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Evaluation of the Historical Sampling Error for Global Models |
SHEN Si1,2, LIU Juan-Juan1, WANG Bin1,3 |
1State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
2College of Earth Science, University of Chinese Academy of Sciences, Beijing 100049, China
3Ministry of Education Key Laboratory for Earth System Modeling, Center of Earth System Science (CESS), Tsinghua University, Beijing 100084, China |
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Abstract Various ensemble-based schemes are employed in data assimilation because they can use the ensemble to estimate the flow-dependent background error covariance. The most common way to generate the real-time ensemble is to use an ensemble forecast; however, this is very time-consuming. The historical sampling approach is an alternative way to generate the ensemble, by picking some snapshots from historical forecast series. With this approach, many ensemble-based assimilation schemes can be used in a deterministic forecast environment. Furthermore, considering the time that it saves, the method has the potential for operational application. However, the historical sampling approach carries with it a special kind of sampling error because, in a historical forecast, the way to integrate the ensemble members is different from the way to integrate the initial conditions at the analysis time (i.e., forcing and lateral boundary condition differences, and ‘warm start’ or ‘cold start’ differences). This study analyzes the results of an experiment with the Global Regional Assimilation Prediction System-Global Forecast System (GRAPES-GFS), to evaluate how the different integration configurations influence the historical sampling error for global models. The results show that the sampling error is dominated by diurnal cycle patterns as a result of the radiance forcing difference. Although the RMSEs of the sampling error are small, in view of the correlation coefficients of the perturbed ensemble, the sampling error for some variables on some levels (e.g., low-level temperature and humidity, stratospheric temperature and geopotential height and humidity), is non-negligible. The results suggest some caution must be applied, and advice taken, when using the historical sampling approach.
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Received: 04 January 2015
Revised: 07 April 2015
Accepted: 07 April 2015
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Corresponding Author:
LIU Juan-Juan
E-mail: ljjxgg@mail.iap.ac.cn
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