Baroclinic instabilities in Jupiter's zonal flow

The baroclinic stability of Jupiter's zonal flow is investigated using a model consisting of two continuously stratified fluid layers. The upper layer, containing a zonal shear flow and representing the Jovian cloudy regions above p ～ 5 bars, is the same as Eady's (1949) model for the Earth. The lower layer has a relatively large but finite depth with a quiescent basic state, representing the deep Jovian fluid bulk below p ～ 5 bars. Due to the presence of the lower layer, the linearized non-dimensional growth rates are drastically reduced from the O(1) growth rates of the original Early model. Only very long wavelengths relative to the upper fluid's radius of deformation L₁ are unstable. Eddy transports of heat are also reduced relative to estimates based on scaling arguments alone. Since the hydrostatic approximation for the lower-layer perturbation breaks down at great depths, a second model is presented in which energy propagates downward in an infinitely deep lower fluid obeying the full linearized fluid equations. In this model, the growth rates are again very small, but now all wavelengths are unstable with maximum growth rates occurring for wavelengths O(1) relative to L₁. These results illustrate the importance for the upper-layer meteorology of the interface boundary condition with the lower fluid, which is radically different from the rigid lower boundary of the Earth's troposphere.     

Monte Carlo Radiative Transfer Modeling of Lightning Observed in Galileo Images of Jupiter

We study lightning on Jupiter and the clouds illuminated by the lightning using images taken by the Galileo orbiter. The Galileo images have a resolution of ∼25 km/pixel and are able to resolve the shape of single lightning spots, which have half widths (radii) at half the maximum intensity in the range 45-80 km. We compare the shape and width of lightning flashes in the images with simulated flashes produced by our 3D Monte Carlo light-scattering model.The model calculates Monte Carlo scattering of photons in a 3D opacity distribution. During each scattering event, light is partially absorbed. The new direction of the photon after scattering is chosen according to a Henyey-Greenstein phase function. An image from each direction is produced by accumulating photons emerging from the cloud in a small range (bins) of emission angles. The light source is modeled either as a point or a vertical line.A plane-parallel cloud layer does not always fit the data. In some cases the cloud over the light source appears to resemble cumulus clouds on Earth. Lightning is estimated to occur at least as deep as the bottom of the expected water cloud. For the six flashes studied, we find that the clouds above the lightning are optically thick (τ>5). Jovian flashes are more regular and circular than the largest terrestrial flashes observed from space. On Jupiter there is nothing equivalent to the 30-40-km horizontal flashes that are seen on Earth.     

Submarine fan facies of the Upper Cretaceous Great Valley sequence,northern and central California

The Upper Cretaceous part of the Great Valley Sequence provides a unique opportunity to study deep-marine sedimentation within an arc-trench gap. Facies analysis delineates submarine fan facies similar to those described from other ancient basins. Fan models and facies of Mutti and Ricci-Lucchi allow reconstruction of the following depositional environments: basin plain, outer fan, midfan, inner fan, and slope. Basin plain deposits are characterized by hemipelagic mudstone with randomly interbedded thin sandstone beds exhibiting distal turbidite characteristics. Outer fan deposits are characterized by regularly interbedded sandstone and mudstone, and commonly exhibit thickening-upward (negative) cycles that constitute depositional lobes. The sandstone occurs as proximal to distal turbidites without channeling. Midfan deposits are characterized by the predominance of coarse-grained, thick, channelized sandstone beds that commonly are amalgamated. Thinning-upward (positive) cycles and braided channelization also are common. Inner fan deposits are characterized by major channel-fill complexes (conglomerate, pebbly sandstone, and pebbly mudstone) enclosed in mudstone and siltstone. Positive cycles occur within these channel-fill complexes. Much of the fine-grained material consists of levee (overbank) deposits that are characterized by rhythmically interbedded thin mudstone and irregular sandstone beds with climbing and starved ripples. Slope deposits are characterized by mudstone with little interbedded sandstone; slumping and contortion of bedding is common. Progressions of fan facies associations can be described as retrogradational and progradational suites that correspond, respectively, to onlapping and offlapping relations in the basin. The paleoenvironments, fan facies associations, and tectonic setting of the Late Cretaceous fore-arc basin are similar to those of modern arc—trench systems.     

