Ice Crystals In Cirrus Clouds Research Articles

Backscattering Characteristics of Optical and Electromagnetic Waves in Joint Sensing of Cirrus Clouds by a Polarizing Lidar (0.355 µm) and a 94-GHz Radar

In this work, the problem of radiation scattering by ice crystals typical for cirrus clouds is solved for a 94-GHz radar (wavelength 3189 µm) and a lidar (wavelength 0.355 µm); the ice refractive indices are 1.7864 + 0.0032i and 1.3249 + 0i, respectively. The scattering matrices are calculated within the physical optics approximation and the discrete dipole approximation for the case of randomly oriented particles with sizes from 4 to 1000 µm. The ratio of the radar and lidar backscattering signals in the backward direction (the so-called radar–lidar ratio) is calculated for a wide range of the particle size for typical shapes of cirrus cloud ice crystals. It is shown that this ratio can be used for estimating the size of ice crystals in cirrus clouds.

Destruction of Stratospheric Ozone: Role of Aerosolized Coal Fly Ash Iron

We have previously provided compelling evidence that aerosolized coal fly ash particles, not chlorofluorocarbons (CFC’s) are the primary cause of stratospheric ozone depletion [European Journal of Applied Sciences, 2022, 10(3), 586-603]. Here we turn our attention to iron, one of the most reactive elements among the others in coal fly ash that destroy ozone.  Iron in primary and secondary aerosols plays a crucial role in the formation of ice crystals in cirrus clouds and in the polar stratospheric clouds that are involved in ozone hole formation. Iron is associated with reactive oxygen species, like the hydroxyl radical (OH) that destroys ozone in the stratosphere. Iron is known to activate halogens including chlorine, bromine, and iodine. Ozone is destroyed by adsorption onto iron-containing particles and by photochemical reactions with other constituents of coal fly ash. The inability of the scientific community to break out of the CFC paradigm and address the true cause of stratospheric ozone depletion is a potentially fatal mistake. We already face a ghastly future with severely depleted stratospheric ozone levels and devastation from deadly ultraviolet radiation. All sources of coal fly ash aerosols must be reduced or eliminated; first and foremost, the deliberate, covert, and near global jet-spraying of coal fly ash particulates into the upper troposphere.

Edge effect correction formula for superspheroids using the Debye series.

Accurate quantification of the effects of nonspherical particles (e.g., ice crystals in cirrus clouds and dust aerosol particles) on the radiation budget in the atmosphere-earth coupled system requires a robust characterization of their light scattering and absorption properties. Recent studies have shown that it is feasible to compute the single-scattering properties of all sizes of arbitrary nonspherical atmospheric particles by combining the numerically exact invariant imbedding T-matrix (IITM) method and the approximate physical geometric optics method (PGOM). IITM cannot be implemented for very large-sized particles due to its tremendous demand on computational resources. While either method is usable for moderate sized particles, PGOM does not include the edge effect contributions to the extinction and absorption efficiencies. Unfortunately, we can only rigorously calculate the edge effect contributions to the extinction and absorption efficiencies for spheres and spheroids. This study develops empirical formulas for the edge effect contributions to the extinction and absorption efficiencies in the case of a special superspheroid called a superegg by modifying the formulas for the extinction and absorption efficiencies of a spheroid to account for the changes in roundness. We use the superegg edge effect correction formulas to compare the optical properties of supereggs and simple, convex particles, as an initial approximation to more complex atmospheric aerosols. This study is the first step towards quantifying the edge effect contributions to the extinction and absorption efficiencies of a wide range of natural nonspherical particles.

Radar-lidar ratio for ice crystals of cirrus clouds.

Simultaneous measurement of lidar and radar signals returned from the same cirrus clouds is a prospective method for retrieving the cloud microphysics, i.e. size and shape of the ice crystals constituting the clouds. In this study, the ratio of the backscattered signals of lidar and radar called the radar-lidar ratio has been calculated for the first time for typical shapes of ice crystals and wide distribution of the crystals over their sizes. It is shown that it is the lidar-radar ratio that is most sensitive to crystal sizes while the lidar depolarization ratio is most sensitive to crystal shapes.

