| GRAPES变分同化系统中动力平衡约束的统计求解 |
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| 引用本文： | 王瑞春,龚建东,张林.GRAPES变分同化系统中动力平衡约束的统计求解[J].应用气象学报,2012,23(2):129-138. |
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| 作者姓名： | 王瑞春 龚建东 张林 |
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| 作者单位： | 1.南京信息工程大学大气科学学院，南京 210044 |
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| 基金项目： | 资助项目：公益性行业(气象)科研专项（GYHY201106008） |
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| 摘 要： | 该文在GRAPES (Global/Regional Assimilation and PrEdiction System) 模式面三维变分 (3D_Var) 框架中引入了一种描述不同控制变量之间动力平衡约束的新方案。新方案采用统计得到的流函数和模式气压变量 ( π ) 之间的回归系数代替原方案中的线性平衡方程，来表达旋转风和质量场之间的平衡关系；采用流函数和势函数之间的回归系数，补充表达了原方案中所没有的旋转风和散度风之间的平衡关系。与原方案相比，新方案算法简单，避免了垂直方向的反复插值，减少了插值误差的引入。通过随机扰动试验和单点试验可以发现，在地转关系成立较好的区域，新方案中旋转风和质量场的耦合程度与原方案接近一致；而在地转关系不适用区域，新方案可以有效减小两者的耦合程度。此外，由于新方案中添加了旋转风和散度风之间的动力平衡约束，边界层的风场分析也更加接近大气真实状况。
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| 关 键 词： | GRAPES 三维变分同化 动力平衡约束 线性回归 |
| 收稿时间： | 2011-07-20 |
| 修稿时间： | 1/9/2012 12:00:00 AM |
Statistical Estimation of Dynamic Balance Constraints in GRAPES Variational Data Assimilation System |
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| Wang Ruichun,Gong Jiandong and Zhang Lin.Statistical Estimation of Dynamic Balance Constraints in GRAPES Variational Data Assimilation System[J].Quarterly Journal of Applied Meteorology,2012,23(2):129-138. |
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| Authors: | Wang Ruichun Gong Jiandong and Zhang Lin |
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| Affiliation: | 1.School of Atmospheric Sciences, Nanjing University of Information Science & Technology, Nanjing 2100442.National Meteorological Center, Beijing 100081 |
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| Abstract: | Dynamic balance constraints that govern the atmospheric circulation play very important roles in the analysis of atmospheric state.These constraints indicate how it might be possible to determine one variable from another.As a result,they could help to avoid noise caused by gravity waves,and enable maximum information to be extracted from the observations. The existing GRAPES three-dimensional variational data assimilation system,which is defined on sig- ma coordinates,uses linear balance equation to ensure that mass and wind analysis increments to be geo- strophically coupled.In this formulation,to deal with difficulties in solving the balance equation at sigma levels,analysis variables need to be interpolated to a series of auxiliary isobaric surface to calculate balanced components.A new formulation of estimating dynamic balance constraints is developed in the variation assimilation system.In the new scheme,regression coefficients between stream function and dimensionless pressure(Exner function),instead of the geostrophic balance equation in the original scheme,is used to describe the balance relationship between rotational wind and mass field.In addition,the balance relationship between rotational wind and divergent wind is similarly described by regression coefficients between stream function and velocity potential.Compared to the old scheme,the new formulation avoids repeated interpolations along the vertical direction,which would make the estimation simpler and more accurate. In the new formulation,the balanced coefficients are computed using NMC method,which is found to produce a useful approximation to the true balance constraints in atmosphere.Based on 24 h,48 h GRAPES forecast differences,linear regression is carried out for each level and for each latitude.By doing this an implied latitude-dependent structure in the dimensionless pressure is directly included in the analy- sis.Statistical results show that the explained variance of dimensionless pressure is primarily in the extrat- ropics with the variance best explained below 100 hPa in this new formulation.And the explained velocity potential ratio has a maximum in the middle and high latitudes near the surface.Results of randomization and single-observation experiments indicate that,in regions where geostrophic balance is appropriate,the new formulation behaves similarly to the original scheme.However,in regions where geostrophic balance is not appropriate,the new formulation could allow for a smooth decoupling of stream function and dimen- sionless pressure,while the original scheme can not.Such properties of the new formulation could help variational data assimilation system get more reasonable analysis results in tropics and tropopause.Moreo- ver,by adding the balance relationship between rotational wind and divergent wind,the new formulation could derive a more reasonable wind field in boundary layer. |
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| Keywords: | 3D_Var system dynamical balance constraint linear regression |
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