engleski

New initial condition perturbations for a convection-permitting EPS

One of the main challenges presented by a Limited Area Model Ensemble Prediction System (LAMEPS) concerns the limited capacity for its initial condition perturbations to correctly represent large-scale flow uncertainties. In addition, a mismatch between LAMEPS (initial condition) and host EPS lateral boundary perturbations can form spurious gravity waves at the boundaries. In the present work, an ensemble Jk blending method is proposed for improving representation of large-scale uncertainties and for addressing consistent initial conditions and lateral boundary perturbations. Our approach involves employing Jk blending within a framework of 3D-Var ensemble data assimilation (EDA). In such a system, small-scale perturbations are generated from 3D-Var EDA, while large-scale perturbations are generated from the host ensemble via Jk blending. The ensemble Jk method is implemented to the C-LAEF (Convection-permitting Limited-Area Ensemble Forecasting) system and is compared to the standard perturbed-observation EDA approach. The comparison shows that the ensemble Jk method gives a more skillful and reliable EPS, especially for upper-air variables, surface pressure and precipitation. Also, perturbation mismatch at the boundaries is reduced.

Blending, ensemble data assimilation, initial condition perturbations, Jk, limited-area ensemble

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