Dynamical Properties of MOS Forecasts: Analysis of the ECMWF Operational Forecasting System

Abstract

The dynamical properties of ECMWF operational forecasts corrected by a (linear) model output statistics (MOS) technique are investigated, in light of the analysis performed in the context of low-order chaotic systems. Based on the latter work, the respective roles of the initial condition and model errors on the forecasts can be disentangled. For the temperature forecasted by the ECMWF model over Belgium, it is found that (i) the error amplification arising from the presence of uncertainties in the initial conditions dominates the error dynamics of the ?free? atmosphere and (ii) the temperature at 2 m can be partly corrected by the use of the (linear) MOS technique (as expected from earlier works), suggesting that model errors and systematic initial condition biases dominate at the surface. In the latter case, the respective amplitudes of the model errors and systematic initial condition biases corrected by MOS depend on the location of the synoptic station. In addition, for a two-observables MOS scheme, the best second predictor is the temperature predicted at 850 hPa in the central part of the country, while for the coastal zone, it is the sensible heat flux entering in the evolution of the surface temperature. These differences are associated with a dominant problem of vertical temperature interpolation in the central and east parts of the country and a difficulty in assessing correctly the surface heat fluxes on the coastal zone. Potential corrections of these problems using higher-resolution models are also discussed.