Optical Properties Of Ice Crystals Research Articles

The Use of Superspheroids as Surrogates for Modeling Electromagnetic Wave Scattering by Ice Crystals

Electromagnetic wave scattering by ice particles is commonly modeled by defining representative habits, including droxtals, columns, plates, and aggregates, although actual particles in the atmosphere can be even much more complex. In this study, we examined a superspheroidal approximation method for modeling electromagnetic wave scattering by ice crystals. Superspheroid can be associated with a shape index (SI) defined by the particle volume and average projected area. Corresponding to realistic ice crystals, suitable superspheroid models with the same SI (that means, identical volume and average projected area) and aspect ratio can be identified as surrogates for optical property calculations. We systematically compared the optical properties of ice crystals and superspheroids at 33 microwave bands in the range of 3–640 GHz and at three representative visible or infrared wavelengths (0.66, 2.13, and 11 μm). It was found that the single-scattering properties of compact ice crystal habits and their superspheroidal model particles were quite close. For an aggregate with sparse distribution of elements, a superspheroid model produces relatively large errors because the aspect ratio may not be sufficient to describe a particle shape. However, the optical similarity of a superspheroid and an aggregate is still encouraging.

A Flexible Parameterization for Shortwave and Longwave Optical Properties of Ice Crystals and Derived Bulk Optical Properties for Climate Models

AbstractWe provide a parameterization of the extinction efficiency, single-scattering albedo, and asymmetry parameter of single ice crystals with any combination of particle volume, projected area, component aspect ratio, and crystal distortion at any wavelength between 0.2 and 100 μm. The parameterization is an extension of the one previously published by van Diedenhoven et al. In addition, the parameterized optical properties are integrated over size distributions yielding bulk extinction efficiencies, single-scattering albedos, and asymmetry parameters for large ranges of effective radii, particle component aspect ratios, and crystal distortion values. The parameterization of single-particle optical properties is evaluated with a reference database. The bulk optical properties are evaluated against the ice model selected for the Moderate Resolution Imaging Spectroradiometer (MODIS) collection 6 products, for which accurate optical properties are available. Mean absolute errors in parameterized extinction efficiency, asymmetry parameter, and single-scattering albedo are shown to be 0.0272, 0.008 90, and 0.004 68, respectively, for shortwave wavelengths, while they are 0.0641, 0.0368, and 0.0200 in the longwave. Shortwave and longwave asymmetry parameters and single-scattering albedos are shown to vary strongly with particle component aspect ratio and distortion, resulting in substantial variation in shortwave fluxes, but relatively small variations in longwave cloud emissivity. The parameterization and bulk optical properties are made publicly available.

New Optical Properties of Ice Crystals for Multiclass Cloud Microphysics

AbstractNew optical properties of ice crystals were implemented in a general circulation model. Ebert and Curry's (1992, https://doi.org/10.1029/91JD02472) and Fu's (1996, https://doi.org/10.1175/1520‐0442(1996)009<2058:AAPOTS>2.0.CO;2) parameterizations are widely used in the general circulation model radiation scheme. However, the validity of the data is limited to an effective radius (size) of 130 or 140 μm. This limit exceeds the median of the effective radius of snow (~200 μm), which is computed from the multiclass cloud microphysics scheme. A comparable amount of snow exists in the upper atmosphere near the lower part of ice clouds. Although the cross section of snow (extinction probability) is smaller than cloud ice, its impact on radiation has not been properly considered so far. We constructed a new lookup table of optical properties based on recent data for an extended effective radius up to 500 μm. The old data (<130 μm) also need to be updated with a more recent and elaborate method. We processed extensive data provided by Yang et al. (2013, https://doi.org/10.1175/JAS‐D‐12‐039.1) and linked to the radiation module of the Korean Integrated Model (KIM). The new optical properties change the shortwave scattering features, thus increasing the heating rate. This tendency is checked in both one‐dimensional and global weather forecast simulations. With the new optical properties, KIM shows better performance by reducing bias with respect to the Integrated Forecasting Systems (IFS) analysis data.

