Abstract
This paper discusses the scale-dependent growth of the global forecast uncertainties simulated by the operational ensemble prediction system of the European Centre for Medium-Range Weather Forecasts. It is shown that the initial uncertainties are largest in the tropics and have biggest amplitudes at the large scales. The growth of forecast uncertainties (ensemble spread) takes place at all scales from the beginning of forecasts. The growth is nearly uniform in the zonal wavenumbers 1–5 and strongly scale-dependent in the larger wavenumbers. Moreover, the growth from initial uncertainties at large scales appears dominant over the impact of errors cascading up from small scales. A decomposition of the ensemble spread in components associated with the balanced and unbalanced dynamics shows that the initial uncertainties are primarily in the unbalanced modes, especially at the subsynoptic scales. The growth of uncertainties is found to be faster in the balanced than in the unbalanced modes and after 0.5–1 day of forecasts the balanced errors become dominant except at the subsynoptic scales.
Highlights
We assessed the scale-dependent growth of forecast uncertainties in a state-of-the-art global forecasting system, the operational European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble prediction system
Presented results complement previous studies of the forecast error growth that focused on the extra-tropical quasi-geostrophic dynamics and often considered the error-free large-scale initial state
The operational numerical weather prediction (NWP) systems are characterized by uncertainties in the initial state at all scales
Summary
In spite of the progress in global numerical weather prediction (NWP) (e.g. Magnusson and Källén, 2013) and the progress in the simulation of tropical variability (e.g. Bechtold et al, 2008; Vitart and Molteni, 2010), the tropics remain a region with the largest uncertainties in the initial state (analysis) for NWP (e.g. Park et al, 2008) as well as the largest forecast errors in the short range (e.g. Žagar et al, 2013). The large-scale errors in NWP models were previously presented by Dalcher and Kalnay (1987) who compared the ECMWF forecasts with their verifying analyses for various global wavenumbers and showed that the forecast errors grow from the start of the forecasts in all scales. Their results have been reaffirmed by the recent paper of Žagar et al (2015) who analyzed 7-day long ensemble forecasts from the operational ensemble prediction system of ECMWF (ENS).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
