Improvements in Shortwave Bulk Scattering and Absorption Models for the Remote Sensing of Ice Clouds

Abstract

This study summarizes recent improvements in the development of bulk scattering/absorption models at solar wavelengths. The approach combines microphysical measurements from various field campaigns with single-scattering properties for nine habits including droxtals, plates, solid/hollow columns, solid/hollow bullet rosettes, and several types of aggregates. Microphysical measurements are incorporated from a number of recent field campaigns in both the Northern and Southern Hemisphere. A set of 12 815 particle size distributions is used for which T cld # 2408C. The ice water content in the microphysical data spans six orders of magnitude. For evaluation, a library of ice-particle single-scattering properties is employed for 101 wavelengths between 0.4 and 2.24 mm. The library includes the full phase matrix as well as properties for smooth, moderately roughened, and severely roughened particles. Habit mixtures are developed for generalized cirrus, midlatitude cirrus, and deep tropical convection. The single-scattering properties are integrated over particle size and wavelength using an assumed habit mixture to develop bulk scattering and absorption properties. In comparison with global Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) data, models built with severely roughened particles compare best for all habit mixtures. The assumption of smooth particles provided the largest departure from CALIOP measurements. The use of roughened rather than smooth particles to infer optical thickness and effective diameter from satellite imagery such as the Moderate Resolution Imaging Spectroradiometer (MODIS) will result in a decrease in optical thickness and an increase in particle size.

Dimensions and aspect ratios of natural ice crystals

“…There is no doubt that the systems sampled had a large influence on these habit fractions. Sampling of more clouds at a variety of locations associated with different meteorological forcings would give more information on controls of these habit mixtures that need to be known for use in satellite retrieval schemes (e.g., Baum et al, 2011) and parameterizations of simple scattering properties (e.g., McFarquhar et al, 2002). Table 7 also separates the relative fraction of occurrence (numbers in parentheses) of each habit for anvil and nonanvil clouds, respectively.…”

“…Ice clouds consist almost exclusively of non-spherical ice crystals with various shapes (i.e., habits) and sizes (e.g., Liou, 1986). To determine the influences of ice clouds on solar and infrared radiation as required for numerical models and remote sensing retrievals, knowledge of single-scattering properties of ice crystals is required (e.g., Fu, 1996Fu, , 2007McFarquhar et al, 2002;Baum et al, 2011;Baran, 2012;DeVore et al, 2012;Yang et al, 2013;Yi et al, 2013;Um and McFarquhar, 2013;Cole et al, 2014;van Diedenhoven et al, 2014a).…”

Ice particle habit and surface roughness derived from PARASOL polarization measurements

“…The calculations are provided by the ADDA (Amsterdam Discrete Dipole Approximation, Yurkin and Hoekstra 2011) and the IGOM (Improved Geometric Optics Method, Yang et al, 2013) methods and the results are stored in a database and then used to calculate the bulk scattering properties. As shown in Baum et al ( , 2011, the bulk scattering properties are obtained by averaging over a particle size distribution, instrument spectral response function, solar spectrum, and any habit distribution (none if considering a single habit).…”

Photopolarimetric retrievals of snow properties

“…Ice crystals are found to vary within a virtually countless set of shapes, but, limited by the lack of constraints on the correct optical properties, very few studies have departed from the common practice of assuming spherical shapes for the snow grains, an approximation which can negatively impact the quality of the albedo products (Tedesco and Kokhanovsky, 2007;Aoki et al, 2000). Among the available optical models for non-spherical grains are for example those of Baum et al (2011Baum et al ( , 2014 and Yang et al (2013). Incidentally, Räisänen et al (2015) recently attempted to fit measured phase functions of blowing snow by using phase functions of several complex habits provided by Yang et al (2013) in addition to phase functions of other habits.…”