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Peter Wendling

Abstract

The Monte Carlo method has been applied to the transfer of solar radiation through clouds with horizontal inhomogeneities which are assumed to consist of horizontally periodic striations. At a wave- length of γ = 0.55 µm the cloud albedo and reflected radiance are calculated as functions of cloud drop size distribution, optical thickness, solar geometry and type of cloud striations. The resulting cloud albedo is shown to be lower for striated clouds than for a plane parallel cloud with the same mean optical thickness. The largest differences of about 20% occur for a striated cloud with deep striations when the sun is in the zenith. A change in the cloud drop size distribution has nearly the same influence on the albedo of striated clouds as a change in the type of striations, the optical thickness remaining unchanged. A comparable influence results for the radiance reflected from striated clouds, although in this case it depends upon solar geometry and angle of the emerging radiance. It is shown that the radiance emerging from striated clouds can reach even larger values than that reflected by a plane parallel cloud with a thickness equal to the largest thickness of the striated cloud. This effect is due to the backscattering from the vertical walls of the cloud columns occuring at intermediate sun elevation angles in the antisolar direction. On the other hand, the reflected radiance of striated clouds is shown to be reduced compared to that of a plant parallel cloud by the effect of shadows. The magnitude of this effect is mainly determined by the optical thickness.

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Jörg E. Finger
and
Peter Wendling

Abstract

Results are presented from a detailed case study of an Arctic stratus cloud over the Fram Strait that is based on aircraft measurements and model calculations. The measurements have been performed during MIZEX 1984 (Marginal Ice Zone Experiment) and include high frequency data of meteorological parameters and low frequency measurements of radiation fluxes and cloud microphysical data. The vertical mean structure of the Arctic cloud-topped-planetary boundary layer and the turbulence structure are analyzed and discussed. The main processes that contribute to the turbulent kinetic energy are identified by comparison of the measurements with the results of a one dimensional turbulence model with second-order closure. The radiative cooling at cloud top is identified to be the dominant process controlling the whole turbulence structure for the case of a quasi steady state boundary layer. In this fully developed regime the energy consuming entrainment is sustained by the shear-produced horizontal velocity variance via pressure velocity correlation.

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Jean-François Gayet
,
Guy Febvre
,
Gerard Brogniez
,
Helene Chepfer
,
Wolfgang Renger
, and
Peter Wendling

Abstract

During the intensive International Cirrus Experiment conducted over the North Sea during fall 1989, natural cirrus and contrail-induced cirrus were analyzed from in situ and remote sensing measurements (lidar and infrared radiometer). These two cloud types primarily formed at the same range of altitude (8200 m, −37°C). Analysis of the measurements depicts distinctive microphysical and optical properties in the two types of cirrus. Natural cirrus exhibits sheared fallstreaks of ice crystals up to 750 µm in size near the base level. From the top to the base of this cloud the mean values of ice water content and particle concentration increase from 15 to 50 mg m−3 and from 26 to 60 L−1, respectively. The corresponding visible optical depth is around 2.0. Greatest particle concentration and smallest ice crystals are measured at all levels in contrails leading to an optical depth of 0.8 in the denser cloud despite an ice water content that never exceeds 18 mg m−3. These results are consistent with remote measurements from which the backscattering to extinction ratio k is deduced. The largest values of k (0.047 sr −1) are found in a young-life contrail and can be theoretically explained by a spherical shape of small ice crystals. Nonspherical ice particles with larger mean diameter are found in natural cirrus and lead to lower values of k (around 0.02 sr−1).

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