Investigating Ice Particle Size Distribution Characteristics Using Collocated Aircraft in-Situ Measurements and Radar Observations

Radar retrievals involving ice-phased hydrometers are challenging. One reason is the large uncertainties in ice hydrometer particle size distributions (PSDs). In this study, the PSDs of ice hydrometers in North China are examined using aircraft in-situ observations on 22 May 2017. A new habit classification algorithm based on decision tree method for High-Volume Precipitation Spectrometer (HVPS) images is introduced, which can automatically classify hydrometer into pristine ice crystals, aggregate snow, and rain, and can detect particle maximum dimension and aspect ratio. Based on this classification algorithm, the binned PSDs and aspect ratio distributions of hydrometers along the flight track are calculated. To verify the observed PSDs, polarimetric radar variables are simulated by radar simulator that uses the in-situ observed PSDs along the flight track and single scattering properties of hydrometers calculated using the T-matrix method. The simulated radar reflectivity shows good agreement with a nearby operational S-band radar observations, with a mean absolute error of 24% over the entire flight. This difference may be caused by different sample volume of the airborne observations and radar observations. The observed binned PSDs are fitted into Gamma distributions, and the statistical relations between Gamma distribution parameters are examined, which can be used as constraints in developing radar retrieval algorithms. The collocated observed PSD characteristics and radar observations in North China can help to improve the retrieval algorithms of ice PSDs with polarimetric radars, which can improve the analysis of the ice phase microphysical processes, and would help improve the microphysical parameterization of numerical modeling in the future.