Vertical distribution of microphysical properties of Arctic springtime low-level mixed-phase clouds over the Greenland and Norwegian seas

Abstract. This study aims to characterize the microphysical and optical properties of ice crystals and supercooled liquid droplets within low-level Arctic mixed-phase clouds (MPCs). We compiled and analyzed cloud in situ measurements from four airborne spring campaigns (representing 18 flights and 71 vertical profiles in MPCs) over the Greenland and Norwegian seas mainly in the vicinity of the Svalbard archipelago. Cloud phase discrimination and representative vertical profiles of the number, size, mass and shape of ice crystals and liquid droplets are established. The results show that the liquid phase dominates the upper part of the MPCs. High concentrations (120 cm−3 on average) of small droplets (mean values of 15 µm), with an averaged liquid water content (LWC) of 0.2 g m−3 are measured at cloud top. The ice phase dominates the microphysical properties in the lower part of the cloud and beneath it in the precipitation region (mean values of 100 µm, 3 L−1 and 0.025 g m−3 for diameter, particle concentration and ice water content (IWC), respectively). The analysis of the ice crystal morphology shows that the majority of ice particles are irregularly shaped or rimed particles; the prevailing regular habits found are stellars and plates. We hypothesize that riming and diffusional growth processes, including the Wegener–Bergeron–Findeisen (WBF) mechanism, are the main growth mechanisms involved in the observed MPCs. The impact of larger-scale meteorological conditions on the vertical profiles of MPC properties was also investigated. Large values of LWC and high concentration of smaller droplets are possibly linked to polluted situations and air mass origins from the south, which can lead to very low values of ice crystal size and IWC. On the contrary, clean situations with low temperatures exhibit larger values of ice crystal size and IWC. Several parameterizations relevant for remote sensing or modeling studies are also determined, such as IWC (and LWC) – extinction relationship, ice and liquid integrated water paths, ice concentration and liquid water fraction according to temperature.

Disk and circumsolar radiances in the presence of ice clouds

Abstract. The impact of ice clouds on solar disk and circumsolar radiances is investigated using a Monte Carlo radiative transfer model. The monochromatic direct and diffuse radiances are simulated at angles of 0 to 8° from the center of the sun. Input data for the model are derived from measurements conducted during the 2010 Small Particles in Cirrus (SPARTICUS) campaign together with state-of-the-art databases of optical properties of ice crystals and aerosols. For selected cases, the simulated radiances are compared with ground-based radiance measurements obtained by the Sun and Aureole Measurements (SAM) instrument. First, the sensitivity of the radiances to the ice cloud properties and aerosol optical thickness is addressed. The angular dependence of the disk and circumsolar radiances is found to be most sensitive to assumptions about ice crystal roughness (or, more generally, non-ideal features of ice crystals) and size distribution, with ice crystal habit playing a somewhat smaller role. Second, in comparisons with SAM data, the ice cloud optical thickness is adjusted for each case so that the simulated radiances agree closely (i.e., within 3 %) with the measured disk radiances. Circumsolar radiances at angles larger than ≈ 3° are systematically underestimated when assuming smooth ice crystals, whereas the agreement with the measurements is better when rough ice crystals are assumed. Our results suggest that it may well be possible to infer the particle roughness directly from ground-based SAM measurements. In addition, the results show the necessity of correcting the ground-based measurements of direct radiation for the presence of diffuse radiation in the instrument's field of view, in particular in the presence of ice clouds.

