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Petr Chýlek

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Petr Chylek and V. Ramaswamy

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We have derived a simple approximation for the emissivity and flux emissivity of water clouds inside the atmospheric window between 8 and 14 m. In our approximation the emissivity in the 8–11.5 m band is a function only of the cloud's liquid water content and cloud thickness. When compared with the exact radiative transfer calculations the broad-band flux emissivities (in the 8–11.5 m region) differ by less than 10%. At wavelengths > 11.5 m the emissivity is a function of the droplet size distribution as well. By considering a typical droplet size distribution for stratus, altostratus and cumulus clouds, we have shown that the effect of the size distribution on the broad-band flux emissivity in the 8–14 m band is about 35%. Our approximation should be useful for treatment of cloud infrared properties in climate models.

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J. Li and Petr Chylek

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The global horizontal structure of atmospheric entropy has been investigated. In energy balance models, the horizontal distribution of the atmospheric internal entropy production rate has been obtained. Based on the entropy balance relation, this work is of rigorous thermodynamics foundation. In the models, the radiation entropy has been evaluated through the effective temperature method. It is found that with the increase of latitude, the internal entropy production decreases and the entropy production corresponding to the thermal conduction increases. In addition, the atmospheric entropy structure problem under ice age conditions is discussed.

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Jiangnan Li and Petr Chylek

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Atmospheric entropy and its association with climate dissipation are investigated. The balance equation for entropy is derived through the mean and transient thermal and moisture equations. The entropy production contains the internal and external parts. The external entropy production, due to small-scale diabatic heating, can be evaluated by the surface entropy flux. Using NCEP data from 1998 to 2007, it is found that the surface entropy flux is much larger in the tropics than in the extratropics. In the December–February (DJF) Northern Hemisphere, there are two strong positive centers of boundary layer supply of entropy: one is in the northwestern Pacific and the other is in the western Atlantic. The external entropy production, due to large-scale eddy flow, can be evaluated by the convergence of eddy entropy flow. It is found that the large-scale eddy entropy flow is divergent in the midlatitudes and convergent in the higher latitudes. The internal entropy production shows the dissipation to the orderly thermal structure. For the internal entropy production due to a large-scale eddy, it is shown that in the Northern Hemisphere during DJF there are three maxima, located in the western Pacific, western Atlantic, and northern polar regions. This illustrates the dissipation of the highly organized thermal structure in such regions. An interesting finding is that the large-scale eddy internal entropy production is negative in the lower stratosphere. It is found that the long-time-averaged global mean of the internal entropy production is 0.037 49 W m−2 K−1. By including the entropy sink from radiation, the total entropy production is close to balance.

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Peter Damiano and Petr Chýlek

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The dependence of cloud shortwave radiative properties on the details of the droplet size distribution is minimized when the effective radius is chosen as a variable characterizing the size distribution. The shortwave radiative properties of clouds (extinction coefficient, single scattering albedo, and asymmetry parameter) are determined for a given value of the effective radius with an accuracy of better than 10% for effective radii above 10 μm. However, the shortwave radiative properties of clouds composed of small droplet size distributions with effective radii below 5 μm can vary up to 40% (with the change of effective variance) at the given value of the effective radius. The spectral albedo of optically thick clouds at wavelengths shorter than 1 μm is determined very accurately (within 1%) by the effective radius of the droplet size distribution. At longer wavelengths, the albedo of optically thick clouds is a sensitive function of the effective variance at all values of the effective radius and at all wavelengths. Typical variations are within 10% to 50%.

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Petr Chýlek and Gorden Videen

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With the use of the anomalous diffraction approximation, analytical expressions for the absorption and scattering coefficients and for the single scattering albedo of a polydispersion of horizontally oriented hexagonal columns and hexagonal plates are derived. By comparison of the anomalous diffraction results with those obtained using the dipole superposition method, it is estimated that the accuracy of the derived expressions are within 5% in the two narrow bands from 2.77 to 3.02 and from 10.6 to 11.1 μm and within 15% from 0.8 to 3.05 from 5.4 to 6.1 and from 8.3 to 11.9 μm. The equivalent spheres of equal volume or of equal surface area do not provide a suitable approximation for the scattering properties of the polydispersion of horizontally oriented hexagonal columns or plates. The errors due to the use of equivalent spheres in calculation of the absorption and scattering coefficients and in single scattering albedo are typically of the order of several hundred percent. Equal volume or equal surface area cylinders approximate well the scattering proprieties of hexagonal columns; however, they are not suitable as equivalent particles for the case of hexagonal plates.

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Petr Chýlek and J. T. Kiehl

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We have compared sensitivities of four different radiative-convective climate models. Although surface temperature sensitivities with respect to changes in solar constant and atmospheric CO2, concentration are almost the same in all models, sensitivity with respect to some other climate variables varies up to a factor of 2. We have found that the surface, temperature sensitivity with respect to changes of the lapse rate is high in all models, and we emphasize the importance of a lapse rate-surface temperature feedback.

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Petr Chýlek and J. Steven Dobbie

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The Monte Carlo method is used to study the impact of various cloud morphologies (roughness, voids, waves, and horizontal spreading) on radiative properties of finite, thin, model cirrus clouds. The cloud-top reflectance is calculated for various cloud-top structures and is compared to reflectance of finite homogeneous cloud of the same ice crystal size and ice water content. Cloud roughness, voids, and waves generally decrease cloud reflectance as well as cloud absorption. Although the local horizontal variations in the reflectance can be quite large (several hundred percent), variation in the total reflectance, integrated over the top surface of a cloud, is in the range of a few percent. For overhead incident radiation, the decrease in cloud reflectance due to the considered morphological changes remains under 5%, as compared to a finite homogeneous cloud. A comparable reduction in cloud reflectivity is achieved by about a 5% increase of the effective size of ice crystals. The reflectivity of an optically thin cloud is primarily determined by the total ice content and the effective ice crystal size. The morphological structure of a cloud plays a secondary role.

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Petr Chýlek, V. Ramaswamy, and W. J. Wiscombe

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An expression for the surface area of a nonspherical particle described by the equationr = r 0[1 ± εTn(cos θ)]is derived, and radii of various equivalent spheres are calculated.

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J. Li, Petr Chýlek, and G. B. Lesins

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The vertical atmospheric entropy structure has been investigated using one-dimensional radiative–convective models. A method for evaluating radiation entropy is proposed. In the models, the entropy radiation is dealt with in a way parallel to the energy radiation. The profiles of the radiative entropy fluxes, the vertical distributions of the internal entropy production rates of the atmosphere in different seasons, and the entropy budget for a column atmosphere are obtained. The influence of clouds and the possible change of atmospheric constituents on the atmospheric entropy structure are also discussed.

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