Research Of Mass Conservation On The Dynamic Core Of GRAPES Model

For the long term integration of numerical models,the conservation of mass is very important.GRAPES?GFS of China Meteorological Administration has made great progress in recent years,but also save the following basic problems:1)the Quasi-Monotone Semi-Lagrangian scheme which adopted in GRAPES?GFS suffers a lower accuracy and cannot strictly conserved the mass.2)The conservation of total mass of dry atmosphere can not be strictly guaranteed in the process of long term integration.In view of the above problems,the research work of this paper is divided into the following two parts:Conservation of wet dynamic process: How to better simulate the advection and the distribution of water substance,is of great significance for the improvement of numerical weather prediction,especially for precipitation.The computation of advection process in a semi-lagrangian model requires high accuracy,conservation of mass,positive and preserve of shape.But the QMSL(Quasi-Monotone Semi-Lagrangian)scheme which adopted in GRAPES?GFS suffers a lower accuracy in the strong gradient and discontinuity regional of the scalar field,and cannot be strictly conserved the mass.This study draws on research progress in the field of computational fluid dynamics,introduce a material advection scheme PRM(Piecewise Rational Method)which based on piecewise continuous rational function into GRAPES?GFS,solve the water vapor equation in the flux form,and treat the polar regions with a mix technical.The two advection schemes were compared by a series of stand-alone and in-model ideal test,proofed that compare to QMSL scheme,PRM scheme is of higher precision,especially in the area of large water vapor gradient,the dispersion and dissipation error is smaller,the conservation and shape preserving is also better.Through the examine of batch prediction experiments in GRAPES?GFS,verified that the PRM scheme can effectively improve the simulation of the advection and distribution of water substance,promote the effect of precipitation forecast,and also plays a significant role to the enhancement of the comprehensive model performance.Conservation of dry dynamic process: How to ensure the conservation of mass in the long term is one of the fundamental problems in the dynamic framework of the numerical model,for the semi-lagrangian dynamic core,it is difficult to satisfy the mass conservation in the theory.The GRAPES?GFS face a problem that the mass loss is obviously in the process of long term integration.in order to solve this problem,based on the extensive investigation in a wide range of operational centers,this part of research work introduced density as a new forecast variable of GRAPES?GFS dynamical core,and calculated the continuous equation in the Euler flux form,according to the new method to solve the whole dynamic process,hoped to get a new dynamical core with mass conservation in theory,but it has not been properly tested for the time being.On the other hand,The research work of this part draws on the ideas of C-CAM(Climate-Community Atmosphere Model)model,to achieve mass conservation through the correction of surface air,developed a new method to achieve mass conservation through the adjustment of mass in each grid box according to different weight coefficient.The feasibility of this method is verified by a series of experiments,include ideal test,real case prediction and a batch of cycle prediction,illustrated that the method can effectively reduce the bias of height field under the premise of ensuring the mass conservation,and alleviate the problem of the weakness of weather system.It shows that this method has a certain application value in actual operational forecast.