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Keith T. Griffith, Stephen K. Cox, and Robert G. Knollenberg

Abstract

Longwave emissivities and the vertical profile of cooling rates of tropical cirrus clouds are determined using broadband hemispheric irradiance data. Additionally, a broadband mass absorption coefficient is defined and used to relate emissivity to water content. The data used were collected by the National Center for Atmospheric Research (NCAR) Sabreliner during the GARP Atlantic Tropical Experiment (GATE) in the summer of 1974.

Three case studies are analyzed showing that these tropical cirrus clouds approached an emissivity of 1.0 within a vertical distance of 1.0 km. Broadband mass absorption coefficients ranging from 0.076 to 0.096 m2 g−1 are derived. A comparison of these results with other work suggests that tropical cirrus cloud emissivities may be significantly larger than heretofore believed.

Ice water content of the clouds was deduced from data collected by a one-dimensional particle spectrometer. Analyses of the ice water content and the observed particle size distributions are presented.

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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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Bruce A. Albrecht, Stephen K. Cox, and Michael Prokofyev

Significant differences in U.S. and U.S.S.R. aircraft measurements of hemispherical infrared irradiance were noted during GATE in-flight intercomparisons. In specific instances the downward irradiance measured by the U.S.S.R. instrument (a Kozyrev pyrgeometer) was as much as 1.5 times greater than the irradiance measured with the U.S. instrument (an Eppley pyrgeometer). A post-GATE intercomparison at Colorado State University verified these differences; the pyrgeometer measurements were compared with independent measurements obtained with an infrared bolometer and with a radiative transfer calculation. The differences noted during GATE and post-GATE intercomparisons may be attributed to differences in calibration techniques and the accurate determination of the temperature of the instrument's thermopile reference junctions. When corrections based upon this analysis were applied to the U.S.S.R. data, the maximum intercomparison differences between the U.S. and the U.S.S.R. data were <5%.

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David W. Reynolds, Thomas H. Vonder Haar, and Stephen K. Cox

Abstract

Data from an experiment to measure the upward and downward components of solar radiation from aircraft during the Barbados Oceanographic and Meteorological Experiment (BOMEX) have been analyzed in the present study. Two groups of results were found.

In the cloud-free tropical troposphere: 1) Absorption of solar radiation in the entire troposphere can be twice as large as previous estimates of 13% given by Manabe and Strickler. Comparison of observed heating rates to calculations shows that the increase in attenuation may be due to non-gaseous constituents in the atmosphere. 2) The vertical profile of solar radiative heating was particularly variable in the lowest layers while in the mean, the profile suggests a slight maximum near 700 mb. 3) In using the solar radiation observations of this study in an energy budget of the tropics a hypothesis regarding a nighttime maximum of precipitation in the tropical regions was formed.

Findings from a radiative study of certain cloud type cases show (i) selective vertical absorption in stratocumulus acts to destabilize the clouds' environment; (ii) large cumulus or cumulonimbus clouds occasionally decrease the solar insolation reaching the surface to only 3% of that incoming at cloud top; and (iii) cirrus clouds are possible stabilizing mechanisms in the tropical environment since they act to warm the local environment at high altitudes while suppressing solar warming from cloud base to surface.

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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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Graeme L. Stephens, Stephen K. Cox, Paul W. Stackhouse Jr., John Davis, and the AT622 Class

This paper describes a classroom project that exposes students to research data collected during the Cirrus II First ISCCP (International Satellite Cloud Climatology Program) Regional Experiment Information Systems Office from Parsons, Kansas, during November and December 1991. The data employed in this project were primarily those obtained from a Michelson interferometer. The students were assigned a number of tasks that were aimed at (i) providing them with a basic understanding of a Michelson interferometer and, most importantly, an appreciation of the importance of calibration, (ii) understanding the spectral distribution of clear-sky emission and identifying major gaseous absorption features, (iii) understanding the effects of cirrus clouds on the emission spectrum, and finally (iv) learning how these spectra may be used to derive certain properties of the clouds and in so doing appreciate some of the limitations and ambiguities of this particular type of remote sensing.