On the Use of Scattering Kernels to Calculate Ice Cloud Bulk Optical Properties

AbstractNonspherical ice crystal optical properties are of fundamental importance to atmospheric radiative transfer through an ice cloud and the remote sensing of its properties. In practice, the optical properties of individual ice crystals need to be integrated over particle size distributions to derive the bulk optical properties of ice clouds. Given a particle size distribution represented in terms of size bins, the conventional approach uses the microphysical and optical properties of ice crystals at the bin centers as approximations to the bin-averaged values. However, errors are incurred when the size bins are large. To reduce the potential errors, a kernel technique is utilized to calculate the bulk optical properties of ice clouds by computing the bin-averaged values instead of using the bin-center values. Comparisons between the solutions based on the conventional method and the kernel technique for different numbers of size bins from in situ measurements demonstrate that the results computed from the kernel technique are more accurate. The present study illustrates that, for a given size distribution, 40 or more size bins should be used to calculate the bulk optical properties of ice clouds by the conventional method. Although the accuracy of bulk-scattering properties can be improved by using fine bin resolutions in the single-scattering property computation, the advantage of using a precomputed database of scattering kernels allows efficient computation of ice cloud bulk optical properties without losing the accuracy.

Computation of Solar Radiative Fluxes by 1D and 3D Methods Using Cloudy Atmospheres Inferred from A-train Satellite Data.

This study used realistic representations of cloudy atmospheres to assess errors in solar flux estimates associated with 1D radiative transfer models. A scene construction algorithm, developed for the EarthCARE mission, was applied to CloudSat, CALIPSO and MODIS satellite data thus producing 3D cloudy atmospheres measuring 61 km wide by 14,000 km long at 1 km grid-spacing. Broadband solar fluxes and radiances were then computed by a Monte Carlo photon transfer model run in both full 3D and 1D independent column approximation modes. Results were averaged into 1,303 (50 km)2 domains. For domains with total cloud fractions A c < 0.7 top-of-atmosphere (TOA) albedos tend to be largest for 3D transfer with differences increasing with solar zenith angle. Differences are largest for A c > 0.7 and characterized by small bias yet large random errors. Regardless of A c, differences between 3D and 1D transfer rarely exceed ±30 W m−2 for net TOA and surface fluxes and ±10 W m−2 for atmospheric absorption. Horizontal fluxes through domain sides depend on A c with ∼20% of cases exceeding ±30 W m−2; the largest values occur for A c > 0.7. Conversely, heating rate differences rarely exceed ±20%. As a cursory test of TOA radiative closure, fluxes produced by the 3D model were averaged up to (20 km)2 and compared to values measured by CERES. While relatively little attention was paid to optical properties of ice crystals and surfaces, and aerosols were neglected entirely, ∼30% of the differences between 3D model estimates and measurements fall within ±10 W m−2; this is the target agreement set for EarthCARE. This, coupled with the aforementioned comparison between 3D and 1D transfer, leads to the recommendation that EarthCARE employ a 3D transport model when attempting TOA radiative closure.

Properties of cirrus and subvisible cirrus from nighttime Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), related to atmospheric dynamics and water vapor

We map cirrus and subvisible cirrus clouds (SVC, optical depth <0.03) on a global scale, detecting optically thin clouds in 2.5 years of Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) spaceborne lidar observations. Cirrus clouds are mostly concentrated around strong convection areas in the tropics (cloud fractions (CF) 50%-60%, up to 90%), while SVC spread over higher latitudes (CF 30%-40%). We document cloud properties (geometrical thickness, top altitude, and midlayer temperature) and ice crystal depolarization ratios. SVC are thin (<1 km), are 2°C-3°C colder than cirrus clouds, and produce depolarization ratio lower by 0.03 on average, suggesting that the shapes of their crystals deviate from other cirrus clouds. We investigate correlations between retrieved properties and vertical and horizontal wind speed from European Centre for Medium-Range Weather Forecasts reanalyses characterizing vertical air motions in the tropics and jet streams in midlatitudes. In the tropics, cloud occurrence is correlated with vertical motions: cirrus CF goes from 5%-15% in subsidence to 30%-50% in updraft conditions, where clouds are 0.6 km thicker, ~1 km higher, and ~3° colder than in subsidence. In updraft conditions, cirrus CF is double the SVC CF (15%-25%). Optical properties of ice crystals do not change with vertical motions. In midlatitudes, horizontal winds faster than 30 m/s lead to higher CF, clouds ~8°C warmer (i.e., 1.8 km lower), and particulate depolarization ratio 0.1 lower. Changes in wind speeds affect SVC and cirrus clouds alike. Where CALIOP detects cirrus and SVC clouds, upper tropospheric water vapor concentrations from collocated MLS observations increase by 15-30 ppmv (cirrus) and 5-10 ppmv (SVC). Copyright 2011 by the American Geophysical Union.

