临界理查森数对不稳定状态下边界层模拟的影响

Influence of Critical Richardson Number on Boundary Layer Simulation in Unstable State

在边界层参数化方案中,临界理查森数可以用来判断湍流状态和计算边界层高度。大部分模式会将临界理查森数设置为常数,但实际上它会发生改变。基于WRF(weather research and forecasting)中尺度数值预报模式和YSU(Yonsei University)边界层参数化方案(以下简称WRF/YSU),以一次夏季的晴好天气过程为背景,研究了当临界理查森数发生变化,会对气象场和边界层湍流产生的影响。结果表明:当临界理查森数增加时,大范围区域的边界层的高度会增加。在时空上能够体现出来。在空间上,边界层高度分布不均。从时间上看,在下午边界层高度受到的影响最大;理查森数的临界值增加,湍流扩散系数也增大,水汽混合比增加,并且地面热通量总体增大,使边界层高度增大;YSU方案中湍流扩散方程由一个局部项、一个非局部项和一个夹卷项组成。对于热通量,夹卷项对总热通量的贡献与局部项和非局部项的贡献相当。对于水汽和动量通量,局地项及夹卷通量项在边界层中最大。

In the boundary layer parameterization scheme of numerical model, the critical Richardson number is diagnosed for turbulence state and boundary layer height. In most models, the critical Richardson number is set to be constant, but in practice it is changed. Based on the WRF(weather research and forecasting) mesoscale numerical forecast model and the YSU(Yonsei University) boundary layer parameterization scheme(hereinafter referred to as WRF/YSU), a fine weather process in summer was used as the background. The effects on the meteorological field and boundary layer turbulence were studied when the critical Richardson number changes. The results Show as follows. When the critical Richardson number was increased, the height of the boundary layer in a large area was increased. It can be manifested in time and space. Spatially, the boundary layer height was unevenly distributed. In terms of time, the height of the boundary layer was most affected in the afternoon. The critical Richardson number was increased, the turbulent diffusion coefficient was increased, the water-vapor mixing ratio was increased, and the surface heat flux was generally increased, so that the boundary layer height was increased. The turbulent diffusion equation in the YSU scheme was composed of a local term, a nonlocal term and an entrainment term. For heat flux, the contribution of the entrainment term to the total heat flux is comparable to that of the local and nonlocal terms. For water vapor and momentum fluxes, the local terms and entrainment flux terms are largest in the boundary layer.