Revealing Layers of Pristine Oriented Crystals Embedded within Deep Ice Clouds using Differential Reflectivity and the Co-Polar Correlation Coefficient.

Pristine ice crystals typically have high aspect ratios (>>1), have a high density and tend to fall preferentially with their major axis aligned horizontally. Consequently, they can, in certain circumstances, be readily identified by measurements of differential reflectivity (Z_DR), which is related to their average aspect ratio. However, because Z_DR is reflectivity weighted, its interpretation becomes ambiguous in the presence of even a few, larger aggregates or irregular polycrystals. An example of this is in mixed-phase regions that are embedded within deeper ice cloud. Currently, our understanding of the microphysical processes within these regions is hindered by a lack of good observations.

A novel technique is presented that removes this ambiguity using measurements from the 3 GHz Chilbolton Advanced Meteorological Radar in Southern England. By combining measurements of Z_DR and the co-polar correlation coefficient (ρ_hv), we show that it is possible to retrieve both the relative contribution to the radar signal and “intrinsic" Z_DR (Z^P_DRI) of the pristine oriented crystals, even in circumstances where their signal is being masked by the presence of aggregates. Results from two case studies indicate that enhancements in Z_DR embedded within deep ice clouds are typically produced by pristine oriented crystals with Z^P_DRI values between 3 and 7 dB (equivalent to 5-9 dB at horizontal incidence) but with varying contributions to the radar reflectivity. Vertically pointing 35 GHz cloud radar Doppler spectra and in-situ particle images from the FAAM BAe-146 aircraft support the conceptual model used and are consistent with the retrieval interpretation.