Systematic errors in the medium range prediction of the Asian summer monsoon circulation

The present study describes an analysis of Asian summer monsoon forecasts with an operational general circulation model (GCM) of the European Centre for Medium Range Weather Forecasts (ECMWF), U.K. An attempt is made to examine the influence of improved treatment of physical processes on the reduction of systematic errors. As some of the major changes in the parameterization of physical processes, such as modification to the infrared radiation scheme, deep cumulus convection scheme, introduction of the shallow convection scheme etc., were introduced during 1985–88, a thorough systematic error analysis of the ECMWF monsoon forecasts is carried out for a period prior to the incorporation of such changes i.e. summer monsoon season (June–August) of 1984, and for the corresponding period after relevant changes were implemented (summer monsoon season of 1988). Monsoon forecasts of the ECMWF demonstrate an increasing trend of forecast skill after the implementation of the major changes in parameterizations of radiation, convection and land-surface processes. Further, the upper level flow is found to be more predictable than that of the lower level and wind forecasts display a better skill than temperature. Apart from this, a notable increase in the magnitudes of persistence error statistics indicates that the monsoon circulation in the analysed fields became more intense with the introduction of changes in the operational forecasting system. Although, considerable reduction in systematic errors of the Asian summer monsoon forecasts is observed (up to day-5) with the introduction of major changes in the treatment of physical processes, the nature of errors remain unchanged (by day-10). The forecast errors of temperature and moisture in the middle troposphere are also reduced due to the changes in treatment of longwave radiation. Moreover, the introduction of shallow convection helped it further by enhancing the vertical transports of heat and moisture from the lower troposphere. Though, the hydrological cycle in the operational forecasts appears to have enhanced with the major modifications and improvements to the physical parameterization schemes, certain regional peculiarities have developed in the simulated rainfall distribution over the monsoon region. Hence, this study suggests further attempts to improve the formulations of physical processes for further reduction of systematic forecast errors.     

A SIMPLE PROGNOSTIC CLOSURE ASSUMPTION TO DEEP CONVECTIVE PARAMETERIZATION:I

In this work,the problem of dependency of the predicted rainfall upon the grid-size in mesoscale numerical weatherprediction 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 maybe 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 100km,have a capacity to resolve motions with characteristic scales close to the ones of the convective motions.Wehypothesize that a possible way to eliminate this problem is (i) to take a prognostic approach to the parameterization ofdeep 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 numericalmodel in order to damp large-scale circulation usually too induced when the grid size becomes smaller (from 100 km to10 kin).We propose an implementation of this idea in the frame of one existing scheme,already tested and used for along time at the French Weather Service.The results of the test through one-dimensional experiments with the Phase Ⅲof GATE data are reported in this paper;and the ones on its implementation in the three-dimensional model with theOSCAR data will be reported in a companion paper.     

THE FORMATION AND THE DEVELOPMENT OF CAMS TWO-DIMENSIONAL DYNAMICAL-PHYSICAL CLIMATE MODEL

In the paper,we have developed a 2-D physical-dynamical coupled climate model.Some sensitive experiments have been done by use of this model.First of all,we have studied the effects of different results by two radiational calculation schemes on circulation variation.The calculated results have shown that the different radiation parameterization schemes give different results,therefore the variational effects of wind,temperature,and humidity field are presented on the medium-range circulation variation.Besides,we have also studied the role of the meridional eddy momentum fluxes in formation of the monsoon over East Asia.The results of study have shown that on the average,the roles of meridional eddy flux of momentum formative processes of monsoon which is added to momentum equations by using the scheme of moist process parameterization with plateau are manifest.     

A SIMPLE PROGNOSTIC CLOSURE ASSUMPTION TO DEEP CONVECTIVE PARAMETERIZATION:Ⅱ

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.     

The vertical transport of air pollutants by convective clouds Part II: Transport of soluble gases and sensitivity tests

A two-dimensional, non-reactive convective cloud transport model is used to simulate in detail the vertical transport and wet scavenging of soluble pollutant gases by a deep thunderstorm system, Simulations show that for gases with not very high solubility, a deep and intense thunderstorm can still rapidly and efficiently transport them from boundary layer (PBL) up to mid and upper troposphere, resulting in a local significant increase of concentration in the upper layer and a reduction in PBL. Dissolution effects decrease both the incloud gas concentration and the upward net fluxes. The higher the solubility is, the more remarkable the decrease is. However, for very low soluble gases (H < 102 M atm-1), the influences are very slight. In addition, the effects of irreversible dissolution and aqueous reactions in drops on the vertical transport of gaseous pollutants are estimated in extreme.     

