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Kuo-Nan Liou

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Kuo-Nan Liou

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A retrieval technique is presented for the determination of the surface temperature, the thickness and transmissivity of cirrus clouds, and the fraction of the cirrus cloudiness by means of four observed upwelling radiances in the 10 μm window region. On the basis of radiative transfer calculations for mean wavenumbers of 900, 950, 1100 and 1150 cm−1, assumptions are made in the theoretical retrieval analyses that water vapor effects above cirrus clouds are negligible and that ratios of the transmissivities are linear functions of the cloud thickness. Error analyses employing climatological data reveal that independent random errors in temperature and humidity profiles introduce insignificant errors in the four resulting parameters. The resulting errors caused by random errors in the expected upwelling radiances, however, depend upon their standard deviations. Once the thickness and the transmissivity at a given wavenumber of a cirrus cloud have been determined, we illustrate that the vertical ice content may be estimated assuming that ice particles are randomly oriented in a horizontal plane.

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Kuo-Nan Liou

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Transmission, emission and reflection characteristics of cirrus clouds in the 800–1200 cm−1 window region are obtained by means of solving the transfer of infrared radiation in the realistic moist and dry model atmospheres. These model atmospheres include the water vapor within, above and below the cirrus clouds which are assumed to be composed of randomly oriented cylinders. Results of the radiation parameters are presented in terms of the geometrical thickness of circus as well as the model atmospheres. The discrepancies between the transmission and emission for flux and those for the vertically emergent intensity are pointed out and discussed. It is shown that the radiative properties of cirrus depend strongly upon the particle concentration. For a typical cirrus with a thickness of 1 km whose concentration is 0.05 cm−3, the intensity transmission is about 0.65 with an emissivity of about 0.35. Further, based upon the theoretical analyses, the reductions in the effective temperature when cirrus clouds fill in the field of view of the radiometer reveal values from 0 to 60K. Having knowledge of the transmission and emission properties of cirrus clouds, it is demonstrated that the temperatures of the cloud and the underlying surface may be derived with reliable accuracy from meteorological satellites.

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Kuo-Nan Liou

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Current understanding and knowledge of the composition and structure of cirrus clouds are reviewed and documented in this paper. In addition, the radiative properties of cirrus clouds as they relate to weather and climate processes are described in detail. To place the relevance and importance of cirrus composition, structure and radiative properties into a global perspective, we present pertinent results derived from simulation experiments utilizing models with varying degrees of complexity, which have been carried out for the investigation of the influence of cirrus clouds on the thermodynamics and dynamics of the atmosphere. In light of these reviews, suggestions are outlined for cirrus-radiation activities aimed toward the development and improvement of weather and climate models for a physical understanding of cause and effect relationships and for prediction purposes.

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Kuo-Nan Liou

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Computations of the intensity and linear polarization for single scattering by ice clouds have been made based on the assumption that the particles in ice clouds can be approximated by long circular cylinders which are allowed to be polydispersive as well as arbitrarily oriented in space. The results of two models of optically thin ice clouds are presented and compared with those for polydisperse ice spheres. The two models for ice cylinders are assumed to be either uniformly or randomly oriented in a horizontal plane. Four different wavelengths, 0.7, 3, 3.5 and 6.05 μ, are employed in the light scattering computations.

It is found that, compared to ice spheres, long ice cylinders scatter more light in the region with scattering angles near 90°, at the expense of scattering in both the forward and backward directions. The glory and cloudbows, which occur in light scattered by spherical particles, are either lost (the glory) or largely reduced and distorted (the cloudbows) in the case of cylinders. It is probable that for more irregular particles the cloudbows would also disappear. These differences in scattering by spherical and non-spherical scatterers therefore provide useful information for the differentiation between the ice and liquid phase of cloud particles.

The light scattering computations performed for ice cylinders in this paper represent a new theoretical approach in an attempt to understand the radiation scattered by ice crystals. Hence, results of the angular scattering patterns for ice cylinders could be of use in the evaluation of the transfer of visible or infrared radiation through thick ice clouds, especially cirrus.

