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James B. Pollack and Richard Young

Abstract

Modelling the atmosphere in accord with recent spacecraft and ground-based observations, we have carried out accurate, multiple scattering calculations to determine the solar energy deposition profile in the atmosphere of Venus. We find that most of the absorbed energy is deposited in the main cloud layer region, located at altitudes above 35 km, and that the ground receives approximately 3% of the energy absorbed in toto by Venus. Using these results we have computed vertical temperature profiles under conditions of pure radiative equilibrium and radiative-convective equilibrium. Since the latter results satisfactorily match the temperature structure determined from various spacecraft observations, we infer that the greenhouse effect can account for the high surface temperature. Aerosols make an important contribution to the infrared opacity in these calculations. Finally, we discuss preliminary three-dimensional calculations of the general circulation of the atmosphere that incorporate the results of the radiative calculations.

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James E. Hansen and James B. Pollack

Abstract

Calculations of the reflectivity of water clouds (liquid and ice particles) are compared to observations of terrestrial clouds in the near infrared. The presentation is divided into four parts which may be consulted individually. Section 3 presents new Mic scattering calculations of general interest, Sections 4–7 compare multiple-scattering results to cloud observations, Section 8 suggests a revision in the optical constants of ice for λ ≈ 3 μ, and the Appendix details several methods which substantially reduce the work load in multiple-scattering computations.

Our results indicate that it is possible to use the spectral variation of the reflectivity to derive the size of the cloud particles and their phase (liquid or solid) as well as the total optical depth of the clouds. Typical results show dense cirrus clouds to have an optical depth ≥10 and to be composed of ice particles of mean radius 15–20 μ the cumulus clouds which were analyzed showed a more variable, but usually smaller, particle size.

In spectral regions where the single-scattering albedo is high it is found that most of the gas absorption takes place within the clouds rather than above them.

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Owen B. Toon and James B. Pollack

Abstract

A global average model of the size distribution, chemical composition and optical thickness of stratospheric and tropospheric aerosols is proposed. The uncertainties involved in making the model are emphasized, and some of the model's implications are discussed. The model is designed for, and biased toward, global average radiative transfer calculations.

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Richard E. Young and James B. Pollack

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Not available.

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Richard E. Young and James B. Pollack

Abstract

Three–dimensional calculations of the circulation of the Venus atmosphere have resulted in mean zonal winds in the same direction and of the same magnitude as those observed, i.e., retrograde with speeds ∼100 m s−1. The solutions exhibit other observed properties of the circulation: small horizontal temperature contrasts with the larger variations being between equator and pole, meridional velocities at mid and low latitudes less than 10 m s−1, and the existence of planetary waves which at certain times show vertical flow field contours in a horizontal Y configuration.

The mechanism maintaining the large zonal winds is a nonlinear instability involving both the mean meridional circulation and planetary–scale eddies. The meridional circulation is the principal means by which zonal momentum is transported vertically. Planetary–scale eddies are the principal means by which potential energy is released, and they are also significant in transporting angular momentum horizontally. Planetary rotation plays an important role in initially generating the mean zonal winds starting from rest. Initial conditions affect the characteristics of the solutions, including the magnitude of the mean zonal velocity and whether or not planetary waves are generated.

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James B. Pollack and Jeffrey N. Cuzzi

Abstract

We propose an approximate method for evaluating the interaction Of randomly oriented. nonspherical particles with the total intensity component of electromagnetic radiation. When the particle size parameter x (the ratio of particle circumference to wavelength) is less than some upper bound x O (∼5) theory is used. For x>x O the interaction is divided into three components: diffraction, external reflection and transmission. Physical optics theory is used to obtain the first of these components; geometrical optics theory is applied to the second; and a simple parameterization is employed for the third. The predictions of this theory are found to be in very good agreement with laboratory measurements for a wide variety of particle shapes, sizes and refractive indices. Limitations of the theory are also noted.

As an application of the theory, we consider the influence of the shape of tropospheric aerosols on their contribution to Earth's global albedo. Irregularly shaped tropospheric particles generally have larger single-scattering albedos and smaller scattering asymmetry factors than their equal volume spherical counterparts. Hence, for a fixed optical depth, the former cause a larger increase in the global albedo than the latter. Explicit calculations of the contribution of tropospheric aerosols to the global albedo for a variety of values of the particles’ imaginary index of refraction and shape parameters indicate that size able differences occur between cases involving nonspherical particles and ones involving their spherical counterparts.

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James B. Pollack and Christopher P. McKay

Abstract

We have computed the perturbation to the infrared radiative heating rates of the lower stratosphere due to the occurrence of polar stratospheric clouds (PSCs) during the winter season in the Antarctic and Arctic regions. The calculations were made with a multispectral radiative transfer code that allows for scattering, absorption, and thermal emission by particles and gases. We investigated perturbations due to both particulate opacity of the PSCs (direct effect), and to the partial condensation, and hence, decrease of H20 vapor accompanying their formation (indirect effect).

For plausible values of model parameters, the direct effect is always one of increased radiative cooling, while the indirect effect is always one of decreased cooling. On a synoptic time scale of a single PSC event (∼ days), the net effect is probably one of enhanced cooling, with its magnitude having an important impact on stratospheric heating rates only for the most optically thick PSCs (extinction coefficient > 10−1 km−1). On the time scale of the winter season (∼ mouths), the cumulative radiative effect of multiple PSC formation may be significant for the heat budget and temperatures in the Antarctic region, but probably not so in the Arctic, where PSCs are both optically thinner and less frequent. In cases where the short- and long-term radiative effects of PSCs are significant, they act to alter the growth rate of individual PSC events and the frequency of occurrence of PSCs, respectively.

