Abstract
This study applies a multigrid beta filter (MGBF) for covariance
localization in ensemble-variational (EnVar) data assimilation instead
of the conventional recursive filter (RF) to achieve faster computation
in a large number of processors. The parallelization efficiency of the
MGBF is higher than that of the RF because all-to-all communication to
change the computational region of each processor is not necessary.
However, the MGBF-based localization additionally requires horizontal
variable exchange between processors; its computational cost is
proportional to the number of grid points and to the ensemble size, and
is generally more expensive than the RF. In this study, we implement the
MGBF-based localization both for the single-scale localization and for
the scale-dependent localization in the regional atmospheric EnVar data
assimilation system. In addition, we clarify that applying a coarser
filter grid and omitting filtering except for the coarsest resolution
make the computation of the MGBF-based localization several times faster
than that of the RF-based one without significantly changing the EnVar
analysis.