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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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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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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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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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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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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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