Assessment of the accuracy of the conventional ray-tracing technique: Implications in remote sensing and radiative transfer involving ice clouds

Abstract

A fundamental problem in remote sensing and radiative transfer simulations involving ice clouds is the ability to compute accurate optical properties for individual ice particles. While relatively simple and intuitively appealing, the conventional geometric-optics method (CGOM) is used frequently for the solution of light scattering by ice crystals. Due to the approximations in the ray-tracing technique, the CGOM accuracy is not well quantified. The result is that the uncertainties are introduced that can impact many applications. Improvements in the Invariant Imbedding T-matrix method (II-TM) and the Improved Geometric-Optics Method (IGOM) provide a mechanism to assess the aforementioned uncertainties. The results computed by the II-TMþ IGOM are considered as a benchmark because the II-TM solves Maxwell's equations from first principles and is applicable to particle size parameters ranging into the domain at which the IGOM has reasonable accuracy. To assess the uncertainties with the CGOM in remote sensing and radiative transfer simulations, two independent optical property datasets of hexagonal columns are developed for sensitivity studies by using the CGOM and the II-TMþ IGOM, respectively. Ice cloud bulk optical properties obtained from the two datasets are compared and subsequently applied to retrieve the optical thickness and effective diameter from Moderate Resolution Imaging Spectroradiometer (MODIS) measurements. Additionally, the bulk optical properties are tested in broadband radiative transfer (RT) simulations using the general circulation model (GCM) version of the Rapid Radiative Transfer Model (RRTMG) that is adopted in the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM, version 5.1). For MODIS retrievals, the mean bias of uncertainties of applying the CGOM in shortwave bands (0.86 and 2.13 μm) can be up to 5% in the optical thickness and as high as 20% in the effective diameter, depending on cloud optical thickness and effective diameter. In the MODIS infrared window bands centered at 8.5, 11, and 12 μm, biases in the optical thickness and effective diameter are up to 12% and 10%, respectively. The CGOM-based simulation errors in ice cloud radiative forcing calculations are on the order of 10 W m À 2 .

Cloud thermodynamic phase detection with polarimetrically sensitive passive sky radiometers

“…Figure 6 shows P 12 of drops and ice crystals with varying aspect ratios. Here, variations in ice size are not considered since ice crystals are generally large enough for their scattering properties to be simulated with geometric optics (Bi et al, 2014), and thus size has a minimal impact on P 12 . Note, however, that aspect ratio could depend on size (Auer Jr and Veal, 1970;Um et al, 2014).…”

On Averaging Aspect Ratios and Distortion Parameters over Ice Crystal Population Ensembles for Estimating Effective Scattering Asymmetry Parameters

“…This study focuses on the calculation and retrieval of effective asymmetry parameters for particles that are large enough for geometric optics to apply, which is generally assumed to be true for size parameters larger than about 100 (Bi et al 2014). Small ice crystals are generally found to be compact and thus variation in crystal geometry is of less importance (Baum et al 2011).…”

“…Small ice crystals are generally found to be compact and thus variation in crystal geometry is of less importance (Baum et al 2011). Effective extinction coefficients and single scattering albedos are not considered here, since, for size parameters larger than about 100, extinction coefficients are close to 2 and independent of particle geometry (e.g., Bi et al 2014), while single scattering albedos only minimally depend on particle geometry and are not affected by crystal distortion (van Diedenhoven et al 2014a). Finally, although an evaluation of effective phase functions from ensemble-average aspect ratio and distortion parameter is beyond the scope of this paper, of primary concern for such effective phase functions is the accuracy of their asymmetry parameters.…”

Formation of atmospheric halos and applicability of geometric optics for calculating single-scattering properties of hexagonal ice crystals: Impacts of aspect ratio and ice crystal size

“…More sophisticated versions of GOM have been developed to overcome drawbacks of conventional GOMs, such as the discontinuity of scattered energy in the far field, the use of a localization principle, and the use of Kirchhoff boundary conditions for formulating diffraction [71][72][73][74]. Although several studies have estimated the low size limit of conventional GOM (e.g., [63,75]), the range of applicability of GOM has not been well established and should be determined through comparison against numerically exact methods [76]. This is an important distinction because numerically exact solutions, such as T-matrix, FDTD, PSTD, and discrete dipole approximation (DDA), are computationally very expensive in the geometric optics region and, thus, the use of GOMs for large nonspherical particles is unavoidable.…”