Analysis of planetary cloud images considering local rotation

<p>Cloud tracking has been used to measure motions of planetary atmospheres remotely without direct observations. Cloud tracking is a method to track the movements of cloud parcels using temporally-continuous cloud images to obtain cloud motion vectors. Since it is considered in most of the cases that clouds move at the same speed and the same direction as the surrounding atmosphere, the wind direction and wind velocity can be obtained by tracking the movement of clouds. This method has been applied to the atmospheres of the planets, such as Venus and Jupiter, where direct observation is difficult as well as that of the Earth's atmosphere.</p> <p>In the cloud tracking methods developed so far, only the parallel movement of the characteristic pattern is assumed, and the rotation of the pattern is not directly measured. Here we developed a new algorithm to track the parallel movement and the rotation of cloud patterns using the rotation invariant phase-only correlation method. In this method, the tracking region is Fourier-transformed before applying the phase correlation method for measuring parallel movement, and logarithmic polar coordinate conversion is performed to the amplitude spectra so that the rotation and enlargement/reduction motions can be obtained as parallel movements. With this method, not only the parallel movement but also the rotational movement of the characteristic pattern can be detected at the same time.</p> <p>We first applied the newly-developed method to simulated image pairs. The rotation rate of the cloud pattern and the vorticity derived from the velocity field were compared in three velocity patterns: solid body rotation, irrotational vortex, and sinusoidal velocity field in the latitude and longitude directions. As a result, in the case of a solid body rotation, the wind speed field and the rotation angle were determined correctly. Large-scale rotations can be measured more accurately by the new method than by the calculation of vorticity from the cloud-tracked velocity. When the scale of the velocity structure is decreased, the number of missing cloud tracking vectors increases, and thus the spatial pattern of the vorticity becomes difficult to obtain. Even in such cases, the spatial pattern of the rotation rate can be relatively well retrieved although its amplitude is underestimated.</p> <p>The new method was applied to Jupiter and Venus images based on the results above. For Jupiter, many small eddies were found to be distributed in the equatorial region. The spatial scales and the strengths of the eddies resemble those seen in numerical simulations. The observed vortex chains can contribute to the formation of Jupiter's equatorial jet. For Venus, though small-scale eddies are less prominent, a planetary-scale distribution of the rotation rate with a north-south reflection symmetry was seen, such that anti-clockwise rotation occurs in the northern hemisphere and clockwise rotation in the southern hemisphere. Since the large-scale rotation pattern is consistent with the latitudinal shear of the mean zonal wind, the result means that the rotation of small-scale clouds is caused by the large-scale wind. This result suggests that the small-scale streaky features at mid-latitudes, whose origin is poorly understood, are created by the deformation of clouds by large-scale winds.</p> <p>The newly-developed method can extract smaller scale eddies than those observed in the previous studies; the method has enabled investigation of the interaction between different scales in a wider wavelength range. The method would also enable studies of mesoscale weather systems such as deep convection and also studies of upward energy cascade from small-scale convective storms to planetary scale motions in planetary atmospheres.</p>

Brightness modulations of our nearest terrestrial planet Venus reveal atmospheric super-rotation rather than surface features

Terrestrial exoplanets orbiting within or near their host stars’ habitable zone are potentially apt for life. It has been proposed that time-series measurements of reflected starlight from such planets will reveal their rotational period, main surface features and some atmospheric information. From imagery obtained with the Akatsuki spacecraft, here we show that Venus’ brightness at 283, 365, and 2020 nm is modulated by one or both of two periods of 3.7 and 4.6 days, and typical amplitudes <10% but occasional events of 20–40%. The modulations are unrelated to the solid-body rotation; they are caused by planetary-scale waves superimposed on the super-rotating winds. Here we propose that two modulation periods whose ratio of large-to-small values is not an integer number imply the existence of an atmosphere if detected at an exoplanet, but it remains ambiguous whether the atmosphere is optically thin or thick, as for Earth or Venus respectively. Multi-wavelength and long temporal baseline observations may be required to decide between these scenarios. Ultimately, Venus represents a false positive for interpretations of brightness modulations of terrestrial exoplanets in terms of surface features.

