Title: The impact of re-centring perturbations for ensemble forecasts

Author: S. T. K. Lang (ECMWF)
Massimo Bonavita (ECMWF)
Martin Leutbecher (ECMWF)

It is common practice in many weather forecasting centres to generate the initial conditions for their ensemble prediction system by adding a set of perturbations to a single analysis. At the European Centre for Medium-Range Weather Forecasts (ECMWF), initial conditions from an ensemble of data assimilations (EDA) are re-centred on the deterministic high-resolution analysis. In this study we assesses how the re-centring step affects the ensemble forecasts and what advantages are associated with starting directly from the perturbed EDA members. We find that the re-centring leads to an artificial increase of the climatological variance of the initial conditions for variables and spatial scales that are not well constrained in the analysis, increases the jumpiness of the ensemble forecasts and alters the precipitation frequency during the first 12 hours of the forecasts.

The benefit for the ECMWF ensemble of starting directly from EDA initial conditions is sensitive to the difference in analysis accuracy between the unperturbed EDA control and the high-resolution analysis, the number of EDA members and the EDA spread. At present, the forecast skill still benefits from centring the ensemble initial conditions on the high-resolution analysis but future changes to the ECMWF ensemble data assimilation system may change the balance.

GMAO Head: Steven Pawson Global Modeling and Assimilation Office NASA Goddard Space Flight Center

Curator: Nikki Privé Last Updated: Feb 9 2015