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Graeme L. Stephens

Abstract

The problem of radiative transfer in a horizontally infinite cloud layer possessing anisotropy with respect to volume extinction and other single-scattering properties was solved using the method of discrete space theory. The model was applied to a hypothetical ice crystal cloud composed of long cylinders displaying preferential orientation (in the horizontal) to provide the gross radiative properties of shortwave reflection, shortwave absorption, and longwave emission and reflection. These results were directly compared to clouds with the assumed microstructure of cylinders randomly orientated in three dimensions and of equivalent (by area) spheres. Generally, the gross radiative properties for clouds composed of equivalent spheres are substantially different than those for either of the cylinder models. The relative differences between the three assumed microstructures suggests that equivalent spheres cannot be employed to approximate the gross radiative properties determined for clouds composed of long cylinders. The preferential orientation of the long cylinders does affect significantly the estimates of cloud albedo, shortwave absorption and cloud emission when compared to the three-dimensional randomly orientated case. Thus it may be necessary to incorporate preferential crystal orientation into detailed multiple-scattering calculations which may eventually be employed to develop some parameterization of the gross radiative properties of ice crystal clouds.

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Graeme L. Stephens

Abstract

This paper offers a critical review of the topic of cloud–climate feedbacks and exposes some of the underlying reasons for the inherent lack of understanding of these feedbacks and why progress might be expected on this important climate problem in the coming decade. Although many processes and related parameters come under the influence of clouds, it is argued that atmospheric processes fundamentally govern the cloud feedbacks via the relationship between the atmospheric circulations, cloudiness, and the radiative and latent heating of the atmosphere. It is also shown how perturbations to the atmospheric radiation budget that are induced by cloud changes in response to climate forcing dictate the eventual response of the global-mean hydrological cycle of the climate model to climate forcing. This suggests that cloud feedbacks are likely to control the bulk precipitation efficiency and associated responses of the planet’s hydrological cycle to climate radiative forcings.

The paper provides a brief overview of the effects of clouds on the radiation budget of the earth–atmosphere system and a review of cloud feedbacks as they have been defined in simple systems, one being a system in radiative–convective equilibrium (RCE) and others relating to simple feedback ideas that regulate tropical SSTs. The systems perspective is reviewed as it has served as the basis for most feedback analyses. What emerges is the importance of being clear about the definition of the system. It is shown how different assumptions about the system produce very different conclusions about the magnitude and sign of feedbacks. Much more diligence is called for in terms of defining the system and justifying assumptions. In principle, there is also neither any theoretical basis to justify the system that defines feedbacks in terms of global–time-mean changes in surface temperature nor is there any compelling empirical evidence to do so. The lack of maturity of feedback analysis methods also suggests that progress in understanding climate feedback will require development of alternative methods of analysis.

It has been argued that, in view of the complex nature of the climate system, and the cumbersome problems encountered in diagnosing feedbacks, understanding cloud feedback will be gleaned neither from observations nor proved from simple theoretical argument alone. The blueprint for progress must follow a more arduous path that requires a carefully orchestrated and systematic combination of model and observations. Models provide the tool for diagnosing processes and quantifying feedbacks while observations provide the essential test of the model’s credibility in representing these processes. While GCM climate and NWP models represent the most complete description of all the interactions between the processes that presumably establish the main cloud feedbacks, the weak link in the use of these models lies in the cloud parameterization imbedded in them. Aspects of these parameterizations remain worrisome, containing levels of empiricism and assumptions that are hard to evaluate with current global observations. Clearly observationally based methods for evaluating cloud parameterizations are an important element in the road map to progress.

Although progress in understanding the cloud feedback problem has been slow and confused by past analysis, there are legitimate reasons outlined in the paper that give hope for real progress in the future.

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Graeme L. Stephens

Abstract

A new treatment of absorption in the window region depending on water vapor pressure is incorporated into the IR cooling rate model of Rodgers [for review see Rodgers and Walshaw (1966). The appropriate mean diffuse transmission gradient is derived. The results are presented in the form of hemispheric cross sections and indicate that for tropical climates, the new absorption has a considerable effect on radiative transfer in the lower troposphere. The results obtained are very encouraging, showing a reduction in the differences between the measured cooling rates of Cox (1969) and the calculated cooling rates in the lower troposphere.

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Graeme L. Stephens

Abstract

A multiple-scattering radiative transfer model is employed to evaluate the 11 μm and the broad-band infrared (IR) fluxes, cooling rates and emittances in model cirrus clouds for a number of standard vertical atmospheric profiles of temperature and moisture. The single-scattering properties for scattering by mono- and polydispersed randomly orientated long ice columns and for the associated polydispersed equivalent spheres are used in the calculation.

The results reveal IR reflectance at the cloud base of 4% (spheres) and 6% (cylinders). This reflectance modifies significantly the cloud effective emittances, cloud cooling rates and the emission by the total atmospheric column. It is shown that the radiative properties of model cirrus clouds determined under the equivalent sphere approximation represents well the properties determined for scattering by randomly orientated columns. The largest difference between the sphere and cylinder models is for reflectance which is a function of the degree of anisotropy of the scatter. It is shown that the relative contribution to the downward radiative flux at the cloud base (and within the cloud) varied according to the temperature differences between the cloud and the effective radiative temperature of the warmer atmosphere below the cloud. For example, the reflection contribution to the downward effective emittance varies for cylinders (spheres) from 25% (15%) of the total effective emittance in a model tropical atmosphere to 10% (5%) in a subarctic summer model atmosphere.

