Further observations of regional dust storms and baroclinic eddies in the northern hemisphere of Mars

We have investigated the near-surface meteorology in the northern hemisphere of Mars through detailed analysis of data obtained with Mars Global Surveyor in January-August 2005. The season in the northern hemisphere ranged from midsummer through winter solstice of Mars Year (MY) 27. We examined composite, wide-angle images from the Mars Orbiter Camera and compiled a catalog of the dust storms that occurred in this interval. As in previous martian years, activity in the northern hemisphere was dominated by regional “flushing” dust storms that sweep southward through the major topographic basins, most frequently in Acidalia Planitia. We also used atmospheric profiles retrieved from radio occultation experiments to characterize eddy activity near the surface at high northern latitudes. There are strong correlations between the two sets of observations, which allowed us to identify three factors that influence the timing and location of the regional dust storms: (1) transitions among baroclinic wave modes, which strongly modulate the intensity of meridional winds near the surface, (2) storms zones, which impose strong zonal variations on the amplitude of some baroclinic eddies, and (3) stationary waves, which further modulate the wind field near the surface. The flushing dust storms ceased abruptly in midautumn, possibly in response to source depletion, CO₂ condensation, a shift in the period of the baroclinic eddies, and changes in the tidal wind field near the surface. Our results extend the meteorological record of the northern hemisphere, substantiate the findings of previous investigations, and further illuminate the climatic impact of baroclinic eddies.     

The stability of non-separable barotropic and baroclinic shear flows

The linear stability with respect to three-dimensional perturbations of unbounded barotropic and baroclinic shear flows depending linearly on both transverse coordinates is studied. The Boussinesq approximation is used, but the usual hydrostatic approximation in the vertical is relaxed. Dissipation effects are ignored. A baroclinic flow can always be destabilized by sufficiently large horizontal anticyclonic shear. The results are relevant for the stability of differential rotation in radiative stars and accretion disks.     

Exact density–potential pairs from the holomorphic Coulomb field

We show how the complex-shift method developed by Appell to study the gravitational field of a point mass (and used in electrodynamics by, among others, Newman, Carter, Lynden-Bell, and Kaiser to determine some remarkable properties of the electromagnetic field of rotating charged configurations) can be extended to obtain new and explicit density–potential pairs for self-gravitating systems departing significantly from spherical symmetry. The rotational properties of two axisymmetric baroclinic gaseous configurations derived with the proposed method are illustrated.     

Baroclinic instability in differentially rotating stars

A linear analysis of baroclinic instability in a stellar radiation zone with radial differential rotation is performed. The instability sets in at a very small rotation inhomogeneity, ΔΩ ～ 10^?3Ω. There are two families of unstable disturbances corresponding to Rossby waves and internal gravity waves. The instability is dynamical: its growth time is several thousand rotation periods but is short compared to the stellar evolution time. A decrease in thermal conductivity amplifies the instability. Unstable disturbances possess kinetic helicity. Magnetic field generation by the turbulence resulting from the instability is possible.     

The deep wind structure of the giant planets: Results from an anelastic general circulation model

The giant gas planets have hot convective interiors, and therefore a common assumption is that these deep atmospheres are close to a barotropic state. Here we show using a new anelastic general circulation model that baroclinic vorticity contributions are not negligible, and drive the system away from an isentropic and therefore barotropic state. The motion is still aligned with the direction of the axis of rotation as in a barotropic rotating fluid, but the wind structure has a vertical shear with stronger winds in the atmosphere than in the interior. This shear is associated with baroclinic compressibility effects. Most previous convection models of giant planets have used the Boussinesq approximation, which assumes the density is constant in depth; however, Jupiter's actual density varies by four orders of magnitude through its deep molecular envelope. We therefore developed a new general circulation model (based on the MITgcm) that is anelastic and thereby incorporates this density variation. The model's geometry is a full 3D sphere down to a small inner core. It is nonhydrostatic, uses an equation of state suitable for hydrogen-helium mixtures (SCVH), and is driven by an internal heating profile. We demonstrate the effect of compressibility by comparing anelastic and Boussinesq cases. The simulations develop a mean state that is geostrophic and hydrostatic including the often neglected, but significant, vertical Coriolis contribution. This leads to modification of the standard thermal wind relation for a deep compressible atmosphere. The interior flow organizes in large cyclonically rotating columnar eddies parallel to the rotation axis, which drive upgradient angular momentum eddy fluxes, generating the observed equatorial superrotation. Heat fluxes align with the axis of rotation, and provide a mechanism for the transport of heat poleward, which can cause the observed flat meridional emission. We address the issue of over-forcing which is common in such convection models and analyze the dependence of our results on this; showing that the vertical wind structure is not very sensitive to the Rayleigh number. We also study the effect of rotation, showing how the transition from a rapidly to a slowly rotating system affects the dynamics.     