Saturn eddy momentum fluxes and convection: First estimates from Cassini images

We apply an automated cloud feature tracking algorithm to estimate eddy momentum fluxes in Saturn's southern hemisphere from Cassini Imaging Science Subsystem near-infrared continuum image sequences. Voyager Saturn manually tracked images had suggested no conversion of eddy to mean flow kinetic energy, but this was based on a small sample of <1000 wind vectors. The automated procedure we use for the Cassini data produces an order of magnitude more usable wind vectors with relatively unbiased sampling. Automated tracking is successful in and around the westward jet latitudes on Saturn but not in the vicinity of most eastward jets, where the linearity and non-discrete nature of cloud features produces ambiguous results. For the regions we are able to track, we find peak eddy fluxes and a clear positive correlation between eddy momentum fluxes and meridional shear of the mean zonal wind, implying that eddies supply momentum to eastward jets and remove momentum from westward jets at a rate . The behavior we observe is similar to that seen on Jupiter, though with smaller eddy-mean kinetic energy conversion rates per unit mass of atmosphere (). We also use the appearance and rapid evolution of small bright features at continuum wavelengths, in combination with evidence from weak methane band images where possible, to diagnose the occurrence of moist convective storms on Saturn. Areal expansion rates imply updraft speeds of over the convective anvil cloud area. As on Jupiter, convection preferentially occurs in cyclonic shear regions on Saturn, but unlike Jupiter, convection is also observed in eastward jet regions. With one possible exception, the large eddy fluxes seen in the cyclonic shear latitudes do not seem to be associated with convective events.     

作为热机的自然对流:CAPE理论

在许多行星上,不断有热量供给地面,且热量也不断通过大气向太空发出红外辐射.由于在高压区是热源不是热汇,因此,该系统能够做机械功.大气对流是可以在此系统中运转的自然热机.基于这种热机结构,提出了一个大气对流的简单理论,预报在统计平衡状态下干、湿对流的浮力、垂直速度及其覆盖区域.在对流热机一次循环期间,来自地面层(热源,热区)的热,其中一部分被报废到空间自由对流层(热汇,冷区),并从那里辐射到太空.其余部分变为机械功.此机械功用于克服机械耗散维持对流运动上.最终,机械能耗尽,转变成热能.消耗掉的部分能量被辐射到太空,而其余的通过对流气块进入再循环.在温度较低能量消耗部分降低的情况下,温度较高能量消耗部分增加,提高了对流热机的视效率.对流热机所做的功对气柱积分,给出了存在于行星大气中克服粘性耗散、维持对流运动的对流有效位能(CAPE)统计平衡量的大小.此积分被认为是准平衡条件下行星状态的一个全球性数字.对于地球目前的气候,热机结构预报热带地区CAPE值的大小为1000 J·kg-1,该预报值与观测值一致.从结果也可得出,存在于对流大气的CAPE总量应随着全球地表温度的升高(或大气对红外辐射的暗度)的增大而增大.     

Supersonic meteorology of Io: Sublimation-driven flow of SO2

The horizontal flow of SO₂ gas from day side to night side of Io is calculated. The surface is assumed to be covered by a frost whose vapor pressure at the subsolar point is orders of magnitude larger than that on the night side. Temperature of the frost is controlled by radiation. The flow is hydrostatic and turbulent, with velocity and entropy per particle independent of height. The vertically integrated conservation equations for mass, momentum, and energy are solved for atmospheric pressure, temperature, and horizontal velocity as functions of solar zenith angle. Formulas from boundary layer theory govern the interaction between atmosphere and surface. The flow becomes supersonic as it expands away from the subsolar point, as in the theory of rocket nozzles and the solar wind. Within 35° of the subsolar point atmospheric pressureis less than the frost vapor pressure, and the frost sublimes. Elsewhere, atmospheric pressure is greater than the frost vapor pressure, and the frost condenses. The two pressures seldom differ by more than a factor of 2. The sublimation rate at the subsolar point is proportional to the frost vapor pressure, which is a sensitive function of temperature. For a subsolar temperature of 130°K, the sublimation rate is 10¹⁵ molecules/cm²/sec. Diurnally averaged sublimation rates at the equator are comparable to the 0.1 cm/year resurfacing rate required for burial of impact craters. At the poles where both the vapor pressures and atmospheric pressures are low, the condensation rates are 100 times smaller. Surface pressures near the terminator are generally too low to account for the ionosphere discovered by Pioneer 10. The possibility of a noncondensable gas in addition to SO₂ must be seriously considered.     

