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B. Albrecht and S. K. Cox

Abstract

Infrared beating rates determined from the Line Islands Experiment radiometersonde measurements vary from −4 to −0.5°C day−1 at 250 nib. Correspondingly, the beating rates at 500 mb vary from +0.5 to −3°C day−1. These variations result from high-middle cloudiness occurring every 6–8 days.

The structure of waves forced by the observed differences in radiative beating is determined through the use of a large-scale diagnostic model. In this model, the diabatic heating is specified and the response of the atmosphere to this forcing is computed. When the radiative beating is combined with convective heating, the vertical structure of the forced motions is shown to be very sensitive to the phase difference between the convective and the radiative beating. The amplitude of the weridional wind perturbations at 200 mb varies from 1 m s−1 for the use when radiatively active upper tropospheric clouds am centered over the convective beating to 5.5 m s−1 when the cloud is centered a half wavelength from the convective heating. For the same cloud configurations, the mass divergence at 200mb varies from 3×10−6 to 1×10−6s−1 and the convergence at 900 mb varies from 3×10−6s−1 at the region of the maximum convective heating.

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B. Albrecht and S. K. Cox

Abstract

The actual performance of an Eppley pyrgeometer is compared to the desired theoretical performance. Several systematic errors are identified and evaluated in detail. The three most significant errors identified are due to 1) battery voltage uncertainties, 2) nonlinearity of circuitry at extreme temperature and 3) differential heating of the instrument. The elimination of the error due to differential heating is found to be essential to the successful calibration of the instrument. A pyrgeometer laboratory calibration technique is described.

Pyrgeometer measurements made from aircraft are shown to have potential errors as large as 60 W m−2. These errors, however, do not significantly affect the net radiation provided the upward and downward facing pyrgeometers are at the same equilibrium temperature, and may be largely eliminated by making accurate temperature measurements of the KRS-5 dome and the cold junctions of the thermopile. The corrections considered in this paper not only reduce the absolute errors but significantly decrease the transient response of the instrument. The feasibility of using an empirical expression to correct errors due to solar heating is also demonstrated for aircraft measurements.

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P. M. Kuhn and S. K. Cox

Abstract

By varying the amount of water vapor as input to the radiative power transfer equation, assuming a constant carbon dioxide and varying ozone distribution, it is possible to infer stratospheric water vapor from broadband observations of downward irradiance. The procedure is iterative in that downward observed and calculated irradiances, at several levels for each of several radiometric soundings, are brought within the limits of a convergence criterion. This is accomplished by successively reducing an initial over-estimate of the stratospheric mixing ratio, defined by a power law, until the sum of the squared differences of observed and calculated irradiances is minimized. The sum includes all levels of the sounding.

Results for a continental area during winter months indicate that the stratospheric water vapor content from 50 mb upward to 10 mb decreases from approximately 20 to 3 parts per million. For tropical Guam and Canton Island the corresponding magnitudes are larger, decreasing from 21 to 4 ppm. The standard deviation of the mean for all pressure levels is approximately 1.0 ppm. Adding deviation to the values inferred should give an upper bound to the water vapor content. The average mixing ratio for the continental stations between 25 and 10 mb is 5.7 ppm with a standard deviation of the mean of 0.8 ppm. Since the infrared radiative emission and attenuation of aerosols is inseparable from emission and attenuation of the atmospheric gases when measured with a broad response radiometer, these mixing ratio results would be reduced by the presence of aerosols. In view of apparent aerosol contamination we have made no inferences below 50 mb (21 km). The results may be said to be an upper bound to the actual quantity of water vapor, favoring an increasingly dry stratospheric profile.

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R. A. Raschke and S. K. Cox

Abstract

The feasibility of using a photodiode radiometer to infer optical depth of thin clouds from solar intensity measurements is examined. Data were collected by a photodiode radiometer which measured incident radiation at angular fields of view of 2, 5, 10, 20 and 28°. Values of normalized annular radiance and transmittance were calculated from the observations and compared to similar calculations from a Monte Carlo radiative transfer model. The Monte Carlo results were for cloud optical depths of 1 through 6 over a spectral bandpass of 0.3 to 2.8 μm.

Eight case studies including high, middle and low clouds were examined. Experimental values of cloud optical depth were determined by comparing plots of transmittance versus field of view with the model calculated curves and from the average of the five optical depths calculated for each field of view. Analysis of the case study results indicates that the photodiode radiometer can be used effectively to determine the optical depth of thin clouds.

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T. M. Thompson and S. K. Cox

Abstract

A climatology of direct beam irradiance has been compiled for Mauna Loa Observatory. A broadband transmittance, calculated from the direct-beam data, has been stratified into clear sky and optically thin and thick cloud regimes; statistics of this stratification may represent a means of monitoring possible climate change.