密近双星子星间的照射吸收对具有椭率子星表面温度的改变

照射吸收引起的子星表面温度不均匀分布是子星包层大气斜压性的原因,同时又 会影响到光变曲线的形态．要了解子星包层大气斜压性形成的详细图景,要计算双星 照射吸收对光变曲线的影响,首先要计算出经过照射吸收后双星表面温度的改变,本 文在最简单的双球模型的基础上,采用椭球－球模型就椭球受球的照射吸收后所改变 的表面温度进行了计算．     

An ensemble Kalman filter data assimilation system for the martian atmosphere: Implementation and simulation experiments

The local ensemble transform Kalman filter (LETKF) is applied to the GFDL Mars general circulation model (MGCM) to demonstrate the potential benefit of an advanced data assimilation method. In perfect model (aka identical twin) experiments, simulated observations are used to assess the performance of the LETKF-MGCM system and to determine the dependence of the assimilation on observational data coverage. Temperature retrievals are simulated at locations that mirror the spatial distribution of the Thermal Emission Spectrometer (TES) retrievals from the Mars Global Surveyor (MGS). The LETKF converges quickly and substantially reduces the analysis and subsequent forecast errors in both temperature and velocity fields, even though only temperature observations are assimilated. The LETKF is also found to accurately estimate the magnitude of forecast uncertainties, notably those associated with the phase and amplitude of baroclinic waves along the boundary of the polar ice cap during Northern Hemisphere winter.     

Hydromagnetic stability of a rotating baroclinic star

The magnetohydrodynamic stability of rotating baroclinic star has been studied in the presence of poloidal and toroidal magnetic fields taking into account the diffusive effects. Necessary conditions for stability have been found by normal mode analysis under small wavelength approximation.     

Planetary-scale wave structure in the Martian atmosphere

Wave-like perturbations are found in the Mariner 9 IRIS atmospheric temperature data during late Northern Hemisphere winter in a latitude band between 45°N and 65°N. The nature of the data base prevents a unique separation of spatial and temporal behavior, but Fourier analysis of the data constrains the waves to discrete combinations of planetary wavenumber and period. One major spectral component possesses a meridional amplitude cross section with a maximum near the 1-mbar level at 60°N and is strongly correlated with the circumpolar jet observed in thermal winds calculated from the mean meridional temperature cross section. This feature is consistent with the low-wavenumber baroclinic waves observed in Viking Lander data, and the vertical structure reflects the behavior anticipated for a vertically penetrating quasi-geostrophic disturbance. Other possible origins for the wave cannotbe ruled out, however. Among these is a stationary wave forced by wavenumber-2 topographic relief.     

Tidal dynamical considerations constrain the state of an ocean on Enceladus

In previous work, solutions to the non-dissipative Laplace Tidal Equations (LTE) were used to provide bounds on the heat generated by the response of a subsurface ocean on Enceladus to an obliquity component of tidal forces. Here we improve these bounds using solutions from the LTE with a generic dissipation term explicitly added. We find solutions for a wider range of ocean tidal responses that include both unstratified (barotropic) and stratified (baroclinic) flow responses to obliquity as well as eccentricity components of the tidal forces. We consolidate the results in three ocean tidal scenarios on Enceladus that can explain the high heat fluxes (∼7 mW/m² globally averaged) inferred from measurements by the Cassini spacecraft: (1) a deep (1-50 km) barotropic ocean responding to obliquity tidal forces, where obliquity is at least 0.1°; (2) a shallow (∼360 m) barotropic ocean responding to eccentricity tidal forces; (3) a stratified (baroclinic) ocean responding to eccentricity tidal forces where the density-weighted “equivalent depth” (typically much smaller than the ocean’s physical depth) is near 360 m. The ocean is assumed to be global, but extensions for a semi-global case are also described. A more general result which is independent of the specific scenarios proposed is that an ocean attempting to freeze (with an associated decrease in its liquid depth, which affects the ocean’s dynamical response to the tidal forcing) must first pass through resonant configurations with a greatly increased generation of ocean tidal heat (exceeding 1 W/m² to 1 kW/m²) that would act to halt further freezing and stagnate the ocean state in this configuration so long as there is still orbital energy to provide the tidal forces. With an additional assumption that the ocean has evolved from a more energetic state where the depth of the liquid ocean was greater, we obtain the three scenarios proposed.     

