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Thomas B. McKee and Stephen K. Cox

Abstract

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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Douglas A. Wesley and Stephen K. Cox

Abstract

The diffusional mass evolution of hydrometeors in upper tropospheric clouds for various radiative conditions in the cloud and for varying ambient moisture Supply is simulated using a time dependent microphysical model. Radiation can play an important role in this mass evolution when only one phase is present. When both liquid droplets and ice crystals are situated in a typical upper tropospheric environment and the moisture supply is limited, radiation produces only a minor effect on the mass evolution of the hydrometeors. In these cases ice crystals grow quickly at the expense of the droplets and the droplets evaporate within several minutes, even under water supersaturation conditions. Radiation does not significantly influence the evaporation rates of the droplets in the coexisting cases. In the absence of ice crystals and under certain radiative conditions, the droplets can evaporate when the environment is supersaturated.

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James R. Fleming and Stephen K. Cox

Abstract

The divergence of net radiation in a tropical atmosphere with cirrus clouds has been examined in terms of two bulk radiative properties of the cloud: effective shortwave optical thickness τ* and broad–band infrared emissivity ε&ast. The effective shortwave optical thickness of the cirrus cloud is the primary factor controlling the radiative energy budget at the earth's surface while the cloud's broad–band infrared emissivity is the primary factor influencing the heat budget of the atmosphere.

The net radiative energy budget of the tropical atmosphere over an ocean surface in the presence and absence of a cirrus cloud has been examined. The total net radiative energy at the top of the atmosphere is relatively unchanged from the clear sky value by the presence of a cirrus cloud layer while the surface energy budget shows a significant decrease in the shortwave component. The total net radiative energy loss of the atmosphere when a cirrus layer is present is shown to be 22 to 78% of the loss from a cloudless sky. This reduction of divergence when cirrus are present generates in 24 hr an amount of energy which is 32 to 100% of the estimated net latent heat released to the atmosphere by precipitation processes in a cumulonimbus cloud.

The divergence of net radiation through three layers encompassing the atmosphere is shown to be linearly dependent on the broad–band infrared emissivity and effective shortwave optical thickness of cirrus clouds.

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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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David O'C. Starr and Stephen K. Cox

Abstract

A two-dimensional (x, z), time-dependent, numerical cloud model is developed for the purpose of investigating the role of various physical processes involved in the maintenance of cirriform clouds. In addition to accounting for dynamic and thermodynamic processes including phase changes of water, effects due to microphysical composition and radiative processes are also explicitly incorporated into the model. Diagnostic parameterizations for the local radiative properties of cloudy volumes and the gravity induced relative fall speed of the contained ice water are presented. Results of a simulation of a thin cirrostratus cloud are given. Features of the simulated cloud structure are quite realistic. Quantitative agreement is found between the simulated ice water contents and vertical motions and comparable observations. It is shown that radiative effect may be very significant in the maintenance of cirrus. The effects of the gravity-induced relative fall speed of ice crystals are found to be of critical importance in the evolution of the cloud layer.

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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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John M. Davis, Stephen K. Cox, and Thomas B. McKee

Abstract

A band absorption model is used in conjunction with a Monte Carlo scattering model to calculate the amount of solar radiation absorbed above, below, within and adjacent to cubic, finite clouds. Horizontally and vertically nonhomogeneous values of absorption within the finite cloud range from 0.16 to 6.4 times the corresponding values in horizontally infinite clouds of the same optical thickness, which were calculated using the same model. Absorption values in the regions adjoining the finite cloud on the solar and anti-solar sides, converge to clear sky values within a distance of two cloud dimensions from the side walls of the cloud.

Absorption below the finite cloud ranges from 1.4 to 4.5 times that below the infinite cloud volume element. Values of absorption above the two cloud types are nearly identical when normalized to the cross-sectional area of the incident beam. If the absorption in the atmospheric column containing the finite cloud is normalized with respect to the horizontal area of the parallel radiation incident on the top plus the side of the cloud, the resulting value is within 3% of the absorption value in the column containing an element of infinite cloud. Thus, infinite cloud total column absorption values may be used to compute areal averages of absorption for a region partially covered by widely separated finite clouds, whose height to width ratio is near unity, if the fractional cloud cover is adjusted in the appropriate manner.

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Ronald M. Welch, Stephen K. Cox, and John M. Davis

Abstract

No Abstract available.

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William L. Smith Jr., Paul F. Hein, and Stephen K. Cox

Abstract

On 28 October 1986 the NCAR Sabreliner observed a cirrus cloud layer in the vicinity of Green Bay, Wisconsin. A portion of each flight leg was conducted over western Lake Michigan and over the adjacent western shore. The cirrus layer would be qualitatively described as optically thin and tenuous, yet broadband infrared effective emittances were found between about 0.4 and 0.6 while broadband shortwave extinction values ranged from as low as 5% to 32%. This investigation examines the bulk radiative properties of the cirrus layer and the horizontal variability of these radiative properties. In addition, the microphysical characteristics and the dynamic properties of the layer are presented and analyzed. The broadband infrared volume absorption coefficients were deduced for the cirrus layer and found to be very similar in terms of a dependence on temperature to results recently presented by other authors. Infrared radiative heating rates were calculated and found to be typical of the optically thin cirrus layer examined here. The horizontal structures of the radiative properties of the cirrus cloud layer and the vertical velocity observations were very similar. Both showed a smaller scale variation at the top of the cirrus layer which merged into larger scale common elements near the bases of the layer. Power spectra analyses of along-wind and cross-wind components near the base of the clouds sampled exhibited a steep spectral slope of k −3 at the smaller wave numbers (scalelengths greater than 1 km). This k −3 slope is characteristic of two-dimensional eddies. The same k −3 slope is present in the power spectra of the radiative properties. It is probable that these radiative properties, which are modulated by the cloud elements, have their scales determined by the eddies detected in the analysis of wind components.

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Stephen K. Cox, James A. Maynard, and Verner E. Suomi

Abstract

An analysis of tropical radiosonde temperature measurements made during the Line Island Experiment suggests that conventional radiosonde preflight procedures are inadequate in a remote tropical environment. Temperatures computed from conventional and modified baseline techniques are compared at five pressure surfaces, 1000, 800, 600, 400 and 200 mb. Temperatures obtained from the two baseline techniques showed an average deviation at 1000 mb of 0.96C for 62 soundings. These comparisons indicate that a careful examination of radiosonde calibration techniques is needed before large investments are made in future global experiments.

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