R. Lee Panetta

Zonal Jets in Wide Baroclinically Unstable Regions: Persistence and Scale Selection

Abstract Extremely persistent, equivalent barotropic zonal jets are observed in statistically steady quasigeostrophic two-layer beta-plane turbulence. Flows are forced by an imposed unstable vertical shear, horizontally uniform over domains several tens of Rossby radii wide. Damping is by surface drag, small-scale mixing, and for some runs, radiative relaxation. When dissipation is weak, zonal jets emerge with a meridional scale related to beta and the equilibrated eddy energy level as suggested by Rhines. Spinup behavior suggests a priori prediction of this level will be difficult. The scale of energy conversion also cannot be determined a priori, and while upscale energy transfer is important, (reverse) energy cascading ranges of any significant extent do not occur. Time scales considerably longer than those simply related to model parameters are prominent. The choices of doubly periodic boundary conditions and spatially homogeneous forcing and dissipation emphasize that the low-frequency behavior is du...

Representation of the Equatorial Stratospheric Quasi-Biennial Oscillation in EOF Phase Space

Abstract A 35-year record of monthly mean zonal wind data for the equatorial stratosphere is represented in terms of a vector (radius and phase angle) in a two-dimensional phase space defined by the normalized expansion coefficients of the two leading empirical orthogonal functions (E0Fs) of the vertical structure. The tip of the vector completes one nearly circular loop during each cycle of the quasi-biennial oscillation (QBO). Hence, its position and rate of progress along the orbit of the point provide a measure of the instantaneous amplitude and rate of phase progression of the QBO. Although the phase of the QBO bears little if any relation to calendar month, the rate of phase progression is strongly modulated by the first and second harmonics of the annual cycle, with a primary maximum in April/May, in agreement with previous studies based on the descent rates of easterly and westerly regimes. A simple linear prediction model is developed for the rate of phase progression, based on the phase of the Q...

First-Principles Modeling Of Electromagnetic Scattering By Discrete and Discretely Heterogeneous Random Media

A discrete random medium is an object in the form of a finite volume of a vacuum or a homogeneous material medium filled with quasi-randomly and quasi-uniformly distributed discrete macroscopic impurities called small particles. Such objects are ubiquitous in natural and artificial environments. They are often characterized by analyzing theoretically the results of laboratory, in situ, or remote-sensing measurements of the scattering of light and other electromagnetic radiation. Electromagnetic scattering and absorption by particles can also affect the energy budget of a discrete random medium and hence various ambient physical and chemical processes. In either case electromagnetic scattering must be modeled in terms of appropriate optical observables, i.e., quadratic or bilinear forms in the field that quantify the reading of a relevant optical instrument or the electromagnetic energy budget. It is generally believed that time-harmonic Maxwells equations can accurately describe elastic electromagnetic scattering by macroscopic particulate media that change in time much more slowly than the incident electromagnetic field. However, direct solutions of these equations for discrete random media had been impracticable until quite recently. This has led to a widespread use of various phenomenological approaches in situations when their very applicability can be questioned. Recently, however, a new branch of physical optics has emerged wherein electromagnetic scattering by discrete and discretely heterogeneous random media is modeled directly by using analytical or numerically exact computer solutions of the Maxwell equations. Therefore, the main objective of this Report is to formulate the general theoretical framework of electromagnetic scattering by discrete random media rooted in the Maxwell-Lorentz electromagnetics and discuss its immediate analytical and numerical consequences. Starting from the microscopic Maxwell-Lorentz equations, we trace the development of the first-principles formalism enabling accurate calculations of monochromatic and quasi-monochromatic scattering by static and randomly varying multiparticle groups. We illustrate how this general framework can be coupled with state-of-the-art computer solvers of the Maxwell equations and applied to direct modeling of electromagnetic scattering by representative random multi-particle groups with arbitrary packing densities. This first-principles modeling yields general physical insights unavailable with phenomenological approaches. We discuss how the first-order-scattering approximation, the radiative transfer theory, and the theory of weak localization of electromagnetic waves can be derived as immediate corollaries of the Maxwell equations for very specific and well-defined kinds of particulate medium. These recent developments confirm the mesoscopic origin of the radiative transfer, weak localization, and effective-medium regimes and help evaluate the numerical accuracy of widely used approximate modeling methodologies.

Stationary External Rossby Waves in Vertical Shear

Abstract The structure of stationary Rossby waves in the presence of a mean westerly zonal flow with vertical shear is examined. There is typically only one stationary vertical mode, the external mode, trapped within the troposphere. For more than one tropospheric mode to exist, we find that vertical shears must be smaller than those usually observed in extratropical latitudes. The vertical structure, horizontal wavenumber and group velocity of the external mode, and the projection onto this mode of topographic and thermal forcing are studied with continuous models (a linear shear profile as well as more realistic basic states), and a finite-differenced model with resolution and upper boundary condition similar to that used in GCMs. We point out that the rigid-lid upper boundary condition need not create artificial stationary resonances, as the artificial stationary vertical modes that are created are often horizontally evanescent. The results are presented in a form which allows one to design the equival...