Interaction between eddies and mean flow in Jupiter's atmosphere: Analysis of Cassini imaging data

Beebe et al. [Beebe, R.F., et al., 1980. Geophys. Res. Lett. 17, 1-4] and Ingersoll et al. [Ingersoll, A.P., et al., 1981. J. Geophys. Res. 86, 8733-8743] used images from Voyagers 1 and 2 to analyze the interaction between zonal winds and eddies in Jupiter's atmosphere. They reported a high positive correlation between Jupiter's eddy momentum flux, , and the variation of zonal velocity with latitude, . This correlation implied a surprisingly high rate of conversion of energy from eddies to zonal flow: , a value more than 10% of Jupiter's thermal flux emission. However, Sromovsky et al. [Sromovsky, L.A., et al., 1982. J. Atmos. Sci. 39, 1413-1432] argued that possible biases in the analysis could have caused an artificially high correlation. In addition, significant differences in the derived eddy flux between datasets put into question the robustness of any one result. We return to this long-standing puzzle using images of Jupiter from the Cassini flyby of December 2000. Our method is similar to previous analyses, but utilizes an automatic feature tracker instead of the human eye. The number of velocity vectors used in this analysis is over 200,000, compared to the 14,000 vectors used by Ingersoll et al. We also find a positive correlation between and and derive a global average power per unit mass, , ranging from . Utilizing Ingersoll et al.'s estimate of the mass per unit area involved in the transport, this would imply a rate of energy conversion of . We discuss the implications of this result and employ several tests to demonstrate its robustness.     

Solar heating and internal heat flow on Jupiter

Models of convection in Jupiter's interior are studied to test the hypothesis that internal heat balances the absorbed sunlight at each latitude. Such a balance requires that the ratio of total emitted heat to absorbed sunlight be above a critical value 4/π ≈ 1.27. The necessary horizontal heat transport then takes place in the interior instead of in the atmosphere. Regions of stable stratification can arise in the interior owing to the effects of solar heating and rotation. In such regions, upward heat transfer takes place on sloping surfaces, as in the Earth's atmosphere, provided there are horizontal temperature gradients. Potential temperature gradients are found to be small, and the time constant for the pattern to reach equilibrium is found to be short compared to the age of the solar system. It is suggested that Jupiter and Saturn owe their axisymmetric appearance to internal heat flow, which eliminates differential heating in the atmosphere that would otherwise drive meridional motions.     

Lateral inhomogeneities in the Venus atmosphere: Analysis of thermal infrared maps

The thermal infrared maps of Venus published by Murray, Wildey, and Westphal (1963) and Westphal, Wildey, and Murray (1965) have been analyzed systematically in order to separate the observed intensity into a limb-darkening component and a solar-associated component representing fixed patterns of intensity corotating with the earth and sun, respectively. Interesting new results are obtained for the solar-associated component. Regions near the subsolar point and the poles are not covered in the original maps or in the analysis.The solar-associated pattern of intensity is very nearly symmetric about the equator. In both northern and southern hemispheres, an intensity minimum seems to occur near the morning terminator at middle to high latitudes, slightly beyond the limit of the maps. An intensity maximum occurs on the equator slightly to the east of the antisolar point. Three broad ridges of relatively high intensity radiate away from this point, one pointing to the west along the equator, the others pointing to the northeast and southeast, respectively. The eastward tilt of the latter two ridges may indicate that horizontal exchange is important in maintaining the equatorial maximum of zonal momentum which is associated with the 4-day circulation of the Venus atmosphere.