Equilibrium and decay of sunspots

We suggest a quantitative sunspot model developed in terms of mean-field magnetohydrodynamics (MHD). The model consistently describes the distributions of magnetic field, fluid velocity, and thermodynamic parameters in a sunspot and the surrounding matter. Two versions of the model allow the MHD equilibrium in sunspots and their slow decay to be analyzed. The baroclinic flow converging to the sunspot plays an important role in the equilibrium. Several calculated characteristics—almost uniform distributions of brightness and magnetic field inside sunspots, their abrupt changes at the boundary, and nearly linear decreases in the area and magnetic flux of decaying sunspots with time—qualitatively agree with the observations.     

Tropospheric effects of variable solar activity

Within 12 hr after strong solar flares at middle to high latitudes a cooling at tropopause level is observed along with changes in temperature and height of isobaric levels in the troposphere. Delayed tropospheric effects are reported to occur at 2–4 days after a flare. It is suggested that the early effect through changing the baroclinic instability conditions of the troposphere is responsible for the occurrence of the late effects. Computations show that the observed early effect may amplify the growth rate of the most unstable wave by at least 20% and perhaps by a factor of 1.5 to 2.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.     

A first look at atmospheric dynamics and temperature variations on Titan

Pollack (1973) has used a radiative equilibrium model to match radiometric data for Titan and infers the atmospheric mass, composition, opacity, and gross vertical thermal structure. These results are used to estimate the atmospheric temperature variations by means of scaling analysis, taking into account dynamics both for a baroclinic wave regime and for an axially symmetric circulation regime. Horizontal temperature variations of the atmosphere and surface are found to be very small, and the circulation is found to be weak and probably axially symmetric. The small temperature variations appear to preclude the storage of volatiles in polar caps, so that the present atmospheric methane content may be due to a balance between outgassing and photodissociation.     

Dust storms originating in the northern hemisphere during the third mapping year of Mars Global Surveyor

Data from the third Mars Global Surveyor (MGS) mapping year (MY 26, 2003-2005) are used to investigate dust storms originating in the northern hemisphere. Flushing dust storms, which originate as frontal dust storms at the northern polar vortex edge and propagate southward through topographic channels, are observed immediately before and after a quiescent period that occurs around the northern winter solstice (240°<Ls<300°). Both the pre- and post-solstice active periods can be further divided into two sub-periods. The most vigorous of these flushing storms occurred during L_s 210-220° and L_s 310-320°. The lifted dust crossed the equator and accumulated in the southern hemisphere. These major dust storms enhanced the Hadley circulation and suppressed the lower-level baroclinic eddies in the northern mid and high latitudes. The 2-3 sol wave number m=3 traveling waves show the best correlation with flushing dust storms and can combine with other wave modes to produce storm tracks and fronts within individual sub-periods.     

Solitary waves on an unsymmetrical shear flow with applications to Jupiter's Great Red Spot

A baroclinic model of the Jovian atmosphere is assumed. The zonal flow in the neighborhood of the Great Red Spot has been modeled by an unsymmetrical shear layer. The calculated solitary wave flow fields consist of various combinations of regions of closed streamlines and regions of reversed flow. Some of these flows fields closely resemble those observed around the Great Red Spot but the lack of accurate atmospheric data makes it difficult to determine the type of flow field that would be predicted by the theory.     

