Searcharxiv⌕ Search

arXiv subjects

Anthony B. Davis

Publications and source records attributed to Anthony B. Davis.

11 recordsLinked to original sources
arXiv2023

Radiometry for Nighttime Sub-Cloud Imaging of Venus' Surface in the Near-InfraRed Spectrum

Does radiometry (e.g., signal-to-noise ratio) limit the performance of near-IR subcloud imaging of our sister planet's surface at night? It does not. We compute subcloud radiometry using above-cloud observations, an assumed ground temperature, sub-cloud absorption and emission modeling, and Rayleigh scattering simulations. We thus confirm both archival and recent studies that deployment of a modest subcloud camera does enable high-resolution surface imaging.

astro-ph.EP↗
arXiv2022

Imaging Venus' surface at night in the near-IR from above its clouds: New analytical models for the effective spatial resolution, illustrated with new Parker Solar Probe data

There are a handful of spectral windows in the near-IR through which we can see down to Venus' surface on the night side of the planet. The surface of our sister planet has thus been imaged by sensors on Venus-orbiting platforms (Venus Express, Akatsuki) and during fly-by with missions to other planets (Galileo, Cassini). The most tantalizing finding, so far, is the hint of possible active volcanism. However, the thermal radiation emitted by Venus' searing surface (c. 475 degrees C) has to get through the opaque clouds between 50 and 70 km altitude, as well as the sub-cloud atmosphere. In the clouds, the light is not absorbed but scattered, indeed, many times. This results in blurring the surface imagery to the point where the smallest discernible feature is roughly 100 km in size, full-width half-max (FWHM), and this has been reproduced using numerical models. We propose a new analytical modeling framework for predicting the width of the atmospheric point-spread function (APSF) that determines the effective resolution of surface imaging from space. Our best estimates of the APSF width for the 1-to-1.2 micron spectral range are clustered around 130 km FWHM. Interestingly, this is somewhat larger than the accepted value of 100 km based on both visual image inspection and detailed numerical simulations. Lastly, we apply the new modeling framework to the fly-by imaging by the Parker Solar Probe in a somewhat shorter wavelength band.

astro-ph.EP↗
arXiv2021

GEOSCAN: Global Earth Observation using Swarm of Coordinated Autonomous Nanosats

The climate crisis we are facing calls for significant improvements in our understanding of natural phenomena, with clouds being identified as a dominant source of uncertainty. To this end, the emerging field of 3D computed cloud tomography (CCT) aims to more precisely characterize clouds by utilizing multi-dimensional imaging to reconstruct their outer and inner structure. In this paper, we propose a future Earth observation mission concept, driven by the needs of CCT, that operates constellation of NanoSats to provide multi-angular, spectrally-resolved, spatial and temporal scientific measurements of natural atmospheric phenomena. Our proposed mission, GEOSCAN, will on-board active steering capability to rapidly reconfigure networked swarm of autonomous Nanosats to track evolving phenomena of interest, on-demand, in real-time. We present the structure of the GEOSCAN constellation and discuss details of the mission concept from both science and engineering perspectives. On the science side, we outline the types of remote Earth observation measurements that GEOSCAN enables beyond the state-of-the-art, and how such measurements translate to improvements in CCT that can lead to reduction in uncertainty of the global climate models (GCMs). From the engineering side, we investigate feasibility of the concept starting from hardware components of the NanoSat that form the basis of the constellation. In particular, we focus on the active steering capability of the GEOSCAN with algorithmic approaches that enable coordination from new software. We identify technology gaps that need to be bridged and discuss other aspects of the mission that require in-depth analysis to further mature the concept.

eess.SP↗
arXiv2021

Moments of Sunlight Pathlength in Water and Aerosol Clouds from O$_2$ Spectroscopy: Exploitable Parameter Sensitivities

The last IPCC assessment states that clouds and aerosols remain a challenge in climate prediction with Global Climate Models. Therefore, NASA's 2017 Decadal Survey has made them, along with convection and precipitation, a priority target for future missions under the ACCP banner, now Atmospheric Observing System. Atmospheric science is now more than ever driving renewal in remote sensing to probe clouds and aerosols more accurately and with improved sampling. Here, we bring new results that support Differential Optical Absorption Spectroscopy (DOAS) using oxygen absorption features in the visible/near-IR spectrum. With known concentration and cross-section, the remaining unknown in O$_2$ DOAS is the path the light followed through the absorbing gas. In presence of scattering by cloud or aerosol particles, that path is broken at each interaction. Cumulative pathlength through the gas is a random variable, and its probability distribution function (PDF) will contain desirable information about the clouds or aerosols. Here, we compute statistical moments of the that PDF, showing that mean and variance convey different pieces of information, namely, size and opacity of the medium. We also extend the connection between geometric thickness and mean pathlength in presence of absorption by the scattering particles. We view pathlength moments as intermediate products that can be obtained from O$_2$ spectroscopy, and describe an algorithm to do that. In turn, these moments yield cloud or aerosol profile parameters of interest: layer geometric and optical thicknesses. One normally thinks about active sensors, radars and lidars, for such atmospheric profiling. Here, the profile is parameterized and representative of a horizontal average determined by multiple scattering. However, there are advantages in passive vs active instrumentation, starting with the possibility of imaging over a large swath.

