Data Assimilation Meetings at Reading

3 May 2012 DARC seminar Jean-Francois Caron (Met Office at Reading)
Mismatching perturbations at the lateral boundaries in limited-area ensemble forecasting (ppt)
An experimental convection-permitting ensemble prediction system (EPS) has recently been developed at the Met Office where the analysis uncertainty is estimated by means of an ensemble transform Kalman filter (ETKF). We will present a case study where gravity wave activity triggered by mismatches between the analysis perturbations and the perturbations coming from the lateral boundaries led to the generation of significant spurious perturbations in the ensemble forecasts of the surface pressure. To alleviate ensemble perturbation mismatches originating from the ensemble technique, we tested a so-called scale-selective ETKF where a revised transform matrix is applied only to the small-scale component of the high resolution forecasts while the large scale component of the analysis perturbations is taken from the driving EPS. Results showed that the new approach successfully removed the spurious perturbations in the surface pressure fields and also allowed some benefits in the precipitation forecasts for the case studied. An examination of ensemble-derived forecast error covariances revealed that ensemble perturbation mismatches at the lateral boundaries tend to decrease the degree of balance between the mass field and the rotational wind field and to produce more compact horizontal and vertical correlations. Finally, the limitations of the scale-selective approach and future directions will be discussed.

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