Anthony J. Baran

A New Parameterization of Single Scattering Solar Radiative Properties for Tropical Anvils Using Observed Ice Crystal Size and Shape Distributions

Parameterizations of single scattering properties currently used in cloud resolving and general circulation models are somewhat limited in that they typically assume the presence of single particle habits, do not adequately account for the numbers of ice crystals with diameters smaller than 100 mm, and contain no information about the variance of parameterization coefficients. Here, new parameterizations of mean single scattering properties (e.g., single scatter albedo, asymmetry parameter, and extinction efficiency) for distributions of ice crystals in tropical anvils are developed. Using information about the size and shape of ice crystals acquired by a twodimensional cloud probe during the Central Equatorial Pacific Experiment (CEPEX), a self-organized neural network defines shape based on simulations of how the particle maximum dimension and area ratio (ratio of projected area to that of circumscribed circle with maximum dimension) vary for random orientations of different idealized shapes (i.e., columns, bullet rosettes, rough aggregates, and particles represented by Chebyshev polynomials). The size distributions for ice crystals smaller than 100 mm are based on parameterizations developed using representative samples of 11 633 crystals imaged by a video ice particle sampler (VIPS). The meanscattering properties for distributions of ice crystals are then determined by weighting the single scattering properties of individual ice crystals, determined using an improved geometric ray-tracing method, according to number concentration and scattering cross section. The featureless nature of the calculated phase function, averaged over all observed sizes and shapes of ice crystals, is similar to that obtained using other schemes designed to account for variations in sizes and shapes of ice crystals. The new parameterizations of single scatter albedo, asymmetry parameter, and extinction efficiency are then determined by functional fits in terms of cloud particle effective radius; there was no statistically significant dependence on either ice water content or temperature. Uncertainty estimates incorporated into the parameterization coefficients are based upon a Monte Carlo approach. Comparisons with previously used parameterizations and with parameterizations developed using single crystal habits are made to show that the determination of representative crystal habits is still a major unknown in the development of parameterization schemes.

Aircraft measurements of the solar and infrared radiative properties of cirrus and their dependence on ice crystal shape

We present aircraft measurements of the radiative transfer properties of thin cirrus cloud sampled off the east coast of Scotland on November 9, 1995. Downwelling radiances were measured from below the cirrus at 0.87, 1.61, 3.7, 8.55, and 11.0 μm, thereby covering a large range of size parameter and ice refractive index and enabling information on cirrus optical thickness, effective crystal size, and scattering phase function to be deduced. The sensitivity of these quantities to the ice crystal shape assumed in the calculations is examined, and the results are compared with in situ data. We find that a randomized polycrystal shape produces effective sizes that are consistent with the in situ data across all wavelengths considered and performs better in this respect than the other crystal shapes analyzed. However, the optical thicknesses retrieved from the 0.87 μm radiances for this shape are considerably less than those derived from the 11.0 μm data, implying that the phase function at solar wavelengths is in error for this shape over a significant portion of the full scattering angle range. An empirical phase function derived from laboratory measurements produces optical thicknesses which are more consistent with the 11.0 μm and in situ data and matches the angular distribution of scattered radiance more accurately than that calculated using any of the model crystal shapes. The anomalous diffraction approximation is found to produce good agreement with the measurements at 8.55 and 11.0 μm for the crystal sizes relevant to the present case study.

Sensitivity of retrieved POLDER directional cloud optical thickness to various ice particle models

The ”directional” values of cloud optical thickness (or cloud spherical albedo) retrieved from ADEOS-POLDER data constitute a strong constraint on the microphysical model used in the retrieval algorithm. In this paper, we focus on ice clouds. We quantify the departure of the directional values of spherical albedo from their averaged value. By so doing, we can assess the suitability of different ice particle scattering models (i.e., spheres, fractal polycrystal, inhomogeneous hexagonal monocrystal, or a synthesized phase function). The liquid water droplet and the ice fractal polycrystal models appear to be inappropriate since they induce a notable dependence of the ice cloud spherical albedo on scattering angle. On the contrary, much better agreement is achieved by using a synthesized phase function or an inhomegeneous hexagonal monocrystal model. Using these models, the retrieved ice cloud optical thickness is then found to be 40% smaller than using the droplet model.

