TY  - JOUR
A1  - Amezcua, Javier
A1  - Ide, Kayo
A1  - Kalnay, Eugenia
A1  - Reich, Sebastian
T1  - Ensemble transform Kalman-Bucy filters
JF  - Quarterly journal of the Royal Meteorological Society
N2  - Two recent works have adapted the Kalman-Bucy filter into an ensemble setting. In the first formulation, the ensemble of perturbations is updated by the solution of an ordinary differential equation (ODE) in pseudo-time, while the mean is updated as in the standard Kalman filter. In the second formulation, the full ensemble is updated in the analysis step as the solution of single set of ODEs in pseudo-time. Neither requires matrix inversions except for the frequently diagonal observation error covariance.
 We analyse the behaviour of the ODEs involved in these formulations. We demonstrate that they stiffen for large magnitudes of the ratio of background error to observational error variance, and that using the integration scheme proposed in both formulations can lead to failure. A numerical integration scheme that is both stable and is not computationally expensive is proposed. We develop transform-based alternatives for these Bucy-type approaches so that the integrations are computed in ensemble space where the variables are weights (of dimension equal to the ensemble size) rather than model variables.
 Finally, the performance of our ensemble transform Kalman-Bucy implementations is evaluated using three models: the 3-variable Lorenz 1963 model, the 40-variable Lorenz 1996 model, and a medium complexity atmospheric general circulation model known as SPEEDY. The results from all three models are encouraging and warrant further exploration of these assimilation techniques.
KW  - Kalman-Bucy Filter
KW  - Ensemble Kalman Filter
KW  - stiff ODE
KW  - weight-based formulations
Y1  - 2014
U6  - https://doi.org/10.1002/qj.2186
SN  - 0035-9009
SN  - 1477-870X
VL  - 140
IS  - 680
SP  - 995
EP  - 1004
PB  - Wiley-Blackwell
CY  - Hoboken
ER  -