Breakdown of hydrostatic balance at convective scales in the forecast errors in the Met Office Unified Model
Breakdown of hydrostatic balance at convective scales in the forecast errors in the Met Office Unified Model
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英国气象局统一模型中对流尺度的静水力平衡预报误差的分解
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发表时间：
2012
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通讯作者：
S. Ballard
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作者：
Sanita Vetra;M. Dixon;S. Migliorini;N. Nichols;S. Ballard
对于数值天气预报中的数据同化，需要初始预报误差协方差矩阵Pf。对于变分同化，规定一个精确的初始矩阵Pf是特别重要的，因为Pf在每个同化窗口的开始处要么是静态的（在3D-Var情况下），要么是常数（在4D-Var情况下）。在大尺度上，大气流动很好地近似于流体静力平衡，这种平衡在气象局等业务变分同化系统中使用的初始矩阵Pf中得到了很好的实施。然而，在对流尺度上，这种平衡不一定再保持下去。在这里，我们研究了流体静力平衡在高分辨率模式的垂直预报误差协方差中的有效程度，以确定是否需要在对流尺度数据同化中放松这种平衡约束。我们使用气象局的全球和区域的包围预测系统（MOGREPS）和1.5公里分辨率版本的统一模式的一个案例研究，其特征在于对流活动的存在。生成对流开始后三小时内有效的高分辨率预报集合。我们发现，在1.5公里的分辨率流体静力平衡不成立的对流区域的预报误差。这表明，在对流的存在下，流体静力平衡不应该强制执行的协方差矩阵变分资料同化在这个尺度上。结果表明，需要调查协方差模型，可能更适合对流尺度数据同化。最后，我们给出了一个衡量的平衡存在于预测扰动的水平尺度（从3-90公里）的函数，使用一组诊断。版权所有© 2012皇家气象学会和英国皇家气象局版权所有
For data assimilation in numerical weather prediction, the initial forecast‐error covariance matrix Pf is required. For variational assimilation it is particularly important to prescribe an accurate initial matrix Pf, since Pf is either static (in the 3D‐Var case) or constant at the beginning of each assimilation window (in the 4D‐Var case). At large scales the atmospheric flow is well approximated by hydrostatic balance and this balance is strongly enforced in the initial matrix Pf used in operational variational assimilation systems such as that of the Met Office. However, at convective scales this balance does not necessarily hold any more. Here we examine the extent to which hydrostatic balance is valid in the vertical forecast‐error covariances for high‐resolution models in order to determine whether there is a need to relax this balance constraint in convective‐scale data assimilation. We use the Met Office Global and Regional Ensemble Prediction System (MOGREPS) and a 1.5 km resolution version of the Unified Model for a case study characterized by the presence of convective activity. An ensemble of high‐resolution forecasts valid up to three hours after the onset of convection is produced. We show that at 1.5 km resolution hydrostatic balance does not hold for forecast errors in regions of convection. This indicates that in the presence of convection hydrostatic balance should not be enforced in the covariance matrix used for variational data assimilation at this scale. The results show the need to investigate covariance models that may be better suited for convective‐scale data assimilation. Finally, we give a measure of the balance present in the forecast perturbations as a function of the horizontal scale (from 3–90 km) using a set of diagnostics. Copyright © 2012 Royal Meteorological Society and British Crown Copyright, the Met Office