Light Backscattering by Ice Crystals of Cirrus Clouds Within the Physical Optics Approximation

Lidar technologies are widely used for retrieving microphysics of cirrus clouds, i.e. sizes, shapes and spatial orientation of ice crystals constituting the clouds. Interpretation of the lidar signals are based on the backscattered light. However, properties of the light backscattered by the ice crystals have not been well known and understood yet. The reason of this is that the problem of light scattering by the ice crystals much larger that incident wavelengths has not been solved satisfactorily yet because of great demands to computer resources. In this contribution we review the physical optics approximation as a prospective method to solve the light scattering problem on large nonspherical particles in lidar application.

Comparison of Modeled and Measured Ice Nucleating Particle Composition in a Cirrus Cloud

Abstract The contribution of heterogeneous ice nucleation to the formation of cirrus cloud ice crystals is still not well quantified. This results in large uncertainties when predicting cirrus radiative effects and their role in Earth’s climate system. The goal of this case study is to simulate the composition, and thus activation conditions, of ice nucleating particles (INPs) to evaluate their contribution to heterogeneous cirrus ice formation in relation to homogeneous ice nucleation. For this, the regional model COSMO—Aerosols and Reactive Trace Gases (COSMO-ART) was used to simulate a synoptic cirrus cloud over Texas on 13 April 2011. The simulated INP composition was then compared to measured ice residual particle (IRP) composition from the actual event obtained during the NASA Midlatitude Airborne Cirrus Properties Experiment (MACPEX) aircraft campaign. These IRP measurements indicated that the dominance of heterogeneous ice nucleation was mainly driven by mineral dust with contributions from a variety of other particle types. Applying realistic activation thresholds and concentrations of airborne transported mineral dust and biomass-burning particles, the model implementing the heterogeneous ice nucleation parameterization scheme of Ullrich et al. is able to reproduce the overall dominating ice formation mechanism in contrast to the model simulation with the scheme of Phillips et al. However, the model showed flaws in reproducing the IRP composition.

Power laws for backscattering by ice crystals of cirrus clouds.

The backscattering Mueller matrix for the typical shapes of ice crystals of cirrus (hexagonal columns and plates, bullets and droxtals) in the case of their random orientations has been approximately presented as the power functions of the crystal size from 10 µm to 1000 µm. The coefficients of the power functions have been found. Four commonly used backscatter ratios used in lidar research (depolarization, lidar, color and backscatter-to-IWC ratios) have been calculated for the first time as the functions of the modal crystal size at the conventional lidar wavelengths of 0.355, 0.532 and 1.064 µm.

Backscatter ratios for nonspherical ice crystals in cirrus clouds calculated by geometrical-optics-integral-equation method

Backscatter ratios for two kinds of ice crystal models, an irregularly shaped Voronoi aggregates (VA) and a regularly shaped hexagonal columns (COL), are calculated using the geometrical-optics-integral-equation (GOIE) method for volume equivalent sphere radii (Req) ranging from 2.8 to 202.0μm. Ice crystals are assumed to be randomly oriented in three-dimensional (3D) space. Characteristics of the depolarization ratio (δ) and the lidar ratio (L) at a wavelength of 0.532μm and the color ratio (χ) at a wavelength pair of 1.064 and 0.532μm are investigated.The backscatter ratios calculated for VA are generally larger than those for COL. For example, L, δ, and χ for VA are approximately 50~60, 0.5~0.6, and 0.8~0.9, respectively, in the range of Req>10μm; 4~15, 0.2~0.3, and 0.45~0.5 for COL. Effects of deformation of COL on the backscatter ratios are examined by using hexagonal columns with hollowness. It is shown that the L, δ, and χ values significantly increase due to the hollowness. Furthermore, the degree of increase of the backscatter ratios is considerably sensitive to the crystal size and the depth of hollow. On the other hand, backscatter ratios calculated for VA show rather steady values with respect to the number and size of facets.

Light scattering by ice crystals of cirrus clouds: From exact numerical methods to physical-optics approximation

The problem of light scattering by ice crystals of cirrus clouds is considered in the case of a hexagonal ice plate with different distributions over crystal orientations. The physical-optics approximation based on (E, M)-diffraction theory is compared with two exact numerical methods: the finite difference time domain (FDTD) and the discontinuous Galerkin time domain (DGTD) in order to estimate its accuracy and limits of applicability. It is shown that the accuracy of the physical-optics approximation is estimated as 95% for the averaged backscattering Mueller matrix for particles with size parameter more than 120. Furthermore, the simple expression that allows one to estimate the minimal number of particle orientations required for appropriate spatial averaging has been derived.

