Evaluation of the differential Doppler velocity approach for sizing particles in ice clouds

Measurements from ground-based collocated Ka- and W-band vertically pointing Doppler radars were used to evaluate the differential Doppler velocity (DDV) approach to retrieve a size parameter of the aggregate particle distributions in ice clouds. This approach was compared to a more traditional method based on the dual-frequency reflectivity ratio (DFR) using case study observations in different clouds. Due to measurement errors and other uncertainties, meaningful DDV based retrievals were generally available for the size slope parameter interval of 9 cm-1< Λ < 25 cm-1. The DFR were generally available for particle populations with Λ up to about 45 cm-1. Expected retrieval errors in the Λ interval between 10 cm-1 and 25 cm-1 were about 30-40% for the DFR-based estimates and about a factor of 2 larger for the DDV method. Comparisons of the DDV- and DFR- inferred values of Λ when both retrievals were available revealed their general consistency with a relative standard deviation between results being within retrieval uncertainties. While the DFR approach appears to be more accurate, it requires a 0 dB constraint near cloud tops, which mitigates uncertainties in absolute radar calibrations and differing attenuation paths. The DDV approach generally does not require such a constraint if radar beams are perfectly aligned in vertical (which might not be exactly a case during some observations). Given this, DDV measurements may potentially allow ice particle sizing in situations when DFR constraining is not effective or available (e.g., in precipitating clouds and in clouds with substantial amounts of supercooled water).