In this work,the problem of dependency of the predicted rainfall upon the grid-size in mesoscale numerical weather prediction models is addressed.We argue that this problem is due to (i) the violation of the quasi-equ...In this work,the problem of dependency of the predicted rainfall upon the grid-size in mesoscale numerical weather prediction models is addressed.We argue that this problem is due to (i) the violation of the quasi-equilibrium assump- tion,which is underlying most existing convective parameterization schemes,and states that the convective activity may be considered in instantaneous equilibrium with the larger-scale forcing;and (ii) the violation of the hydrostatic approx- imation,made in most mesoscale models,which would induce too large-scale circulation in occurrence of strong con- vection.On the contrary,meso-β and meso-α scale models,i.e.models with horizontal grid size ranging from 10 to 100 km,have a capacity to resolve motions with characteristic scales close to the ones of the convective motions.We hypothesize that a possible way to eliminate this problem is (i) to take a prognostic approach to the parameterization of deep convection,whereby the quantities that describe the activity of convection are no longer diagnosed from the instan- taneous value of the large-scale forcing,but predicted by time-dependent equations,that integrate the large-scale forc- ing over time;(ii)to introduce a mesoscale parameter which varies systematically with the grid size of the numerical model in order to damp large-scale circulation usually too induced when the grid size becomes smaller (from 100 km to 10 kin).We propose an implementation of this idea in the frame of one existing scheme,already tested and used for a long time at the French Weather Service.The results of the test through one-dimensional experiments with the Phase Ⅲ of GATE data are reported in this paper;and the ones on its implementation in the three-dimensional model with the OSCAR data will be reported in a companion paper.展开更多
A series of 3D predictions,dealing with the development of a heavy storm observed during the OSCAR experiment, were carried out by utilizing the PERIDOT model,and introducing alternatively the cumulus parameterization...A series of 3D predictions,dealing with the development of a heavy storm observed during the OSCAR experiment, were carried out by utilizing the PERIDOT model,and introducing alternatively the cumulus parameterization scheme of Bougeault (1985) and the prognostic one (Chen,1989;Chen and Bougeault,1993),with three different grid sizes: 160 km,80 km,40 km.The feasibility of the new prognostic scheme and its improvement on the problem of dependency of the predicted rainfall upon the grid size of the numerical model were verified by comparison of the rainfall observed and those predicted. The results demonstrate that,in general,the predicted rainfall increases when the grid size decreases for both diagnostic and prognostic schemes.However,with the new prognostic scheme,the numerical model is capable,on the one hand,for the larger grid sizes,to increase the rainfall,which is under-estimated with the scheme of Bougeault (1985);on the another hand,for the smaller grid sizes,to reduce the rainfall,which is usually over-estimated.In other word,there is an obvious improvement on the problem under study.展开更多
文摘In this work,the problem of dependency of the predicted rainfall upon the grid-size in mesoscale numerical weather prediction models is addressed.We argue that this problem is due to (i) the violation of the quasi-equilibrium assump- tion,which is underlying most existing convective parameterization schemes,and states that the convective activity may be considered in instantaneous equilibrium with the larger-scale forcing;and (ii) the violation of the hydrostatic approx- imation,made in most mesoscale models,which would induce too large-scale circulation in occurrence of strong con- vection.On the contrary,meso-β and meso-α scale models,i.e.models with horizontal grid size ranging from 10 to 100 km,have a capacity to resolve motions with characteristic scales close to the ones of the convective motions.We hypothesize that a possible way to eliminate this problem is (i) to take a prognostic approach to the parameterization of deep convection,whereby the quantities that describe the activity of convection are no longer diagnosed from the instan- taneous value of the large-scale forcing,but predicted by time-dependent equations,that integrate the large-scale forc- ing over time;(ii)to introduce a mesoscale parameter which varies systematically with the grid size of the numerical model in order to damp large-scale circulation usually too induced when the grid size becomes smaller (from 100 km to 10 kin).We propose an implementation of this idea in the frame of one existing scheme,already tested and used for a long time at the French Weather Service.The results of the test through one-dimensional experiments with the Phase Ⅲ of GATE data are reported in this paper;and the ones on its implementation in the three-dimensional model with the OSCAR data will be reported in a companion paper.
文摘A series of 3D predictions,dealing with the development of a heavy storm observed during the OSCAR experiment, were carried out by utilizing the PERIDOT model,and introducing alternatively the cumulus parameterization scheme of Bougeault (1985) and the prognostic one (Chen,1989;Chen and Bougeault,1993),with three different grid sizes: 160 km,80 km,40 km.The feasibility of the new prognostic scheme and its improvement on the problem of dependency of the predicted rainfall upon the grid size of the numerical model were verified by comparison of the rainfall observed and those predicted. The results demonstrate that,in general,the predicted rainfall increases when the grid size decreases for both diagnostic and prognostic schemes.However,with the new prognostic scheme,the numerical model is capable,on the one hand,for the larger grid sizes,to increase the rainfall,which is under-estimated with the scheme of Bougeault (1985);on the another hand,for the smaller grid sizes,to reduce the rainfall,which is usually over-estimated.In other word,there is an obvious improvement on the problem under study.
