Abstract
AbstractPristine 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 (ZDR), which is related to their average aspect ratio. However, because ZDR 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. In this paper, 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 ZDR and the copolar correlation coefficient (ρhv), we show that it is possible to retrieve both the relative contribution to the radar signal and “intrinsic” ZDR ( ) 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 ZDR embedded within deep ice clouds are typically produced by pristine oriented crystals with 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 Facility for Airborne Atmospheric Measurements BAe‐146 aircraft support the conceptual model used and are consistent with the retrieval interpretation.
Highlights
Microphysical processes occurring within mixed-phase clouds dictate the clouds’ radiative properties (Comstock et al, 2007; Solomon et al, 2007), evolution, and lifetime (Morrison et al, 2012) and are fundamental to the production of precipitation (Mülmenstädt et al, 2015)
An example of this is in mixed-phase regions that are embedded within deeper ice cloud
By combining measurements of ZDR and the copolar correlation coefficient, we show that it is possible to retrieve both the relative contribution to the radar signal and “intrinsic” ZDR (ZDPRI) of the pristine oriented crystals, even in circumstances where their signal is being masked by the presence of aggregates
Summary
Microphysical processes occurring within mixed-phase clouds dictate the clouds’ radiative properties (Comstock et al, 2007; Solomon et al, 2007), evolution, and lifetime (Morrison et al, 2012) and are fundamental to the production of precipitation (Mülmenstädt et al, 2015). In mixed-phase conditions, pristine crystals are known to grow rapidly via the Bergeron-Findeison process; their habit is a function of the environmental temperature and supersaturation in which they form and grow. Their shape determines their scattering properties, growth rate, and fall speeds, cloud scattering properties, microphysical evolution, precipitation rate, and cloud lifetime. Estimates of ice water content or number concentration require accurate knowledge of ice particle shape (Westbrook & Heymsfield, 2011) These pristine crystals typically have a high density and tend to fall with their major axes aligned horizontally (Cho et al, 1981; Sassen, 1980; Westbrook et al, 2010). This property makes dual-polarization radar a powerful tool for investigating the microphysical properties and processes within mixed-phase clouds, as the preferential alignment of these crystals produces a larger backscatter in the horizontal (H) than the vertical (V) polarization
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