Trace gas/aerosol boundary concentrations and their impacts on continental-scale AQMEII modeling domains

Copyright 2011 Elsevier B.V., All rights reserved.Over twenty modeling groups are participating in the Air Quality Model Evaluation International Initiative (AQMEII) in which a variety of mesoscale photochemical and aerosol air quality modeling systems are being applied to continental-scale domains in North America and Europe for 2006 full-year simulations for model inter-comparisons and evaluations. To better understand the reasons for differences in model results among these participating groups, each group was asked to use the same source of emissions and boundary concentration data for their simulations. This paper describes the development and application of the boundary concentration data for this AQMEII modeling exercise. The European project known as GEMS (Global and regional Earth-system Monitoring using Satellite and in-situ data) has produced global-scale re-analyses of air quality for several years, including 2006 (http://gems.ecmwf.int). The GEMS trace gas and aerosol data were made available at 3-hourly intervals on a regular latitude/longitude grid of approximately 1.9° resolution within 2 "cut-outs" from the global model domain. One cut-out was centered over North America and the other over Europe, covering sufficient spatial domain for each modeling group to extract the necessary time- and space-varying (horizontal and vertical) concentrations for their mesoscale model boundaries. Examples of the impact of these boundary concentrations on the AQMEII continental simulations are presented to quantify the sensitivity of the simulations to boundary concentrations. In addition, some participating groups were not able to use the GEMS data and instead relied upon other sources for their boundary concentration specifications. These are noted, and the contrasting impacts of other data sources for boundary data are presented. How one specifies four-dimensional boundary concentrations for mesoscale air quality simulations can have a profound impact on the model results, and hence, this aspect of data preparation must be performed with considerable care.Peer reviewedFinal Accepted Versio

A wildland fire model with data assimilation

A wildfire model is formulated based on balance equations for energy and fuel, where the fuel loss due to combustion corresponds to the fuel reaction rate. The resulting coupled partial differential equations have coefficients that can be approximated from prior measurements of wildfires. An ensemble Kalman filter technique with regularization is then used to assimilate temperatures measured at selected points into running wildfire simulations. The assimilation technique is able to modify the simulations to track the measurements correctly even if the simulations were started with an erroneous ignition location that is quite far away from the correct one.Comment: 35 pages, 12 figures; minor revision January 2008. Original version available from http://www-math.cudenver.edu/ccm/report

Non-equilibrium model of spray-stratified atmospheric boundary layer under high wind conditions

Magnetite-based ferrofluids are manufactured magneto-polarisable nanofluids that magnetize in an external magnetic field in a similar way to natural paramagnetic fluids(e.g. oxygen), however to a much higher degree. Paramagnetic and ferrofluid flows are described by similar equations and it is expected that they would exhibit a similar behaviour. Indeed we show that in both type of fluids the most prominent instability structures align with the in-layer field component and the onset of magnetoconvection is delayed by the field inclination. However we find that in contrast to paramagnetic fluids the instabilities arising in differentially heated ferrofluids placed in a uniform external oblique magnetic field are oscillatory. This is traced back to the nonlinearity of the magnetic field distribution induced inside the ferrofluid layer that arises whenever the direction of the applied magnetic field is not normal. Given that the magnetic field inclination with respect to the plane of the layer is inevitable near its edges the obtained stability results shed light on the possible reasons for the existnce of unsteady patterns that have been detected in the normal field experiments we reported previously

Spatial reduction algorithm for atmospheric chemical transport models

Numerical modeling of global atmospheric chemical dynamics presents an enormous challenge, associated with simulating hundreds of chemical species with time scales varying from milliseconds to years. Here we present an algorithm that provides a significant reduction in computational cost. Because most of the fast reactants and their quickly decomposing reaction products are localized near emission sources, we use a series of reduced chemical models of decreasing complexity with increasing distance from the source. The algorithm diagnoses the chemical dynamics on-the-run, locally and separately for every species according to its characteristic reaction time. Unlike conventional time-scale separation methods, the spatial reduction algorithm speeds up not only the chemical solver but also advection–diffusion integration. Through several examples we demonstrate that the algorithm can reduce computational cost by at least an order of magnitude for typical atmospheric chemical kinetic mechanisms

Resolving intercontinental pollution plumes in global models of atmospheric transport

Synoptic-scale pollution plumes in the free troposphere can preserve their identity as well-defined structures for a week or more while traveling around the globe. Eulerian chemical transport models (CTMs) have difficulty reproducing these layered structures due to numerical plume dissipation. We show that this dissipation is much faster than would be expected from the order of the advection scheme because of interaction between numerical diffusion and the nonuniformity of the atmospheric flow. The nonuniform flow stretches out the plume, enhancing the effect of numerical diffusion. For sufficiently strong stretching, the numerical decay of the plume is independent of the model grid resolution and is set instead by the flow Lyapunov exponent l. In this regime, conventional numerical methods are not convergent: upon increasing grid resolution, the plume still decays with the same decay rate. The critical plume size below which the numerical scheme does not converge is set by the geometric mean of the grid spacing and the characteristic length scale l = v/l over which the flow varies, where v is the wind speed. Above this critical plume size the numerically induced decay rate of the plume scales like the square root of the grid spacing. Application to an intercontinental pollution plume in a global CTM with realistic atmospheric flow shows that proper simulation of such a plume would require an impractical increase in grid resolution. Novel methods such as adaptive grids or embedded Lagrangian plumes are needed.Engineering and Applied Science