一个基于Eta垂直坐标的新WRF动力框架及其数值试验

A New Eta-Coordinate-Based WRF Dynamic Core and Its Numerical Experiments

针对我国陡峭地形数值预报难题,本文在国际先进WRF(Weather Research and Forecasting)模式动力框架中引入阶梯地形垂直坐标,以期为改进复杂地形区域数值天气预报提供模式发展可选方案。设计气柱质量变换方法,实现阶梯地形和追随地形两种垂直坐标下动力方程组的形式一致,从而简化方程组离散及程序实现的复杂过程。阶梯地形的构造借鉴了AREM(Advanced Regional Eta-coordinate Model)的设计思路,即首先构造参考大气,而后定义阶梯地形网格特征量,最后进行阶梯地形表达。在Eta坐标下的WRF动力框架搭建完成后,采用山脉波理想试验和实例试验对新动力框架的正确性和有效性进行了检验。结果表明:由于阶梯地形固有的孤立分布特征,可造成不同台阶处气流的分离,从而使得阶梯地形在较粗垂直分辨率情形下,不能很好模拟山脉波;但如果垂直分层足够精细,或模拟时间足够长,可通过减弱气流分离现象或调整气流分布状态,对地形波进行有效刻画。

This paper mainly focuses on resolving the numerical weather prediction problem in the steep orographic region of China. The stepped terrain vertical coordinate(known as Eta) is introduced into the dynamic core of the weather research and forecasting(WRF) model to improve numerical forecasting in the complex orographic region. We designed the atmospheric column mass transformation scheme to give the Eta WRF core the same model equation system formulation as the Sigma WRF core, which can facilitate discretization and programming. The design of the stepped terrain follows the computational scheme of the advanced regional Eta-coordinate model(AREM), including the introduction of the reference atmosphere, the adoption of mesh description indicators, and the representation of stepped mountains. We performed 2D idealized mountain wave simulations and 3D real-case experiments to test the correctness and robustness of the new dynamic frame. Inherent isolation of the stepped topography can result in airflow separation,which accounts for inadequate simulation of the mountain wave with a coarse vertical layering using the new Eta WRF core. In this case, the mountain wave simulation will improve using the new dynamic frame either because of the fine vertical resolution or the long simulation time, which is considered alleviation on the airflow isolation in stepped mountains.