Backscatter by azimuthally oriented ice crystals of cirrus clouds.

The backscattering Mueller matrix has been calculated for the first time for the hexagonal ice columns and plates with both zenith and azimuth preferential orientations. The possibility of a vertically pointing polarization lidar measuring the full Mueller matrix for retrieving the orientation distributions of the crystals is considered. It is shown that the element m44 or, equivalently, the circular depolarization ratio distinguishes between the low and high zenith tilts of the crystals. Then, at their low or high zenith tilts, either the element m22 or m34, respectively, should be measured to retrieve the azimuth tilts.

Light scattering by ice crystals of cirrus clouds: comparison of the physical optics methods

The physical optics approximations are derived from the Maxwell equations. The scattered field equations by Kirchhoff, Stratton-Chu, Kottler and Franz are compared and discussed. It is shown that in the case of faceted particles, these equations reduce to a sum of the diffraction integrals, where every diffraction integral is associated with one plane–parallel optical beam leaving a particle facet. In the far zone, these diffraction integrals correspond to the Fraunhofer diffraction patterns. The paper discusses the E-, M- and (E, M)-diffraction theories as applied to ice crystals of cirrus clouds. The comparison to the exact solution obtained by the discontinuous Galerkin time domain method shows that the Kirchhoff diffraction theory is preferable.

The technique for solving the problem of light backscattering by ice crystals of cirrus clouds by the physical optics method for a lidar with zenith scanning

The technique for solving the problem of light backscattering by the physical optics method is considered. Recommendations on carrying out a preliminary estimation of the contribution of geometrical optics beams are given to reduce the list of beams that are necessary for the calculation by a factor of hundreds. The presented empirical estimating formulas and recommendations on choosing the optimum step of numerical integration make it possible to considerably reduce the resource intensity of the physical optics method for specified microphysical models of hexagonal crystalline particles. The obtained results of solving the light scattering problem are freely available in the form of a databank of Mueller matrices.

Методика решения задачи рассеяния света на ледяных кристаллах перистых облаков в направлении рассеяния назад методом физической оптики для лидара с зенитным сканированием

Методика решения задачи рассеяния света на ледяных кристаллах перистых облаков в направлении рассеяния назад методом физической оптики для лидара с зенитным сканированием

Interference phenomena at backscattering by ice crystals of cirrus clouds.

It is shown that light backscattering by hexagonal ice crystals of cirrus clouds is formed within the physical-optics approximation by both diffraction and interference phenomena. Diffraction determines the angular width of the backscattering peak and interference produces the interference rings inside the peak. By use of a simple model for distortion of the pristine hexagonal shape, we show that the shape distortion leads to both oscillations of the scattering (Mueller) matrix within the backscattering peak and to a strong increase of the depolarization, color, and lidar ratios needed for interpretation of lidar signals.

Limits to applicability of geometrical optics approximation to light backscattering by quasihorizontally oriented hexagonal ice plates

Quasihorizontally oriented ice crystals of cirrus clouds became an object of active study in recent times. Experimental observations are made with the use of multiwavelength and polarization lidars; their signals are interpreted on the basis of solutions obtained in the approximation of physical or geometrical optics. In this work, we compare these approximations for solution of the problem of light backscattering by quasihorizontally oriented hexagonal ice plates. Special attention is paid to the limits to applicability of geometrical optics approach to solution of such problems.

Backscatter ratios for arbitrary oriented hexagonal ice crystals of cirrus clouds

Three dimensionless ratios widely used for interpretation of lidar signals, i.e., the color ratio, lidar ratio, and depolarization ratio, have been calculated for hexagonal ice crystals of cirrus clouds as functions of their spatial orientation. The physical-optics algorithm developed earlier by the authors is applied. It is shown that these ratios are minimal at the horizontal crystal orientation. Then these quantities increase with the effective tilt angle approaching the asymptotic values of the random particle orientation. The values obtained are consistent with the available experimental data.

Layers of quasi-horizontally oriented ice crystals in cirrus clouds observed by a two-wavelength polarization lidar.

Layers of quasi-horizontally oriented ice crystals in cirrus clouds are observed by a two-wavelength polarization lidar. These layers of thickness of several hundred meters are identified by three attributes: the backscatter reveals a sharp ridge while the depolarization ratio and color ratio become deep minima. These attributes have been justified by theoretical calculations of these quantities within the framework of the physical-optics approximation.