Scattering of light from atmospheric ice analogues

New analogues replacing atmospheric ice crystals in light scattering measurements have been developed. These include thin, hexagonal glass fibres and both simple and complex microcrystals resembling cirrus ice, including columns, plates, rosettes and aggregates with a variety of sizes. Results of angle-dependent scattering measurements on the fibres and on levitated crystals are presented, including phase functions, polarization properties and the asymmetry parameter.

Testing the coherence of cirrus microphysical and bulk properties retrieved from dual‐viewing multispectral satellite radiance measurements

In this paper the coherence of retrieved cirrus microphysical and bulk properties using data from a satellite-based dual-viewing and multispectral instrument is tested using different ice crystal models. Radiance data from the dual-viewing Along Track Scanning Radiometer (ATSR-2) instrument is used to show that coherent retrievals are possible between nonabsorbing (visible) and infrared wavelengths if an appropriate ice crystal model is employed. The dominating crystal habit is estimated by finding the ice crystal model that best fits the dual-view 0.87 μm reflectance data. The ice crystal models tested are hexagonal plates, hexagonal columns, bullet-rosettes (six branched), and randomized polycrystals, all of which are assumed to be randomly oriented in space. Given the best fit crystal shape other cirrus properties, such as optical depth, crystal maximum dimension, and an estimate of ice water path, are retrieved by contrasting reflectance data at the wavelengths of 0.55 and 1.6 μm. To demonstrate probable retrieval errors in terms of optical depth and crystal maximur dimension, if the wrong crystal habit is applied, a tropical convective case in the western Pacific Ocean is used as a typical example. It is found that the more complex particles as represented by the bullet-rosette and randomized polycrystal best fit the ATSR-2 radiance data, while the pristine geometries represented by the hexagonal plate and column do not. These results indicate that phase functions that are relatively flat at backscattering angles should be employed in satellite remote sensing of cirrus. Moreover, if hexagonal plates or columns were assumed as the habit in the radiative transfer model, then this would lead to retrieval errors of about a factor of 2 for optical depth(overestimate) and crystal maximum dimension(underestimate). To test if the retrieved optical depth at the wavelength of 0.55 μm is coherent, the extinction optical depth at the wavelength of 10.8 μm is also retrieved to test for a one-to-one relationship between the two wavelengths. To validate this procedure the same test is applied to a near coincident aircraft and ATSR-2 midlatitude case study.

Calculation of the single-scattering properties of randomly oriented hexagonal ice columns: a comparison of the T-matrix and the finite-difference time-domain methods.

We calculated the scattering and absorption properties of randomly oriented hexagonal ice columns using T-matrix theory, employing analytic orientation averaging, and the finite-difference time-domain method, which uses a numerical procedure to simulate random orientation. The total optical properties calculated are the extinction efficiency, absorption efficiency, single-scattering albedo, and the asymmetry parameter. The optical properties are calculated at the wavelengths of 0.66, 8.5, and 12 mum, up to a size parameter of 20 at 0.66 mum and 15 at the two other wavelengths. The phase-matrix elements P11, P12, and P22 are also calculated and compared, up to a size parameter of 20 at 0.66 mum and 15 at 12.0 mum. The scattering and absorption solutions obtained from the two independent electromagnetic methods are compared and contrasted, as well as the central processing unit time and memory load for each size parameter. It is found that the total optical properties calculated by the two methods are well within 3% of each other for all three wavelengths and size parameters. In terms of the phase-matrix elements it is found that there are some differences between the T-matrix and the finite-difference time-domain methods appearing in all three elements. Differences between the two methods for the P11 element are seen particularly at scattering angles from approximately 120 degrees to 180 degrees ; and at the scattering angle of 180 degrees , relative differences are less than 16%. At scattering angles less than 100 degrees , agreement is generally within a few percent. Similar results are also found for the P12 and P22 elements of the phase matrix. The validity of approximating randomly oriented hexagonal ice columns by randomly oriented equal surface area circular cylinders is also investigated in terms of the linear depolarization ratio.