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Kuo-Nan Liou

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Kuo-Nan Liou

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Band-by-band calculations have been carried out to evaluate the reflection, absorption and transmission of solar radiation by cloud layer and model cloudy atmospheres in the entire solar spectrum. The radiation transfer program is based on the discrete-ordinate method with applications to inhomogencous atmospheres. The gaseous absorption in scattering atmospheres is taken into account by means of exponential fits to the total band absorption based on laboratory measurements. Thick clouds such as nimbostratus. and cumulonimbus reflect 80–90% and absorb 10–20% of the solar radiation incident upon them. The reflection and absorption of a fair weather cumulus with a thickness of 0.45 km are about 68–85% and 4–9%7p, respectively. A thin stratus, whose thickness is 0.1 km, reflects about 45–72% and absorbs about 1–6% of the solar flux incident on the cloud top. The reflection of a 0.6 km thick altostratus is about 57–77%, with a larger absorption of 8–15%. A number of aircraft observations reveal that clouds may absorb as much as 30–40% of the solar flux incident upon them. Since the maximum absorption of clouds resulting from theoretical calculations is only 20%, certain clouds in the atmosphere ore likely to consist of hydrophobic absorbing aerosol particles.

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Kuo-nan Liou

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Kuo-Nan Liou

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The discrete-ordinate method for radiative transfer introduced originally by Chandrasekhar has been theoretically developed and numerically verified for use in solving the transfer of both solar and thermal infrared radiation through cloudy and hazy atmospheres. This method differs from other radiative transfer approaches in the sense that the solution of the transfer equation can be explicitly derived by employing a finite set of discrete-streams representing the emergent angles in the integral term. Hence such a method is practical for deriving a simplified but reliable radiative transfer approximation for meteorological applications involving clouds and aerosols. Comprehensive comparisons with other rigorous means are carried out for the transmitted and reflected intensity and flux associated with isotropic, Rayleigh and anisotropic scattering. The comparisons reveal that close agreement of radiation computations can be achieved by using discrete-streams of 16. For flux calculations, it was found that values obtained by discrete-streams of 4 yield an accuracy of about 1% for typical phase functions of clouds and haze in either solar or IR radiation, whereas the method of discrete-streams of 2 introduces errors on the order of 3–10%. The solutions for these two simplified cases can be expressed analytically.

Applications have been made by employing wavelengths of 0.7, 1.5 and 10 µm to denote the transfer of solar and thermal IR irradiance through cloudy and hazy atmospheres. For solar radiation, the reflection (albedo), transmission. and absorption are obtained as functions of the zenith angle and optical thickness. It is shown that a single-scattering albedo of 0.99 produces absorption of about 30% for a cloud with an optical thickness of 20, and subsequently decreases the albedo by about 20%. The dependence of the transmitted and reflected infrared radiation on the temperatures of particulate layers and ground is illustrated for a number of thicknesses. A cloud at a temperature of −10C would normally reduce the outgoing radiation by about 40%. Interference effects due to thin haze appear to be unimportant. In addition, the net flux and the heating and cooling rates within cloud layers are presented for wavelengths of 1.5 and 10 µm. For a cloud 1 km in thickness, the solar beating takes place near the upper portion at a rate of about 0.3C hr−1 µm−1 at normal incidence, while it experiences base-warming and top-cooling at a comparable rate resulting from the thermal IR radiation. Although monochromatic wavelengths aye used in this study, the method can be extended to include the entire solar and infrared spectrum.

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Kuo-Nan Liou

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Multiple backscattering of pulsed light is formulated for linearly polarized incident radiation. The method is based on the selection of a particular geometry in which the primary and nth order backscattering return to the receiver simultaneously.

It is shown that the returned power from water clouds at a range of 1 km due to orders of scattering higher than the second may be neglected in a collimated pulsed lidar system whose field of view is less than 10−2 rad.

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