Comparison of ice cloud properties simulated by the Community Atmosphere Model (CAM5) with in-situ observations

Abstract. Detailed measurements of ice crystals in cirrus clouds were used to compare with results from the Community Atmospheric Model Version 5 (CAM5) global climate model. The observations are from two different field campaigns with contrasting conditions: Atmospheric Radiation Measurements Spring Cloud Intensive Operational Period in 2000 (ARM-IOP), which was characterized primarily by midlatitude frontal clouds and cirrus, and Tropical Composition, Cloud and Climate Coupling (TC4), which was dominated by anvil cirrus. Results show that the model typically overestimates the slope parameter of the exponential size distributions of cloud ice and snow, while the variation with temperature (height) is comparable. The model also overestimates the ice/snow number concentration (0th moment of the size distribution) and underestimates higher moments (2nd through 5th), but compares well with observations for the 1st moment. Overall the model shows better agreement with observations for TC4 than for ARM-IOP in regards to the moments. The mass-weighted terminal fall speed is lower in the model compared to observations for both ARM-IOP and TC4, which is partly due to the overestimation of the size distribution slope parameter. Sensitivity tests with modification of the threshold size for cloud ice to snow autoconversion (Dcs) do not show noticeable improvement in modeled moments, slope parameter and mass weighed fall speed compared to observations. Further, there is considerable sensitivity of the cloud radiative forcing to Dcs, consistent with previous studies, but no value of Dcs improves modeled cloud radiative forcing compared to measurements. Since the autoconversion of cloud ice to snow using the threshold size Dcs has little physical basis, future improvement to combine cloud ice and snow into a single category, eliminating the need for autoconversion, is suggested.

Numerical Modeling of Ice Fog in Interior Alaska Using the Weather Research and Forecasting Model

An ice microphysics parameterization scheme has been modified to better describe and understand ice fog formation. The modeling effort is based on observations in the Sub-Arctic Region of Interior Alaska, where ice fog occurs frequently during the cold season due to abundant water vapor sources and strong inversions existing near the surface at extremely low air temperatures. The microphysical characteristics of ice fog are different from those of other ice clouds, implying that the microphysical processes of ice should be changed in order to generate ice fog particles. Ice fog microphysical characteristics were derived with the NCAR Video Ice Particle Sampler during strong ice fog cases in the vicinity of Fairbanks, Alaska, in January and February 2012. To improve the prediction of ice fog in the Weather Research and Forecasting model, observational data were used to change particle size distribution properties and gravitational settling rates, as well as to implement a homogeneous freezing process. The newly implemented homogeneous freezing process compliments the existing heterogeneous freezing scheme and generates a higher number concentration of ice crystals than the original Thompson scheme. The size distribution of ice crystals is changed into a Gamma distribution with the shape factor of 2.0, using the observed size distribution. Furthermore, gravitational settling rates are reduced for the ice crystals since the crystals in ice fog do not precipitate in a similar manner when compared to the ice crystals of cirrus clouds. The slow terminal velocity plays a role in increasing the time scale for the ice crystals to settle to the surface. Sensitivity tests contribute to understanding the effects of water vapor emissions as an anthropogenic source on the formation of ice fog.

Scattering of partially coherent electromagnetic beams by water droplets and ice crystals

The conventional Lorenz–Mie theory is generalized for a case when the light source is partially spatially coherent. The influence of the degree of coherence of the incident field on the generalized Mueller matrix and the spectral degree of coherence of the scattered light is analytically studied by using the vector field instead of the scalar field to extend previous results on the angular intensity distribution. The results are compared with the Mueller matrix obtained from the Discrete Dipole Approximation (DDA) method, which is an average over an ensemble of stochastic incident beams. Special attention is paid to the Mueller matrix elements in the backward direction, and the results show some Mueller matrix elements, such as P22, depend monotonically on the coherence length of the incident beam. Therefore, detecting back scattering Mueller matrix elements may be a promising method to measure the degree of coherence. The new formalism is applied to cases of large spherical droplets in water clouds and hexagonal ice crystals in cirrus clouds. The corona and glory phenomena due to spheres and halos associated with hexagonal ice crystals are found to disappear if the incident light tends to be highly incoherent.