Abstract
Abstract. In-situ Polar Nephelometer (PN) measurements of unusual ice crystal scattering phase functions, obtained near the cloud-top of a mid-latitude anvil cloud, at a temperature of about −58 °C, were recently reported by Gayet et al. (2012). The ice crystal habits that produced the phase functions consisted of aggregates of ice crystals and aggregates of quasi-spherical ice particles. The diameters of the individual quasi-spherical ice particles were estimated to be between about 15 μm and 20 μm. The measured-averaged scattering phase functions were featureless, at scattering angles less than about 100°, but an ice bow-like feature was noted between the scattering angles of about 120° to 160°. The estimated asymmetry parameter was 0.78 ± 0.04. In this paper, the averaged scattering phase function is interpreted in terms of a weighted habit mixture model. The model that provides the best overall fit to the measured scattering phase function comprises of highly distorted ten-element hexagonal ice aggregates and quasi-spherical ice particles. The smaller quasi-spherical ice crystals are represented by Chebyshev ice particles of order 3, and were assumed to have equivalent spherical diameters of 24 μm. The asymmetry parameter of the best overall model was found to be 0.79. It is argued that the Chebyshev-like ice particles are responsible for the ice bow-like feature and mostly dominate the scattered intensity measured by the PN. The results from this paper have important implications for climate modelling (energy balance of anvils), cloud physics and the remote sensing of cirrus properties.
Highlights
The most-recent report of the Intergovernmental Panel on Climate Change (IPCC, 2007) concluded that radiative coupling, between clouds of all types, and the Earth’s atmosphere is still one of the greatest uncertainties in predicting climate change
The full model scattering phase function is plotted, to demonstrate that in order to discriminate between models, Polar Nephelometer (PN) instruments that measure the scattered intensity over a more complete range of scattering angle are required
G predicted by model 1 is 0.84, which is outside the upper range of uncertainty estimated by the PN
Summary
The most-recent report of the Intergovernmental Panel on Climate Change (IPCC, 2007) concluded that radiative coupling, between clouds of all types, and the Earth’s atmosphere is still one of the greatest uncertainties in predicting climate change. This is because cirrus is composed of highly irregular ice crystals, which generally exist as various habit mixtures, and their sizes can vary between less than 10 μm toward the cloud-top, to several centimetres toward the cloud-bottom (Korolev et al, 2006; Baran, 2009) Due to this variability in ice crystal size and shape, using climate models to predict the radiative effect of cirrus has proven to be problematic (Zhang et al, 1999; Kristjansson et al, 2000; Edwards et al, 2007; Gu et al, 2011, Baran 2012). In recent years there has been a large amount of research that has focused on habit mixture models of cirrus and their bulk-scattering properties (Macke et al, 1996a; Mishchenko et al, 2002; McFarquhar et al, 2002; Baum et al, 2005, 2011; Baran and Labonnote, 2007; Baran 2012, and references therein)
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