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Yuan Lian

Publications and source records attributed to Yuan Lian.

14 recordsLinked to original sources
arXiv2026

Juno Microwave Radiometer Observations Reveal A Warmer Polar Atmosphere on Jupiter

The intriguing circumpolar cyclone pattern at Jupiter's poles raises fundamental questions about how these systems are organized vertically and, further, how the planet's internal heat shapes and sustains them in the absence of solar insolation. We report recent close-in observations of Jupiter's north pole acquired by NASA's Juno Microwave Radiometer (MWR), which achieved comprehensive microwave mapping of the region at an unprecedentedly high resolution. Using six-channel measurements from eleven perijove passes (PJ51-PJ61) poleward of 75N, we derive polar-mean nadir brightness temperatures and limb-darkening spectra that together point to two equally plausible atmospheric scenarios: (1) a dry-adiabatic profile with slightly depleted ammonia gas at a few bars, or (2) a moist-adiabatic profile with uniform ammonia. Markov chain Monte Carlo retrievals yield a deep ammonia abundance of 354.8+12.0/-11.0 ppmv (3+/-0.1 x solar) and a water abundance of 1.8+1.5/-1.1 x 1000 ppmv (2.1+1.8/-1.3 x solar), resembling previous estimates at lower latitudes. Remarkably, the north pole is found to be 6-7 K warmer than the equator at the 1-bar level, although the inferred difference is close to the 1-sigma uncertainty level. If confirmed, this result would suggest an enhanced internal heat flux toward the poles, which is consistent with the more intense lightning activity observed at high latitudes.

astro-ph.EP↗
arXiv2026

Internal Knowledge Without External Expression: Probing the Generalization Boundary of a Classical Chinese Language Model

We train a 318M-parameter Transformer language model from scratch on a curated corpus of 1.56 billion tokens of pure Classical Chinese, with zero English characters or Arabic numerals. Through systematic out-of-distribution (OOD) testing, we ask whether the model distinguishes known from unknown inputs, and whether it expresses that distinction in its generated text. We find a clear dissociation between internal and external uncertainty. Internally, the model exhibits a perplexity jump ratio of 2.39x between real and fabricated historical events (p = 8.9e-11, n = 92 per group), with semi-fabricated events (real figures + fictional actions) showing the highest perplexity (4.24x, p = 1.1e-16), demonstrating genuine factual encoding beyond syntactic pattern matching. Externally, however, the model never learns to express uncertainty: classical Chinese epistemic markers appear at lower rates for OOD questions (3.5%) than in-distribution ones (8.3%, p = 0.023), reflecting rhetorical conventions in the training data rather than genuine metacognition. We test both findings across three languages (Classical Chinese, English, Japanese), three writing systems, and eight models from 110M to 1.56B. The internal factual-encoding effect replicates in six of the eight models, emerging with scale (the two smallest Japanese models do not yet separate real from fabricated history), while the external absence of uncertainty expression holds across all eight. We further show that uncertainty expression frequency is determined entirely by training data conventions -- not epistemic states -- with Classical Chinese models showing a "humility paradox" (more hedging for known topics), while Japanese models almost never hedge. We argue that metacognitive expression -- the ability to say "I don't know" -- does not emerge from language modeling alone and requires explicit training signals such as RLHF.

cs.CL↗
arXiv2025

Reconciling Jupiter's Vertical Motions with the Observed Cloud Structure in the Upper Troposphere

The eddy fluxes of angular momentum in Jupiter's upper troposphere are known to converge in prograde jets and diverge in retrograde jets. Away from the equator, this implies convergence of the Eulerian mean meridional flow in zones (anticyclonic shear) and divergence in belts (cyclonic shear). It indicates lower-tropospheric downwelling in zones and upwelling in belts because the mean meridional circulation almost certainly closes at depth. Yet the observed banded structure of Jupiter's clouds and hazes suggests that there is upwelling in the brighter zones and downwelling in the darker belts. Here, we show that this apparent contradiction can be resolved by considering not the Eulerian but the transformed Eulerian mean circulation, which includes a Stokes drift owing to eddies and is a better approximation of the Lagrangian mean transport of tracers such as ammonia. The potential vorticity structure inferred from observations paired with mixing length arguments suggests that there is transformed Eulerian mean upwelling in zones and downwelling in belts. Simulations with a global circulation model of Jupiter's upper atmosphere demonstrate the plausibility of these inferences and allow us to speculate on the band structure at deeper levels.

astro-ph.EP↗
arXiv2025

The Shannon-McMillan-Breiman theorem of random dynamical systems for amenable group actions

