Light scattering by complex ice-analogue crystals

Abstract

Original article can be found at : http://www.sciencedirect.com/ Copyright ElsevierAngle-dependent light scattering measurements on single ice analogues crystals are described. Phase functions and degree of linear polarization are measured for electrodynamically levitated crystals. A procedure for randomizing particle orientation during levitation is demonstrated. The dependence of scattering on the shape, complexity and surface roughness of the crystals is examined. The phase functions from complex crystals with smooth surfaces show little dependence on shape. There is close agreement between the measured functions and the analytic phase function for ice clouds. However, rosettes with rough surfaces have qualitatively different phase functions, with raised side and back scattering. The asymmetry parameter is typically about 0.8??0.04 and 0.63??0.05 for smooth and rough crystals, respectively. The 22o halo peak is present for smooth rosettes and aggregates but absent for rough rosettes. Two-dimensional scattering patterns from several crystals in fixed orientations are also shown. The results suggest that it may be possible to use such patterns to discriminate not only between crystals of different shape but also to obtain some information on surface properties

The Impact of Two Coupled Cirrus Microphysics–Radiation Parameterizations on the Temperature and Specific Humidity Biases in the Tropical Tropopause Layer in a Climate Model

“…The difference in reflected shortwave flux is at most about 7 W m 22 , assuming an area-averaged incident solar flux of 330 W m 22 , at least for the case considered here. These differences found for the g parameterizations are far less than the range in g found by several studies (Ulanowski et al 2006;Fu 2007;Garrett 2008;Baran 2012;van Diedenhoven et al 2014;Yang et al 2015). Indeed, in the case of Ulanowski et al (2006), experimentally derived g values were found to vary between 0.80 6 0.04 and 0.63 6 0.05 for smooth and rough ice analog rosettes, respectively, and this difference results in a shortwave flux uncertainty of about 256 W m 22 .…”

“…These differences found for the g parameterizations are far less than the range in g found by several studies (Ulanowski et al 2006;Fu 2007;Garrett 2008;Baran 2012;van Diedenhoven et al 2014;Yang et al 2015). Indeed, in the case of Ulanowski et al (2006), experimentally derived g values were found to vary between 0.80 6 0.04 and 0.63 6 0.05 for smooth and rough ice analog rosettes, respectively, and this difference results in a shortwave flux uncertainty of about 256 W m 22 . Furthermore, the calculated asymmetry parameter values using the two parameterizations compare well against radiometrically derived asymmetry parameter values using POLDER observations from van Diedenhoven et al (2014).…”

“…(3). However, ADT is a soft particle approximation (i.e., assumes real refractive indices near unity), originally due to van de Hulst (1957); by using this approximation, v 0 values presented in Murray et al (2015) may be overestimated (due to the neglect of reflection, refraction, and particle edge effects, which all tend to increase absorption; see, e.g., Mitchell et al 2006). The latter limitations of ADT were noted by Murray et al (2015), who also called for more accurate computations of the single-scattering properties of trigonal particles.…”

“…Figure 4c shows «(l) for g and, as can be seen from the figure, «(l) is within 62.5% for about 83% of the database, which is also acceptable. Theoretical and in situ uncertainty in the asymmetry parameter value is far greater than the error in the g parameterization (Ulanowski et al 2006;Fu 2007;Garrett 2008;Baran 2012;van Diedenhoven et al 2014). We compare our parameterization of g to the g parameterization developed by Fu (2007) by assuming q i and T c values of 1.0 3 10 24 kg kg 21 and 190 K, respectively.…”

“…However, it is uncertain as to whether these particles are actually quasi-spherical due to the limiting resolving power of the microphysics instrumentation used at the time; therefore, the appearance of quasi-sphericity could be due to diffractive and optical effects, as noted by Cotton et al (2010) and references therein. On the other hand, these particles could be quasispherical, but instruments are required that can adequately resolve these ice crystals of an uncertain shape, such as the small ice detector described in Ulanowski et al (2006). For crystal sizes greater than 65 mm, Lawson et al (2008) found habit mixtures comprising mostly hexagonal plates and irregular ice crystals.…”