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James B. Pollack, David Rind, Andrew Lacis, James E. Hansen, Makiko Sato, and Reto Ruedy

Abstract

The authors have used the Goddard Institute for Space Studies Climate Model II to simulate the response of the climate system to a spatially and temporally constant forcing by volcanic aerosols having an optical depth of 0.15. The climatic changes produced by long-term volcanic aerosol forcing are obtained by differencing this simulation and one made for the present climate with no volcanic aerosol forcing. These climatic changes are compared with those obtained with the same climate model when the C02 content of the atmosphere was doubled (2×C02) and when the boundary conditions associated with the peak of the last ice age were used (18 K). In all three cases, the absolute magnitude of the change in the globally averaged air temperature at the surface is approximately the same, ∼5 K.

The simulations imply that a significant cooling of the troposphere and surface can occur at times of closely spaced, multiple, sulfur-rich volcanic explosions that span time scales of decades to centuries, such as occurred at the end of the nineteenth and beginning of the twentieth centuries. The steady-state climate response to volcanic forcing includes a large expansion of sea ice, especially in the Southern Hemisphere; a resultant large increase in surface and planetary albedo at high latitudes; and sizable changes in the annually and zonally averaged air temperature, ΔT; ΔT at the surface (ΔTs) does not sharply increase with increasing latitude, while ΔT in the lower stratosphere is positive at low latitudes and negative at high latitudes.

In certain ways, the climate response to the three different forcings is similar. Direct radiative forcing accounts for 30% and 25% of the total ΔTs in the volcano and 2×C02 runs, respectively. Changes in atmospheric water vapor act as the most important feedback, and are positive in all three cases. Albedo feedback is a significant, positive feedback at high latitudes in all three simulations, although the land ice feedback is prominent only in the 18 K run.

In other ways, the climate response to the three forcings is quite different. The latitudinal profiles of ΔTs for the three runs differ considerably, reflecting significant variations in the latitudinal profiles of the primary radiative forcing. Partially as a result of this difference in the ΔTs profiles, changes in eddy kinetic energy, beat transport by atmospheric eddies, and total atmospheric heat transport are quite different in the three cases. In fact, atmospheric beat transport acts as a positive feedback at high latitudes in the volcano run and as a negative feedback in the other two runs. These results raise questions about the ease with which atmospheric heat transport can be parameterized in a simple way in energy balance climate models.

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James B. Pollack, Owen B. Toon, Andrey Summers, Warren Van Camp, and Betty Baldwin

Abstract

Aircraft and Space Shuttles flying through the stratosphere over the next several decades will add sulfuric acid and aluminum oxide particles, respectively, to this region of the atmosphere. To evaluate the effect of these additional aerosols on the global heat balance, we have performed solar and terrestrial radiative transfer calculations. The solar calculations employed an accurate numerical method for solving the multiple-scattering problem for unpolarized light to determine the dependence of the global (spherical) albedo on the optical depth perturbation Δτ. Correct allowance was made for absorption by gases. Using these results, and those obtained from calculations of the terrestrial thermal flux at the top of the atmosphere, we determined the resulting change in the mean surface temperature, ΔT, as a function of Δτ. In both calculations, we used the measured optical constants of the aerosol species.

To apply these results to the problem of interest, we used engine exhaust properties of the various types of vehicles to estimate their optical depth perturbation and examined the record of past climate changes to set a threshold value, 0.1 K, on the mean surface temperature change, below which no significant impact is to be expected. Using the above information, we find that no significant climate change should result from the aerosols produced by Space Shuttles, SST's, and other high flying aircraft, operating at traffic levels projected for the next several decades. However, the effect of SST's is sufficiently close to our threshold limit to warrant a reevaluation as their characteristics are updated.

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James B. Pollack, Conway B. Leovy, Paul W. Greiman, and Yale Mintz

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A three-layer general circulation model, used to simulate the Martian atmosphere, is described and results are presented. The model assumes a dust-free pure C02 atmosphere and allows for a diurnally- varying convective boundary layer. Smoothed Martian topography and albedo variations are incorporated. The simulation described is for the period near southern winter solstice, season of the Viking landings. The zonally-averaged circulation, mass, heat and momentum balances, and properties of stationary and transient waves are described in some detail, and are compared with results of previous simulations of the Martian general circulation, with related features of the Earth's general circulation, and with observed characteristics of the Martian atmosphere.

The principal conclusions are the following: 1) The simulated zonally-averaged circulation is not very sensitive to differences between this model and the earlier general circulation model of Leovy and Mintz (1969), and compares reasonably well with observations, except for differences attributable to dust and season. 2) The meridional mass flow produced by the seasonal condensation of CO2, in the winter polar region has a major influence on the circulation, but, because of the weak influence of atmospheric heat transport, it is controlled almost entirely by radiation. 3) Quasi-barotropic stationary waves forced kinematically by the topography and resembling topographically-forced terrestrial planetary waves, are generated by the model in the winter hemisphere region of strong eastward flow, while baroclinic stationary waves are thermally forced by topography in the tropics and summer subtropics. 4) Transient baroclinically unstable waves, of somewhat lower dominant wavenumber than those found on the Earth, are generated in winter midlatitudes and their amplitudes, wavenumbers and phase speeds closely agree with what has been deduced from the Viking lander observations.

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