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Makiko Sato and James E. Hansen

Abstract

The spectrum of sunlight reflected by Jupiter is analyzed by comparing observations of Woodman et al. (1979) with multiple-scattering computations. The analysis yields information on the vertical cloud structure at several latitudes and on the abundance of CH4 and NH3 in the atmosphere of Jupiter.

The abundance of CH4, is (1.8±0.4) × 10−3 for [CH4]/[H2], which corresponds to a carbon abundance 2±0.4 times that in the atmosphere of the sun for currently accepted values of the solar composition. The quoted limits for the abundance include the effects of uncertainties in the cloud and haze structure. The abundance of NH3 is (2.8±1.0) × 10−4 for [NH3]/[H2] in the region between 1 bar and 3–5 bars, corresponding to a nitrogen abundance 1.5±0.5 times that in the atmosphere of the sun. Thus nitrogen is at least as abundant on Jupiter as on the sun, and it may exceed the abundance in the solar atmosphere by a factor as great as that for carbon. These abundances suggest that all ices (and rocks) are overabundant on Jupiter by a factor approximately 2 or more, providing an important constraint on models for the formation of Jupiter from the primitive solar nebula.

Clouds of mean visible optical depth approximately 10 exist in both belts and zones at a pressure level of several hundred millibars. The pressure level of the clouds, the gaseous NH3 abundance, the mean temperature profile and the Clausius-Clapeyron relation together suggest that these clouds are predominantly ammonia crystals and place the cloud bottom at 600–700 mb. Beneath this “ammonia” cloud region is an optically thick cloud layer at 3–5 bars; this cloud may be composed of H20. The region between these two cloud layers is relatively transparent. Thus NH4SH clouds, assumed to be optically thick in all previous multi-layered cloud models for Jupiter, are optically thin or broken, if they exist.

A diffuse distribution of aerosols (“haze”) exists between approximately 150 and 400–500 mb, i.e., above the main ammonia cloud region. These aerosols are at least 1 μm in diameter. The ultraviolet absorption occurs in both the haze region and the ammonia cloud region. The decreasing absorption with increasing wavelength is due to an increasing single scattering albedo rather than a decreasing aerosol optical depth as in the “Axel dust” model. Thus the spectral variation of albedo reflects a changing bulk absorption coefficient of the material composing the aerosols and is diagnostic of the aerosol composition.

Ratio spectra of the North Tropical Zone (NTrZ) and North Equatorial Belt (NEB) imply that the scatterers in the 150–500 mb haze region (which may include ammonia “cirrus") reach to higher altitudes over the NTrZ than over the NEB. But the tops of the more optically dense main “cloud” layer appear to reach to higher altitudes over the NEB, implying that the usual picture of the zones as regions of rising motions and enhanced ammonia cloudiness is too simple. The total optical thickness of aerosols in the haze and cloud regions is greater in the zone than in the belt, but there is more ultraviolet-absorbing aerosol in the belt. Ten parameters are needed to describe the vertical distribution of aerosol properties to satisfy only the spectra of Woodman et al., suggesting that the atmospheric dynamics and cloud physics an Jupiter are extremely complex.

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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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Gavin A. Schmidt, Reto Ruedy, James E. Hansen, Igor Aleinov, Nadine Bell, Mike Bauer, Susanne Bauer, Brian Cairns, Vittorio Canuto, Ye Cheng, Anthony Del Genio, Greg Faluvegi, Andrew D. Friend, Tim M. Hall, Yongyun Hu, Max Kelley, Nancy Y. Kiang, Dorothy Koch, Andy A. Lacis, Jean Lerner, Ken K. Lo, Ron L. Miller, Larissa Nazarenko, Valdar Oinas, Jan Perlwitz, Judith Perlwitz, David Rind, Anastasia Romanou, Gary L. Russell, Makiko Sato, Drew T. Shindell, Peter H. Stone, Shan Sun, Nick Tausnev, Duane Thresher, and Mao-Sung Yao

Abstract

A full description of the ModelE version of the Goddard Institute for Space Studies (GISS) atmospheric general circulation model (GCM) and results are presented for present-day climate simulations (ca. 1979). This version is a complete rewrite of previous models incorporating numerous improvements in basic physics, the stratospheric circulation, and forcing fields. Notable changes include the following: the model top is now above the stratopause, the number of vertical layers has increased, a new cloud microphysical scheme is used, vegetation biophysics now incorporates a sensitivity to humidity, atmospheric turbulence is calculated over the whole column, and new land snow and lake schemes are introduced. The performance of the model using three configurations with different horizontal and vertical resolutions is compared to quality-controlled in situ data, remotely sensed and reanalysis products. Overall, significant improvements over previous models are seen, particularly in upper-atmosphere temperatures and winds, cloud heights, precipitation, and sea level pressure. Data–model comparisons continue, however, to highlight persistent problems in the marine stratocumulus regions.

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