The existence of IR reflectance from high clouds may account for the previously reported discrepancies between the broad band (effective) emittances derived from observed flux profiles and the theoretical values. The results suggest that the reflectance of high clouds should definitely be included in any parameterization scheme. It appears that the effective emittance is not as useful for high-cloud parameterization as for low-level cloud because of its more pronounced dependence on the temperature structure of the atmosphere.

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Graeme L. Stephens

Abstract

A theoretical study was carried out to investigate the effect of radiative heating and cooling on the mass and heat budgets of an ice crystal. Equations describing the radiative budget of an ice crystal were derived and particle absorption efficiencies were calculated from the scattering theories for spherical and cylindrical particles. The radiation budget equation was solved in terms of upper limits of warming and cooling. By the introduction of the cloud blackbody depth, these limits were shown to apply over depths of several kilometres for typical ice clouds. The effects of radiation on the growth and evaporation rates of ice crystals were shown to be significant. Particle growth (evaporation) is enhanced (suppressed) in a radiatively cooled (heated) environment. It was further demonstrated that the effects of radiative cooling in the upper regions of the cloud greatly enhances the particle fall distances. In addition, particle growth and evaporation with and without radiation exchange are discussed in terms of their effect on the total expected heating of the cloud environment. It is demonstrated that radiation is the principal component in the diabatic heating of the cloud environment especially when the ice particle dimensions are large.

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Graeme L. Stephens

Abstract

This paper presents a formulation of the radiative transfer equation which allows for the distinction between various groups of spatial scales of variation that comprise the radiance field. Such a formulation provides a convenient means for studying the effects of spatial inhomogeneity and scale interaction on the radiative transfer. Notions of scale hierarchy and closure are introduced into the radiative transfer equation, and it is demonstrated how the customary treatment of partial cloudiness based on cloud amount as a weighting parameter is a special form of closure. Discussion of this particular closure and other assumptions relevant to this partial cloud treatment are presented. Another simple example of closure is described which allows for the treatment of spatial inhomogeneities as a new form of optical property. This concept is introduced into a two-stream model to demonstrate, in a gross way, the effects of inhomogeneities on radiative transfer. Comparisons with the more formal calculations of Part I are presented.

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Graeme L. Stephens

Abstract

This paper presents a review of the various methods used to compute both the fluxes and the rate of heating and/or cooling due to atmospheric radiation for use in numerical models of atmospheric circulation. The paper does not follow, step by step, the solution to the relevant radiative transfer problem but rather concentrates on providing the reader with the physical basis underlying the various methods. The paper discusses, separately, the various parameterizations for the absorptions by water vapor, carbon dioxide and ozone and for the scattering and absorption associated with cloud (and hazes) and also provides some indication of their accuracy.

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Graeme L. Stephens

Abstract

A general transform method is presented for studying problems of radiative transfer through absorbing, emitting and anisotropically scattering media exposed to arbitrary radiation conditions on its boundaries. The method permits quite arbitrary horizontal and vertical variability in the scattering and extinction properties of the medium bounded by a surface whose albedo and bidirectional reflection function varies from point to point. The technique developed incorporates a two-dimensional Fourier transform of the radiative transfer equation and a full Fourier expansion in azimuth. The general solution is based on the use of invariant imbedding principles in the form of doubling and adding algorithms. In developing these algorithms the principles of invariance are derived for three-dimensional geometry. Differences and similarities to the one-dimensional transfer problem are highlighted throughout. The method is applied to two special problems, namely the reflection by an atmosphere overlying or surface possessing an albedo step function and the transfer through an inhomogeneous Gaussian shaped medium.

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Graeme L. Stephens

Abstract

Monthly mean precipitable water data obtained from passive microwave radiometry (SMMR) are correlated with NMC-blended sea surface temperature data. It is shown that the monthly mean water vapor content of the atmosphere above the oceans can generally be prescribed from the sea surface temperature with a standard deviation of O.36 g cm−2. The form of the relationship between precipitable water and sea surface temperature in the range Ts gt; 15°C also resembles that predicted from simple arguments based on the Clausius-Clapeyron relationship. The annual cycle of the mass of SMMR water vapor integrated over the global oceans is shown to differ from analyses of fully global water vapor data in both phase and amplitude, and these difference paint to a significant influence of the continents on water vapor. Regional scale analyses of water vapor demonstrate that monthly averaged water vapor data, when contrasted with the bulk sea surface temperature relationship developed in this study, reflect various known characteristics of the time mean large-scale circulation over the oceans. A water vapor parameter is introduced to highlight the effects of large-scale motion on atmospheric water vapor. Based on the magnitude of this parameter, it is shown that the effects of large-scale flow on precipitable water vapor are regionally dependent, but for the most part, the influence of circulation is generally less than about ±20% of the seasonal mean.

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Cristian Mitrescu and Graeme L. Stephens

Abstract

Cirrus clouds play an important role in the climate through their optical and microphysical properties. The problem with measuring the optical properties of these clouds can be partially addressed by using lidar systems. The calibration of backscatter lidar systems, in particular, typically relies on the known molecular (Rayleigh) backscatter, which is a function of temperature, pressure, and chemical composition of the air. This paper presents an improved method for determining the cloud transmittance, and thus optical depth, derived from backscatter lidar measurements. A system of equations is developed in terms of a proposed metric that is required to possess a minima, and has a unique solution for the gain, offset, and transmittance. The new method is tested on a synthetic case as well as using data from two different lidar systems that operate at two different wavelengths. The method is applied to lidar data collected by the lidar operating at the central Pacific island of Nauru under the auspices of the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program.

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