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G. L. Stephens

Abstract

A detailed multiple-scattering model has been employed to investigate the sensitivity of radiation profiles and flux divergences to changes in macrostructure and microstructure of basic water cloud types. The study has been performed on a range of cloud types including variable distributions of liquid water content (LWC) and drop-size distributions. Total shortwave heating rates vary from 1 to 5°C h−1 and are larger in higher clouds. IR cooling rates in the upper regions of cloud also increase with increasing elevation and are dominated by the atmospheric window contribution. Thus the typical instrument discriminating the IR radiation between 7 and 14,μm will measure almost the entire IR radiative cooling or heating of low-level water clouds. Both shortwave heating and IR cooling within cloud layers are primarily dependent on LWC and its vertical distribution and are more or less independent of drop-size distribution. Cloud albedo does vary with drop-size distribution but is virtually independent of LWC distribution for fixed total water.

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G. L. Stephens

Abstract

No abstract available.

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G. L. Stephens

Abstract

The shortwave absorption, albedo and longwave emissivity of water clouds are parameterized for use in operational and climatic models of the atmosphere. The parameterization also provides the shortwave heating and longwave cooling rates within the cloud. The scheme presented in this paper assumes a prior knowledge of the broadband spectral fluxes incident on the cloud and further assumes that the atmospheric models will provide the surface albedo, solar zenith angle, cloud temperature and total vertical liquid water path. The last parameter was chosen because it likely to be available in atmospheric circulation models and both observational and theoretical evidence suggest that it is strongly related to the radiative properties of clouds (Paltridge, 1974; Platt, 1976).

The parameterization of shortwave radiation resembles a two-stream approximation which has been “tuned” to match the results from a detailed theoretical model. The longwave scheme simply involves the parameterization of effective emissivity. Both schemes have been tested and the errors investigated. The shortwave radiative properties of clouds when compared against calculations can generally be estimated by the parameterized scheme to within 5% of the incident flux at the cloud top. The longwave cooling rates are well within 0.5°C h−1 of the theoretical beating rate profiles. The errors in longwave cooling and shortwave absorption are much smaller than the uncertainties that may arise from variations of cloud liquid water distribution.

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Starley L. Thompson and Stephen G. Warren

Abstract

Correction to Volume 39, Issue 12, Article 2667.

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C. M. R. Platt and G. L. Stephens

Abstract

The scattering and reflection components of the remotely measured effective beam emittance of high clouds are calculated using a detailed model of radiative transfer through cirrus. Two atmospheric profiles of temperature and humidity are used representing tropical and midlatitude summer atmospheres respectively. The scattering and reflection components of the measured beam emittance are shown to be appreciable, particularly for tropical atmospheres where for example the reflection component at the ground for vertical viewing is 20% of the total emittance.

Computed values of the broad-band effective flux emittance are compared with equivalent values of the narrow-band effective flux emittance at 11 μm wavelength and the narrow-band beam emittance at 11μm. It is shown that the two former quantities are well correlated and approximately equal in magnitude.

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R. J. Engelen and G. L. Stephens

Abstract

Information theory is used to study the capabilities of the new-generation satellite infrared sounders [Atmospheric Infrared Sounder (AIRS) and Infrared Atmospheric Sounding Interferometer (IASI)] for retrieving atmospheric carbon dioxide (CO2) and for contrasting these new instruments with the current system of infrared sounders [Television and Infrared Observation Satellite (TIROS) Operational Vertical Sounder/High-Resolution Infrared Radiation Sounder (TOVS/HIRS)]. It is shown that instruments like AIRS and IASI will be able to retrieve column-averaged CO2 mixing ratios with high enough accuracy (order of 1–2 ppmv) to be useful for atmospheric CO2 inversion studies that try to estimate sources and sinks of CO2. On the other hand, the TOVS/HIRS system is only able to retrieve column-averaged CO2 mixing ratios with an accuracy of the same order as the seasonal amplitude of atmospheric CO2 variations (order of 10 ppmv). It is also shown that the constraining a priori covariance matrix has an important effect on what information can be extracted from the observations.

