A multiple-cell flat-level tracer dispersion model is developed for atmospheric pollution study. The horizontal domain may be constructed with multiple-sized cells for varied resolution. The sequence of cells is arbitrary, as in unstructured grids, as long as no holes are left in the horizontal domain, which may be tailored in shape according to local orography. The vertical levels are truly flat and the level spacing may vary from level to level. The surface orography is included by removing cells from the bottom of the three-dimensional cell block. The arrangement of wind velocity and tracer concentration is similar to the Arakawa C grid. Advection and horizontal diffusion are formulated on each cell face, using tracer concentrations in the two cells that share the cell face. Pointer-orientated numerical loops are used to facilitate the arbitrary horizontal cell arrangement and orographic variation of vertical levels. A second-order upstream upper limiter advection scheme is developed for this model and numerically tested to be positive-definite and mass conserving. Vertical diffusion is solved with an implicit scheme and simplified vertical diffusivity, which is parameterised as a function of the mixing layer depth. The model is fast, compact, easy to implement and highly portable. It is suitable for studies ofmesoscale and small-scale atmospheric tracer dispersion over complex terrain, including steep slopes. The model is used to simulate traffic pollution in London, UK, and is compared with available observations. 相似文献
The Loire River, with one of the largest watersheds in France, has been monitored just outside the city of Orleans since 1994. Physico-chemical parameters and major and trace elements were measured between 2-day and 1-week intervals according to the river flow. The sampling site represents 34% of the total Loire watershed with 76% silicate rocks and 24% carbonate rocks.
Elements are transported mainly in the dissolved phase with the ratio of total dissolved salts (TDS) to suspended matter (SM) ranging between 1.6 and 17.4. Chemical weathering of rocks and soils are thus the dominant mechanisms in the Loire waters composition. The highest TDS/SM ratios are due to dissolved anthropogenic inputs. The database shows no link between NO3− content and river flow. The Na+, K+, Mg2+, SO42−, and Cl− concentrations are seen to decrease with increasing discharge, in agreement with a mixing process involving at least two components: the first component (during low flow) is concentrated and may be related with input from the groundwater and sewage station water, the second component (during high flow) is more dilute and is in agreement with bedrock weathering and rainwater inputs. A geochemical behaviour pattern is also observed for HCO3− and Ca2+ species, their concentrations increase with increasing discharge up to 300 m3/s, after which, they decrease with increasing discharge. The Sr isotopic composition of the dissolved load is controlled by at least five components — a series of natural components represented by (a) waters draining the silicate and carbonate bedrock, (b) groundwater, and (c) rainwaters, and two kinds of anthropogenic components.
The aim of this study is to describe the mixing model in order to estimate the contribution of each component. Finally, specific export rates in the upper Loire watershed were evaluated close to 12 t year−1 km−2 for the silicate rate and 47 t year−1 km−2 for the carbonate rate. 相似文献