physics.ao-ph↗
arXiv2021

Toward Cloud Tomography from Space using MISR and MODIS: The Physics of Image Formation for Opaque Convective Clouds

3D convective cloud images form via two intertwined radiative diffusion processes. Sunlight starts in the anti-solar direction and ends in toward-sensor ones, but repeated forward-peaked scattering smears the well-collimated beams across_direction_ space. This loss of directional memory in the cloud's "outer shell" (OS) is modeled as a random walk (RW) on the sphere. We show that, for typical cloud phase functions, 5 or 6 scatterings suffice for severe degrading of directionality. Simultaneously, a RW unfolds in_standard_ 3D space where steps are angularly-correlated, hence a drift in the original direction and an associated lateral dispersion. Any distinctive cloud image "feature" originates in the OS, and the shallower the better. That is also why we previously found that the optical depth of the "veiled core" (VC) is ~5. Significant amounts of sunlight thus arrive at the VC as a diffuse irradiance, and leave it even more isotropic. The diffusion limit of 3D radiative transfer (RT) is therefore valid inside the VC. Consequently, the underlying RW in the VC unfolds in 3D space, now with isotropic steps since extinction is scaled back to account for forward scattering. We show that the VC optical thickness controls cloud-scale brightness contrast between the illuminated and self-shaded sides of the cloud. Full cloud image formation thus involves diffusions (i.e., RWs) in both Euclidian and spherical/direction spaces. 3D cloud tomography based on MISR and MODIS multi-angle/-spectral data is an emerging technique in passive VNIR-SWIR sensing that will make judicious use this spatial separation of RT regimes to accelerate forward modeling without significant loss in accuracy.

physics.ao-ph↗
arXiv2020

Multi-view polarimetric scattering cloud tomography and retrieval of droplet size

Tomography aims to recover a three-dimensional (3D) density map of a medium or an object. In medical imaging, it is extensively used for diagnostics via X-ray computed tomography (CT). Optical diffusion tomography is an alternative to X-ray CT that uses multiply scattered light to deliver coarse density maps for soft tissues. We define and derive tomography of cloud droplet distributions via passive remote sensing. We use multi-view polarimetric images to fit a 3D polarized radiative transfer (RT) forward model. Our motivation is 3D volumetric probing of vertically-developed convectively-driven clouds that are ill-served by current methods in operational passive remote sensing. These techniques are based on strictly 1D RT modeling and applied to a single cloudy pixel, where cloud geometry is assumed to be that of a plane-parallel slab. Incident unpolarized sunlight, once scattered by cloud-droplets, changes its polarization state according to droplet size. Therefore, polarimetric measurements in the rainbow and glory angular regions can be used to infer the droplet size distribution. This work defines and derives a framework for a full 3D tomography of cloud droplets for both their mass concentration in space and their distribution across a range of sizes. This 3D retrieval of key microphysical properties is made tractable by our novel approach that involves a restructuring and differentiation of an open-source polarized 3D RT code to accommodate a special two-step optimization technique. Physically-realistic synthetic clouds are used to demonstrate the methodology with rigorous uncertainty quantification.

physics.comp-ph↗
arXiv2019

Cloud Tomography from Space using MISR and MODIS: Locating the "Veiled Core" in Opaque Convective Clouds

For passive satellite imagers, current retrievals of cloud optical thickness and effective particle size fail for convective clouds with 3D morphology. Indeed, being based on 1D radiative transfer (RT) theory, they work well only for horizontally homogeneous clouds. A promising approach for treating clouds as fully 3D objects is cloud tomography, and this has been demonstrated for airborne observations. For cloud tomography from space, however, more efficient forward 3D RT solvers are required. Here, we present a path forward, acknowledging that optically thick clouds have "veiled cores." Photons scattered into and out of this deep region do not contribute significant information to the observed imagery about the inner structure of the cloud. We investigate the location of the veiled core for the MISR and MODIS imagers. While MISR provides multi-angle imagery in the visible and near-IR, MODIS includes channels in the short-wave IR, albeit at a single view angle. This combination will enable future 3D retrievals to disentangle the cloud's effective particle size and optical thickness. We find that, in practice, the veiled core is located at an optical distance of $\approx$5 starting from the cloud boundary along the line-of-sight. For MODIS' absorbing wavelengths the veiled core covers a larger volume, starting at smaller optical distances. This result makes it possible to reduce the number of unknowns for the cloud tomographic reconstruction, and opens up new ways to increase the efficiency of the 3D RT solver at the heart of the reconstruction algorithm.

