热带风压场平衡特征及其对GRAPES系统中同化预报的影响研究Ⅰ:平衡特征分析

Tropical Balance Characteristics between Mass and Wind Fields and Their Impact on Analyses and Forecasts in GRAPES System Part Ⅰ: Balance Characteristics

本文基于GRAPES全球模式的短期预报误差样本，利用赤道波动正规模态研究了热带风、压场平衡特征，并根据这些特征分析了线性平衡方程（LBE）在该区域应用时存在的问题。结果表明：（1）赤道波动能成功解释热带短期预报误差样本的大部分分量，对流层中层为60%～80%，对流层顶和平流层低层为80%以上。（2）在可解释的误差方差中，赤道罗斯贝波（ER）占比仅为30%～55%，其他赤道波动的作用不可忽视。（3）在ER模态基础上引入其他赤道波动会大幅削弱原有风、压场平衡约束，重力惯性波与Kelvin波的作用最为显著。此时，对流层中层位势高度h与u风、v风间的约束接近于零，而平流层低层h–u的平衡特征由Kelvin波主导。（4）LBE主要表达了 ER 模态下的风、压场平衡特征，与实际情形相比高估了热带风、压场的耦合程度，进一步的改进中需削弱这一虚假平衡，使得热带风、压场分析变得更加独立。

AbstractUsing equatorial wave modes and short-range forecast error samples of the global GRAPES （global/regional assimilation and prediction system） model, this study investigated the balance characteristics of mass and wind fields in the tropics and analyzed the problem of imposition of the Linear Balance Equation （LBE） in the tropics based on such characteristics. The results show the following： （1） Equatorial wave modes can explain a substantial fraction of the tropical forecast error variance, which ranges from 60%–80% in the mid-troposphere to more than 80% in the tropopause and lower stratosphere. （2） Only 30%–55% of the explained variance can be represented by the Equatorial Rossby wave （ER） modes, suggesting that contributions of other equatorial waves should not be ignored. （3） Based on the ER modes, introducing other equatorial waves, especially inertial-gravity and Kelvin wave modes, greatly reduces the coupling between the mass and wind. In this situation, the correlation between geopotential height and wind is close to zero in the mid-troposphere, and the correlation between geopotential height and zonal wind in the lower stratosphere is dominated by Kelvin wave modes. （4） The coupling between mass and wind was overestimated when using LBE in the tropics, because LBE mainly expresses the balance characteristics of the ER modes. Therefore, when LBE is used as the balance constraint, extra steps should be taken to reduce the fallacious correlation and allow for a decoupling of mass and wind in the tropics.