Mismatching Perturbations at the Lateral Boundaries in Limited-Area Ensemble Forecasting: A Case Study

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). In this paper, the author reports on a case study where mismatches between the analysis perturbations and the perturbations coming from the lateral boundaries lead 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, he tests 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 show that the new approach successfully removes the spurious perturbations in the surface pressure fields and also provides some benefits in the precipitation forecasts for the case studied. An examination of ensemble-derived forecast error covariances reveals 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 are discussed.