physics.geo-ph↗
arXiv2014

A Generalized Linear Transport Model for Spatially-Correlated Stochastic Media

We formulate a new model for transport in stochastic media with long-range spatial correlations where exponential attenuation (controlling the propagation part of the transport) becomes power law. Direct transmission over optical distance $τ(s)$, for fixed physical distance $s$, thus becomes $(1+τ(s)/a)^{-a}$, with standard exponential decay recovered when $a\to\infty$. Atmospheric turbulence phenomenology for fluctuating optical properties rationalizes this switch. Foundational equations for this generalized transport model are stated in integral form for $d=1,2,3$ spatial dimensions. A deterministic numerical solution is developed in $d=1$ using Markov Chain formalism, verified with Monte Carlo, and used to investigate internal radiation fields. Standard two-stream theory, where diffusion is exact, is recovered when $a=\infty$. Differential diffusion equations are not presently known when $a<\infty$, nor is the integro-differential form of the generalized transport equation. Monte Carlo simulations are performed in $d=2$, as a model for transport on random surfaces, to explore scaling behavior of transmittance $T$ when transport optical thickness $τ_\text{t} \gg 1$. Random walk theory correctly predicts $T \propto τ_\text{t}^{-\min\{1,a/2\}}$ in the absence of absorption. Finally, single scattering theory in $d=3$ highlights the model's violation of angular reciprocity when $a<\infty$, a desirable property at least in atmospheric applications. This violation is traced back to a key trait of generalized transport theory, namely, that we must distinguish more carefully between two kinds of propagation: one that ends in a virtual or actual detection, the other in a transition from one position to another in the medium.

physics.optics↗
arXiv2007

A General Strategy for Physics-Based Model Validation Illustrated with Earthquake Phenomenology, Atmospheric Radiative Transfer, and Computational Fluid Dynamics

Validation is often defined as the process of determining the degree to which a model is an accurate representation of the real world from the perspective of its intended uses. Validation is crucial as industries and governments depend increasingly on predictions by computer models to justify their decisions. In this article, we survey the model validation literature and propose to formulate validation as an iterative construction process that mimics the process occurring implicitly in the minds of scientists. We thus offer a formal representation of the progressive build-up of trust in the model, and thereby replace incapacitating claims on the impossibility of validating a given model by an adaptive process of constructive approximation. This approach is better adapted to the fuzzy, coarse-grained nature of validation. Our procedure factors in the degree of redundancy versus novelty of the experiments used for validation as well as the degree to which the model predicts the observations. We illustrate the new methodology first with the maturation of Quantum Mechanics as the arguably best established physics theory and then with several concrete examples drawn from some of our primary scientific interests: a cellular automaton model for earthquakes, an anomalous diffusion model for solar radiation transport in the cloudy atmosphere, and a computational fluid dynamics code for the Richtmyer-Meshkov instability. This article is an augmented version of Sornette et al. [2007] that appeared in Proceedings of the National Academy of Sciences in 2007 (doi: 10.1073/pnas.0611677104), with an electronic supplement at URL http://www.pnas.org/cgi/content/full/0611677104/DC1. Sornette et al. [2007] is also available in preprint form at physics/0511219.

physics.comp-ph↗
arXiv2005

Celebrating the Physics in Geophysics

As 2005, the International Year of Physics, comes to an end, two physicists working primarily in geophysical research reflect on how geophysics is not an applied physics. Although geophysics has certainly benefited from progress in physics and sometimes emulated the reductionist program of mainstream physics, it has also educated the physics community about some of the generic behaviors of strongly nonlinear systems. Dramatic examples are the insights we have gained into the ``emergent'' phenomena of chaos, cascading instabilities, turbulence, self-organization, fractal structure, power-law variability, anomalous scaling, threshold dynamics, creep, fracture, and so on. In all of these examples, relatively simple models have been able to explain the recurring features of apparently very complex signals and fields. It appears that the future of the intricate relation between physics and geophysics will be as exciting as its past has been characterized by a mutual fascination. Physics departments in our universities should capitalize on this trend to attract and retain young talent motivated to address problems that really matter for the future of the planet. A pressing topic with huge impact on populations and that is challenging enough for both physics and geophysics communities to work together like never before is the understanding and prediction of extreme events.

physics.geo-ph↗