Retrieval of cirrus ice crystal sizes from 8.3 and 11.1 μm emissivities determined by the improved initialization inversion of TIROS-N Operational Vertical Sounder observations

The improved initialization inversion (31) algorithms convert TIROS-N Operational Vertical Sounder (TOVS) observations from the NOAA polar orbiting environmental satellites into atmospheric temperature and water vapor profiles as well as cloud and surface properties. Because of their relatively high spectral resolution, infrared vertical sounders are especially useful for the identification of cirrus clouds. Differences in cirrus emissivity between the wavelengths 8.3 and 11.1 μm are used to retrieve ice crystal size; the radiative transfer model is based on the anomalous diffraction approximation applied to different crystal morphologies. We present sensitivity studies of ice crystal sizes estimated on a global scale to uncertainty factors in the retrieval as well as to assumptions in the model. On average, cirrus ice crystal mean maximum dimensions lie between 80 and 150 μm or effective ice crystal sizes between 35 and 45 μm. Correlations between estimated cirrus ice crystal mean maximum dimensions and cloud-top temperature seem to be positive in the tropics and midlatitude winter but depend on assumed temperature-dependent ice crystal morphology and size distribution function. These estimates can be helpful for the evaluation of general circulation models. With satellite measurements, one estimates mean ice crystal sizes mostly on the top of the cirrus clouds. However, when the clouds are thinner, the IR sounder can reach deeper into the cloud. Yet a quantitative relation between cloud thickness (effective cloud emissivity) and retrieval height inside the cloud has still to be investigated.

Testing and Comparing the Modified Anomalous Diffraction Approximation

Abstract The modified anomalous diffraction approximation (MADA) is used to predict absorption and extinction in water and ice clouds, but it does not predict the scattering phase function or asymmetry parameter g. In conjunction with g parameterizations, it has been used in satellite remote sensing and to treat the radiative properties of ice clouds in global climate models. However, it has undergone only limited testing. This study 1) compares extinction efficiencies (Qext) predicted by MADA for a laboratory grown ice cloud against corresponding Qext measurements over the wavelength range 2–14 μm; 2) tests absorption efficiencies (Qabs) and Qext predicted by MADA against those predicted by T-matrix theory and the finite difference time domain (FDTD) method; and 3) compares MADA with three popular schemes used for predicting the radiative properties of cirrus clouds. In addition, the photon tunneling process may contribute up to 45% of the absorption in water clouds at some terrestrial wavelengths, but i...

A Coupled Cloud Physics–Radiation Parameterization of the Bulk Optical Properties of Cirrus and Its Impact on the Met Office Unified Model Global Atmosphere 5.0 Configuration

AbstractA new coupled cloud physics–radiation parameterization of the bulk optical properties of ice clouds is presented. The parameterization is consistent with assumptions in the cloud physics scheme regarding particle size distributions (PSDs) and mass–dimensional relationships. The parameterization is based on a weighted ice crystal habit mixture model, and its bulk optical properties are parameterized as simple functions of wavelength and ice water content (IWC). This approach directly couples IWC to the bulk optical properties, negating the need for diagnosed variables, such as the ice crystal effective dimension. The parameterization is implemented into the Met Office Unified Model Global Atmosphere 5.0 (GA5) configuration. The GA5 configuration is used to simulate the annual 20-yr shortwave (SW) and longwave (LW) fluxes at the top of the atmosphere (TOA), as well as the temperature structure of the atmosphere, under various microphysical assumptions. The coupled parameterization is directly compar...

Performance assessment of a five‐channel estimation‐based ice cloud retrieval scheme for use over the global oceans

[1] This work determines the performance of a five-channel ice cloud retrieval scheme in context of numerical synthetic experiments and real-world data and examines the implications of these results on the global retrieval of ice cloud microphysical properties over the global oceans. This estimation-based scheme, designed from information content principles, uses a rigorous, state-dependent error analysis to combine measurements from the visible, near-infrared, and infrared spectral regions. In the synthetic experiments, the five-channel scheme performed as well or better in terms of retrieval bias and random error than the traditional split-window and Nakajima and King bispectral retrieval techniques for all states of the atmosphere. Although the five-channel scheme performed favorably compared to the other methods, the inherently large uncertainties associated with ice cloud physics dictate typical retrieval uncertainties in both IWP and effective radius of 30–40%. These relatively large uncertainties suggest caution in the strict interpretation of small temporal or spatial trends found in existing cloud products. In MODIS and CRYSTAL-FACE applications, the five-channel scheme exploited the strengths of each of the bispectral approaches to smoothly transition from a split-window type approach for thin clouds to a Nakajima and King type approach for thick clouds. Uniform application of such a retrieval scheme across different satellite and field measurement campaigns would provide a set of consistent cloud products to the user community, theoretically allowing the direct comparison of cloud properties for the climate processes studies found throughout the literature.