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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

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No abstract available.

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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

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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Gregory P. Byrd
and
Stephen K. Cox

Abstract

Tropospheric radiative convergence profiles from Cox and Griffith are used to assess the radiative forcing upon a tropical cloud cluster located in the vicinity of the GATE A/B-scale array during 4–6 September 1974. A background discussion summarizes some of the previous investigations that served as motivation for the present study. The atmospheric response to differential radiative cooling between the cluster and its surrounding environment is examined by means of “slab” and cross section analyses over the Cox-Griffith array. A radiatively derived vertical motion model is constructed to investigate the role of radiation with respect to larger-scale dynamics during a daytime (0600–1200 LST 5 September) and nighttime (1800–2400 LST 5 September) period of the cluster life cycle.

Radiative forcing is found to be strongest during the initial stages of cluster development. Throughout the cluster life cycle, the radiative forcing is consistently strongest in the middle troposphere (400–700 mb). As the cluster system intensifies, daytime shortwave warming superimposed upon the longwave cooling lessens the total radiative cooling in the surrounding cloud-free region, resulting in a lessening of the differential radiative cooling. Increased amounts of middle and high cloud remnants also contribute to the observed weakening of radiative forcing during the mature and dissipating disturbance stages. Cross section analyses reveal that E-W gradients of radiative convergence between the cluster and its surroundings are comparable in magnitude to the N-S gradients.

The radiatively derived vertical motion model yields a qualitatively realistic total area of cluster influence for a nighttime case, 1800–2400 GMT on 5 September. The model assumption of a closed mass system breaks down during the daytime (0600–1200 LST, 5 September) period, yielding an unrealistically 1arge total area of cluster influence. This suggests the occurrence of significant cluster-scale interactions with large-scale circulations during the daytime period. Radiative forcing appears to play a more significant role in dynamical interactions during the nighttime period, when circulations seem to be somewhat more localized.

The maximum in-cluster precipitation intensity lags the incidence of strong radiative forcing by 6–8 h, in general agreement with GATE composite observations. Continental oceanic differential beating must also play a significant role in modulating cluster- and large-scale dynamical interactions, accounting for the anomalously long precipitation lag observable in the GATE cluster. The interpretations presented herein are based solely upon this single case study and may not necessarily be representative of cluster disturbances as a whole.

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Steven A. Ackerman
and
Stephen K. Cox

Abstract

Tropospheric radiative convergence profiles are derived for an easterly wave composite during Phase III of the GATE. The easterly waves observed during this period were generally well developed. The profiles also represent the magnitudes and the spatial distribution of atmospheric radiative convergence of the Intertropical Convergence Zone in the GATE area. The 12 h mean daytime and nighttime profiles are presented. Cloud-top pressure distributions as a function of wave position are also presented.

The results of this research indicate three possible radiative induced mechanisms which contribute to the observed diurnal cycle in large-scale mass convergence: 1) radiative convergence differences between the ITCZ and the surrounding regions; 2) mesoscale radiative convergence differences between clear and cluster regions, and 3) a nighttime upper level tropospheric cooling maximum that is centered one-half a wavelength from the region of maximum convective activity.

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Timothy L. Alberta
and
Stephen K. Cox

Abstract

Results of experiments conducted using the Cloud Field Optical Simulator (CFOS) to examine the variability in reflectance properties of cloud fields with fixed cloud amount but different cloud patterns are presented. Angular reflectance data from 20 cloud fields with a common cloud amount of 30 percent were analyzed. The experiment demonstrated the problem of changing spot size as a function of view angle for a fixed field of view detector.

Seven different incident solar zenith angles were analyzed for variations in reflected irradiances arising from different cloud field patterns. Results show irradiance variations as great as 31% at large incident zenith angles. Also indicated are increased irradiances and increased anisotropy at large incident zenith angles.

Radiances and irradiances of interacting cloud elements were compared to those of noninteracting clouds. Interacting cloud fields produced larger radiance and irradiance values than noninteracting cloud fields. The differences between interacting and noninteracting cloud fields were greater at smaller source zenith angles. Maximum radiances were found at photodiode locations measuring backscattered radiation in the interacting cloud fields.

Reflectances were integrated as a function of zenith angle to produce daily reflectances for five different latitude/date combinations. Analysis of this study demonstrated the importance of the sides of clouds, and verified irradiance differences due to cloud patterns when examined on a daily scale.

Irradiances calculated utilizing 195 radiances, each measured at different local zenith and azimuth angles, were compared with irradiances calculated from a single radiance assuming isotropy. The isotropic assumption produced overestimates of the measured irradiances at large local zenith angles when the photodiode detectors measured backscattered radiation, and underestimates when the detectors measured forward scattered radiation. Minimum errors were found at small local zenith angles.

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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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