Mesoscale eddy characteristics in the Labrador Sea from observations and a 1/60° numerical model

&lt;p&gt;Mesoscale eddies play an important role in lateral property fluxes. Observational studies often use sea level anomaly maps from satellite altimetry to estimate eddy statistics (incl. eddy kinetic energy). Recent findings suggest that altimetry derived eddy characteristics may suffer from the low spatial resolution of past and current satellite-tracks in high-latitude oceans associated with small Rossby radii. Here we present results of an eddy reconstruction based on a nonlinear damping Gauss-Newton optimisation algorithm using ship based current profiler observations from two research expeditions in the Labrador Sea in 2014 and 2016. Overall we detect 14 eddies with radii ranging from 7 to 35 km.&lt;/p&gt;&lt;p&gt;In order to verify the skill of the reconstruction we used the submesoscale permitting NATL60 model (1/60&amp;#176;) as a reference data set. Spectral analysis of the horizontal velocity implies that the mesoscale regime is well represented in NATL60 compared with the observations. The submesoscale regime in the model spectra shows deviations to the observations at scales smaller than 10km near the ocean surface. The representation of the submesoscale flow further decreases in the model with increasing depth.&lt;/p&gt;&lt;p&gt;By subsampling the NATL60 model velocities along artificial ship tracks, applying our eddy reconstruction algorithm, and comparing the results with the full model field, a skill assessment of the reconstruction is done. We show that the reconstruction of the eddy characteristics can be affected by the location of the ship track through the velocity field.&lt;/p&gt;&lt;p&gt;In comparison with the observed eddies the NATL60 eddies have smaller radii and higher azimuthal velocities and thus are more nonlinear. The inner core velocity structure for observations and NATL60 suggests solid body rotation for 2/3 of the radius. The maximum azimuthal velocity may deviate by up to 50% from solid body rotation.&lt;/p&gt;&lt;p&gt;The seasonality of the submesoscale regime can be seen in the data as the power spectrum is reduced from spring to summer in both the ship-based measurements and model.&lt;/p&gt;

Unveiling kinematic structure in the starburst heart of NGC 253

ABSTRACT We observed the Brackett α emission line (4.05 μm) within the nuclear starburst of NGC 253 to measure the kinematics of ionized gas, and distinguish motions driven by star formation feedback from gravitational motions induced by the central mass structure. Using NIRSPEC on Keck II, we obtained 30 spectra through a $0^{\prime \prime }_{.}5$ slit stepped across the central ∼5 arcsec × 25 arcsec (85 × 425 pc) region to produce a spectral cube. The Br α emission resolves into four nuclear sources: S1 at the infrared core (IRC), N1 at the radio core, and the fainter sources N2 and N3 in the northeast. The line profile is characterized by a primary component with Δvprimary ∼90–130 $\rm km\, s^{-1}$ (full width at half-maximum) on top of a broad blue 2wing with Δvbroad ∼300–350 $\rm km\, s^{-1}$, and an additional redshifted narrow component in the west. The velocity field generated from our cube reveals several distinct patterns. A mean NE–SW velocity gradient of +10 $\rm km\, s^{-1}$ arcsec−1 along the major axis traces the solid-body rotation curve of the nuclear disc. At the radio core, isovelocity contours become S-shaped, indicating the presence of secondary nuclear bar of total extent ∼5 arcsec (90 pc). The symmetry of the bar places the galactic centre, and potential supermassive black hole, near the radio peak rather than the IRC. A third kinematic substructure is formed by blueshifted gas near the IRC. This feature likely traces a ∼100–250 $\rm km\, s^{-1}$ starburst-driven outflow, potentially linking the IRC to the galactic wind observed on kpc scales.

Nonlinear Effects on the Precessional Instability in Magnetized Turbulence

By means of direct numerical simulations (DNS), we study the impact of an imposed uniform magnetic field on precessing magnetohydrodynamic homogeneous turbulence with a unit magnetic Prandtl number. The base flow which can trigger the precessional instability consists of the superposition of a solid-body rotation around the vertical ( x 3 ) axis (with rate Ω ) and a plane shear (with rate S = 2 ε Ω ) viewed in a frame rotating (with rate Ω p = ε Ω ) about an axis normal to the plane of shear and to the solid-body rotation axis and under an imposed magnetic field that aligns with the solid-body rotation axis ( B ‖ Ω ) . While rotation rate and Poincaré number are fixed, Ω = 20 and ε = 0.17 , the B intensity was varied, B = 0.1 , 0.5 , and 2.5 , so that the Elsasser number is about Λ = 0.1 , 2.5 and 62.5 , respectively. At the final computational dimensionless time, S t = 2 ε Ω t = 67 , the Rossby number Ro is about 0.1 characterizing rapidly rotating flow. It is shown that the total (kinetic + magnetic) energy ( E ) , production rate ( P ) due the basic flow and dissipation rate ( D ) occur in two main phases associated with different flow topologies: (i) an exponential growth and (ii) nonlinear saturation during which these global quantities remain almost time independent with P ∼ D . The impact of a "strong" imposed magnetic field ( B = 2.5 ) on large scale structures at the saturation stage is reflected by the formation of structures that look like filaments and there is no dominance of horizontal motion over the vertical (along the solid-rotation axis) one. The comparison between the spectra of kinetic energy E ( κ ) ( k ⊥ ) , E ( κ ) ( k ⊥ , k ‖ = 1 , 2 ) and E κ ) ( k ⊥ , k ‖ = 0 ) at the saturation stage reveals that, at large horizontal scales, the major contribution to E ( κ ) ( k ⊥ ) does not come only from the mode k ‖ = 0 but also from the k ‖ = 1 mode which is the most energetic. Only at very large horizontal scales at which E ( κ ) ( k ⊥ ) ∼ E 2 D ( κ ) ( k ⊥ ) , the flow is almost two-dimensional. In the wavenumbers range 10 ≤ k ⊥ ≤ 40 , the spectra E ( κ ) ( k ⊥ ) and E ( κ ) ( k ⊥ , k ‖ = 0 ) respectively follow the scaling k ⊥ − 2 and k ⊥ − 3 . Unlike the velocity field the magnetic field remains strongly three-dimensional for all scales since E 2 D ( m ) ( k ⊥ ) ≪ E ( m ) ( k ⊥ ) . At the saturation stage, the Alfvén ratio between kinetic and magnetic energies behaves like k ‖ − 2 for B k ‖ / ( 2 ε Ω ) < 1 .