We have shown how direct beam irradiance observations may be used to infer a climatology of optically thin cloudiness. These techniques have been applied to a five-year data set from Mauna Loa Observatory. Clear sky frequency of occurrence was 56% with a mean clear sky transmittance of 0.72. Clouds influenced the direct solar beam irradiance 44% of the time; this resulted in a mean transmittance of 0.49. Clear skies exhibited an annual cycle with less direct beam attenuation in the winter months.

Although no trends in optically thin cloudiness were apparent in the five-year data set, the variability of the data allows one to estimate the length of record required to detect a given magnitude trend. For example, trends of 0.1 and 1.0% per year would require monitoring times of 212 and 46 months, respectively.

After removing zenith-angle dependence from the data through a vertical transformation, thin cloud occurrence is uniformly distributed between the 0.12-0.60 transmittance range.

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A. A. Rockwood and S. K. Cox

Abstract

A technique has been developed from simultaneous satellite and aircraft data that allows the magnitude and gradient of the earth's surface albedo to be inferred from satellite measurements of the earth-atmosphere system brightness. The technique uses the visible brightness observations from the SMS-1 geosynchronous satellite made during the GARP Atlantic Tropical Experiment (GATE) in 1974. Direct albedo measurements from aircraft verified an albedo range of from 50% over the Sahara Desert to 7% over the wet tropical rainforests. The technique is used to study the changes in the physical characteristics of the surface of northwestern Africa's Sahel region between July and September, 1974. Changes in the gradient of surface albedo and areal extent of various vegetation types are inferred. The albedo values calculated through the application of this technique should be incorporated into the numerical and analytic models of the atmosphere that have recently been used to study local circulation patterns and precipitation trends.

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R. M. Welch, M. G. Ravichandran, and S. K. Cox

Abstract

There is considerable controversy in the literature concerning fog formation. One set of observations suggests that fog forms during a lull in turbulence, white another set of observations suggests that increased turbulence leads to fog formation.

A number of first-order closure techniques are applied to numerical simulations. The results show that fog formation and development is directly correlated with the magnitude of the eddy mixing coefficients. Larger turbulence generation leads to more rapid fog development and to larger liquid water contents. The rate at which the fog top grows is directly related to the rate at which turbulence lifts the inversion.

During the mature fog stage, a series of fog dissipation and redevelopment episodes occur. Liquid water develops in the upper regions of the fog during the turbulently quiet periods. Subsequent destabilization of the atmosphere increases turbulence generation and mixes the upper-level liquid water to the surface, creating surface fog intensification. Quasi-periodic oscillations in fog parameters are largest in the upper regions of the fog and become progressively damped in the lower regions of a thick fog.

These results are in qualitative agreement with the observations reported by Jiusto and Lala and support the hypothesis that there are distinct stages of fog development.

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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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Gordon H. Beck, John M. Davis, and S. K. Cox

Abstract

Beam transmittance, emittance, reflectance, and outgoing radiance are inferred from interferometric measurements in the infrared window region for 14 temperate continental and 12 subtropical cirrus cloud cash observed during FIRE II at Parsons, Kansas (37°18′N, 95°07′W), and the ASTEX at Porto Santo, Madeira (33°5′N, 16°21′W). Cirrus emittances were found to span nearly the entire range from 0 to 1 for cloud systems in each location. Spectrally averaged volume extinction coefficients of 0.19 and 0.62 km−1 were found for the respective continental and subtropical samples. A delta-Eddington routine was incorporated into the inference technique to examine the sensitivity of the inferences to the upwelling surface and subcloud-layer emission reflected by the cloud assuming spherical and nonspherical cloud particles. Including reflectance had only a small effect on the spectrally averaged values of the radiative parameters; however, the slope of outgoing longwave radiation across the window region was altered with the introduction of smaller particles. The iterative method is structured in a manner that does not constrain the transmittances of the clear atmosphere to line-by-line model results. Inferred emittances and extinction coefficients are compared to previously published results.

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J. M. Davis, S. K. Cox, and T. B. McKee

Abstract

A concept and an apparatus designed to investigate the reflected and transmitted distributions of light from optically thick clouds is presented. The Cloud Field Optical Simulator (CFOS) is a laboratory device which utilizes an array of incandescent lamps as a source, simulated clouds made from cotton or styrofoam as targets, and an array of silicon photodiodes as detectors. The device allows virtually any source-target-detector geometry to be examined. Similitude between real clouds and their CFOS cotton or styrofoam counterparts is established by relying on a linear relationship between optical depth and the ratio of reflected to transmitted light for a semi-infinite layer. Comparisons of principal plane radiances observed by the CFOS with Monte Carlo computations for a water cloud at 0.7 μm show excellent agreement. Initial applications of the CFOS are discussed.

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