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Nicole L. Jones and Stephen G. Monismith

Abstract

The vertical distribution of the turbulent kinetic energy dissipation rate was measured using an array of four acoustic Doppler velocimeters in the shallow embayment of Grizzly Bay, San Francisco Bay, California. Owing to the combination of wind and tide forcing in this shallow system, the surface and bottom boundary layers overlapped. Whitecapping waves were generated for a significant spectral peak steepness greater than 0.05 or above a wind speed of 3 m s−1. Under conditions of whitecapping waves, the turbulent kinetic energy dissipation rate in the upper portion of the water column was greatly enhanced, relative to the predictions of wind stress wall-layer theory. Instead, the dissipation followed a modified deep-water breaking-wave scaling. Near the bed (bottom 10% of the water column), the dissipation measurements were either equal to or less than that predicted by wall-layer theory. Stratification due to concentration gradients in suspended sediment was identified as the likely cause for these periods of production–dissipation imbalance close to the bed. During 50% of the well-mixed conditions experienced in the month-long experiment, whitecapping waves provided the dominant source of turbulent kinetic energy over 90% or more of the water column.

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Starley L. Thompson and Stephen G. Warren

Abstract

State-of-the-art radiative transfer models can calculate outgoing infrared (IR) irradiance at the top of the atmosphere (F) to an accuracy suitable for climate modeling given the proper atmospheric profiles of temperature and absorbing gases and aerosols. However, such sophisticated methods are computationally time consuming and ill-suited for simple vertically-averaged models or diagnostic studies. The alternative of empirical expressions for F is plagued by observational uncertainty which forces the functional forms to be very simple. We develop, a parameterization of climatological F by curve-fitting the results of a detailed radiative transfer model. The parameterization comprises clear-sky and cloudy-sky terms. Only two parameters are used to predict clear-sky outgoing IR irradiance: surface air temperature (Ts) and 0–12 km height-mean relative humidity (^RH). With this choice of parameters (in particular, the use of ^RH instead of precipitable water) the outgoing IR irradiance can be estimated without knowledge of the detailed temperature profile or average lapse rate. Comparisons between the clear-sky parameterization and detailed model show maximum errors of ∼10 W m−2 with average errors of only a few watts per square meter. Single-layer “black” clouds are found to reduce the outgoing IR irradiance (relative to clear-sky values) as a function of TsTc, Tc and ^RH, where Tc is the cloud-top temperature. Errors in the parameterization of the cloudy-sky term are comparable to those of the clear-sky term.

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G. L. Stephens, G. W. Paltridge, and C. M. R. Platt

Abstract

Six case studies of “uniform” planetary boundary layer clouds are reported where the solar and infrared radiation fields, liquid water content, drop-size distributions and temperature and humidity profiles were measured simultaneously. The measurements are compared with theoretical prediction from a detailed radiative transfer model in an attempt to verify the performance of the model and its associated parameterization schemes (Parts 1 and 2). The measurements support the parameterization of both shortwave and longwave radiative characteristics in terms of vertical liquid water path (LWP) i.e., without the need to define cloud drop-size distributions. Within experimental error, there are no significant discrepancies between theory and measurement. However, there is some evidence in the present study, supported by measurements of others in (generally) thicker and denser clouds that solar absorption is in excess of theoretical prediction.

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P. M. Gabriel, S-C. Tsay, and G. L. Stephens

Abstract

The three-dimensional equation of radiative transfer is formally solved using a Fourier-Riccati approach while calculations are performed on cloudy media embedded in a two-dimensional space. An extension to Stephens’ work, this study addresses the coupling between space and angle asserted by the equation of transfer. In particular, the accuracy of the computed radiation field as it is influenced by the angular resolution of the phase function and spatial discretization of the cloudy medium is discussed. The necessity of using a large number of quadrature points to calculate fluxes even when the phase function is isotropic for media exhibiting vertical and horizontal inhomogeneities is demonstrated. Effects of incorrect spatial sampling on both radiance and flux fields are also quantified by example. Radiance and flux comparisons obtained by the Fourier-Riccati model and the independent pixel approximation for inhomogeneous cloudy media illustrate the inadequacy of the latter even for tenuous clouds.

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