PRM标量平流方案在GRAPES全球预报系统中的应用

如何更好地模拟水物质的分布,对于数值天气预报效果的改进,特别是对于更好地模拟降水过程,具有重要的意义。半拉格朗日模式中的标量平流计算要求做到高精度、守恒、正定和保形,但GRAPES_GFS (Global-Regional Assimilation and PrEdiction System, Global Forecast System) 中采用的QMSL(Quasi-Monotone Semi-Lagrangian)平流方案在水汽的强梯度、不连续区域计算精度较低,且不能做到严格守恒。本研究借鉴计算流体力学领域的研究进展,将一个基于分段有理函数的物质平流方案PRM(Piecewise Rational Method)引入GRAPES_GFS中,按照通量形式求解水汽方程,并对极区进行了混合等技术处理。通过一系列理想试验对两种平流方案进行了对比,证明了PRM方案精度较高,特别是在水汽梯度大的区域优势明显,频散、耗散误差较小,守恒、保形性也要好于QMSL方案。通过对GRAPES_GFS中批量预报试验效果的检验,验证了PRM方案可以有效地改进模式对水物质分布的模拟,提高了降水的预报效果,对模式综合预报性能的提升也有明显作用。

Application of PRM Scalar Advection Scheme in GRAPES Global Forecast System

Better simulation of the advection and distribution of moisture is highly significant for the improvement of numerical weather predictions, especially for precipitation. The computation of the advection process in a semi-Lagrangian model requires high accuracy, conservation of mass, positive and preserves the shape or spatial distribution of the advection quantity. However, the Quasi-Monotone Semi-Lagrangian (QMSL) scheme adopted in the Global-Regional Assimilation and PrEdiction System, Global Forecast System (GRAPES_GFS) suffers from lower accuracy in the strong gradient and discontinuity region of the scalar field and cannot strictly conserve the mass. This study draws on research progress in computational fluid dynamics; it introduces a material advection scheme, the Piecewise Rational Method (PRM), which is based on a piecewise continuous rational function, into the GRAPES_GFS, solves the flux form of the water vapor equation, and treats the polar regions with a mixing technique. The two advection schemes were compared in a series of stand-alone and in-model ideal tests. The results proved that the PRM scheme is more precise than the QMSL scheme; in particular, in the area of large water vapor gradient, the dispersion and dissipation error is smaller, and the conservation and shape preservation are also better. An examination of batch prediction experiments in the GRAPES_GFS verified that the PRM scheme can effectively improve the simulation of the advection and distribution of moisture, improve the accuracy of precipitation forecasts, and also play a significant role in enhancing the comprehensive model performance.