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Harshvardhan

Abstract

A study has been made of the effect of brokenness on the infrared and albedo feedback of clouds in climate models using a simplified treatment of broken cloudiness. It is shown that the individual feedback terms computed using the plane-parallel assumption differ markedly from the computations performed for a regular array of cuboidal clouds.

If the cloudiness changes such that clear areas become cloudy but the nature of the broken cloud field remains unchanged, then both the feedbacks are magnified, the infrared more so than the albedo. However, if individual elements in the cloud field are increased horizontally in a constrained area, the feedbacks can be enhanced or diminished depending on the cloud fraction and aspect ratio of the elements. It is also shown that, in the global average, the ratio of the change in outgoing infrared flux to global albedo, as deduced from satellite measurements, will be larger than model simulators using planiform clouds.

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Harshvardhan and David A. Randall

Abstract

No abstract available.

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Harshvardhan and James A. Weinman

Abstract

A study has been made of infrared radiative transfer through a regular array of cuboidal clouds which considers the interaction of the sides of the clouds with each other and the ground. The theory is developed for black clouds and is extended to scattering clouds using a variable azimuth two-stream (VATS) approximation (Harshvardhan et al., 1981). It is shown that geometrical considerations often dominate over the microphysical aspects of radiative transfer through the clouds. For example, the difference in simulated 10 μm brightness temperature between black isothermal cubic clouds and cubic clouds of optical depth 10, is <2 K for zenith angles <50° for all cloud fractions when viewed parallel to the array.

The results show that serious errors are made in flux and cooling rate computations if broken clouds are modeled as planiform. Radiances computed by the usual practice of area-weighting cloudy- and clear-sky radiances are in error by 2–8 K in brightness temperature for cubic clouds over a wide range of cloud fractions and zenith angles. It is also shown that the lapse rate does not markedly affect the exiting radiances for cuboidal clouds of unit aspect ratio and optical depth 10.

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Michael D. King and Harshvardhan

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Computational results have been obtained for the plane albedo, total transmission and fractional absorption of plane-parallel atmospheres composed of cloud droplets. These computations, which were obtained using the doubling method, are compared with comparable results obtained using selected radiative transfer approximations. Both the relative and absolute accuracies of asymptotic theory for thick layers and delta-Eddington, Meador–Weaver and Coakley–Chýlek approximations are compared as a function of optical thickness, solar zenith angle and single scattering albedo. Asymptotic theory is found to be accurate to within 5% for all optical thickness greater than about 6. On the other hand, the Coakley–Chýlek approximation is accurate to within 5% for thin atmospheres having optical thickness less than about 0.2 for most values of the solar zenith angle. Though the accuracies of delta-Eddington and Meador-Weaver approximations are less easily summarized it can generally be concluded that the delta-Eddington approximation is the most accurate for conservative scattering when the solar zenith angle is small, while the Meador–Weaver approximation is the most accurate for nonconservative scattering (ω0 ≤ 0.9). Selected results from the Eddington approximation are presented to illustrate the effect of delta function scaling in the delta-Eddington approximation. In addition, selected results from the single scattering approximation and asymptotic theory are presented in order to help explain the strengths and limitations of the various approximations.

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Harshvardhan and Michael D. King

Abstract

Computational results have been obtained for the spherical albedo, global transmission, and global absorption of plane-parallel layers composed of cloud droplets. These computations, obtained using the doubling method for the entire range of single scattering albedos (0 ≤ ω 0 ≤ 1) and for optical depths between 0.1 and 100, are compared with corresponding results obtained using selected multiple scattering approximations. Both the relative and absolute accuracies of asymptotic theory for thick layers, three diffuse two-stream approximations, and two integrated two-stream approximations are presented as a function of optical thickness and single scattering albedo for a scattering phase function representative of cloud droplets at visible wavelengths. The spherical albedo and global absorption computed using asymptotic theory are found to be accurate to better than 5% for all values of the single scattering albedo, provided the optical thickness exceeds about 2. The diffuse two-stream approximations have relative accuracies that are much worse than 5% for the spherical albedo over most of the parameter space, yet are accurate to within 5% in the global absorption when the absorption is significant. The integrated delta-Eddington scheme appears to be the most suitable model over the entire range of variables, generally producing relative errors of less than 5% in both the spherical albedo and global absorption.

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Raymond C. Espinoza Jr. and Harshvardhan

Abstract

The process of finding computationally efficient methods to parameterize the effects of the radiative interactions between water vapor absorption and cloud droplet absorption is fraught with complications. Inside a cloud, scattering greatly enhances the vapor absorption, and the amount of vapor above the cloud layer influences the absorption in a cloud layer. A widely used technique used to treat water vapor and liquid absorption is through the use of the k-distribution method. In the current study, this method is used with a one- and a three-band model to produce absorptances, reflectances, and transmittances of cloudy layers in the near infrared, but unlike standard usage, the single scattering properties are assigned to individual k values from weighting with the k distribution in the limit of semi-infinite and thin clouds, as well as the square root of the co-albedo. While improvement in the accuracy of the radiative parameters is noted for the three-band model as compared to standard three-band models, the one-band model with the square root approximation is very successful in producing absorptances, reflectances, and transmittances that are shown to be on the same order of accuracy as those produced by the three-band models with average single scattering properties. This method shows promise as a useful computational tool in general circulation models since it reduces the number of times the typical two-stream computation needs to be carried out or, alternately, provides more accurate results for the same computational effort as standard models.

