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Stephen K. Cox

Abstract

Cirrus clouds may act to cool or warm the earth's surface, depending upon their infrared emissivity. Direct observation of cirrus cloud emissivities in mid-latitude and tropical environments indicates that cirrus may produce different effects at different latitudes. In the tropics, cirrus emissivities were large enough to cause a significant warming tendency while mid-latitude data showed a cooling effect.

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Stephen K. Cox

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Stephen K. Cox

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Stephen K. Cox

Abstract

This paper presents mean greybody infrared effective emissivity values of clouds deduced from 300 International Quiet Sun Year (IQSY) radiometersonde ascents. The cloud effective emissivity data are presented for two latitude regions: midlatitude and tropical. Mean cloud effective emissivity values for the surface to 300 mb layer ranged from 0.41 to 0.64 for the midlatitude data and from 0.54 to 0.84 for the tropical data. Clouds in the pressure interval from 400 to 600 mb exhibited the largest mean emissivity values. These data should he very useful for incorporation of realistic cloud radiative properties into modeling of atmospheric dynamics and climate.

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STEPHEN K. COX

Abstract

Balloon-borne radiation sonde measurements during 1964 and 1965 are used to form composite, three-dimensional radiative cooling models for the following midlatitude synoptic features: stationary front; nascent cyclone; warm sector cyclone; occluded cyclone; and anticyclone. Composite water vapor distributions for the same synoptic features are used to model the pattern of atmospheric warming by solar radiation.

Thickness tendency analyses of the 1000-500-mb layer for four synoptic features show that radiative cooling and warming may account for 10–30 percent of the observed maximum thickness tendency. The radiative thickness change components are of the same order of magnitude as the latent and the sensible heating terms.

The nascent cyclone case shows a radiatively induced vorticity tendency of 6 × 10−10 sec−2. This compares with a total expected vorticity tendency between 10−9 and 10−10 sec−2. The nascent cyclone, warm sector cyclone, and anticyclone cases show positive cyclonic development from radiative effects, while the occluded cyclone case shows negative cyclonic development.

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Andrew K. Heidinger
and
Stephen K. Cox

Abstract

As numerical weather and climate prediction models demand more accurate treatment of clouds, the role of finite-cloud effects in longwave radiative transfer clearly warrants further study. In this research, finite-cloud effects are defined as the influence of cloud shape, size, and spatial arrangement on longwave radiative transfer. To show the magnitude of these effects, radiometer data collected in 1992 during the Atlantic Stratocumulus Transition Experiment (ASTEX) were analyzed. The ASTEX data showed that radiative transfer calculations that ignored the vertical dimensions of the clouds underestimated the longwave cloud radiative surface forcing by 30%, on average. To study further these finite-cloud effects, a three-dimensional 11-µm radiative transfer model was developed. Results from this model, which neglected scattering, agreed with the measurements taken during ASTEX on 14 June 1992. This model was also used to reiterate that, for optically thick clouds, knowledge of cloud macrophysical properties can be more crucial to the modeling of the transfer of longwave radiation than the detailed description of cloud microphysical properties. Lastly, techniques for the inclusion of these finite-cloud effects in numerical models were explored. Accurate radiative heating rate profiles were achieved with a method that assumed a linear variation of the cloud fraction within the cloud layer. Parameterizations of the finite-cloud effects for the marine stratocumulus observed during ASTEX are presented.

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John M. Davis
and
Stephen K. Cox

Abstract

A set of bi-directional reflectance models is presented for various atmospheric scene types. The models were composited from data collected from an aircraft platform in May-July 1979 during Summer MONEX. The space scale of the composited models is generally from 250 to 1000 km, which corresponds to the scale of interest in climate monitoring and modeling. Composite models for the following scene types are presented: the desert sands of the Saudi Arabian Empty Quarters, the Himalayan mountains, the Arabian Sea with the ever-present fair weather cumulus cloudiness, the semi-arid agricultural land surface of the Indian subcontinent under pre-monsoon conditions, broken middle and low level clouds over ocean, an altostratus cloud deck, and the broken pack-ice fields of Hudson Bay. Nearly all the models display a degree of anisotropy such that serious errors (10–100%) would result in the reflected flux density isotropically inferred from some of the reflected radiances. The features of many of the models are discussed, and all of the models are tabulated in the Appendix. One of the models for altostratus is explicitly compared with theory, and differences between the altostratus and broken cloud models agree with the differences between infinite and finite cloud theory. The models are also compared with models from previous studies. The agreement is generally good (∼100%rms) except in a few cases in which the disagreement may have resulted from natural scene variability or differences between the methods of data collection.

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John M. Davis
and
Stephen K. Cox

Abstract

The results of a laboratory experiment are presented that provide additional verification of the methodology adapted for simulation of the radiances reflected from fields of optically thick clouds using the Cloud Field Optical Simulator (CFOS) at Colorado State University. The comparison of these data with their theoretically derived counterparts indicates that the crucial mechanism of cloud-to-cloud radiance field interaction is accurately simulated in the CFOS experiments and adds confidence to the manner in which the optical depth is scaled.

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Steven A. Ackerman
and
Stephen K. Cox

Abstract

Techniques for normalizing aircraft measurements of solar irradiance to a horizontal surface and a constant solar zenith angle are outlined. The effects of these normalization procedures are a minimum when the data are collected at small solar zenith angles. A method of analysis is discussed which takes into account the effects of the heterogeneous structure of clouds on observations of cloud fractional absorptance in the 0.3–2.8 μm spectral interval. Application of the technique to the observed absorptance, results in a corrected fractional absorptance value which is in better agreement with theoretical calculations than previously reported. In addition, the technique significantly reduces the sampling time required to obtain a representative cloud fractional absorptance.

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Steven A. Ackerman
and
Stephen K. Cox

Abstract

Horizontal cloud coverages derived from a geostationary satellite and all-sky cameras were compared for a 3-month period of the GARP Atlantic Tropical Experiment (GATE). Estimates of total cloud cover using the satellite and all-sky camera were similar for the daytime period. The all-sky cameras also gave reasonable estimates of the 24 h cloud cover due to the small difference in the satellite determined daytime and nighttime total cloud cover in the vicinity of the all-sky cameras. However, other regions in the area of study which were not covered by an all-sky camera revealed large diurnal variations. In these areas the daytime total cloud amount did not yield an accurate representation of the 24 h cloud cover.

A method is presented which enables one to construct a three-dimensional representation of cloud structure by combining surface and satellite observations. The disadvantages of this technique are that it assumes no overlapping cloud tops or cloud bases, as well as the limitations of the satellite and all-sky camera in estimating cloud cover.

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