Filtered Ensemble-based Tangent Linear Model for the Operational EnVar
Data Assimilation System
Abstract
The ensemble-based tangent linear model (ETLM), which evolves the static
background error covariance (BEC) in time in the data assimilation
window, is beneficial for improving the analysis and avoids the need for
developing the tangent linear forecast model and its adjoint as in
four-dimensional variational data assimilation (4DVar). This study
proposes to apply the filtered ETLM (FETLM), which is composed of
ensemble perturbations at limited time slots and localized with a
quasi-Gaussian filter in the way that avoids calculation of the inverse
matrix. Tests with the Lorenz (1996) model showed that FETLM evolved the
static BEC as in 4DVar and improved the analysis. In experiments with
the FETLM implemented in the hourly-updated regional forecast system,
the Rapid Refresh Forecast System, the imbalance of the analysis was
mitigated due to the flow-dependent BEC. Moreover, the cross-variable
covariance in the FETLM made it possible to assimilate radar
reflectivity effectively even without hydrometeors as control variables.