数值模式中地形滤波处理及水平扩散对降雨预报的影响

为克服高分辨率模拟中，对于具有陡峭山峰及深谷的区域，存在不真实降雨场预报问题，本文引入数字滤波器及水平扩散方案分别对地形及计算噪音进行处理.滤波器由不同的一维高阶低通隐式正切滤波器耦合而成，能选择性地过滤由地形坡度所引起的不同尺度的噪音.水平扩散方案是将一个通量受地形限制的线性四阶单调水平扩散项加到预报方程，去控制由数值扩散、非线性不稳定及不连续物理过程等引起的小尺度噪音.试验结果表明：地形滤波处理及水平扩散方案能消除山区降雨预报量集中在山顶，而同时山谷和背风面又无雨的现象.因而，降雨分布更真实.

The impact of topography filter processing and horizontal diffusion on precipitation prediction in numerical model

In high-resolution modeling,there is a problem of the prediction of unrealistic precipitation field in mountainous regions with deep valleys and 'singular' mountain tops.To solve the problem,digital filtering and horizontal diffusion were used in this paper to process topography and computational noise,respectively.The filter consists of different one-dimensional high-order low-pass implicit tangent filters and can selectively remove different scale noises that result from the change of topographical gradient.Horizontal diffusion was used to control small-scale noise that results from numerical dispersion,nonlinear instability,and discontinuous physical processing,etc.by adding a linear forth-order monotonic horizontal diffusion with orographic flux limiting to the prediction equation.Our tests show that the phenomenon is eliminated in which extremely large amount of precipitation occurs directly over mountain tops and at the same time no precipitation occurs in deep valleys and at mountain lee-sides by using both topography filter processing and horizontal diffusion.Therefore,the precipitation patterns appear to be more realistic.