The Shannon-McMillan-Breiman theorem is one of the most important results in information theory, which can describe the random ergodic process, and its proof uses the famous Birkhoff ergodic theorem, so it can be seen that it plays a crucial role in ergodic theory. In this paper, the Shannon-McMillan-Breiman theorem in the random dynamical systems is proved from the perspective of an amenable group action, which provides a boost for the development of entropy theory in the random dynamical systems for amenable group actions.

math.DS↗
arXiv2025

Conditional entropy for Amenable group actions

Let G be an infinite discrete countable amenable group acting continuously on a Lebesgue space X. In this article, using partition and factor-space, the conditional entropy of the action G is defined. We introduction some properties of conditional entropy for amenable group actions and the corresponding decomposition theorem is obtained.

math.DS↗
arXiv2025

The Role of Planetary-Scale Waves on the Stratospheric Superrotation in Titan's Atmosphere

We analyze simulation results from the TitanWRF global circulation model to understand the mechanisms that maintain the equatorial superrotation in Titan's stratosphere. We find that the eddies associated with wave activities can transport angular momentum upgradient to zonal flow, leading to acceleration of the equatorial superrotation. The dominant wave modes identified in this study are consistent with previous studies, with zonal wavenumber 1 being the major contributor to the prograde acceleration. Despite the same conclusion of maintenance of equatorial superrotation via wave-mean interactions, we find that the way waves interact with the zonal flow in TitanWRF is slightly different from some other studies. We confirm our previous findings that in TitanWRF this occurs primarily during a dozen or so annual, short-duration (a few Titan sols) angular momentum "transfer events", which have a repeatable seasonal pattern but differ slightly in timing and magnitude between years. This is not the case in the Titan Atmosphere Model (TAM), which found milder angular momentum transfers that produced the strongest acceleration of superrotation around solstice in the upper stratosphere and more continuous year-around acceleration in the lower stratosphere. Despite differences in angular momentum transfer across models, we further find that, similar to the TAM wave analysis results, eddies generated by Rossby-Kelvin instabilities may be the major source of prograde angular momentum for the equatorial superrotation, although TitanWRF may also include contributions from the absorption of vertically propagating equatorial Kelvin waves. This differs from our previous work, which suggested barotropic waves were responsible for TitanWRF's solsticial transfer event.

astro-ph.EP↗
arXiv2024

A variational principle for entropy of a random dynamical system

In this article, I give a definition of topological entropy for random dynamical systems associated to an infinite countable discrete amenable group action. I obtain a variational principle between the topological entropy and measurable fiber entropy of a random dynamical system.

math.DS↗
arXiv2024

Weyl mean equicontinuity and Weyl mean sensitivity of a random dynamical system

In this article, we introduce the concepts of Weyl mean equicontinuity and Weyl mean sensitivity of a random dynamical system associated to an infinite countable discrete amenable group action. We obtain the dichotomy result to Weyl mean equicontinuity and Weyl mean sensitivity of a random dynamical system when the corresponding skew product transformation is minimal and Ωis finite.

math.DS↗
arXiv2022

Unstructured Grid Dynamical Modeling of Planetary Atmospheres using planetMPAS: The Influence of the Rigid Lid, Computational Efficiency, and Examples of Martian and Jovian Application

We present a new planetary global circulation model, planetMPAS, based on the state-of-the-art NCAR MPAS General Circulation Model. Taking advantage of the cross compatibility between WRF and MPAS, planetMPAS includes most of the planetWRF physics parameterization schemes for terrestrial planets such as Mars and Titan. PlanetMPAS also includes a set of physics that represents radiative transfer, dry convection, moist convection and its associated microphysics for the Jovian atmosphere. We demonstrate that, despite the rigid-lid approximation, planetMPAS is suitable to simulate the climate systems in Martian and Jovian atmospheres with potential application to slow-rotating planets such as Titan. Simulations using planetMPAS show that the new model can reproduce many aspects of the observed features on Mars and Jupiter, such as the seasonal CO2 cycle, polar argon enrichment, zonal mean temperature, and qualitative dust opacity on Mars, as well as the equatorial superrotation and banded zonal wind patterns on Jupiter.

astro-ph.EP↗
arXiv2019

The systems with almost Banach mean equicontinuity for abelian group actions

In this paper, we give the concept of Banach-mean equicontinuity and prove that three concepts, Bnanach-, Weyl- and Besicovitch-mean equicontinuity of a dynamic system with abelian group action are equivalent. Furthermore, we obtain that the topological entropy of a transitive, almost Banach-mean equicontinuous dynamical system with abelain group action is zero. As an application with our main result, we show that the topological entropy of the Banach-mean equicontinuous system under the action of an abelian groups is zero.

math.DS↗
arXiv2013

3D mixing in hot Jupiter atmospheres. I. application to the day/night cold trap in HD 209458b

