airborne triple-frequency radar observations coincident with airborne in situ microphysical measurements to evaluate both the scattering signal discussed and retrievals of characteristic size and effective density. We found that 31% (63%) of the observations from the Olympic Mountains Experiment show nonspheroidal (spheroidal) scattering characteristics. Furthermore, the triple-frequency observations confirm the relationships with observed particle size and effective density outlined in a previous study supporting future use of triple-frequency missions.