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Harshvardhan, James A. Weinman, and Roger Davies

Abstract

The transport of infrared radiation in a single cuboidal cloud has been modeled using a variable azimuth two-stream (VATS) approximation. Computations have been made at 10 μm for a Deirmendjian (1969) C-1 water cloud of single scattering albedo, ω = 0.638 and asymmetry parameter, g=0.865. Results indicate, that the emittance of the top face of the model cloud is always less than that for a plane parallel cloud of the same optical depth. The hemispheric flux escaping from the cloud top has a gradient from the censor to the edges which are warmer when the cloud is over warmer ground. Cooling rate calculations in the 8–13.6 μm region show that there is cooling out of the sides of the cloud at all levels even when there is heating of the core from the ground below.

The radiances exiting from model cuboidal clouds were computed by path integration over the source function obtained with the two-stream approximation. Results suggest that the brightness temperature measured from finite clouds will overestimate the cloud-top temperature.

Some key results of the model have been compared with Monte Carlo simulations. Overall errors in flux and radiance average a few degrees for most cases.

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David A. Randall, Harshvardhan, and Donald A. Dazlich

Abstract

This paper presents an analysis of the diurnal and semidiurnal variability of precipitation, evaporation, precipitable water, horizontal moisture flux convergence, cloudiness, and cloud radiative forcing, as simulated by the Colorado State University General Circulation Model (GCM). In broad agreement with observations, the model produces an afternoon precipitation maximum over land in warm rainy regions, such as the tropics and the midlatitude summer continents, and an early morning maximum over the oceans far from land. The statistical significance of these model results is demonstrated using a chi-square test. The observed diurnal variation of temperature in the oceanic tropical middle troposphere is also realistically simulated.

Encouraged by these results, the model was used to investigate the causes of the diurnal cycle of precipitation over the oceans. For this purpose, experiments have been performed with an all-ocean global model. Results show that an oceanic diurnal cycle of precipitation occurs even in the absence of neighboring continents and tends to have a morning maximum. It is generally weaker than observed, however. When the radiative effects of clouds are omitted, the simulated diurnal cycle of precipitation is much weaker but still present, with essentially the same phase.

Several experiments have also been performed with a one-dimensional version of the GCM, in which time-dependent large-scale vertical motion can be prescribed. The results show that even in the absence of any systematic daily variation of the large-scale vertical motion, the model produces a diurnal cycle of precipitation with an amplitude of about 1 mm day−1, and a morning maximum.

Finally, previously published results have been followed up, which show that the diurnal cycle strongly affects the partitioning of precipitation between land and sea. The new analysis is based on comparison of three nondiurnal June-July integrations with three Julys from a multiyear diurnally forced seasonal simulation. The results show major changes in the time-averaged surface energy budget, and much more precipitation in “summer monsoon” regimes when the diurnal cycle is omitted.

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Michael D. King, Harshvardhan, and Albert Arking

Abstract

An accurate multiple-scattering model has been employed to examine the effect of an aerosol layer at 25 mb, corresponding to the EL Chichon observations, on the reflection, transmission and absorption of radiation by the stratosphere as a function of latitude, optical thickness and aerosol size distribution. Results are presented and parameterized for each of two wavelength intervals in the shortwave region and 17 wavelength intervals in the longwave region for three models of the aerosol size distribution. They include one model representing the unperturbed stratospheric aerosol plus two models based on measurements of the EL Chichon aerosol size distribution. In addition to models of the radiative properties of the aerosol layer, a simple model of the latitudinal distribution of aerosol optical thickness as a function of time is developed, based on diffusive transport in latitude and exponential decay in time. These parameterizations for solar and infrared radiation, together with the dispersion model, permit climate models to account for the evolution of an aerosol size distribution from post-volcanic conditions to background conditions.

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Harshvardhan, David A. Randall, and Donald A. Dazlich

Abstract

Attempts to map the global longwave surface radiation budget from space have been thwarted by the presence of clouds. Unlike the shortwave, there is no physical relationship between the outgoing longwave and the surface longwave under cloudy skies. Therefore, there is no correlation between spatial and temporal averages of the outgoing longwave radiation and not longwave radiation at the surface. However, in regions where a particular cloud regime exists preferentially, a relationship between the mean longwave cloud radiative forcing (CRF) at the top of the atmosphere and at the surface can he shown to exist. Results from a general circulation model suggest that this relationship for monthly means is coherent over fairly large geographical areas. For example, in tropical convective areas, the longwave CRF at the top is very large, but at the surface it is quite small because of the high opacity of the lowest layers of the atmosphere. On the other hand, in areas of stratus over cool ocean surfaces, the longwave CRF at the top is very small but at the surface, it is quite substantial.

To the extent that the cloudiness simulated in the model mimics the real atmosphere, it may be possible to estimate the monthly mean longwave CRF at the surface from the Earth Radiation Budget Experiment cloud forcing at the top. The net longwave radiation at the surface can then be mapped if monthly means of the clear-sky fluxes are obtained by some independent technique.

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