Hot Jupiters exhibit atmospheric temperatures ranging from hundreds to thousands of Kelvin. Because of their large day-night temperature differences, condensable species that are stable in the gas phase on the dayside, such as TiO and silicates, may condense and gravitationally settle on the nightside. Atmospheric circulation may counterbalance this tendency to gravitationally settle. This three dimensional (3D) mixing of chemical species has not previously been studied for hot Jupiters, yet it is crucial to assess the existence and distribution of TiO and silicates in the atmospheres of these planets. We perform 3D global circulation models of HD209458b including passive tracers that advect with the 3D flow, including a source/sink on the nightside to represent condensation and gravitational settling of haze particles. We show that global advection patterns produce strong vertical mixing that can keep condensable species lofted as long as they are trapped in particles of sizes of a few microns or less on the night side. We show that vertical mixing results not from small-scale convection but from the large-scale circulation driven by the day-night heating contrast. Although this vertical mixing is not diffusive in any rigorous sense, a comparison of our results with idealized diffusion models allows a rough estimate of the vertical diffusion coefficient. Kzz=5x10**4/Sqrt(Pbar) m2/s can be used in 1D models of HD 209458b. Moreover, our models exhibit strong spatial and temporal variability in the tracer concentration that could result in observable variations during transit or secondary eclipse measurements. Finally, we apply our model to the case of TiO in HD209458b and show that the day-night cold trap would deplete TiO if it condenses into particles bigger than a few microns on the planet's night side, making it unable to create the observed stratosphere of the planet.

astro-ph.EP↗
arXiv2009

Generation of equatorial jets by large-scale latent heating on the giant planets

Three-dimensional numerical simulations show that large-scale latent heating resulting from condensation of water vapor can produce multiple zonal jets similar to those on the gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune). For plausible water abundances (3-5 times solar on Jupiter/Saturn and 30 times solar on Uranus/Neptune), our simulations produce ~20 zonal jets for Jupiter and Saturn and 3 zonal jets on Uranus and Neptune, similar to the number of jets observed on these planets. Moreover, these Jupiter/Saturn cases produce equatorial superrotation whereas the Uranus/Neptune cases produce equatorial subrotation,consistent with the observed equatorial jet direction on these planets.Sensitivity tests show that water abundance, planetary rotation rate,and planetary radius are all controlling factors, with water playing the most important role; modest water abundances, large planetary radii, and fast rotation rates favor equatorial superrotation, whereas large water abundances favor equatorial subrotation regardless of the planetary radius and rotation rate. Given the larger radii, faster rotation rates, and probable lower water abundances of Jupiter and Saturn relative to Uranus and Neptune, our simulations therefore provide a possible mechanism for the existence of equatorial superrotation on Jupiter and Saturn and the lack of superrotation on Uranus and Neptune. Nevertheless, Saturn poses a possible difficulty, as our simulations were unable to explain the unusually high speed (~400m/s) of that planet's superrotating jet. The zonal jets in our simulations exhibit modest violations of the barotropic and Charney-Stern stability criteria. Overall, our simulations, while idealized, support the idea that latent heating plays an important role in generating the jets on the giant planets.

astro-ph.EP↗
arXiv2009

Atmospheric circulation of hot Jupiters: Coupled radiative-dynamical general circulation model simulations of HD 189733b and HD 209458b

We present global, three-dimensional numerical simulations of HD 189733b and HD 209458b that couple the atmospheric dynamics to a realistic representation of non-gray cloud-free radiative transfer. The model, which we call the Substellar and Planetary Atmospheric Radiation and Circulation (SPARC) model, adopts the MITgcm for the dynamics and uses the radiative model of McKay, Marley, Fortney, and collaborators for the radiation. Like earlier work with simplified forcing, our simulations develop a broad eastward equatorial jet, mean westward flow at higher latitudes, and substantial flow over the poles at low pressure. For HD 189733b, our simulations without TiO and VO opacity can explain the broad features of the observed 8 and 24-micron light curves, including the modest day-night flux variation and the fact that the planet/star flux ratio peaks before the secondary eclipse. Our simulations also provide reasonable matches to the Spitzer secondary-eclipse depths at 4.5, 5.8, 8, 16, and 24 microns and the groundbased upper limit at 2.2 microns. However, we substantially underpredict the 3.6-micron secondary-eclipse depth, suggesting that our simulations are too cold in the 0.1-1 bar region. Predicted temporal variability in secondary-eclipse depths is ~1% at Spitzer bandpasses, consistent with recent observational upper limits at 8 microns. We also show that nonsynchronous rotation can significantly alter the jet structure. For HD 209458b, we include TiO and VO opacity; these simulations develop a hot (>2000 K) dayside stratosphere. Despite this stratosphere, we do not reproduce current Spitzer photometry of this planet. Light curves in Spitzer bandpasses show modest phase variation and satisfy the observational upper limit on day-night phase variation at 8 microns. (abridged)

astro-ph↗