Dynamical controls on the longevity of a non-linear vortex : The case of the Lofoten Basin Eddy

The Lofoten Basin is the largest oceanic reservoir of heat in the Nordic Seas, and the site of important heat fluxes to the atmosphere. An intense permanent anticyclone in the basin impacts the regional hydrography, energetics, and ecosystem. Repeated sampling of this Lofoten Basin Eddy from dedicated cruises, autonomous profiling gliders, and acoustically-tracked subsurface floats enables the documentation of its dynamics and energetics over the course of 15 months. The eddy core, in nearly solid-body rotation, exhibits an unusually low vertical vorticity close to the local inertial frequency and important strain rates at the periphery. Subsurface floats as deep as 800 m are trapped within the core for their entire deployment duration (up to 15 months). The potential vorticity is reduced in the core by two orders of magnitude relative to the surroundings, creating a barrier. In the winter, this barrier weakens and lateral exchanges and heat flux between the eddy and the surroundings increase, apparently the result of dynamical instabilities and a possible eddy merger. Based on a simple energy budget, the dissipation timescale for the eddy energy is three years, during which wintertime convection seasonally modulates potential and kinetic energy.

Dynamics of a perturbed solid-body rotation flow in a finite-length straight rotating pipe

Direct numerical simulations are used to study the three-dimensional, incompressible and viscous flow dynamics of a base solid-body rotation flow with a uniform axial velocity entering a rotating, finite-length, straight circular pipe. Steady in time profiles of the axial, radial and circumferential velocities are prescribed along the pipe inlet. The convective boundary conditions for each velocity flux component is set at the pipe outlet. The simulation results describe the neutral stability line in response to either axisymmetric or three-dimensional perturbations in a diagram of Reynolds number ($Re$, based on inlet axial velocity and pipe radius) versus the incoming flow swirl ratio ($\unicode[STIX]{x1D714}$). This line is in good agreement with the neutral stability line recently predicted by the linear stability theory of Wang et al. (J. Fluid Mech., vol. 797, 2016, pp. 284–321). The computed time history of the velocity components at a certain point in the flow is used to describe three-dimensional phase portraits of the flow global dynamics and its long-term behaviour. They show three types of flow evolution scenarios. First, the Wang & Rusak (Phys. Fluids, vol. 8 (4), 1996, pp. 1007–1016) axisymmetric instability mechanism and evolution to a stable axisymmetric breakdown state is recovered at certain operational conditions in terms of $Re$ and $\unicode[STIX]{x1D714}$. However, at other operational conditions with same $\unicode[STIX]{x1D714}$ but with a higher $Re$, a second scenario is found. The axisymmetric breakdown state continues to evolve and a spiral instability mode appears on it and grows to a rotating spiral breakdown state. Moreover, at higher levels of $\unicode[STIX]{x1D714}$ a third scenario is found where there exists a dominant three-dimensional spiral type of instability mode that agrees with the linear stability theory of Wang et al. (J. Fluid Mech., vol. 797, 2016, pp. 284–321). The growth of this mode leads directly to a spiral type of flow roll-up and nonlinearly saturates on a rotating spiral type of vortex breakdown. The Reynolds–Orr equation is used to reveal the mechanism that drives all the instabilities as well as the nonlinear global flow evolution. At high swirl ratios, the confined kinetic energy in the swirling flow can be triggered to be released through various physical agents, such as the asymmetric inlet–outlet conditions, that eliminate axial homogeneity along the pipe and induce flow instabilities and evolution to breakdown states. It is also shown that local instability analysis or its extension using the assumption of a weakly non-parallel flow to conduct convective instability–absolute instability analyses is definitely not related to any of the instability modes found in the present study. Moreover, a stability study based on the strongly non-parallel flow character, including axial inhomogeneity due to a finite-domain boundary conditions, must be conducted to reveal instabilities in such flows.