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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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Stephen K. Cox

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Stephen K. Cox

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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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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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Thomas McKee and Stephen K. Cox

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Calculated distributions of scattered shortwave radiance are presented for simulated cumulus clouds using a cubic shape. Comparison with similar clouds of semi-infinite horizontal extent is included. For an incident solar zenith angle of 0° the angular distribution of the radiance exiting the cloud top is similar for the cube and the semi-infinite layer, but the radiance from the cube is much smaller for optical depths between 9.8 and 73.5. At an optical depth of 73.5 the vertical radiance from the cube is only 58% of the radiance from the semi-infinite layer cloud. For an incident solar zenith angle of 60°, the angular distribution and the magnitudes of the scattered radiances are similar for the cube top and the semi-infinite layer. A comparison of the total radiance from the cube top and side in the solar plane shows a dramatic change in angular distribution compared with the semi-infinite cloud. Radiances exiting the antisolar side of the cube illustrate the strong forward scattering for short optical paths near cloud edges. The transition from cubic clouds to semi-infinite layers is illustrated for a vertical sun. Results indicate a rapid change for width-to-depth ratios of 1–4 followed by a slower asymptotic change.

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Andrew K. Heidinger and Stephen K. Cox

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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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Michael R. Poellot and Stephen K. Cox

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A computer simulation has been developed to optimize the use of the aircraft platform for the measurement of shortwave irradiances. This model simulates the measurement of radiative fluxes in order to determine the approximate sample sizes required under various conditions of cloudiness.

The simulated required sampling length or averaging distance was found to be inversely proportional to the height of the sensor above or below the cloud field. The magnitude of the averaging distance and the rate of its decrease with height are the result of signal variations on two scales. Near the cloud surface the data have a high variance due to small-scale, large-amplitude variations in the irradiance. These fluctuations are rapidly smoothed as the aircraft-cloud separation increases. The longer period oscillations are not as easily smoothed. When the aircraft is farther from the cloud, large-scale effects become the primary control on the averaging distance.

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Steven A. Ackerman and Stephen K. Cox

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Obsemations of temperature moisture, cloud amount, cloud height and soil-derived aerosols are incorporated into radiative transfer models to yield estimates of the tropospheric and surface radiative energy budgets for the summer Monsoon of 1979. Results are presented for six phases of the monsoon for the region 30°S to 40°N latitude and 30°E to 100°E longitude. The derived radiative fields are significantly different from climatological estimates. The evolution of the radiative energy budgets are discussed in relation to monsoon activity. Total tropospheric convergence (TTC) for the January and February phases exhibits a minimum cooling over the southern Indian Ocean and a maximum tropospheric radiative energy loss over the Arabian Sea and Bay of Bengal. The early May, pre-onset, onset and post-onset periods exhibit cellular patterns in TTC, with maximum cooling over the cloud-free oceanic regions, and minimum cooling associated with continental regions and areas with large amounts of cloud. This cellular structure is still evident when TTC is averaged over 10° regions. Large seasonal variations in TTC are observed over the deserts, due to the presence of dust in the summer. Regions with large seasonal variations in cloud cover (e.g., the Arabian Sea) also display large variations in TTC. Regionally averaged radiative heating profiles also change significantly with period. These variations result primarily from changes in the cloud distribution associated with the evolution of the monsoon.

The net surface radiative flux varies markedly from period to period, and within the same period. As expected, all six periods have a maximum surface radiative energy gain for the cloud-free oceanic regions, while cloudy and continental regions tend to have relative minimae. Large spatial and temporal variations exist in the net surface flux.

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Thomas B. McKee and Stephen K. Cox

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A theoretical model of the scattering of shortwave radiation is applied to clouds finite in horizontal extent. The resulting irradiance patterns are then compared with calculations for horizontally semi-infinite clouds. This analysis shows, that the irradiance fields are dramatically dependent upon energy passing through the vertical sides of the finite sized clouds.

Directional reflectance of individual cubic clouds is shown to be approximately 25% less than for semi-infinite clouds of optical depths ranging from 20 to 80. Directional reflectance from the top of cubic clouds for small solar zenith angle continues to increase at large optical depths (∼70) while the infinite cloud becomes nearly asymptotic at this point. It is shown that for a solar zenith angle of 60°, the directional reflectance for a 2/10 sky cover of cubic clouds is 0.29 while for 2/10 coverage of semi-infinite cloud the directional reflectance is 0.185.

Implications of differences between the cubic cloud results and the semi-infinite cloud case are discussed. These implications include: the effect on calculated planetary albedo; a possible explanation for reported correlations of cloud brightness, cloud height and precipitation; and effects on the surface energy budget.

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