ALBERT

Description: This study investigates and accounts for the influence of various ice cloud parameters on the retrieval of the surface solar radiation budget (SSRB) from reflected flux at the top of the atmosphere (TOA). The optical properties of ice clouds depend on ice crystal shape, size distribution, water content, and the vertical profiles of geometric and microphysical structure. As a result, the relationship between the SSRB and TOA-reflected flux for an ice cloud atmosphere is more complex and differs from that for water cloud and cloudless atmospheres. The sensitivities of the relationship between the SSRB and TOA-reflected flux are examined with respect to various ice cloud parameters. Uncertainties in the retrieval of the SSRB due to inadequate knowledge of various ice cloud parameters are evaluated thoroughly. The uncertainty study is concerned with both pure ice clouds and multiphase clouds (ice cloud above water cloud). According to the magnitudes of errors in the SSRB retrieval caused by different input variables, parameterized correction terms were introduced. If the input variables are known accurately, errors in the retrieval of the SSRB under a wide range of ice cloud conditions are expected to diminish substantially, to less than 10 W m−2 for 91% of the simulated ice cloud cases. In comparison, the same accuracy may be attained for only 19% of the retrievals for the same ice cloud cases using the retrieval algorithm designed for non-ice-cloud conditions.

Description: A method for combining ground-based passive microwave radiometer retrievals of integrated liquid water (LWP), radar reflectivity profiles (Z), and statistics of a cloud model is proposed for deriving cloud liquid water profiles (LWC). A dynamic cloud model is used to determine Z–LWC relations and their errors as functions of height above cloud base. The cloud model is also used to develop an LWP algorithm based on simulations of brightness temperatures of a 20–30-GHz radiometer. For the retrieval of LWC, the radar determined Z profile, the passive microwave retrieved LWP, and a model climatology are combined by an inverse error covariance weighting method. Model studies indicate that LWC retrievals with this method result in rms errors that are about 10%–20% smaller in comparison to a conventional LWC algorithm, which constrains the LWC profile exactly to the measured LWP. According to the new algorithm, errors in the range of 30%–60% are to be anticipated when profiling LWC. The algorithm is applied to a time series measurement of a stratocumulus layer at GKSS in Geesthacht, Germany. The GKSS 95-GHz cloud radar, a 20–30-GHz microwave radiometer, and a laser ceilometer were collocated within a 5-m radius and operated continuously during the measurement period. The laser ceilometer was used to confirm the presence of drizzle-sized drops.

Description: The interaction of clouds with solar and terrestrial radiation is one of the most important topics of climate research. In recent years it has been recognized that only a full three-dimensional (3D) treatment of this interaction can provide answers to many climate and remote sensing problems, leading to the worldwide development of numerous 3D radiative transfer (RT) codes. The international Intercomparison of 3D Radiation Codes (I3RC), described in this paper, sprung from the natural need to compare the performance of these 3D RT codes used in a variety of current scientific work in the atmospheric sciences. I3RC supports intercomparison and development of both exact and approximate 3D methods in its effort to 1) understand and document the errors/limits of 3D algorithms and their sources; 2) provide “baseline” cases for future code development for 3D radiation; 3) promote sharing and production of 3D radiative tools; 4) derive guidelines for 3D radiative tool selection; and 5) improve atmospheric science education in 3D RT. Results from the two completed phases of I3RC have been presented in two workshops and are expected to guide improvements in both remote sensing and radiative energy budget calculations in cloudy atmospheres.

Description: The roles of ice particle size distributions (SDs) and particle shapes in cirrus cloud solar radiative transfer are investigated by analyzing SDs obtained from optical array probe measurements (particle sizes larger than 20–40 μm) during intensive field observations of the International Cirrus Experiment, the European Cloud and Radiation Experiment, the First ISCCP Regional Experiment, and the Central Equatorial Pacific Experiment. It is found that the cloud volume extinction coefficient is more strongly correlated with the total number density than with the effective particle size. Distribution-averaged mean single scattering properties are calculated for hexagonal columns, hexagonal plates, and polycrystals at a nonabsorbing (0.5 μm), moderately absorbing (1.6 μm), and strongly absorbing (3.0 μm) wavelength. At 0.5 μm (1.6 μm) (3.0 μm), the spread in the resulting mean asymmetry parameters due to different SDs is smaller than (comparable to) (smaller than) the difference caused by applying different particle shapes to these distributions. From a broadband solar radiative transfer point of view it appears more important to use the correct particle shapes than to average over the correct size distributions.