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

Abstract

A numerical model which predicts the time variation of the thermodynamic structure of the trade-wind boundary layer is developed. Horizontally homogeneous conditions are assumed and the large-scale divergence, sea surface temperature and surface wind speed are specified externally. The model predicts the average value of mixing ratio and moist static energy in the subcloud and cloud layer and the slopes of these quantities in the cloud layer; the model also predicts the height of the transition layer (the layer which defines the boundary between the cloud and subcloud layer) and the height of the trade inversion. Subcloud layer convective fluxes are specified by using the bulk aerodynamic method for specifying the surface fluxes and a mixed-layer parameterization of the fluxes at the top of the subcloud layer. The moist convective processes are parameterized in terms of a mass flux which varies linearly with height and a cloud-environment difference in thermodynamic quantities which also varies linearly with height. Radiative fluxes are parameterized in terms of a specified cloud cover and vertically averaged boundary-layer heating.

The steady-state model solutions are shown to be relatively insensitive to the specification of closure parameters. The thermodynamic structure below the inversion is shown to be sensitive to the specification of surface wind speed, sea surface temperature, radiative heating and cloud cover. The height of the inversion is shown to be sensitive to these parameters and the large-scale divergence.

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Thomas B. McKee, Mark DeMaria, James A. Kuenning, and Stephen K. Cox

Abstract

Two experimental data sets have been compared with calculations of solar radiation scattered by finite cumulus clouds. The first approach was to observe the radiation exiting the sides of cumulus clouds from 0.8 to 3.0 km in vertical thickness as a function of azimuth angle relative to the sun. Results indicated that small scale feature in the clouds are not accounted for by a cubic model, but observations averaged over the cloud side are well represented by the model. Observations and model computed relative radiances yield a correlation coefficient of 0.91. The second approach was to observe cloud-to-cloud radiative interaction in the laboratory using rayon clouds to simulate optically thick cumulus clouds. These observations also agree well with theoretical predictions.

The results of the two experiments confirm that the Monte Carlo model simulations are compatible with calculations for finite clouds. They also give confidence to the use of physical models such as rayon in exploring radiative properties of clouds. A method has been presented to estimate the effect of cloud-to-cloud interaction on albedo of cloud fields as a function of cloud cover for finite clouds. The method has utility in model and satellite applications.

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Stephen K. Cox, David S. McDougal, David A. Randall, and Robert A. Schiffer
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Wayne H. Schubert, Joseph S. Wakefield, Ellen J. Steiner, and Stephen K. Cox

Abstract

A coupled, convective-radiative, boundary-layer model of marine stratocumulus clouds is presented. The model, which is a slight generalization of the cloud-topped, mixed-layer model of Lilly (1968), has as dependent variables the cloud-top height, cloud-base height, mixed-layer moist static energy and total water content, the turbulent fluxes of moist static energy and total water, the cloud-top jumps of moist static energy and total water, the cloud-top temperature, and the net radiative flux divergence at cloud top and in the mixed layer.

Under horizontally homogeneous steady-state conditions the governing equations reduce to a system of algebraic equations which is easily solved. This system has been solved for sea surface temperatures between 13 and 18°C and large-scale divergences between 1×10−6 and 6×10−6 s−1. These calculations have been performed for the case when all the radiative cooling is confined to the cloud-top jump condition and for the case when some of the cooling is allowed to extend into the mixed layer. The results show that the general pattern of mixed-layer response to sea surface temperature and large-scale divergence is not highly sensitive to the radiation partition. The results also show that the thermodynamic properties of the mixed layer and the surface fluxes of moist static energy and water vapor are sensitive to sea surface temperature but not to large-scale divergence. However, the mixed-layer depth is sensitive to large-scale divergence. Roughly speaking, the depth is inversely proportional to divergence so that halving the divergence approximately doubles the depth of the layer, which means that the cloud top seeks a certain subsidence (entrainment) rate. The turbulent fluxes of heat, water vapor and liquid water are discontinuous at cloud base. These discontinuities are interpreted in terms of convective parcel paths.

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