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We applied a discontinuous Galerkin time domain (DGTD) method, using a fourth-order Runga-Kutta time stepping of the Maxwell equations, to the simulation of the optical properties of dielectric particles in two-dimensional (2D) geometry. As examples of the numerical implementation of this method, the single-scattering properties of 2D circular and hexagonal particles are presented. In the case of circular particles, the scattering phase matrix was computed using the DGTD method and compared with the exact solution. For hexagonal particles, the DGTD method was used to compute single-scattering properties of randomly oriented 2D hexagonal ice crystals, and results were compared with those calculated using a geometric optics method. We consider both shortwave (visible) and longwave (infrared) cases, with particle size parameters 50 and 100. In the hexagonal case, scattering results are also presented as a function of both incident and scattering angles, revealing a structure apparently not reported before. Using the geometric optics method, we are able to interpret this structure in terms of contributions from varying numbers of internal reflections within the crystal. OCIS codes: 290.0290, 080.0080, 260.0260, 010.0010.

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 due to internal dynamics. When the domain size is an integral multiple of the jet scale, jets evolve rather independently of each other, meandering on relatively long time scales. Jet interactions are predominantly pairwise when the implicit quantization is violated. Eddy fluxes occur in intermittent bursts asymmetric about jet axes, producing momentum flux convergences to maintain the jets against surface drag. Heat fluxes are everywhere downgradient, reaching local maxima in jet cores. Potential vorticity homogenization is not seen in these forced-dissipative equilibria. Zonally averaged potential vorticity gradients are bounded away from zero when forcing is strong, and zero crossings occur only in the lower layer at weak forcing (when they may be anticipated from the form of the forcing). The relation between potential vorticity fluxes and their gradients is determined by details of the dissipation rather than by any general principle. As seen earlier with a 'one-dimensional' model, baroclinic adjustment parameterizations are inappropriate in wide domains.

A 35-year record of monthly mean zonal wind data for the equatorial stratosphere is represented in terms of a vector (rxe RWS is important for the steady response in middle and high latitudes. A decomposition of the solution using separately the components of the RWS in tropical and extratropical regions shows that the responses to the tropical and extratropical RWSs tend to cancel in middle and high latitudes. The extratropical component of the RWS may be understood by quasigeostrophic dynamics.

We present some results on the relative performance of the pseudo-spectral time domain, finite difference time domain, and DDA methods in calculating single-particle optical scattering properties. Our interest is in particles with size parameters in excess of 10. Using as test case a homogeneous spherical particle we have found that the pseudospectral time domain method is generally more efficient (uses less cpu time for a given accuracy) than the finite-difference time domain method. The DDA method appears to be superior to the pseudo-spectral method for indices of refraction less than 1.5, but as the index of refraction increases, the pseudo-spectral method becomes superior. We present here some results for particles with size parameters 10 and 30, and indices of refraction 1.3117 and 1.7. [ABSTRACT FROM AUTHOR]

Recent progress in the study of the single-scattering properties of nonspherical ice crystals within cirrus clouds and nonspherical dust particles is reviewed. We have been using the finite-difference time domain (FDTD) method, the discrete dipole approximation (DDA), and an improved geometric optics method (IGOM) to compute the single-scattering properties of nonspherical particles. We have incorporated the so-called edge effect associated with the surface wave into the IGOM extinction and absorption efficiencies. The simulation results in the solar and thermal infrared spectral regimes are presented. Furthermore, the impacts of particle nonsphericity on downstream remote sensing implementations and radiative transfer simulations involving ice clouds and dust aerosols are also summarized. [ABSTRACT FROM AUTHOR]