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Rong Lu and Richard P. Turco

Abstract

Over the southern California coastal region, observations of the vertical distributions of pollutants show that maximum concentrations can occur within temperature inversion layers well above the surface. A mesoscale model is used to study the dynamical phenomena that cause such layers, including sea breezes and mountain flows, and to study the characteristics of air pollutant transport in a coastal environment capped by a temperature inversion. The mathematical and physical structure of the model is described. Two-dimensional simulations corresponding to four configurations of coastal plains and mountains are discussed. The simulations reveal that pollutant transport over a coastal plain is strongly influenced by the topographic configuration, including the height of coastal mountains and their distance from the coastline. Sea breezes induced by land–sea thermal contrast, as well as upslope winds induced along mountain flanks, both create vertical transport that can lead to the formation of elevated pollution layers. The sea-breeze circulation generates pollution layers by undercutting the mixed layer and lofting pollutants into the stable layer. Heating of mountain slopes acts to vent pollutants above the mountain ridge during the day; during the evening, pollutants can be injected directly into the inversion layer from the decaying upslope flows. In a land–sea configuration with mountains close to the coastline, the sea breeze and heated-mountain flow are strongly coupled. In the afternoon, this interaction can produce upslope flow from which polluted air is detrained into the inversion layer as a return circulation. When the mountains lie farther inland, however, pollutants may be trapped aloft when the mixed layer stabilizes in the late afternoon. As the nocturnal boundary layer forms over the coast in the evening, polluted mixed-layer air is effectively left behind in the inversion layer. In the Los Angeles Basin, the formation mechanism for elevated pollution layers is most similar to our cases with inland mountains.

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Sajal K. Kar and Richard P. Turco

Abstract

A lateral sponge layer is designed to minimize the spurious rejections of nondispersive surface gravity waves in a linear, one-dimension shallow-water limited-area model. The formulation selectively damps out the characteristic variables associated with the reflected waves inside the sponge layer. Through normal-mode analyses and subsequent applications to a forced-wave initial-value problem, the effectiveness of the sponge layer is compared to an established formulation of the open boundary condition. It is shown that for moderate thicknesses, the sponge is more effective than the open boundary condition in reducing the reflections of outgoing waves on all spatial scales. Application of the sponge layer to a vertically stratified model atmosphere is accomplished using a matrix procedure that essentially applies the shallow-water formulation to each vertical eigenmode of the model solution. Results of time-integrations using a multilevel quasi-static model of forced gravity waves, once again, demonstrate the relative superiority of the sponge layer over the open boundary condition.

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Donald M. Hunten, Richard P. Turco, and Owen B. Toon

Abstract

A height profile of ablated mass from meteors is calculated, assuming an incoming mass of 10−16 g cm−2 s−1 (44 metric tons per day) and the velocity distribution of Southworth and Sekanina, which has a mean of 14.5 km s−1. The profile peaks at 84 km. The fluxes of micrometeorites and residual meteoroids are also calculated. The coagulation of the evaporated silicates into “smoke” particles is then followed by means of a model adapted from a previous study of the stratospheric sulfate layer. Numerous sensitivity tests are made. Features of the results are a sharp cutoff of the particle distribution above 90 km, and a surface area close to 10−9 cm2 cm−3 all the way from 30 to 85 km. Some confirmation is obtained from balloon studies of condensation nuclei, although the various measurements differ greatly. The optical scattering and extinction am shown to be undetectable. Several potential applications are suggested: nucleation of sulfate particles and noctilucent clouds, scavenging of metallic ions and atoms, and perhaps other aeronomical effects. The latter are limited to processes that can be influenced by a collision time of the order of a day.

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Sajal K. Kar, Richard P. Turco, Carlos R. Mechoso, and Akio Arakawa

Abstract

A splitting method is presented for eliminating the need to directly solve for a two-dimensional Helmholtz-type difference equation in a semi-implicit scheme for a global gridpoint shallow-water model. In the proposed method, the model equations are split so that the gravity-oscillation terms are integrated implicitly in two locally one-dimensional steps. It is required that such splitting must preserve the irrotationality properties of the gradients of pressure in finite-difference form so that no spurious sources of vorticity and divergence are introduced into the flow. The semi-implicit scheme, thus derived, provides a locally one-dimensional method of solving the two-dimensional Helmholtz-type difference equation. This method requires the solutions of two linear tridiagonal systems of equations, which can be solved more easily and efficiently than the original two-dimensional Helmholtz-type equation. Using idealized large-scale flows on the sphere, it is shown that the scheme provides stable and accurate model solutions at considerable computational economy.

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Scott Elliott, Xuepeng Zhao, Richard P. Turco, Chih-Yue Jim Kao, and Mei Shen

Abstract

Atmospheric photochemistry lies at the heart of global-scale pollution problems, but it is a nonlinear system embedded in nonlinear transport and so must be modeled in three dimensions. Total earth grids are massive and kinetics require dozens of interacting tracers, taxing supercomputers to their limits in global calculations. A matrix-free and noniterative family scheme is described that permits chemical step sizes an order of magnitude or more larger than time constants for molecular groupings, in the 1-h range used for transport. Families are partitioned through linearized implicit integrations that produce stabilizing species concentrations for a mass-conserving forward solver. The kinetics are also parallelized by moving geographic loops innermost and changes in the continuity equations are automated through list reading. The combination of speed, parallelization, and automation renders the programs naturally modular. Accuracy lies within 1% for all species in week-long fidelity tests. A 50-species, 150-reaction stratospheric module tested in a spectral GCM benchmarks at 10 min CPU time per day and agrees with lower-dimensionality simulations. Tropospheric nonmethane hydrocarbon chemistry will soon be added, and inherently three-dimensional phenomena will be investigated both decoupled from dynamics and in a complete chemical GCM.

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