长波辐射对大气变化的敏感性和在WRF模式中的应用检验

用RRTM长波辐射 (LWR)参数化方案测试了LWR对大气变化的敏感性。结果表明 :高云对向外长波辐射(OLR)、30 0和 5 0 0hPa净LWR通量的减弱作用较中、低云大 ;低云对 85 0hPa和地表净LWR通量的减弱作用较中、高云大。在云层中 ,LWR冷却率受云影响最大 ;在云层下方 ,云对LWR的影响迅速减小 ;而在云层上方 ,冷却率几乎不受云的影响。当水汽含量减少或增加时 ,地表向下LWR受到相应减弱或增强 ,而净LWR则在一定程度上受到相应增强或减弱 ,并且越接近地面 ,受到水汽变化的影响就越大。O3 对LWR的影响相对云和水汽来说是比较小的。文中介绍了在WRF模式中应用RRTM方案预报LWR不同季节的 2个个例 ,给出了应用NCEP/AVN分析资料预报和验证中国范围 2d之内LWR通量的模拟结果。试验表明 ,OLR和 5 0 0hPa净LWR通量与高度形势场有较好的对应关系 ,而地表净LWR很大程度上还受到地形的影响。     

Theoretical investigation of convective instability in inclined and fluid-saturated three-dimensional fault zones

The convective instability of pore-fluid flow in inclined and fluid-saturated three-dimensional fault zones has been theoretically investigated in this paper. Due to the consideration of the inclined three-dimensional fault zone with any values of the inclined angle, it is impossible to use the conventional linear stability analysis method for deriving the critical condition (i.e., the critical Rayleigh number) which can be used to investigate the convective instability of the pore-fluid flow in an inclined three-dimensional fault zone system. To overcome this mathematical difficulty, a combination of the variable separation method and the integration elimination method has been used to derive the characteristic equation, which depends on the Rayleigh number and the inclined angle of the inclined three-dimensional fault zone. Using this characteristic equation, the critical Rayleigh number of the system can be numerically found as a function of the inclined angle of the three-dimensional fault zone. For a vertically oriented three-dimensional fault zone system, the critical Rayleigh number of the system can be explicitly derived from the characteristic equation. Comparison of the resulting critical Rayleigh number of the system with that previously derived in a vertically oriented one has demonstrated that the characteristic equation of the Rayleigh number is correct and useful for investigating the convective instability of pore-fluid flow in the inclined three-dimensional fault zone system. The related numerical results from this investigation have indicated that: (1) the convective pore-fluid flow may take place in the inclined three-dimensional fault zone; (2) if the height of the fault zone is used as the characteristic length of the system, a decrease in the inclined angle of the inclined fault zone stabilizes the three-dimensional fundamental convective flow in the inclined three-dimensional fault zone system; (3) if the thickness of the stratum is used as the characteristic length of the system, a decrease in the inclined angle of the inclined fault zone destabilizes the three-dimensional fundamental convective flow in the inclined three-dimensional fault zone system; and that (4) the shape of the inclined three-dimensional fault zone may affect the convective instability of pore-fluid flow in the system.     

Refinement Indicators for Optimal Selection of Geostatistical Realizations Using the Gradual Deformation Method

In the analysis of petroleum reservoirs, one of the most challenging problems is to use inverse theory in the search for an optimal parameterization of the reservoir. Generally, scientists approach this problem by computing a sensitivity matrix and then perform a singular value decomposition in order to determine the number of degrees of freedom i.e. the number of independent parameters necessary to specify the configuration of the system. Here we propose a complementary approach: it uses the concept of refinement indicators to select those degrees which have the greatest sensitivity to an objective function quantifying the mismatch between measured and simulated data. We apply this approach to the problem of data integration for petrophysical reservoir charaterization where geoscientists are currently working with multimillion cell geological models. Data integration may be performed by gradually deforming (by a linear combination) a set of these multimillion grid geostatistical realizations during the optimization process. The inversion parameters are then reduced to the number of coefficients of this linear combination. However, there is an infinity of geostatistical realizations to choose from which may not be efficient regarding operational constraints. Following our new approach, we are able through a single objective function evaluation to compute refinement indicators that indicate which realizations might improve the iterative geological model in a significant way. This computation is extremely fast as it implies a single gradient computation through the adjoint state approach and dot products. Using only the most sensitive realizations from a given set, we are able to resolve quicker the optimization problem case. We applied this methodology to the integration of interference test data into 3D geostatistical models.     

GMS5红外卫星云图参数化及在降水预测中的应用

从大气探测的基本原理出发,利用2001—2002年4—10月逐时的气象卫星红外云图和地面观测资料,对云图进行参数化估计,得到一些参数化估计结果,分析了不同视场条件下各类参数与降雨强度的关系,发现测站降水强度和其上空一定视场范围内的平均亮温、亮温方差、等效云量、亮温面积指数(1级、5级、6级)关系较明显,并利用最优子集回归方法建立小区域降水强度的预测方程,检验结果表明:利用红外资料估算未来6h的降雨强度其平均准确率在80%以上,分析视场大小对预测效果影响不大。