Background-error covariances for a convective-scale data-assimilation system: AROME-France 3D-Var

AROME-France is a convective-scale numerical weather prediction system running operationally at Meteo-France since the end of 2008. It uses a 3D-Var assimilation scheme to determine its initial conditions. Climatological background-error covariances of such a system are calculated using differences between forecasts from an AROME ensemble assimilation. These statistics are compared with the lower-resolution ALADIN-France system ones: they provide 3D-Var analysis increments that are more intense and more localized, in accordance with the actual AROME model resolution. AROME ensemble-assimilation (ENS_DA) covariances have also been compared with covariances calculated with an AROME ensemble of forecasts run in spin-up mode(ENS_SU). On the one hand, ENS_SU appears to be a reasonable approximation of ENS_DA compared with ALADIN-France covariances, by representing a large part of the small-scale variance increase. On the other hand, ENS_DA allows for a fully cycled development of small-scale forecast perturbations, which leads to a further enhancement of small-scale covariances. This aspect is shown to be beneficial in terms of assimilation diagnostics and forecast performance and in a case study. Copyright (c) 2011 Royal Meteorological Society