Pau Escribà (Agencia Estatal de Meteorología, AEMET): "A comparison of the schemes LETKF and 4DVAR in the IFS-ECMWF model assimilating only Surface Pressure"

12:00 Meeting Room, IC3
C/ Doctor Trueta, 203
08005 Barcelona

ECMWF has recently developed an Ensemble Kalman Filter (EnKF) for experimental purposes. This enables to do a fairly clean comparison of 4DVAR and EnKF within IFS model. One of the configurations implemented is the Local Ensemble Transform Kalman Filter (LETKF). Several methods for additive and multiplicative inflation and localization of observations have been implemented.

In this seminar five of the analysis systems available at ECMWF are compared for assimilations that only use surface pressure observations. This has relevance for the on-going ECMWF 1900-2010 reanalysis among other activities. It also provides a more stringent test of the quality of background error estimates. The five methods are LETKF, 4DVAR with static background error using 12-hour and 24-hour assimilation windows, hybrid 4DVAR-EDA, and hybrid 4DVAR-LETKF. The comparison looks at the quality of the five analysis states, taking as the truth the operational 4DVAR analysis that runs at ECMWF with the full observing system.

Initial results for a Kalman Smoother LETKF extension will be presented. This technique recomputes the analysis at one assimilatinon cycle by using the weights computed in the next cycle. Using the Kalman Smoother LETKF assimilation method allows the analysis to benefit from observations in the subsequent 6 hours, in a way so the assumption about linearity is respected.