B224 メソ対流系の数値計算における SGS 混合と人工粘性の効果

概要

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Mesoscale cloud simulations (grid spacingο(1km)) employ a turbulence-closure model for subgrid-scale mixing as well as an explicit numerical spatial filter (background mixing) to avoid numerical errors. Using a newly developed cloud model, this study investigates the effects of subgrid mixing and numerical filtering in squall-line simulations by examining the sensitivities to the parameters in subgrid models and to the parameters in the numerical filters. Using the commonly used subgrid model and no numerical filter, the solution is characterized by many poorly resolved grid-scale cells. Such grid-scale noise is avoided by adding a numerical filter which, however, produces excessive damping of the physical small-scale eddies. Without using such a filter, we have found that by increasing the proportionality constant in the eddy viscosity coefficient in the subgrid models, the cells become well-resolved; but that further increases in the constant overly smooth the cells. There should be optimum eddy-viscosity coefficients with which only minimum amount of numerical filter is required.
2002-07-23