Abstract
Abstract. Single-crystal images collected in mid-latitude cirrus are analyzed to provide internally consistent ice physical and optical properties for a size-resolved cloud microphysics model, including single-particle mass, projected area, fall speed, capacitance, single-scattering albedo, and asymmetry parameter. Using measurements gathered during two flights through a widespread synoptic cirrus shield, bullet rosettes are found to be the dominant identifiable habit among ice crystals with maximum dimension (Dmax) greater than 100 µm. Properties are therefore first derived for bullet rosettes based on measurements of arm lengths and widths, then for aggregates of bullet rosettes and for unclassified (irregular) crystals. Derived bullet rosette masses are substantially greater than reported in existing literature, whereas measured projected areas are similar or lesser, resulting in factors of 1.5–2 greater fall speeds, and, in the limit of large Dmax, near-infrared single-scattering albedo and asymmetry parameter (g) greater by ∼ 0.2 and 0.05, respectively. A model that includes commonly imaged side plane growth on bullet rosettes exhibits relatively little difference in microphysical and optical properties aside from ∼ 0.05 increase in mid-visible g primarily attributable to plate aspect ratio. In parcel simulations, ice size distribution, and g are sensitive to assumed ice properties.
Highlights
It is well known that cirrus clouds substantially impact radiative fluxes and climate in a manner that depends upon their microphysical and macrophysical properties (e.g., Stephens et al, 1990)
With respect to microphysical properties, observations of cirrus cloud particle size distributions and underlying ice crystal morphology still remain subject to large uncertainties, in part owing to lack of instrumentation adequate to provide artifact-free and well-calibrated measurements of size-distributed ice particle number and mass concentrations (e.g., Baumgardner et al, 2011; Lawson, 2011; Cotton et al, 2012)
In preparation for large-eddy simulations with size-resolved microphysics for a case study of mid-latitude synoptic cirrus observed on 1–2 April 2010 during the SPARTICUS campaign (Muhlbauer et al, 2015), here we use Cloud Particle Imager (CPI) image analysis to develop ice crystal geometries that are physically continuous over the required crystal size range and suitable to calculate internally consistent physical and optical properties
Summary
It is well known that cirrus clouds substantially impact radiative fluxes and climate in a manner that depends upon their microphysical and macrophysical properties (e.g., Stephens et al, 1990). Because the most accurate representation of cirrus optical properties requires consideration of polycrystal element aspect ratios (e.g., van Diedenhoven et al, 2014a), which are commonly a function of particle size in observations, the polycrystal elements are adopted as the foundation for treating mass and projected area rather than vice versa (as required if area–dimensional and mass–dimensional relationships are instead adopted as the foundation, as most commonly done); a similar approach was taken by Heymsfield and Iaquinta (2000) for the purpose of deriving physically based expressions for cirrus crystal terminal velocities, such as bullet rosettes with varying numbers of arms. Because the derivations here are based on crystal component geometries and do not yield continuous analytic relationships, equations are provided in Appendix A and derived ice properties are provided for download as the Supplement
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