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During 1990 the AMS is conducting a strategic review with the goals of 1) evaluating its mission and practices in view of the new challenges of the 1990s, and 2) identifying the society's priorities for the 1990s. The theme of the review is “Building on Strength”—recognizing the solid organizational base of the society, and its long record of successful journals and meetings. All members of the society are encouraged to propose issues for consideration and to make recommendations for action. A Working Group, representating the wide spectrum of membership interests and organizational elements of the society, will consider all recommendations, and will prepare a report to the council and the entire membership in early 1991.
During 1990 the AMS is conducting a strategic review with the goals of 1) evaluating its mission and practices in view of the new challenges of the 1990s, and 2) identifying the society's priorities for the 1990s. The theme of the review is “Building on Strength”—recognizing the solid organizational base of the society, and its long record of successful journals and meetings. All members of the society are encouraged to propose issues for consideration and to make recommendations for action. A Working Group, representating the wide spectrum of membership interests and organizational elements of the society, will consider all recommendations, and will prepare a report to the council and the entire membership in early 1991.
Abstract
A numerical, advection-diffusion air pollution transport model is used to describe dispersion of emissions from urban area-type sources for several atmospheric stability situations, The estimation of appropriate velocity and diffusivity fields for use as meteorological input data is discussed in detail. Profiles representing the constant-flux surface layer are used in a nondimensional transport equation to obtain concentration fields over a limited horizontal and vertical scale. A wind spiral model and diffusivity profile after Blackadar is used to simulate dispersion under neutral stability conditions on an urban scale and in three dimensions for both a steady source and a single puff. Analysis of the trajectory and spread statistics for the puff suggests that, on an urban scale, the combined effects of vertical diffusion and transverse wind components result in effective cross-wind diffusion coefficients an order of magnitude greater than typical values of vertical diffusion coefficients. Superposition of the concentration fields of a two-dimensional calculation demonstrates the effect of different spatial distributions of sources on ground-level concentrations.
In the last sections, diffusivity profiles are postulated for investigation of the effects of varying stability and mixing height on concentration distributions on an urban scale. Results indicate that ground-level concentrations vary by a factor of three or so over a range of stability conditions from neutral to slightly unstable for infinite mixing heights. The influence of time variations of mixing height on ground-level concentrations is examined in terms of the relative meteorological time scales and corresponding departures from quasi-steady-state concentration values.
Abstract
A numerical, advection-diffusion air pollution transport model is used to describe dispersion of emissions from urban area-type sources for several atmospheric stability situations, The estimation of appropriate velocity and diffusivity fields for use as meteorological input data is discussed in detail. Profiles representing the constant-flux surface layer are used in a nondimensional transport equation to obtain concentration fields over a limited horizontal and vertical scale. A wind spiral model and diffusivity profile after Blackadar is used to simulate dispersion under neutral stability conditions on an urban scale and in three dimensions for both a steady source and a single puff. Analysis of the trajectory and spread statistics for the puff suggests that, on an urban scale, the combined effects of vertical diffusion and transverse wind components result in effective cross-wind diffusion coefficients an order of magnitude greater than typical values of vertical diffusion coefficients. Superposition of the concentration fields of a two-dimensional calculation demonstrates the effect of different spatial distributions of sources on ground-level concentrations.
In the last sections, diffusivity profiles are postulated for investigation of the effects of varying stability and mixing height on concentration distributions on an urban scale. Results indicate that ground-level concentrations vary by a factor of three or so over a range of stability conditions from neutral to slightly unstable for infinite mixing heights. The influence of time variations of mixing height on ground-level concentrations is examined in terms of the relative meteorological time scales and corresponding departures from quasi-steady-state concentration values.
Abstract
A numerical, grid-element model has been developed for the study of air pollution transport from urban area-type sources. This advection-diffusion model is especially useful for the estimation of air pollution concentrations under conditions of spatial and time varying emissions, velocities and diffusion rates. The “pseudo-diffusive” errors associated with conventional finite-difference approximations to advective transport are eliminated by a material-conserving computation procedure involving the zeroth, first and second moments of the concentration distribution within each grid element. Extensions of the procedure are suggested for retention of sub-grid-scale resolution of concentration values necessary in the study of transport of chemically reactive materials, or for the incorporation of emissions from point and line sources. A novel procedure is presented for the numerical simulation of horizontal diffusion from area sources which can be used to model empirically observed dispersive growth rates.
Abstract
A numerical, grid-element model has been developed for the study of air pollution transport from urban area-type sources. This advection-diffusion model is especially useful for the estimation of air pollution concentrations under conditions of spatial and time varying emissions, velocities and diffusion rates. The “pseudo-diffusive” errors associated with conventional finite-difference approximations to advective transport are eliminated by a material-conserving computation procedure involving the zeroth, first and second moments of the concentration distribution within each grid element. Extensions of the procedure are suggested for retention of sub-grid-scale resolution of concentration values necessary in the study of transport of chemically reactive materials, or for the incorporation of emissions from point and line sources. A novel procedure is presented for the numerical simulation of horizontal diffusion from area sources which can be used to model empirically observed dispersive growth rates.
Abstract
Two methods were used to identify the paths of moisture transport that reach the U.S. Intermountain West (IMW) during heavy precipitation events in winter. In the first, the top 150 precipitation events at stations located within six regions in the IMW were identified, and then back trajectories were initiated at 6-h intervals on those days at the four Climate Forecast System Reanalysis grid points nearest the stations. The second method identified the leading patterns of integrated water vapor transport (IVT) using the three leading empirical orthogonal functions of IVT over land that were first normalized by the local standard deviation. The top 1% of the associated 6-hourly time series was used to construct composites of IVT, atmospheric circulation, and precipitation. The results from both methods indicate that moisture originating from the Pacific that leads to extreme precipitation in the IMW during winter takes distinct pathways and is influenced by gaps in the Cascades (Oregon–Washington), the Sierra Nevada (California), and Peninsular Ranges (from Southern California through Baja California). The moisture transported along these routes appears to be the primary source for heavy precipitation for the mountain ranges in the IMW. The synoptic conditions associated with the dominant IVT patterns include a trough–ridge couplet at 500 hPa, with the trough located northwest of the ridge where the associated circulation funnels moisture from the west-southwest through the mountain gaps and into the IMW.
Abstract
Two methods were used to identify the paths of moisture transport that reach the U.S. Intermountain West (IMW) during heavy precipitation events in winter. In the first, the top 150 precipitation events at stations located within six regions in the IMW were identified, and then back trajectories were initiated at 6-h intervals on those days at the four Climate Forecast System Reanalysis grid points nearest the stations. The second method identified the leading patterns of integrated water vapor transport (IVT) using the three leading empirical orthogonal functions of IVT over land that were first normalized by the local standard deviation. The top 1% of the associated 6-hourly time series was used to construct composites of IVT, atmospheric circulation, and precipitation. The results from both methods indicate that moisture originating from the Pacific that leads to extreme precipitation in the IMW during winter takes distinct pathways and is influenced by gaps in the Cascades (Oregon–Washington), the Sierra Nevada (California), and Peninsular Ranges (from Southern California through Baja California). The moisture transported along these routes appears to be the primary source for heavy precipitation for the mountain ranges in the IMW. The synoptic conditions associated with the dominant IVT patterns include a trough–ridge couplet at 500 hPa, with the trough located northwest of the ridge where the associated circulation funnels moisture from the west-southwest through the mountain gaps and into the IMW.
