PDF《热带风压场平衡特征及其对 GRAPES 系统中同化预报》

39 卷Vol.

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1226 of LBE on each level, a vertical regression whose coefficients could vary at different latitudes and model levels was introduced. By analyzing and comparing the implied background error covariance of different schemes, the authors found that the new scheme could efficiently reduce the unreasonable correlations of control variables. Additionally, the forecast error variance of the balanced pressure and its ratio to the whole pressure were also both dramatically reduced in the tropics. Results of the single-observation experiments indicated that, the new scheme had little impacts on the mid- and high-latitude compared to the LBE scheme, but it did successfully decouple the mass/wind analyses in the tropics. Although equatorial wave modes were not explicitly considered in the new scheme, the structure of covariance was consist with the theory analysis results based on those modes in Part I. The results of cycle analysis and forecast experiments in one month showed that, the new scheme could bring slightly positive results in the extratropics and significantly improve the wind analysis and forecast accuracy in the tropics, the abnormally large tropical wind errors in the LBE scheme were dramatically suppressed. Keywords Variational data assimilation, Balance constraint, Linear balance equation, Linear regression, GRAPES

1 引言 研究 I 的结果表明,GRAPES(global/regional assimilation and prediction system)全球变分资料 同化系统(以下简称 GRAPES-VAR)采用的风、压 场平衡约束――线性平衡方程(Linear Balance Equation,以下简称 LBE)在热带地区并不适用, 会造成虚假平衡(王瑞春等, 2015) .LBE 主要表达 了罗斯贝波模态下的风、压场配置,但该模态在热 带区域的短期预报误差中并不占主导,补充考虑其 他赤道波动的影响就需要削弱 LBE 对热带风、 压场 的约束程度,使得两者的分析变得更加独立.作为 研究的第 II 部分,本文致力于在 GRAPES-VAR 中 引入更加合理的平衡约束方案,对热带虚假平衡问 题做针对性修正,以提高该区域的风场分析效果. 热带地区缺少类似中、高纬准地转这样的主导 机制(Holton, 1992) ,基于自身动力学特征构造平 衡约束的研究进展十分缓慢.目前,这方面研究的 一个主要思路是采用相互正交的赤道波动作为特 征分量构造预报(背景)误差协方差矩阵(B 矩阵) (Daley, 1993;

?agar et al., 2004;

K?rnich and K?llén, 2008) ,也即与研究 I 中构造风、压场协相关的方案 类似.然而,由于在波动权重的确定、误差垂直结 构的设定以及如何与中、高纬衔接等问题上仍不十 分清晰,这些研究仅局限于在正压浅水模型中做一 些理想试验,距离业务应用有很大距离.另一个努 力方向是采用比 LBE 更加复杂的平衡方程, 例如非 线性平衡方程.由于考虑了流场曲率的作用,非线 性平衡方程在处理强旋转系统(例如台风)时要更 加精确(Fisher, 2003;

庄照荣等, 2006;

万齐林和薛 纪善, 2007) ,但由于其仍是基于中、高纬大气运动 的尺度特征简化得到,因而在热带也是不适用的 (Daley, 1991;

Barker et al., 2004) . 在业务同化系统中,为减小热带风、压场的平 衡约束,一个可行方法是采用统计方案.该方案采 用线性回归直接统计不同变量间气候态的平衡约 束,其引入的最初目标是解决 Lorenz 垂直离散方案 下求解温度的欠定问题(Parrish et al., 1997) .Derber and Bouttier(1999)将该方法应用于欧洲中期数值 预报中心(ECMWF)变分同化系统时发现,风、压 场分析在中、 高纬与 LBE 相近, 而热带地区两者分 析接近独立.由于构造和计算简单,统计方案在业 务和研究中得以广泛应用(Berre, 2000;