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  • Author or Editor: Stephen K. Cox x
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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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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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Michael R. Poellot and Stephen K. Cox

Abstract

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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Thomas B. McKee and Stephen K. Cox

Abstract

Stlratospheric temperature profiles have been inferred from limb radiance data. The limb radiance observations were made from an Aerobee rocket flown on 7 February 1970 from White Sands Missile Range, N.M. The inferred temperatures were similar to those available from rocket sounding data. Horizontal variations of temperature at the 10-mb surface agreed with radiosonde data; however, the inferred temperatures were systematically warmer at 10 mb with larger increases of temperature from 30 to 10 nab than comparative radiosonde data. Two of the inferred temperature profiles provide an excellent example in which two measurements are sufficiently alike to separate real vertical temperature structure from effects of random radiance noise. Vertical resolution was about 2 km and the largest lapse noted was near dry adiabatic.

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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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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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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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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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Bruce A. Albrecht, Stephen K. Cox, and Wayne H. Schubert

Abstract

The feasibility of measuring in-cloud temperature fluctuations with an infrared radiometer is demonstrated. The results obtained from aircraft measurements in a stratocumulus cloud deck indicate that a radiometer may easily resolve temperature variations within the cloud of less than 0.05°C provided that the cloud is sufficiently opaque. These radiometer measurements of temperature are combined with aircraft observations of vertical velocity to calculate a heat flux within the cloud deck. The heat flux derived using the radiometric observations shows excellent agreement with theoretical predictions of in-cloud heat flux. A similar heat flux calculation using the more conventional temperature transducer data was less by a factor of 5.

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David P. Duda, Graeme L. Stephens, and S. K. Cox

Abstract

Vertical profiles of cloud microphysical data and longwave and shortwave radiation measurements through the marine boundary layer were obtained using an instrument package on the NASA tethered balloon during the FIRE Marine Stratocumulus Experiment. The radiation observations were analyzed to determine heating rates inside the stratocumulus clouds during several tethered balloon flights. The radiation fields in the cloud layer were also simulated by a two-stream radiative transfer model, which used cloud optical properties derived from microphysical measurements and Mie scattering theory.

The vertical profiles of the observed longwave cooling rates were similar in structure and magnitude not only to previous measurements of marine stratocumulus, but also to the cooling rates computed by the two-stream radiative transfer model. The solar heating rates measured in the clouds, however, were systematically much larger than the rates calculated in the model.

Solar albedo measurements showed that the visible spectrum tended to be reflected by the clouds more than the near IR spectrum. This is similar to the results reported by Hignett, although the discrepancies between the observed and calculated near IR to visible albedo ratios were generally much smaller. The results from the flights on 10 and 13 July 1987, however, suggest that the effects of heterogeneities on the radiative transfer through the cloud may be more important in the visible than in the new IR.

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