Parameterization of single scattering properties of mid‐latitude cirrus clouds for fast radiative transfer models using particle mixtures

A new parameterization of single scattering properties has been developed for mid latitude cirrus clouds for application to weather prediction and global circulation models. Ice clouds are treated as a mixture of ice crystals with various habits. The bulk optical properties of ice crystals are parameterized as functions of the effective dimension (De) of measured particle size distributions that are representative of mid latitude cirrus clouds.Deis itself parameterized as a function of temperature and ice water content. The paper describes the results obtained with a stand‐alone version of the radiation routine of the COSMO model and preliminary tests with the full forecast model in various meteorological situations characterized by extended high cirrus clouds over Europe.

Impact of a new scheme for optical properties of ice crystals on climates of two GCMs

A new parameterization scheme for the optical properties of nonspherical ice crystals is implemented in two global climate models. The scheme is based on polynomial fits to the results from a more elaborate scheme recently developed by the third author. The scheme assumes a temperature dependency of ice crystal size and is able to handle five different crystal shapes. We first carry out sensitivity experiments highlighting the direct (“noninteractive”) forcing effect of the new scheme for different assumptions on ice crystal shape and size. It is found that the effects of crystal shape and size are of comparable importance in determining the radiative fluxes at the top of the atmosphere. We have further conducted multiyear simulations with full dynamic coupling. In these experiments we have chosen planar polycrystals as our basic crystal shape. Compared to control integrations with columns in one case and 30 μm spheres in the other, we find a reduction in the cold biases of the models in the upper tropical troposphere. This is shown to be mainly due to enhanced longwave radiative warming there, caused by the presence of smaller ice crystals. The upper tropospheric warming affects tropical convection in such a way that moisture and ice water distributions are modified. Consequently, relative humidity biases are somewhat reduced in both models. In one of the models there is a significant improvement in the longwave radiative budget at the top of the atmosphere, while in the other, the radiative budget remains in fairly close agreement with observations.

A new parameterization scheme for the optical properties of ice crystals for use in general circulation models of the atmosphere

Abstract We present a new parameterization scheme for the optical properties of ice crystals, for use in general circulation models of the atmosphere (GCMs). The scheme consists of separate parameterizations for five different ice crystals shapes: hexagonal columns, hexagonal plates, bullet rosettes, spheres and planar polycrystals. The optical properties are specified as functions of the mean maximum dimension of the ice crystal size distribution using polynomial fits to a more elaborate scheme developed by the last author. The present scheme, based on 10th order fits, gives excellent accuracy and is efficient enough for use in GCMs. First, the scheme is tested in 1-dimensional models, showing a large sensitivity of albedo and emissivity to both ice crystal sizes and shapes. Also, long- and shortwave cloud radiative forcings at the top of the atmosphere are quite sensitive to those two parameters, in such a manner that the two effects do not cancel. It is shown that this may have a significant impact on climate sensitivity, through variations in net cloud forcing.

Aerodynamic and optical properties of ice crystals

Optical and aerodynamic properties of ice crystals were investigated in a cold chamber. The following instruments were used: single-channel photometers for determining the extinction of visible and i.r. laser radiation, a scanning device for measuring the angular distribution of scattered radiation in the visible and i.r. bands, a photoelectric polarization device for measuring the concentration of crystal particles and a remote control sampler. Sedimentation velocities, thickness and orientation of crystals, the asymmetry in the scattering pattern in two orthogonal planes, the angular distribution of scattered radiation and coefficients of radiation extinction by crystal media were determined.