Wednesday, 9 July 2014
Mixed phase clouds are fundamental in the production of precipitation, and are known to exert a key role on Earth's radiation budget. Yet our understanding of the spatial and temporal extent of these conditions remains poorly understood, primarily due to a lack of observations. In the presence of supercooled liquid water (SLW), pristine oriented crystals grow rapidly by the Bergeron-Findeison mechanism and so can be used to infer its presence (Hogan et al, 2003). Since pristine oriented crystals preferentially fall with their major axis horizontally aligned, these crystals are observable from dual polarisation radar as regions of elevated differential reflectivity (ZDR). This is straightforward where they are the dominant crystal type; observing their signal when they exist amongst larger aggregates is challenging because the aggregates mask the signal from the pristine crystals (Bader et al, 1987). Here, we present a method to identify and retrieve the microphysics of these embedded mixed-phase regions using observations from the S-band Chilbolton Advanced Meteorological Radar (CAMRa). The quantitative use of the co-polar correlation coefficient (ρHV) which characterises the mixture of particle shapes in a sample volume to reveal these crystals is demonstrated for the first time. By combining observations of ZDR and ρHV with a two-population forward model, the fraction of horizontal radar reflectivity from the pristine crystals and aggregates, and the intrinsic differential reflectivity (ZDRI) of the pristine crystals can be retrieved. Preliminary retrieval results for ZDRI are consistent with laboratory experiments of pristine crystal growth.
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