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Abstract
In the Ensemble Kalman Filter (EnKF) data assimilation-prediction system, most of the computation
time is spent on the prediction runs of ensemble members. A limited or small ensemble size does reduce the
computational cost, but an excessively small ensemble size usually leads to filter divergence, especially when
there are model errors. In order to improve the efficiency of the EnKF data assimilation-prediction system
and prevent it against filter divergence, a time-expanded sampling approach for EnKF based on the WRF
(Weather Research and Forecasting) model is used to assimilate simulated sounding data. The approach
samples a series of perturbed state vectors from Nb member prediction runs not only at the analysis time
(as the conventional approach does) but also at equally separated time levels (time interval is △t) before
and after the analysis time with M times. All the above sampled state vectors are used to construct the
ensemble and compute the background covariance for the analysis, so the ensemble size is increased from
Nb to Nb+2M£Nb=(1+2M)×Nb) without increasing the number of prediction runs (it is still Nb). This
reduces the computational cost. A series of experiments are conducted to investigate the impact of △t (the
time interval of time-expanded sampling) and M (the maximum sampling times) on the analysis. The results
show that if △t and M are properly selected, the time-expanded sampling approach achieves the similar
effect to that from the conventional approach with an ensemble size of (1+2M)×Nb, but the number of
prediction runs is greatly reduced.
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Citation
YANG Yi, GONG Zhongqiang, WANG Jinyan, LIU Xinhua. 2011: Time-Expanded Sampling Approach for Ensemble Kalman Filter:Experiment Assimilation of Simulated Soundings. Journal of Meteorological Research, 25(5): 558-567.
YANG Yi, GONG Zhongqiang, WANG Jinyan, LIU Xinhua. 2011: Time-Expanded Sampling Approach for Ensemble Kalman Filter:Experiment Assimilation of Simulated Soundings. Journal of Meteorological Research, 25(5): 558-567.
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YANG Yi, GONG Zhongqiang, WANG Jinyan, LIU Xinhua. 2011: Time-Expanded Sampling Approach for Ensemble Kalman Filter:Experiment Assimilation of Simulated Soundings. Journal of Meteorological Research, 25(5): 558-567.
YANG Yi, GONG Zhongqiang, WANG Jinyan, LIU Xinhua. 2011: Time-Expanded Sampling Approach for Ensemble Kalman Filter:Experiment Assimilation of Simulated Soundings. Journal of Meteorological Research, 25(5): 558-567.
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