Strategies for Coupling Global and Limited-Area Ensemble Kalman Filter Assimilation
Strategies for Coupling Global and Limited-Area Ensemble Kalman Filter Assimilation
Files
Publication or External Link
Date
2011
Authors
Merkova, Dagmar
Advisor
Szunyogh, Istvan
Citation
DRUM DOI
Abstract
This thesis compares the forecast performance of four strategies for coupling
global and limited area data assimilation: three strategies propagate information
from the global to the limited area process, while the fourth strategy feeds back
information from the limited area to the global process. All four strategies are
formulated in the Local Ensemble Transform Kalman Filter (LETKF) framework.
Numerical experiments are carried out with the model component of the National
Centers for Environmental Prediction (NCEP) Global Forecast System (GFS)
and the NCEP Regional Spectral Model (RSM). The limited area domain is an extended
North-America region that includes part of the north-east Pacific. The GFS
is integrated at horizontal resolution T62 (about 150 km in the middle latitudes),
while the RSM is integrated at horizontal resolution 48 km. Experiments are carried
out both under the perfect model hypothesis and in a realistic setting. The coupling
strategies are evaluated by comparing their deterministic forecast performance at
12-hr and 48-hr lead times.
The results suggest that the limited area data assimilation system has the
potential to enhance the forecasts at 12-hr lead time in the limited area domain at
the synoptic and sub-synoptic scales (in the global wave number range of about 10
to 40). There is a clear indication that between the forecast performance of the
different coupling strategies those that cycle the limited area assimilation process
produce the most accurate forecasts. In the realistic setting, at 12-hr forecast time
the limited area systems produce more modest improvements compared to the global
system than under the perfect model hypothesis, and at 48-hr forecast time the
global forecasts are more accurate than the limited area forecasts.