Diurnal thermal tides in a non-synchronized hot Jupiter

We perform a linear analysis to investigate the dynamical response of a non-synchronized hot Jupiter to stellar irradiation. In this work, we consider the diurnal Fourier harmonic of the stellar irradiation acting at the top of a radiative layer of a hot Jupiter with no clouds and winds. In the absence of the Coriolis force, the diurnal thermal forcing can excite internal waves propagating into the planet's interior when the thermal forcing period is longer than the sound crossing time of the planet's surface. When the Coriolis effect is taken into consideration, the latitude-dependent stellar heating can excite weak internal waves (g modes) and/or strong baroclinic Rossby waves (buoyant r modes) depending on the asynchrony of the planet. When the planet spins faster than its orbital motion (i.e. retrograde thermal forcing), these waves carry negative angular momentum and are damped by radiative loss as they propagate downwards from the upper layer of the radiative zone. As a result, angular momentum is transferred from the lower layer of the radiative zone to the upper layer and generates a vertical shear. We estimate the resulting internal torques for different rotation periods based on the parameters of HD 209458b.     

Impacts of deforestation and afforestation in the Mediterranean region as simulated by the MPI atmospheric GCM

For two reasons it is important to study the sensitivity of the global climate to changes in the vegetation cover over land. First, in the real world, changes in the vegetation cover may have regional and global implications. Second, in numerical simulations, the sensitivity of the simulated climate may depend on the specific parameterization schemes employed in the model and on the model's large-scale systematic errors. The Max-Planck-Institute's global general circulation model ECHAM4 has been used to study the sensitivity of the local and global climate during a full annual cycle to deforestation and afforestation in the Mediterranean region. The deforestation represents an extreme desertification scenario for this region. The changes in the afforestation experiment are based on the pattern of the vegetation cover 2000 years before present when the climate in the Mediterranean was more humid. The comparison of the deforestation integration to the control shows a slight cooling at the surface and reduced precipitation during the summer as a result of less evapotranspiration of plants and less evaporation from the assumption of eroded soils. There is no significant signal during the winter season due to the stronger influence of the mid-latitude baroclinic disturbances. In general, the results of the afforestation experiment are opposite to those of the deforestation case. A significant response was found in the vicinity of grid points where the land surface characteristics were modified. The response in the Sahara in the afforestation experiment is in agreement with the results from other general circulation model studies.     

Dynamics of the solar tachocline – I. An incompressible study

Gough & McIntyre have suggested that the dynamics of the solar tachocline are dominated by the advection–diffusion balance between the differential rotation, a large-scale primordial field and baroclinicly driven meridional motions. This paper presents the first part of a study of the tachocline, in which a model of the rotation profile below the convection zone is constructed along the lines suggested by Gough & McIntyre and solved numerically. In this first part, a reduced model of the tachocline is derived in which the effects of compressibility and energy transport on the system are neglected; the meridional motions are driven instead by Ekman–Hartmann pumping. Through this simplification, the interaction of the fluid flow and the magnetic field can be isolated and is studied through non-linear numerical analysis for various field strengths and diffusivities. It is shown that there exists only a narrow range of magnetic field strengths for which the system can achieve a nearly uniform rotation. The results are discussed with respect to observations and to the limitations of this initial approach. A following paper combines the effects of realistic baroclinic driving and stratification with a model that closely follows the lines of work of Gough & McIntyre.     

Diagnosis and simulation of a rapidly developing cyclone related to a severe dust storm in East Asia

Mongolian cyclones are important to the outbreaks of severe dust storms in Northeast Asia. In this paper, we conduct a diagnostic study and a numerical simulation of a rapidly developing Mongolian cyclone that produced the April 5 to 7, 2000 severe dust storm in East Asia. The surface pressure of the cyclone decreased rapidly on April 5 to 6 causing strong surface winds. The diagnosis shows that the cyclone was a “dry cyclone”, as the convergence of moisture flux was weak and the baroclinic forcing was strong. The analysis of the Q vectors also reveals that ageostrophic wind was significant in the middle and upper part of the cyclone, resulting in strong vertical motion in the lower part. The three-dimensional structure of the cyclone is characterized by an ascending southern warm current and a descending northern cold current. This structure is favorable to the release of available potential energy and the intensification of the cyclone. In the second part of this study, we develop a coupled system of a dust emission scheme and a mesoscale numerical model (MM5V3) and applied the system to the simulation of the Mongolian cyclone. The main features of the cyclone identified from the diagnostic analysis are successfully reproduced. The predicted dust storm-affected area is found to be consistent with the meteorological observations and satellite remote sensing. It is shown that the coupled system is capable of predicting the spatial and temporal variations of the dust storm.