The Influence of Water Coating on the Optical Scattering Properties of Fractal Soot Aggregates

The effect of water coating of constituent monomers on the optical single-scattering properties of fractal soot aggregates is investigated numerically using core-mantle theory and approximations involving two effective medium theories. A cluster–cluster aggregation algorithm is used to numerically generate fractal aggregates, and the core-mantle Generalized Multi-particle Mie (GMM) method is used to compute the exact single-scattering properties of soot aggregates with water-coated monomers. Comparisons are then made with results obtained using approximations that combine either the GMM method or the Rayleigh–Debye–Gans (RDG) method with either the Maxwell-Garnett or the Bruggeman effective medium approximation (a total of four approximation methods). The optical properties calculated are the extinction and absorption cross sections, the single-scattering albedo, and the phase matrix of water-coated fractal aggregates; these calculations are done for two wavelengths, 0.628 μm and 1.1 μm. Water coating of the fractal aggregates is shown to increase the extinction and absorption cross sections, the single-scattering albedo, and forward scattering, but decrease backward scattering. The combination GMM + Maxwell-Garnett gives approximations that are quite good over a range of coating thicknesses and aggregate size. The combination GMM + Bruggeman performs less well, overestimating the extinction and absorption cross sections and underestimating the single-scattering albedo. In the case of RDG, the better combination is with the Bruggeman approximation, but the errors involved are greater than with the GMM + Maxwell-Garnett combination. Results from simple idealized calculations indicate that the differences between results with the Maxwell-Garnett and Bruggeman approximations should be even larger in cases of aerosol cores that are less absorptive than soot. Copyright 2012 American Association for Aerosol Research

Comparison between the pseudo-spectral time domain method and the discrete dipole approximation for light scattering simulations

The pseudo-spectral time domain (PSTD) and the discrete dipole approximation (DDA) are two popular and robust methods for the numerical simulation of dielectric particle light scattering. The present study compares the numerical performances of the two methods in the computation of the single-scattering properties of homogeneous dielectric spheres and spheroids for which the exact solutions can be obtained from the Lorenz-Mie theory and the T-matrix theory. The accuracy criteria for the extinction efficiency and the phase function are prescribed to be the same for the PSTD and DDA in order that the computational time can be compared in a fair manner. The computational efficiency and applicability of the two methods are each shown to depend on both the size parameter and the refractive index of the scattering particle. For a small refractive index, a critical size parameter, which decreases from 80 to 30 as the refractive index increases from 1.2 to 1.4, exists below which the DDA outperforms the PSTD. For large refractive indices (>1.4), the PSTD is more efficient than the DDA for a wide size parameter range and has a larger region of applicability. Furthermore, the accuracy shown by the two methods in the computation of backscatter, linear polarization, and asymmetry factor is comparable. The comparison was extended to include spheroids with typical refractive indices of ice and dust and similar conclusions were drawn.

Baroclinic Eddy Fluxes in a One-Dimensional Model of Quasi-geostrophic Turbulence

Abstract Statistically steady states of a two-layer quasi-geostrophic model truncated to retain only the zonal mean flow and one nonzero zonal wavenumber, but with high meridional resolution, are described. The model is forced by imposing a time-mean unstable meridional temperature gradient, assuming that deviations from the time-mean are doubly periodic. A comparison is made with a more conventional channel model with the same zonal truncation, in which the flow is forced by radiative relaxation to an unstable temperature gradient. It is shown that the statistics of the channel model approach those of the doubly periodic model as the width of the unstable region in the former is increased. Implications for parameterization theories are discussed.

The effective equivalence of geometric irregularity and surface roughness in determining particle single-scattering properties

This study investigates the effects of geometric irregularity and surface roughness on the single-scattering properties of randomly oriented dielectric particles. Starting from a regular crystal with smooth faces, effects of roughening are compared with effects of perturbing the regular configuration of the smooth faces. Using the same slope distribution for small roughness facets and tilted faces provides a natural way to compare the effects on the single-scattering properties. It is found that the geometric irregularity and surface roughness have similar effects on the single-scattering properties of an ensemble of randomly oriented particles. In other words, particles with irregular geometries and those with surface roughness are optically equivalent if the slope distributions are the same. Furthermore, an ensemble of particles with irregular geometries can be used as an effective approximation for simulation of the scattering properties of roughened particles, and vice versa. This approach also provides new interpretation of the observed, relatively featureless and smooth, scattering phase functions of naturally occurring particles.

Dissipative Destabilization of External Rossby Waves

Abstract External Rossby waves in vertical shear can be destabilized by thermal damping. They can also be destabilized by damping of potential vorticity if this damping is larger in the lower than in the upper troposphere. Results are described in detail for Charneys model. Implications for the effects of diabatic heating and mixing due to smaller scale transients on equivalent barotropic stationary or quasi-stationary long waves are discussed. It is painted out that energy or potential enstrophy budgets may indicate that transients are damping the long waves while, in fact, their presence is destabilizing these waves.

A pseudo-spectral time domain method for light scattering computation

Atmospheric particles, for example ice crystals, dust, soot, or various chemical crystals, play a significant role in the atmosphere by scattering and absorbing radiation, principally in two bands: incident solar, with peak at about 0.5 μm, and terrestrial thermal emission, with peak at about 10 μm. Knowledge of aerosol scattering properties is a fundamental but challenging aspect of radiative transfer studies and remote sensing applications. In this chapter we consider only scattering by single homogeneous particles, but in the atmosphere particles occur both individually and as constituents of such aerosols as homogeneous or heterogeneous aggregates with other particles and sometimes coated with liquids. The pseudo-spectral time domain method (PSTD) for calculating scattering properties that we discuss, like a number of other methods currently in use, can be used to investigate scattering properties of a wide variety of aerosols, homogeneous or heterogeneous, singly or in aggregate.