Description: The solar radiative properties of cirrus clouds depend on ice particle shape, size, and orientation, as well as on the spatial cloud structure. Radiation schemes in atmospheric circulation models rely on estimates of cloud optical thickness only. In the present work, a Monte Carlo radiative transfer code is applied to various cirrus cloud scenarios to obtain the radiative response of uncertainties in the above-mentioned microphysical and spatial cloud properties (except orientation). First, plane-parallel homogeneous (0D) clouds with different crystal shapes (hexagonal columns, irregular polycrystals) and 114 different size distributions have been considered. The resulting variabilities in the solar radiative fluxes are in the order of a few percent for the reflected and about 1% for the diffusely transmitted fluxes. Largest variabilities in the order of 10% to 30% are found for the solar broadband absorptance. However, these variabilities are smaller than the flux differences caused by the choice of ice particle geometries. The influence of cloud inhomogeneities on the radiative fluxes has been examined with the help of time series of Raman lidar extinction coefficient profiles as input for the radiative transfer calculations. Significant differences between results for inhomogeneous and plane-parallel clouds were found. These differences are in the same order of magnitude as those arising from using extremely different crystal shapes for the radiative transfer calculations. From this sensitivity study, the ranking of cirrus cloud properties according to their importance in solar broadband radiative transfer is optical thickness, ice crystal shape, ice particle size, and spatial structure.

Description: Sensible and latent heat fluxes were estimated from turbulence measurements gathered during several Atlantic transects of the R/V Polarstern. The inertial dissipation method was used to analyze the data. Resulting bulk transfer coefficients were then applied to the data from the ship’s meteorological system to get continuous time series of the heat fluxes. Combined to the measured downward solar and longwave radiation fluxes allows for an estimate of the total energy budget at the air-sea interface. Comparing these parameterized energy fluxes to ones based on the COARE 3.0 bulk flux algorithm show very strong agreement.

Description: Combined simultaneous satellite observations are used to evaluate the performance of parameterizations of the microphysical and optical properties of cirrus clouds used for radiative flux computations in climate models. Atmospheric and cirrus properties retrieved from Television and Infrared Observation Satellite (TIROS-N) Operational Vertical Sounder (TOVS) observations are given as input to the radiative transfer model developed for the Met Office climate model to simulate radiative fluxes at the top of the atmosphere (TOA). Simulated cirrus shortwave (SW) albedos are then compared to those retrieved from collocated Scanner for Radiation Budget (ScaRaB) observations. For the retrieval, special care has been given to angular direction models. Three parameterizations of cirrus ice crystal optical properties are represented in the Met Office radiative transfer model. These parameterizations are based on different physical approximations and different hypotheses on crystal habit. One parameterization assumes pristine ice crystals and two ice crystal aggregates. By relating the cirrus ice water path (IWP) retrieved from the effective infrared emissivity to the cirrus SW albedo, differences between the parameterizations are amplified. This study shows that pristine crystals seem to be plausible only for cirrus with IWP less than 30 g m−2. For larger IWP, ice crystal aggregates lead to cirrus SW albedos in better agreement with the observations. The data also indicate that climate models should allow the cirrus effective ice crystal diameter (De) to increase with IWP, especially in the range up to 30 g m−2. For cirrus with IWP less than 20 g m−2, this would lead to SW albedos that are about 0.02 higher than the ones of a constant De of 55 μm.

Description: 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 μm, 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 two-dimensional 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 μm are based on parameterizations developed using representative samples of 11 633 crystals imaged by a video ice particle sampler (VIPS). The mean-scattering 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.